Pathophysiology

The prototypical member of the serpin family of protease inhibitors, α₁-AT binds with and promotes the degradation of serine proteases in the serum and tissues. The most important of these serine proteases is neutrophil elastase, which is inhibited by α₁-AT through formation of a tight 1 : 1 α₁-AT-to-elastase complex. Loss of serum α₁-AT activity, as occurs in the most common form of α₁-AT deficiency, leads to uninhibited neutrophil elastase activity and is one of the primary mechanism for the premature development of pulmonary emphysema in affected patients.⁸ Another proposed mechanism derives from the observation that α₁-ATZ polymers are proinflammatory toward neutrophils and are found in lung lavage fluid of patients with the protease inhibitor (Pi) ZZ phenotype of α₁-AT deficiency (see later) and emphysema; moreover, instillation of α₁-ATZ polymers in murine lung leads to an influx of neutrophils to the lung.^8,9

Allelic α₁-AT mutant variants produce Pi gene products that can be distinguished from the normal product by electrophoretic methods; the normal allelic representation is designated PiM. The PiZ variant produces a mutant α₁-ATZ protein. Homozygosity at the PiZ allele is the most common and classic pathologic form of α₁-AT deficiency and is capable of leading to liver and lung disease. The α₁-ATZ molecule represents a single amino acid replacement of glutamine with a lysine residue as a result of a mutation at position 342 of the α₁-AT (SERPINA1) gene. More than 100 naturally occurring variants of α₁-AT have been described. Although most of these variants are either of no clinical significance or are extremely rare, two additional variants, PiSiiyama and PiMmalton, have been reported to be associated with liver injury and even cirrhosis.^8,10–12

α₁-AT is produced almost exclusively in the rough endoplasmic reticulum (ER) of hepatocytes and is subsequently targeted to the secretory pathway via the Golgi apparatus. Structural misfolding and polymerization of the mutant α₁-ATZ protein causes its aberrant retention in the ER, failure of progression through the secretory pathway, and diminished intracellular degradation. In persons with the phenotype PiZZ, serum α₁-AT activity levels are reduced to less than 15% of normal; this loss of function accounts for the development of pulmonary disease. Studies suggest that the rate of intracellular degradation may itself be genetically determined and may influence the expression of disease; α₁-ATZ appears to be degraded more slowly in the ER of PiZZ patients who are susceptible to liver disease than in PiZZ patients who are not susceptible to liver disease.¹³

The exact mechanism for α₁-ATZ-induced liver injury is not known. Studies of transgenic mice that express the human ATZ gene suggest a gain-of-function mechanism by which retention in the ER and accumulation in hepatocytes of mutant α₁-ATZ is responsible for hepatotoxicity.¹⁴ Multiple intracellular signaling pathways, including caspase activation, ER stress responses, and the autophagic response, are activated by the retention of malformed proteins in the ER.¹⁵ Autophagy is an intracellular degradative pathway that targets proteins and organelles for destruction during development as well as at times of stress or nutrient deprivation. One hypothesis under investigation is that increased stimulation of autophagy by accumulation of α₁-ATZ protein leads to ongoing mitochondrial injury, cellular apoptosis, and a chronic cycle of hepatocellular death and regeneration that may, over time, lead to liver injury and cirrhosis.^7,16 Other genetic and environmental modifiers of the ER “quality control” process for handling mutant α₁-ATZ protein are undoubtedly involved and account for the wide variation in clinical phenotype observed in the hepatic presentation of PiZZ patients.⁷

Clinical Features

Although the prevalence of the classic α₁-AT deficiency allele, PiZ, is highest in populations derived from northern European ancestry, many racial subgroups are affected worldwide, and millions of persons have combinations of deficiency alleles (i.e., PiSS, PiSZ, or PiZZ).^8,17 In the United States, the overall prevalence of deficiency allele combinations is approximately 1 in 490 (i.e., 1 in 1058 for PiSS, 1 in 1124 for PiSZ, and 1 in 4775 for PiZZ).¹⁸ Mounting evidence suggests that heterozygous α₁-AT deficiency states can contribute to the development of cirrhosis and chronic liver failure in adults through mechanisms similar to those encountered with the PiZZ phenotype.^10–1219 In addition, the heterozygous α₁-AT deficiency state may contribute to worsening of chronic liver disease caused by hepatitis C viral infection or nonalcoholic fatty liver disease in adults, as well as cholestatic liver diseases in children.^7,19–22

In the most unbiased study to date, reported by Sveger,²³ on the epidemiology of liver disease in patients with α₁-AT deficiency, 200,000 Swedish infants were screened for α₁-AT deficiency, 184 infants were found to have abnormal allelic forms of α₁-AT (127 PiZZ, 2 PiZnull, 54 PiSZ, and 1 PiSnull), and 6 (5 PiZZ and 1 PiSZ) died in early childhood, although only 2 from cirrhosis.²³ Although about 10% of newborns with α₁-AT deficiency (PiZZ) present with cholestasis and as many as 50% continue to have elevated serum aminotransferase levels at age three months, most are clinically asymptomatic.^23,24 Liver disease does not develop in patients with null α₁-AT phenotypes, whereas early-onset emphysema develops in all of them.²⁵ Therefore, the prognosis for patients with liver disease manifesting in infancy as a result of α₁-AT deficiency (PiZZ) is highly variable. Even those children in whom cirrhosis develops can have a highly variable progression to end-stage liver disease (ESLD), which infrequently leads to liver transplantation.²⁶ Moreover, siblings with PiZZ have a variable degree of liver involvement; in a study reported by Hinds and colleagues, five of seven children with PiZZ requiring liver transplantation had siblings with PiZZ who had no persistent liver involvement.²⁷ This finding suggests that environmental or additional genetic factors must be involved in determining the severity of liver disease in α₁-AT deficiency; this area is under active scientific investigation.²⁸

Of 150 patients with α₁-AT deficiency from Sveger’s original study²³ who subsequently underwent evaluation at age 16 and 18 years, none had clinical signs of liver disease. Elevated serum aminotransferase or gamma glutamyl transpeptidase (GGTP) levels were found in fewer than 20% of patients with a PiZZ phenotype and in fewer than 15% of those with a PiSZ phenotype.²⁴ By the third decade of life, analysis of this same cohort of affected persons showed that 6% of PiZZ and 9% of PiSZ patients had a marginal increase in serum alanine aminotransferase levels.²⁹

Even though liver disease is often (but not always) mild during infancy and childhood, patients with α₁-AT deficiency have an increased risk for development of cirrhosis during adulthood; 42% of all PiZZ patients have histologic evidence of cirrhosis at autopsy.^30,31 Moreover, homozygous α₁-AT deficiency raises the risk for development of hepatocellular carcinoma, especially in men older than 50 years.³¹ The diagnosis of α₁-AT deficiency should be considered in any patient presenting with noninfectious chronic hepatitis, hepatosplenomegaly, cirrhosis, portal hypertension, or hepatocellular carcinoma.

Histopathology

Histopathologic features of α₁-AT deficiency change as the affected patient ages. In infancy, liver biopsy specimens may show bile duct paucity, intracellular cholestasis with or without giant cell transformation, mild inflammatory changes, or steatosis, with few of the characteristic periodic acid–Schiff (PAS)-positive, diastase-resistant globules. These inclusions, which result from polymerized α₁-ATZ protein, are most prominent in periportal hepatocytes, and may also be seen in Kupffer cells. Immunohistochemistry with monoclonal antibody to α₁-ATZ can also be performed to verify the diagnosis. As the patient ages, these changes may resolve completely or progress to chronic hepatitis or cirrhosis.

Diagnosis

α₁-AT is considered a hepatic acute-phase reactant, and its release may be stimulated by stress, injury, pregnancy, or neoplasia. Because these factors can influence α₁-AT production, even in patients with the PiZZ phenotype, the diagnosis of α₁-AT deficiency should be based on phenotype analysis and not solely on the serum α₁-AT level.³² A liver biopsy specimen, although not universally recommended, should confirm the diagnosis. Commercial tests are available to detect the most common mutant alleles by polymerase chain reaction analysis of genomic DNA. In addition, a re-sequencing molecular array chip is available for rapid sequencing of the entire SERPINA1 gene to allow identification of rare mutant alleles.³³

Screening guidelines to diagnose α₁-AT deficiency in asymptomatic persons have been proposed in an effort to reduce the risk of emphysema by counseling patients to avoid smoking.³⁴ Adults with chronic lung disease and siblings of affected patients with lung or liver disease should be targeted for screening, and appropriate education and genetic counseling should be offered to patients with α₁-AT deficiency identified by screening.

Treatment

The initial treatment of α₁-AT deficiency is with symptomatic care. Breast-feeding until the end of the first year of life has been suggested to decrease the manifestations of cholestatic disease, as may the use of ursodeoxycholic acid.³⁵ The importance of providing fat-soluble vitamins, when indicated, adequate nutrition, and counseling to avoid smoking and second-hand smoke cannot be overemphasized. The role of neonatal screening for α₁-AT deficiency is still not settled, although the effect on smoking practices in patients diagnosed at an early age appears to be positive. If effective therapy for liver disease caused by α₁-AT deficiency becomes available, neonatal screening may become more useful for preventing the need for liver transplantation.

Although progression to ESLD is uncommon, α₁-AT deficiency is the most common metabolic liver disease for which liver transplantation is performed. Besides replacing the injured organ, transplantation corrects the metabolic defect, thereby preventing further progression of systemic disease. Between 1995 and 2004, 161 children and 406 adults underwent liver transplantation for ESLD associated with α₁-AT deficiency in the United States; of these, 4.4% of the children and fewer than 1% of the adults were African Americans. Five-year patient survival rates following liver transplantation for the pediatric and adult patients with α₁-AT deficiency were 83% and 90% respectively.³⁶ Liver transplantation with grafts from donors with unrecognized α₁-AT deficiency appears to have a comparable outcome to transplants using grafts without unrecognized liver disease.³⁷

Replacement therapy with purified α₁-AT is the only treatment option approved by the U.S. Food and Drug Administration (FDA) for lung disease associated with α₁-AT deficiency. Patients who received replacement therapy in several studies, including a small randomized placebo-controlled trial, have had a slower rate of decline in lung tissue and function compared with control groups, although clinical efficacy has yet to be conclusively demonstrated.³⁸ This therapy would not be expected to benefit α₁-AT deficiency–associated liver disease, which results from malprocessed α₁-ATZ, not loss of α₁-AT protein function.

Other mechanistically based treatment options aimed at influencing the stability or secretion rates of α₁-ATZ within the hepatocyte ER are being investigated. Chemical chaperones such as phenylbutyric acid (PBA) markedly increase secretion of α₁-ATZ in experimental in vitro and in vivo models of α₁-AT deficiency.³⁹ A small pilot study investigated the potential benefits of PBA in the treatment of children with α₁-AT–deficient liver disease. Unfortunately, no statistically significant increase in serum α₁-AT levels occurred in PBA-treated patients, many of whom experienced unacceptable side effects.⁴⁰ Another pharmacologic approach in the early stages of investigation involves the design of small molecules to inhibit α₁-ATZ protein polymerization and increase clearance of intracellular aggregates.⁴¹

α₁-AT deficiency is one of many diseases for which reconstitution of the normal genotype through gene therapy is being studied. Long-term expression of human α₁-AT in murine liver, at therapeutic levels, has been achieved after hydrodynamics-based intravenous injection of nonviral deoxyribonucleic acid (DNA) constructs.⁴² Also, delivery of a vector carrying a ribozyme designed to target human α₁-ATZ messenger ribonucleic acid (mRNA) in the portal vein of transgenic mice that carry the human PiZ allele led to a 50% reduction in α₁-ATZ mRNA levels. In a separate experiment, administration of a vector carrying the normal PiM allele, which is resistant to ribozyme cleavage, led to long-term expression of human α₁-AT protein in both liver and serum up to one year following injection.⁴³ These results are promising first steps toward the goal of achieving successful gene replacement therapy for α₁-AT deficiency.

Clinical Features

Most patients with GSD type I present in infancy with symptoms of metabolic derangement, such as lethargy, seizures, or coma as a result of profound hypoglycemia or metabolic acidosis, a protruding abdomen caused by hepatomegaly, muscular hypotonia, and delayed psychomotor development.⁴⁹

Physical signs invariably include hepatomegaly, usually with a normal-sized spleen. Patients in whom the disease is poorly controlled for a long time exhibit short stature and growth failure and may be prone to adiposity. Delayed bone age and reduced postpubertal bone mineral density are common.⁵⁰ Xanthomas can appear after puberty and localize to the elbows, knees, buttocks, or nasal septum, the last leading to epistaxis. Patients with GSD type I are susceptible to a wide spectrum of brain damage that may result in epilepsy, hearing loss, and abnormal neuroradiologic findings, most likely as a result of recurrent episodes of hypoglycemia.⁵¹

Other metabolic derangements can be seen. Lactic acid levels can reach four to eight times normal; the accompanying metabolic acidosis may manifest as muscle weakness, hyperventilation, malaise, headache, or recurrent fever. Serum adenosine triphosphate (ATP) and phosphorus levels are low, secondary to an increase in purine synthesis and the inability to release phosphorus from Glu-6-P. Hyperuricemia is common and may lead to gout, arthritis, or progressive nephropathy. Nephromegaly secondary to increased glycogen deposition is common, and with advancing age, progressive renal disease, hypertension, and renal failure requiring dialysis and transplantation may develop.⁴⁹ Because of hypoglycemia, patients have chronically high serum levels of glucagon with depressed levels of insulin. Hypertriglyceridemia and hypercholesterolemia are present in both GSD Ia and GSD Ib (but more prominently in GSD Ia) and may account for the greater frequency of xanthoma formation.⁵²

In addition to the features already noted, patients with GSD type Ib often have severe intermittent neutropenia and neutrophil dysfunction as well as high platelet counts. Crohn’s-like inflammatory bowel disease often occurs in patients with GSD type Ib at the time of severe neutropenia, and patients are prone to the development of severe bacterial infections, with abscess formation at numerous locations throughout the body.⁴⁷ No correlation has been found between the severity of bacterial infections or degree of neutropenia and the molecular defect in Glu-6-P transport activity in humans with GSD type Ib, suggesting that other as yet unknown factors, such as modifying genes, define the ultimate disease phenotype.⁵³

Hepatic Involvement

Hepatomegaly in GSD type I results from increased glycogen storage in the liver as well as a large degree of fatty infiltration; the latter likely develops because of a wide array of perturbations in lipid metabolism, including increased free fatty acid flux into the liver.⁵² Patients demonstrate mild elevations in serum aminotransferase levels but generally do not develop cirrhosis or liver failure.

Hepatic adenomas develop in 22% to 75% of patients, as early as three years of age but most commonly in the second decade of life, and tend to increase in both size and number as the patient ages (see Chapter 94).⁵⁴ Adenomas in rare instances can transform to hepatocellular carcinoma; unfortunately, serum α-fetoprotein and carcinoembryonic antigen levels and features of the lesions on hepatic imaging are not predictive of malignant transformation.^54–56 In the past, reports suggested that the risk of hepatic adenoma was increased in patients who were nonadherent with treatment of GSD type I, but more recent reports have found no differences in the risk of adenoma formation between adherent and nonadherent patients.⁵⁷ In some patients, hepatic adenomas have been demonstrated to regress and disappear after adequate nutritional therapy, but in general, the course is unpredictable, especially in nonadherent patients.^54,55 Because of the unreliability of radiographic imaging and serum marker levels in predicting malignant transformation in this patient population, the better approach—resection of an adenoma or liver transplantation—is uncertain.⁵⁸

Diagnosis

The hepatic glycogen content is elevated in patients with GSD type I, and the most accurate diagnostic measure is direct analysis of enzyme activity performed on fresh, rather than frozen, liver tissue. Analysis of fresh liver tissue is important to avoid disrupting microsomal Glu-6-Pase activity.⁵⁹ Fasting serum glucose and lactate levels, a positive glucagon response test result, and the response to fructose or galactose administration (patients with GSD type I do not show the expected rise in serum glucose concentration after administration of glucagon or either sugar) often provide supportive evidence but may not yield a definitive diagnosis. Intermittent severe neutropenia is noted in most patients with GSD type Ib.^47,53 DNA analysis-based approaches that integrate biochemical features and the presence or absence of persistent neutropenia have been proposed and may provide a diagnostic alternative to liver biopsy.⁶⁰

Treatment

Patients with undiagnosed or undertreated GSD type I are at increased risk of death, usually from hypoglycemic comas, seizures, metabolic acidosis, or, in those with GSD type Ib, sepsis from neutropenia.⁴⁹ Rarely, hepatocellular carcinoma is a cause of death. Management centers on preventing the acute metabolic derangements and potential long-term complications and enabling the patient to attain normal psychological development and a good quality of life.^59,61

Consensus guidelines for the management of GSD type I have been proposed.^59,61 Biomedical targets for good metabolic control include a preprandial blood glucose level higher than 63 to 77 mg/dL (3.5 to 4.3 mmol/L), urine lactate-to-creatinine ratio higher than 0.06, high-normal serum uric acid level, venous blood base excess higher than −5 mmol/L, bicarbonate level higher than 20 mmol/L, serum triglyceride level lower than 6 mmol/L, and body mass index (BMI) between 0 and 2 standard deviations from normal. In addition, for GSD type Ib, demonstrating a normal fecal α₁-AT level is desirable (see Chapter 28).^59,62 Because optimal glycemic control is not always possible and the risk of severe hypoglycemia is high if delivery of glucose is interrupted inadvertently, serum lactate levels should be kept at the high end of normal because lactate is an alternative fuel for the brain.

Nutritional supplementation has become the mainstay of therapy for GSD type I. Frequent, high-carbohydrate, daytime feedings, such as uncooked cornstarch, or continuous nighttime drip feedings, or both, allow the steady release of glucose and lead to improved metabolic control and normalized growth and development.^52,62 A biochemical target is to maintain the serum glucose level above 70 mg/dL (3.9 mmol/L). Uncooked cornstarch in a dose of 2 g/kg every six hours (6 to 8 mg of glucose/kg/minute) has been suggested; however, alternative regimens have been implemented successfully.

For infants, when the diagnosis of GSD type 1 is confirmed, a formula that does not contain fructose or galactose should be prescribed. Frequent daytime feedings and continuous nocturnal administration are required, with the rate of delivery needed to maintain euglycemia being ≈8 mg/kg/minute. Morning feedings should be given quickly after discontinuation of the nighttime drip to avoid hypoglycemia. As solids are introduced, high-carbohydrate foods should be emphasized. These patients require special attention during acute illnesses that may affect oral intake or metabolism because they can become hypoglycemic quickly.

Prophylaxis with antibiotics (e.g., trimethoprim-sulfamethoxazole) is recommended for patients with GSD type 1b and severe neutropenia or recurrent bacterial infections.⁶¹ Granulocyte colony stimulating factor (GCSF) has been used with success in patients with GSD type 1b to improve hematologic parameters and neutrophil function and reduce the morbidity associated with severe bacterial infections.⁶³ Splenomegaly may worsen with GCSF therapy, and bone marrow aspiration before and during GCSF therapy may be prudent, given rare occurrences of acute myelogenous leukemia (AML) in patients with GSD type 1b.⁶¹ Both GCSF and inflammatory bowel disease raise the risk of osteopenia, and monitoring of bone density is advised.

Adenoviral-mediated gene replacement therapy of recombinant Glu-6-Pase in a canine model of GSD type Ia deficiency, which has all of the major features of GSD type 1a in humans, has led to encouraging results and may be an option in humans in the future.⁶⁴ An alternative approach, human hepatocyte transplantation, has been performed in a single patient with GSD type 1a with near-resolution of hypoglycemic episodes; however, hypoglycemia subsequently recurred, likely because of the lack of ongoing immunosuppression to reduce the likelihood of recipient rejection of transplanted donor hepatocytes.⁶⁵ Hepatocyte transplantation, with the use of standard post-transplantation immunosuppression, has been performed successfully in an 18-year-old male adolescent with GSD type 1b, with euglycemia maintained up to 2 years post-transplantation.⁶⁶

Liver transplantation has corrected the metabolic error in patients with GSD type I and permitted catch-up growth, even in patients in the third decade of life.^67,68 Neutrophil counts and function, however, are only variably improved after liver transplantation in patients with GSD type 1b.⁶⁷

Clinical Features

Persons with GSD type III typically exhibit hypoglycemia, hepatomegaly, and growth failure. Liver enlargement results from increased glycogen deposition and not fatty infiltration. The liver may show fibrotic septa that rarely lead to frank cirrhosis and liver failure. Serum lactate and uric acid levels are normal, and aminotransferase levels are increased only moderately until advanced liver disease occurs. Hyperlipidemia may be present but is not as pronounced as in GSD type I. Patients have normal responses to fructose and galactose loading.

Patients with GSD type III may also display progressive muscle weakness, which worsens with activity, and muscle wasting. Nephromegaly is not seen, but cardiomegaly may be present. The diagnosis can be made by direct enzyme analysis of muscle or liver tissue or peripheral leukocytes; mutation analysis of the GDE gene will be increasingly important for diagnosis in the future.^45,70 Hepatic adenomas develop in approximately 25% of patients, and isolated reports of cirrhosis leading to hepatocellular carcinoma have been reported.^55,71

Treatment

A high-protein, low-carbohydrate diet has been suggested to normalize metabolic activity, ensure normal growth, normalize muscle function, and minimize hepatomegaly. This diet provides adequate substrates for gluconeogenesis while reducing the need for glycogen storage. Patients with refractory hypoglycemia or persistent hepatomegaly may require a nighttime continuous infusion or cornstarch therapy, as used for GSD type I.

Pathophysiology

The metabolic pathways of heme synthesis are essentially the same in the two tissues in which heme synthesis primarily occurs, the liver (15% to 20%) and the bone marrow (75% to 80%), although synthetic control may be different in the two tissues. The rate-limiting step in hepatic heme synthesis begins with the conversion of glycine and succinyl coenzyme A (CoA) to 5-aminolevulinic acid (ALA) by the action of ALA synthase (Fig. 76-4). ALA synthase activity is decreased by the end-product of the pathway, heme, and is increased by substances that induce the hepatic cytochrome P450 pathway. Six additional enzymatic steps convert ALA to protoporphyrin IX (see Fig. 76-4). In the final step of the pathway, protoporphyrin IX is coupled to ferrous iron by ferrochelatase to create heme. Enzyme deficiencies arising from any of these eight steps of the heme synthetic pathway lead to the clinically apparent diseases known as the porphyrias.

Figure 76-4. Pathway of heme synthesis. The location of enzymatic deficiency in the various forms of porphyria is noted. On the left, enzymes are shown in italics. On the right, abbreviations for cutaneous porphyrias are shown in italics, and those for acute porphyrias are shown in roman typeface. ADD, 5-aminolevulinic acid (ALA) dehydratase deficiency; AIP, acute intermittent porphyria; CEP, congenital erythropoietic porphyria; EPP, erythropoietic protoporphyria; HCP, hereditary coproporphyria; HEP, hepatoerythropoietic porphyria; PBG, porphobilinogen; PCT, porphyria cutanea tarda; VP, variegate porphyria.

Porphyrias are commonly classified according to clinical features into two main groups, acute porphyrias, which are characterized by dramatic and potentially life-threatening neurologic symptoms, and cutaneous porphyrias, which typically cause few or no neurologic symptoms but instead give rise to a variety of severe skin lesions (Table 76-3). In five of the porphyrias, the liver is the major site of expression; in two others, both the liver and bone marrow are involved; and in one, only the bone marrow contributes.

Acute Porphyrias

The symptoms and signs of the acute neurovisceral attacks that occur in the four acute porphyrias vary considerably. Abdominal pain is present in more than 90% of patients, followed in frequency by tachycardia and dark urine in about 80% of patients. Neuropsychiatric features include hysteria, depression, psychosis, confusion, hallucinations, seizures, and coma, although little evidence suggests that chronic psychiatric illness occurs. Other features are constipation, extremity pain, paresthesias, nausea, vomiting, urinary retention, hypertension, peripheral sensory deficits (often in a “bathing trunk” distribution), and weakness leading to ascending paralysis or quadriplegia. These neurologic attacks appear to be related to the overproduction of ALA and porphobilinogen (PBG), which leads to higher serum and tissue levels of these neurotoxic products.⁹²

Acute episodes are about five times as common in women as in men and may be precipitated by many factors, most commonly drugs, alcohol ingestion, and smoking. Steroids, sex hormones, and medications that stimulate the hepatic cytochrome P450 system, perhaps by increasing the requirements for heme production, are often identified as precipitants.⁹³ Other inciting factors are fasting, infections, and pregnancy; some women report greater problems during the luteal phase of their menstrual cycles.^92,94 The disease is clinically latent in 65% to 80% of patients.

ALA dehydratase deficiency is a rare syndrome with autosomal recessive transmission in which the enzyme activity is less than 3%. The enzyme activity is 50% of normal in carriers, who are asymptomatic. Affected patients have severe, recurrent neurologic attacks that may be life threatening. They excrete large amounts of ALA in their urine. Liver transplantation was reported to result in complete resolution of symptoms in one patient with ALA dehydratase deficiency.⁹⁵

The three remaining acute porphyrias—acute intermittent porphyria (AIP), hereditary coproporphyria (HCP), and variegate porphyria (VP)—result from partial deficiency of the enzymes PBG deaminase, coproporphyrinogen oxidase, and protoporphyrinogen oxidase, respectively. All three disorders are inherited in an autosomal dominant fashion with variable expression. AIP is the most common of the three conditions, occurring in 5 to 10 per 100,000 people, and manifests primarily as derangements in the autonomic nervous system or as a psychiatric disorder.⁹⁶ VP is more common in South Africa than elsewhere. Although HCP and VP give rise to neurologic symptoms similar to those of AIP, cutaneous lesions also occur in HCP and predominate in VP.⁹⁷

Cutaneous Porphyrias

The cutaneous porphyrias differ from the acute porphyrias in that affected patients exhibit few or no neurologic symptoms. In these illnesses, excess porphyrins or porphyrinogens are deposited in the upper dermal capillary walls, where these photoreactive compounds cause tissue damage that manifests as cutaneous vesicles and bullae in areas exposed to light or excessive mechanical manipulation. Scarring, infection, pigment changes, and hypertrichosis can follow and even lead to severe mutilation.⁹⁸

Porphyria cutanea tarda (PCT), the most common of the porphyrias, typically involves a 50% reduction in activity of the enzyme uroporphyrinogen decarboxylase. Patients usually present after 20 years of age. Two types of PCT are recognized. Type I PCT affects 80% of patients and is a sporadic (acquired) form with the enzyme deficiency restricted to the liver. Type II, which affects the other 20% of patients, is familial and inherited in an autosomal dominant fashion with incomplete penetrance; the enzyme deficiency occurs in all tissues.⁹⁹ Symptoms develop in fewer than 10% of patients with type II PCT. Type I PCT is associated strongly with high alcohol intake, estrogen therapy, and systemic illnesses, including systemic lupus erythematosus, diabetes mellitus, chronic kidney disease, and the acquired immunodeficiency syndrome. For unclear reasons, concomitant hepatitis C infection is strongly associated with expression of PCT (see Chapter 79).^100,101 The frequency of mutations of the HFE gene, which causes hereditary hemochromatosis, is increased in patients with types I and II PCT, and these mutations are thus susceptibility factors for clinical expression of the PCT phenotype (see Chapter 74).^100,102,103 This association is consistent with pathologic findings in liver biopsy specimens from patients with PCT, of whom 80% have siderosis, 15% have cirrhosis, and most have evidence of iron overload. A patient in whom hepatocellular carcinoma developed has been reported to have unsuspected underlying PCT and hereditary hemochromatosis with bridging fibrosis.^104,105 Patients usually do not show signs of overt clinical liver disease, apart from elevated serum aminotransferase levels.

Hepatoerythropoietic porphyria (HEP) is a rare form of porphyria with a pathogenesis similar to that of PCT. HEP results from homozygous uroporphyrinogen decarboxylase deficiency, yielding less than 10% of normal enzyme activity. The cutaneous lesions, which resemble those of PCT, are typically severe and mutilating. The disease usually manifests in the first year of life, and as the patient ages, the dermatologic manifestations may subside, but liver disease, characterized by a nonspecific hepatitis, worsens. Mutation analysis of the uroporphyrinogen decarboxylase gene has revealed numerous mutations that are usually unique to an individual family; no clear genotype-phenotype correlations have been observed.⁹⁹

Congenital erythropoietic porphyria (CEP) is a rare form of porphyria with autosomal recessive transmission that is caused by deficiency of uroporphyrinogen III cosynthase, which mainly affects erythropoietic tissue. Affected patients typically present in the first year of life with blisters and disfiguring skin lesions in exposed areas. Infants may present with pink urine and photosensitivity. As patients age, erythrodontia, a pathognomonic red or brownish discoloration of the teeth, is commonly seen. CEP can be distinguished clinically from HEP by the presence in some cases of a Coombs-negative hemolytic anemia, which can be quite severe. Splenomegaly is common.

Erythropoietic protoporphyria (EPP) is caused by partial deficiency of the enzyme ferrochelatase (FECH), the final step in the heme synthetic pathway. EPP is the second most common type of porphyria and is inherited in an autosomal dominant manner with variable penetrance. Patients with EPP and asymptomatic carriers exhibit an FECH enzyme activity of 30% to 40% and 50%, respectively, even though both groups inherit a defective FECH allele. The mechanism for the variable clinical expression is explained by co-inheritance of a “low-expressing” normal FECH allele in symptomatic patients with EPP and a normal FECH allele in asymptomatic patients.^106,107

Although the bone marrow is the predominant source of excess protoporphyrin, with a variable contribution from the liver and other tissues, the skin is the primary site of deposition of this phototoxic compound in patients with EPP. Therefore, the principal clinical manifestation is exquisite photosensitivity, which may present during infancy and can lead to a wide spectrum of symptoms (e.g., itching, burning, or pain) and to scars and lichenification of the skin. Vesicles are rare. Patients with EPP may show a mild hypochromic, microcytic anemia.

Clinical liver disease, which develops in 5% to 10% of patients with EPP, results from progressive hepatic accumulation of protoporphyrin.¹⁰⁸ Liver disease typically occurs after age 30 but has been described in children. The liver appears black and nodular, with hepatocellular necrosis, portal inflammation, cholestasis, and extensive deposits of dark brown pigment in hepatocytes, Kupffer cells, and biliary structures; birefringence of pigment deposits is seen on polarization microscopy.¹⁰⁹ Of 57 patients with EPP followed for more than 20 years in one study,¹¹⁰ 50% had normal serum aminotransferase levels and liver histologic findings. Of the remaining patients, cirrhosis occurred in seven and liver failure developed in two. Liver disease is not believed to be secondary to alcohol consumption, viral infections, or external toxins, although these insults can worsen liver function.¹¹¹ Genetic heterogeneity in the FECH gene has been noted in multiple studies and is seen in patients who need liver transplantation.¹¹²

Diagnosis

The approach to the diagnosis of the porphyrias is summarized in Table 76-3. Clinical features alone are usually not specific enough to confirm a diagnosis or distinguish among the various forms of porphyrias, and biochemical test results must be interpreted correctly for accurate diagnosis and management. The diagnosis of porphyria should be considered in patients with recurrent bouts of severe abdominal pain, dark urine, constipation, and neuropsychiatric disturbances or in patients with typical dermatologic findings. To differentiate among the different porphyrias, urine and stool samples should be obtained for porphyrin studies and a urine specimen collected for quantitative ALA and PBG determinations.

In AIP, excretion of PGB and ALA (PGB more than ALA) in dark urine is common during porphyric attacks, but the levels may be normal during asymptomatic periods and in prepubertal patients.¹¹³ Patients with HCP and VP excrete high levels of ALA and PBG in the urine; in contrast to those with AIP, these patients excrete more ALA than PGB. A rapid “spot” urine test to detect urinary PGB is recommended to diagnose the acute porphyrias, except for the rare patient with ALA dehydratase deficiency.⁹² Fecal coproporphyrins are increased in both HCP and VP, whereas only in VP is the amount of fecal protoporphyrin also increased.

Important advances have been made in the identification of a large number of gene mutations in several of the acute porphyrias, including AIP, VP, and HCP. Given the high degree of genetic heterogeneity, the lack of clear genotype-phenotype correlations, and the failure to find mutations in 5% to 10% of families with current techniques, however, genetic testing is not recommended as a general screening tool. If a mutation that causes porphyria has been identified in a particular subject, however, screening of asymptomatic family members has a sensitivity and specificity of nearly 100% for that family and may be helpful, together with appropriate genetic counseling. Enzyme activity measurement can likewise help to confirm the diagnosis, although to a lesser degree than genetic testing given the variable tissue expression of many of the heme synthetic enzymes.

Hepatic Involvement

Hepatic involvement in porphyria is variable; in general, patients with acute porphyria may have elevations of serum aminotransferase and bile acid levels, with further increases during acute episodes. Liver biopsy specimens may show steatosis and iron deposition. Although these changes are minor, patients with acute porphyria are at increased risk for development of hepatocellular carcinoma.¹¹⁴

PCT and HEP are more commonly associated with hepatic complications, including liver enlargement with fatty infiltration, inflammation, and granulomatous changes. Siderosis and fibrosis may lead to cirrhosis and liver failure. The risk of hepatocellular carcinoma is increased only slightly in patients with these disorders.¹⁰⁵ The patterns of liver injury in CEP are similar to those in PCT and HEP.¹¹⁵

Treatment

The overall survival of patients with acute porphyria is good. Treatment is based on avoidance of drugs and other precipitating factors.⁹³ Generous fluid and glucose administration is recommended during acute attacks and can elicit the “glucose effect” that diminishes ALA synthase activity. Intravenous administration of hematin, a congener of heme, has been shown to decrease the drive for heme synthesis and its excessive by-products. Hematin can also have a dramatic effect on neurologic symptoms, especially if given early in an attack, with clinical improvement often occurring within one to two days.⁹² Women in whom symptoms are affected by the phases of their menstrual cycle can show improvement while taking oral contraceptives. Liver transplantation has been attempted for several of the porphyrias, with mixed results.^92,116,117 Because of the increased frequency of hepatocellular carcinoma, patients with AIP should undergo standard surveillance for this tumor (see Chapter 94).⁹²

Because of the wavelengths of light absorbed by the porphyrins, patients affected by porphyria are at risk from exposure to sunlight as well as to household and fluorescent lights. Patients must use special sunscreen lotions that block rays in the 400- to 410-nm range. Skin trauma should be minimized as much as possible; early treatment of skin infections can decrease scarring. Special screens may be especially useful for protection against indoor lighting. Some patients have incurred severe or lethal internal burns during surgery, including liver transplantation; thus, appropriate precautions must be taken.⁹⁸

Treatment of PCT initially consists of removal of any offending agent. Historically, treatment has included phlebotomy to decrease iron overload and hepatic siderosis. This approach may provide relief of cutaneous symptoms in four to six months. Chloroquine complexes with uroporphyrin and facilitates its excretion, but caution must be used during therapy with chloroquine because the drug is potentially hepatotoxic.⁹⁸ The efficacy of chloroquine has been variable in patients with PCT who are homozygous for mutations in the HFE gene; for these patients, phlebotomy should be first-line therapy.^118,119 Treatment strategies for HEP are similar to, but have not been as successful as, those for PCT.

Blood transfusions and administration of hematin, charcoal, and cholestyramine all have led to clinical improvement in patients with EPP, but long-term resolution has not been demonstrated. One report of a 63-year-old man with severe cholestatic liver disease caused by EPP showed complete biochemical and histologic resolution of cholestasis with the use of blood transfusions, hematin, cholestyramine, and ursodeoxycholic acid. Subsequently, the patient underwent bone marrow transplantation, with apparently complete resolution of EPP.¹²⁰ Liver transplantation has been accomplished in patients with EPP and ESLD, with mixed results; the erythropoietic defect persists.^117,121 A retrospective review of all 20 patients with EPP who have undergone liver transplantation in the United States revealed unique perioperative complications, including light-induced tissue damage in four patients and neuropathy in six, as well as recurrent EPP-associated liver disease in 65% of patients who survived more than two months. Overall patient and graft survival rates were statistically similar to those for all other patients transplanted in the United States during the same period.¹¹⁷ Therefore, liver transplantation must be considered symptomatic therapy, except in patients with fulminant hepatic failure, given the high risk of recurrent disease in the graft and the added risk of intraoperative photodynamic injury to internal organs.^117,122 Bone marrow transplantation after liver transplantation has been performed successfully in a child with EPP-induced ESLD.¹²³

Splenectomy, which lengthens the lifespan of circulating red blood cells and decreases the erythropoietic drive, has been shown to be effective in many patients with CEP. Frequent blood transfusions or hematin infusions inhibit the stimulus for heme production, thereby diminishing or eliminating the cutaneous manifestations of the disease.¹²⁴ Bone marrow transplantation has proved curative in severely affected patients.^124,125

Pathophysiology

The pathway for tyrosine metabolism is shown in Figure 76-5. The enzymatic defect in patients with tyrosinemia has been identified in fumarylacetate hydrolase (FAH), the final step in the degradation process. More than 30 mutations in FAH have been found in patients with HT1, but no clear correlation between FAH genotype and HT1 phenotype has been found.¹²⁷ FAH deficiency leads to accumulation of the upstream metabolites fumarylacetoacetate (FAA) and maleylacetoacetate, which are then converted to the toxic intermediates succinylacetoacetone (SAA) and succinylacetone (SA). FAA has been shown to deplete blood and liver of glutathione, the consequence of which may be augmentation of the mutagenic potential of FAA.^127,128 SA inhibits renal glucose and amino acid transport and the degradation of ALA to PBG, probably via direct modification of amino acids in enzyme active sites. It also inhibits DNA ligase activity in fibroblasts isolated from patients with HT1.^127,129 Over time, the combined effects of high levels of FAA and SA on the integrity of DNA and cellular repair mechanisms may account for increased chromosomal breakage in fibroblasts isolated from patients with HT1, as well as an increased risk of hepatocellular carcinoma.^127,130

Figure 76-5. Pathway of tyrosine metabolism. The location of the enzymatic defect in hereditary tyrosinemia type 1 (HT1) and the site of action of 2-(2-nitro-4-trifluoro-methylbenzoyl)-1,3-cyclohexanedione (NTBC) are shown. Enzymes are shown in italics.

Clinical and Pathologic Features

Patients with HTI present either acutely with liver failure or with chronic liver disease, with or without hepatocellular carcinoma. In the acute form of HTI, patients present with liver disease in the first six months of life; symptoms include those associated with hepatic synthetic dysfunction, such as hypoglycemia, ascites, jaundice, and bleeding diathesis, as well as anorexia, vomiting, and irritability.¹³¹ Laboratory studies show elevations of serum aminotransferase, GGTP, and bilirubin levels and decreased levels of coagulation factors. Serum tyrosine, methionine, and α-fetoprotein levels are elevated substantially. Analysis of the urine reveals phosphaturia, glucosuria, hyperaminoaciduria, renal acidosis, and elevated excretion of ALA and phenolic acids.¹²⁷ The acute form is usually fatal within the first two years of life. In a multicenter study, van Spronsen and associates¹³¹ showed that 77% of patients with tyrosinemia presented before the age of six months. The one- and two-year survival rates were 38% and 29%, respectively, if patients presented between zero and two months, and 74% and 74%, respectively, if they presented between two and six months. Survival for both time intervals rose to 96% if the first symptoms appeared after age six months. The cause of death was usually recurrent bleeding and liver failure (35 of 47 deaths); however, hepatocellular carcinoma (7 of 47) and neurologic crisis (3 of 47) accounted for some deaths.

Patients with the chronic form of HTI classically have symptoms that are similar to but milder than those of the acute presentation; sometimes, serum aminotransferase levels as well as plasma tyrosine and methionine levels can be within the normal range.¹²⁷ These patients usually present after one year of age with hepatomegaly, rickets, nephromegaly, hypertension, and growth retardation. They also are likely to have neurologic problems and hepatocellular carcinoma.

The pathologic changes differ in the acute and chronic forms of the disease. In the acute form, the liver may appear enlarged with a pale nodular pattern or may be shrunken, firm, and brown. Micronodular cirrhosis, fibrotic septa, bile duct proliferation and plugging, steatosis, pseudoacinar and nodular formations, and giant cell transformation may be found on histologic examination. Varying amounts of FAH enzyme activity have been found in liver tissue from patients with HTI as a result of spontaneous reversion of FAH gene mutations. Patients with the chronic form of the disease have a higher level of reversion and a lower frequency of liver dysplasia.¹³² In an analysis of mutations in the FAH gene in members of 13 unrelated families with HT1, no mutation type predominated in the affected families, and no correlation between genotype and phenotype was observed.¹³³

In the chronic form of tyrosinemia, the liver appears enlarged, coarse, and nodular. In histologic specimens, micronodular and macronodular cirrhosis may be present, as may steatosis, fibrotic septa, and a mild lymphoplasmacytic infiltrate. Cholestasis is less pronounced than in the acute form of HT1. Large- or small-cell dysplasia may be present, reflecting premalignant changes. Because of the nodular changes, identification of progression to hepatocellular carcinoma can be difficult. Because the serum α-fetoprotein value is elevated before hepatocellular carcinoma develops, measurement of α-fetoprotein is not helpful in the diagnosis of hepatocellular carcinoma in this setting. Visualization of both low- and high-attenuation hepatic nodules on computed tomography (CT) is thought to be highly suggestive of hepatocellular carcinoma.¹³⁴

Renal involvement is nearly universal in patients with tyrosinemia. Findings include a decreased glomerular filtration rate, proximal renal tubular dysfunction, nephromegaly, phosphaturia (which is responsible for the development of rickets), glucosuria, and aminoaciduria. The toxic metabolites SA and SAA are thought to have a direct effect on kidney function. Some patients progress to renal failure and need renal transplantation.^135,136 One third of patients develop a cardiomyopathy, most commonly interventricular septal hypertrophy, that is reversible with either medical or surgical management of the disease.¹³⁷

The neurologic manifestations may be the most concerning feature in older patients with tyrosinemia. More than 40% of patients experience porphyria-like symptoms.¹³⁸ In a study of 20 children with HTI and 104 neurologic crises, the most common symptoms were pain (96%), hypertonia (76%), vomiting and ileus (69%), weakness (29%), and diarrhea (12%). Eight of the 104 patients required mechanical ventilation.¹³⁸ A neurologic crisis has been considered to be a frequent cause of death, but the onset of a neurologic crisis may not be associated with worsening liver disease. Blockage of the degradation of ALA by SA is thought to be responsible for the neurotoxicity.

Diagnosis

The diagnosis of tyrosinemia should be suspected in any child with neonatal liver disease or a bleeding diathesis or in any child older than one year with undiagnosed liver disease or rickets. The diagnosis is suggested by increased tyrosine, methionine, phenylalanine, and α-fetoprotein levels. Elevated serum and urine SA and urine ALA levels are regarded as pathognomonic for tyrosinemia. The diagnosis can be confirmed with an assay for FAH in lymphocytes, erythrocytes, skin fibroblasts, or liver tissue.¹²⁷

Prenatal diagnosis can be performed by determining SA levels in amniotic fluid or measuring FAH activity in chorionic villus biopsy specimens. If the specific gene mutation in a family is known, early genetic diagnosis can be made from chorionic villus biopsy specimens as well.¹²⁷ Current newborn screening programs measure tyrosine levels on dried blood specimens; unfortunately, neonatal hypertyrosinemia is not specific for a diagnosis of HT1, and the tyrosine levels in newborns with HT1 may overlap substantially with tyrosine levels found in unaffected newborns. Improved newborn screening methodologies have been developed that measure SA in addition to amino acid levels in dried blood specimens; however, these methods are not yet widely employed.^139,140

Treatment

Historically, the treatment of tyrosinemia has been dietary management, with restriction of tyrosine and phenylalanine. Dietary restriction has been shown to reverse renal damage and improve metabolic bone disease, although the liver disease progresses. An adequate intake of these amino acids is needed to ensure normal growth and development.¹²⁷ Few studies of the long-term outcome in tyrosinemia treated with strict dietary management alone are available.

Liver transplantation has become a mainstay of therapy for patients with tyrosinemia. The transplant corrects the phenotype and normalizes FAH activity and liver function. Additionally, the biochemical profiles normalize and kidney disease abates, with improvement in glomerular filtration rate, tubular acidosis, and hypercalcemia in most patients.^141,142 Abnormal renal size and architecture persist after liver transplantation,¹⁴³ and many patients continue to excrete SA despite normal serum values.¹⁴² One report has suggested that patients with tyrosinemia who have undergone liver transplantation and receive tacrolimus-based immunosuppression have a high rate of post-transplant neurologic complications.¹⁴⁴

In 1992, Lindstedt and associates¹⁴⁵ published data on the treatment of tyrosinemia with the herbicide 2-(2-nitro-4-trifluoro-methylbenzoyl)-1,3-cyclohexanedione (NTBC). Later, Holme and Lindstedt¹⁴⁶ published the results of a large long-term study of 220 patients with HT1 who were treated with this agent for up to seven years. NTBC is a potent inhibitor of 4-hydroxyphenylpyruvate dioxygenase, one of the initial steps in tyrosine metabolism. Blocking the degradation of tyrosine to its downstream toxic metabolites (i.e., FAA, SA, and SAA) was postulated to lead to improved hepatic function. Treated patients exhibit improved liver synthetic function, as reflected by a shortening of the prothrombin time, as well as decreased serum aminotransferase levels and a reduction in liver parenchymal heterogeneity and nodules on CT. In addition, serum α-fetoprotein and ALA levels are diminished and renal tubular dysfunction reverses.¹⁴⁷ Therefore, elevated α-fetoprotein levels in a patient receiving NTBC therapy should raise concern about the patient’s nonadherence to therapy or the development of hepatocellular cancer.^127,148 Long-term results show continued improvement in all parameters noted in the earlier report as well as a lower risk for the development of hepatocellular carcinoma in patients who start therapy and were free of hepatocellular carcinoma before the age of two years.¹⁴⁶ No patient withdrew from the study because of adverse side effects of the drug. Transient thrombocytopenia and neutropenia as well as ocular symptoms suggestive of corneal irritation have been noted rarely. Cognitive impairment resulting in learning problems may be a long-term complication of chronic use of NTBC in this patient population, possibly from the effects of chronic hypertyrosinemia.¹⁴⁹

In another study from Quebec,¹⁵⁰ only 4 of 35 patients treated with NTBC underwent liver transplantation; 1 patient received a transplant because of concern about the heterogeneous texture of the liver (suggestive of cirrhosis) shown by ultrasonography coupled with a persistent moderate elevation in the serum α-fetoprotein level. At resection, this child was found to have a small nodule with hepatocellular dysplasia. The 31 remaining patients were monitored while receiving NTBC therapy for up to three years, and none experienced neurologic crises or deterioration of liver disease.¹⁵⁰

Therefore, therapy with NTBC significantly improves the clinical course of patients treated at an early age. For those in whom therapy is initiated at a later age, NTBC offers a palliative benefit, although the risk of hepatocellular carcinoma is still high in this group of patients. Because the strategy of treating patients with HT1 with NTBC is relatively new, greater experience is required to enable assessment of the relative costs and long-term outcome and recognition of any possible adverse effects of long-term NTBC therapy.¹⁴⁹ Early diagnosis, achieved by inclusion of HT1 in neonatal screening programs, may allow the prompt initiation of effective therapy with NTBC and avoidance of liver transplantation.

Pathophysiology

The steps of the urea cycle are illustrated in Figure 76-6. Carbamyl phosphate synthetase (CPS) I forms carbamyl phosphate from ammonium and bicarbonate. This step requires N-acetyl glutamate as a cofactor, which is synthesized from N-acetyl CoA and glutamic acid by N-acetyl glutamate synthetase. Ornithine transcarbamylase (OTC) combines carbamyl phosphate with ornithine to form citrulline. A second nitrogen enters the cycle as aspartate, which combines with citrulline by the action of argininosuccinate synthetase (AS) to form argininosuccinate. Argininosuccinate is converted to arginine and fumarate by argininosuccinase, or argininosuccinate lyase (AL). Arginase then catalyzes the breakdown of arginine to urea and ornithine in the final step of the pathway. Several amino acid transporters, such as citrin, an aspartate/glutamate carrier protein that supplies aspartate to the urea cycle, are involved in shuttling metabolites into the urea cycle.¹⁵²

Figure 76-6. The urea cycle. Alternative pathways that are used therapeutically for waste nitrogen disposal are also illustrated. Enzymes are shown in italics.

CPS II, through the pyrimidine synthetic pathway, leads to the formation of orotic acid. Excess carbamyl phosphate can be used by this pathway if a block occurs distal to OTC in the metabolic pathway. Excess nitrogen in the form of amino acids can be shunted to alternative pathways of waste-nitrogen excretion by the medicinal use of sodium benzoate and sodium phenylacetate, leading to the generation of hippurate and phenylacetylglutamine, respectively.

Enzymatic defects have been identified in all five steps involved in the urea cycle. Deficiency of four of these enzymes is transmitted through autosomal recessive inheritance, whereas OTC deficiency is transmitted as an X-linked trait. More than 340 different mutations in the OTC gene give rise to OTC deficiency, the most common urea cycle defect.¹⁵³ Numerous defects in the other enzymes or amino acid transporters of the cycle (e.g., N-acetylglutamate synthetase or citrin) have been characterized as well.^154–156 Moreover, numerous mRNA instability mutations have been found in patients with CPS I deficiency.¹⁵⁷

A UCD has two main biochemical consequences: Arginine becomes an essential amino acid (except in arginase deficiency), and nitrogen accumulates in a variety of molecules, some of which can have deleterious toxic effects.

Clinical Features

The spectra of clinical presentations in patients with any of the UCDs are virtually identical; in the neonatal period, these disorders usually manifest as acute life-threatening events. Contrary to the commonly held belief that UCDs are primarily disorders of the newborn period, a large report of the clinical presentation and outcome data of 260 patients with UCDs revealed that two thirds of patients present at greater than 30 days of age, even when female OTC carriers are excluded from the analysis.¹⁵⁸ Another large cross-sectional study of 183 patients with UCDs had similar findings.¹⁵⁹ Late-onset adult presentations also have been described. Affected infants appear normal for the first 24 to 72 hours until they are exposed to their first feeding, which provides the initial protein load that fosters ammonia production. Symptoms include irritability, poor feeding, vomiting, lethargy, hypotonia, seizures, coma, and hyperventilation, all secondary to hyperammonemia.¹⁵⁸ Initially, neonates are often mistakenly thought to have sepsis, despite the absence of perinatal risk factors, and diagnostic laboratory testing is delayed.¹⁶⁰ Plasma ammonia levels should be obtained whenever an evaluation for sepsis is initiated in a neonate; levels may exceed 2000 µmol/L (3400 mg/dL), normal levels being 50 µmol/L (85 mg/dL) or less.

For all age groups, overall survival decreases as the peak plasma ammonia level rises for a given episode of hyperammonemia with survival rates of 98% and 47% for peak ammonia levels of less than 200 µmol/L and greater than 1000 µmol/L, respectively.¹⁶¹ Newborns have a survival rate of 73% after their presenting episode of hyperammonemia, whereas patients older than 30 days of age have a survival rate of 98%. Male patients with OTC deficiency have a survival rate of 91% following an episode of hyperammonemia, a rate significantly less than those (93% to 98%) for all other forms of UCDs. Blood gas analysis shows respiratory alkalosis secondary to the hyperventilation caused by the effects of ammonia on the central nervous system. Blood urea nitrogen levels are typically low but can be elevated during times of dehydration or hypoperfusion. Serum levels of liver enzymes are usually normal or minimally elevated. Severe hepatomegaly can occur in early-onset forms of AL deficiency.¹⁵¹

As stated earlier, one third of patients with UCDs present in the neonatal period; the remainder are diagnosed at variable times from infancy to adulthood.^158,159 OTC deficiency is the most common UCD (55%), followed by argininosuccinic aciduria (AL deficiency, 16%) and citrullinemia (AS deficiency, 14%).¹⁵⁹ Male patients with OTC deficiency have been diagnosed as late as 40 years of age with varied phenotypic presentations.¹⁶² As many as 10% of female carriers of OTC deficiency can have symptoms, which may be severe and fatal, although most female carriers have no symptoms or report only nausea after high-protein meals.¹⁶³ Late-onset CPS deficiency has also been described,¹⁶⁴ and the adult form of AS deficiency is relatively common in Japan.¹⁵⁵

Symptoms and signs of late-onset UCDs, especially OTC and CPS deficiencies, include episodic irritability, lethargy, or vomiting; self-induced avoidance of protein such as milk, eggs, and meats; and short stature or growth delays. Neurologic symptoms, which can also be episodic, include ataxia, developmental delays, behavioral abnormalities, combativeness, biting, confusion, hallucinations, headaches, dizziness, visual impairment, diplopia, anorexia, and seizures.¹⁵⁸ Acute hyperammonemic episodes can resemble Reye’s syndrome (see Chapter 86). Such episodes can be precipitated by high-protein meals, viral or bacterial infections, medications, trauma, or surgery. Infants may present after being weaned from breast milk to infant formulas, which have a higher protein content. Patients with OTC and CPS deficiencies have been reported to present in the postpartum period with acute decompensation and death.^165,166

Citrin deficiency is caused by mutations in the SLC25A13 gene and is associated with both adult-onset type 2 citrullinemia and neonatal intrahepatic cholestasis resulting from citrin deficiency (NICCD), a syndrome that primarily affects newborns in East Asia.^155,167,168 A case of citrin deficiency in a white infant of European descent who presented with poor weight gain and a bleeding diathesis, but without cholestasis, has been reported, expanding the clinical spectrum of presentations of citrin deficiency.¹⁶⁹ NICCD is associated with hyperaminoacidemia (e.g., elevated citrulline, methionine, and tyrosine levels) and cholestasis.¹⁵⁵ Hypergalactosemia and elevated acylcarnitine levels also may be observed.¹⁷⁰ Hepatic steatosis may be seen in the liver histologically.¹⁷¹ In most patients with NICCD, all biochemical abnormalities resolve spontaneously or with minimal dietary restrictions (e.g., the use of lactose-free formulas); however, several affected infants have required liver transplantation before age one year.¹⁶⁷ Therefore, jaundiced infants with multiple abnormal newborn metabolic screen results (e.g., elevation of blood phenylalanine, methionine, or galactose levels) must be observed closely because of the risk for development of ESLD caused by NICCD; a chubby face outside of the realm of normal may be a diagnostic clue.¹⁷² The diagnosis can be made by sequence analysis of the SLC25A13 gene or by failure to detect citrin protein in peripheral lymphocytes.^168,173

Diagnosis

A high index of suspicion is required for prompt diagnosis of UCDs. Symptoms can mimic those of other acute neonatal problems, such as infections, seizures, and pulmonary or cardiac disease.^158,174 Later presentations can mimic other behavioral, psychiatric, or developmental disorders. The first clue may be an elevated serum ammonia level with normal serum aminotransferase levels and without metabolic acidosis. Therefore, if a UCD is considered, the following laboratory measurements should be obtained: serum ammonia, arterial blood gases, urine organic acids, serum amino acids, and urinary orotic acid; Table 76-4 reviews the expected laboratory results.

Urinary organic acid profiles are typically normal in UCDs. The plasma amino acid profiles are distinctive, with abnormal levels of arginine, ornithine, and citrulline. Citrulline levels are barely detectable in OTC or CPS deficiencies but markedly raised in AS and AL deficiencies. AS deficiency can be distinguished from AL deficiency by the finding of argininosuccinic acid in the plasma and urine of patients with AL deficiency. OTC deficiency is differentiated from CPS deficiency by the excessive urinary excretion of orotic acid in OTC deficiency. Direct enzyme analysis can be performed and can be useful in patients who have a partial deficiency or who present in adulthood. Prenatal enzyme and genetic linkage analysis can be carried out in family members of known carriers.¹⁷⁵ Early neonatal diagnosis leads to improved survival. An N-carbamoyl-glutamic acid test has been advocated for all patients presenting with a suspected UCD while further testing is carried out; patients with N-acetylglutamate synthase deficiency, one of the less common UCDs, have a significant drop in plasma ammonia levels within eight hours of receiving an oral dose of N-carbamoyl glutamic acid, a structural analog of N-acetyl glutamic acid.^176,177 An allopurinol loading test, which leads to excretion of orotic acid in amounts that are 10- to 20-fold greater than normal in heterozygote female carriers of OTC deficiency, is nonspecific, and its results must be interpreted with caution; the result can be positive in some patients with mitochondrial disease or defects in pyrimidine metabolism.¹⁷⁸ Liver biopsy specimens typically show minimal fatty infiltration; fibrosis or cirrhosis are uncommon findings but have been reported.^151,179

Treatment

All external protein intake should be discontinued in infants with UCDs presenting acutely. Serum ammonia levels should be restored to normal. The use of oral lactulose to lower the nitrogen load has not been studied in this patient population. Given the extremely high ammonia levels often encountered, continuous arteriovenous hemodialysis or hemofiltration is often required, but exchange transfusions and peritoneal dialysis are ineffective. Alternative pathways for waste nitrogen disposal should be employed, specifically intravenous administration of sodium benzoate and sodium phenylacetate; however, sodium benzoate should be used with caution in patients with cirrhosis because a paradoxical rise in blood ammonia levels has been observed.¹⁸⁰ Arginine, carnitine, and long-chain fatty acids are usually present in low levels in these patients and should be supplemented.^181,182 Once the patient stabilizes, low levels of dietary protein, 0.5 to 1.0 gm/kg, may be introduced, with progressive increases as tolerated to provide sufficient protein for growth and tissue repair while minimizing urea production. Long-term therapy and protein restriction are then tailored to the patient; patients with a severe disorder may need essential amino acids to supplement their protein intake.¹⁸² Oral phenylbutyrate can be substituted for phenylacetate to improve palatability.

The outcome for patients who present with hyperammonemic coma and a delayed diagnosis is poor.¹⁶¹ The level of ammonia at the time of the first hyperammonemic episode is a rough guide to the eventual neurodevelopmental outcome.¹⁸³ The sooner the hyperammonemia is treated and the correct diagnosis is made, the better the long-term survival, although for patients who survive the neonatal period, the median survival without liver transplantation is less than four years and is associated with severe developmental delay and neurologic morbidity.¹⁶⁰ With optimal dietary and medical management, patients may still have repeated hyperammonemic crises, often during intercurrent viral infections. Symptomatic female OTC heterozygotes also benefit from therapy, which leads to fewer hyperammonemic episodes and a reduced risk of further cognitive decline.¹⁸⁴

Patients with a UCD and a deterioration or lack of improvement despite therapy have undergone either total or auxiliary liver transplantation successfully, with normalization of enzyme activity and ammonia levels, the ability to tolerate a normal diet, and five-year survival rates of 90%.^185,186 Liver transplantation, if considered, should be done before neurologic damage is permanent because the patient’s neurologic status does not improve after transplantation. The metabolic condition of the patients normalizes completely, and neurologic status does not worsen after transplantation.¹⁶⁴ In addition, hepatocyte transplantation has been successful in improving metabolic function in patients with UCDs and has been applied successfully as a bridge to auxiliary liver transplantation.^186,187 For patients without severe neurologic compromise before liver transplantation, this therapeutic approach is worthwhile. In addition, the annual cost of care for this group of patients is likely to be reduced dramatically after liver transplantation.

The importance of identifying the deleterious mutation in a patient with a UCD will likely become increasingly important not only as a means of allowing carrier testing and prenatal diagnosis, but also as an aid to treatment decisions. For example, patients with the mutations of OTC deficiency that result in the most severe disease (e.g., abolished enzyme activity) may benefit preferentially from immediate liver transplantation to prevent severe mental retardation or death, whereas those with mutations that lead to milder disease may be better managed medically with dietary restrictions and ammonia scavengers to facilitate growth before possible liver transplantation.