ENCEPHALOPATHIES

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CHAPTER 107 ENCEPHALOPATHIES

Juan M. Pascual

CONCEPT AND CONTEXT OF NEUROMETABOLIC DISEASES

Metabolic disorders constitute an expanding group of diseases comprising such heterogeneous conditions that a uniform introductory definition becomes necessary. Strictly speaking, neurometabolic diseases arise from genetic deficiencies of intermediary metabolism enzymes. Thus, mutation of genes encoding cytostructural or regulatory proteins or proteins involved in cell division, immunity, excitability, cell-to-cell communication, secretion, or movement do not give rise to metabolic diseases sensu stricto. Nevertheless, careful reflection on the molecular mechanisms of these latter disease categories leads to the recognition of intermediary metabolism abnormalities virtually in all of them, allowing at least some of them to be included with neurometabolic diseases.

Whether involving carbohydrate, lipid, or protein metabolism, the manifestations of neurometabolic diseases are pleomorphic and can manifest at any stage during a life span. Regardless of their time and mode of manifestation, the approach to the potential patient with a neurometabolic disease includes a customized but systematic series of evaluations as well as a thorough assessment of the ancestry and family structure aimed at identifying all relatives at risk for a heritable trait. At the conclusion of the clinical interview, it should be possible to suspect the pattern of inheritance of a familial disease or, alternatively, the probability of a de novo, that is, noninherited, mutation. In the case of a potentially heritable trait, apparently unaffected relatives should be questioned and examined for the presence of specific abnormalities indicative of an incompletely penetrant trait. Then, a series of analytical investigations are tailored to confirm the diagnosis or, at least, to circumscribe the metabolic abnormality as much as possible according to the patient’s clinical syndrome. The biochemical analysis of banked patient tissues, such as biopsied muscle, and cellular elements, such as cultured fibroblasts, is often necessary to confirm a specific enzyme deficiency and should be followed, whenever possible, by genotyping. A variety of genetic test batteries and panels are available to screen genes associated with diseases that share a similar phenotype. When successful, genotyping allows for fully informed genetic counseling, for screening of at-risk relatives, and, in an increasing number of instances, for prenatal diagnosis via amniocentesis.

MANIFESTATIONS UNFOLD IN TIME

The apparent age at onset of metabolic disorders is variable, as the manifestations of neurometabolic diseases are intimately linked to the development of the nervous system. During infancy and childhood, several genetic expression programs come into play and become quiescent as the organism grows and matures. Thus, the effects of a pathogenic mutation may not be noticeable until the mutant gene is activated, and this may occur well after birth. For example, the fetal brain consumes preferentially lipid byproducts such as ketone bodies. At birth, the cerebral metabolic rate for glucose is minimal; it increases gradually during childhood, when it exceeds the neonatal rate by three-fold. By early adolescence, glucose consumption decreases and reaches the adult level, which is about twice the newborn rate. It is not surprising, then, that fetal and neonatal brains tolerate hypoglycemia relatively well. This is exemplified by the manifestations of glucose transporter type 1 deficiency, caused by mutation of the glucose carrier of the blood-brain barrier, which is usually unnoticeable in the newborn and becomes most pronounced during childhood; furthermore, they can be circumvented, to some degree, by a diet rich in ketogenic substrates, as the brain always retains the capacity to metabolize ketone bodies. In some occasions, the converse phenomenon is true, allowing for the restoration of a normal phenotype as development progresses and new genes replace the function of abnormal ones. This is illustrated by a transient or reversible form of cytochrome c oxidase–deficient myopathy that manifests in infancy with hypotonia and profound weakness, followed by a return of enzyme activity in muscle and normal strength later in childhood. In disorders causing an accumulation of a metabolite, symptoms may remain latent until the stored metabolite interferes with cellular organelles or forms deleterious aggregates. Yet, in other occasions, a precipitant factor triggers the sudden decompensation of a precarious cellular machinery. For example, a nutritional excess of fat may aggravate a fatty acid oxidation defect, inducing severe hypoglycemia and coma, or a protein load may result in hyperammonemia and mental disturbances in urea cycle defects (see Chapter 110). In some cases, well exemplified by Leigh syndrome, a “free interval” of several years may lapse before a trivial intercurrent respiratory or infectious illness precipitates cerebral necrosis, leading to fulminant disability. In such cases, meticulous questioning and review of developmental milestones often uncover a preceding history of subtle but lifelong neurological dysfunction.

STATIC, EPISODIC, OR PROGRESSIVE DISEASES?

Generally speaking, metabolic diseases are lifelong, permanent diseases, like their causal genetic mutations. However, they follow any imaginable temporal course even in the absence of environmental or nutritional precipitating factors, with some conditions manifesting only periodically and others exhibiting a static or apparently immutable course, which is in contrast with the still common notion of metabolic diseases as unrelenting, continuously symptomatic processes. The basis for the apparently paradoxical static and episodic manifestations of neurometabolic diseases is provided by the compartmentalization of metabolism.¹ Because all cellular functions are spatially limited and regulated by membranes, metabolic reactions occur at rates governed not only by the kinetics of their corresponding enzymes but also (and often mainly) by substrate availability and product abundance. The former process, dependent on enzyme structure, is dictated by the gene; the latter, by the ability of cells and organelles to distribute and clear substrates and products, a process that is inherently dependent on the function of the membranes and membrane compartments of the various cell types that carry out the reaction in question.² Thus, for example, an enzymatic deficiency affecting a reaction that is constantly active may be associated with the accumulation of a substrate that interferes with other reactions, causing inhibition and resulting in abnormal cell function. Such a substrate may be eliminated from the cell after it reaches a certain threshold level at a rate that is dependent on its concentration. After the substrate accumulates for the first time, production and elimination proceed indefinitely, maintaining a constant (elevated) concentration in the cell. This may constitute the basis for the permanent, immutable clinical manifestations that are sometimes associated with a static metabolic abnormality. Episodic diseases can also be modeled using the same mechanistic framework: a compound may accumulate without causing any abnormalities until a threshold concentration is reached or until another slowly fluctuating cellular process becomes vulnerable to it, such that an additional reaction is triggered, causing a decompensation that may propagate into a clinical crisis, only later followed by the restoration of the original (unstable) equilibrium. In this case, rare fluctuations of cellular metabolism may coincide and compound one another to cause a crisis in the setting of a permanently abnormal enzyme function. Metabolic control analysis computations are accommodating and predictive of such fluctuations. Of course, these scenarios may be complemented with other, higher-complexity hypothetical mechanisms that need not act exclusively of one another.

A COMPLEX GENETIC LANDSCAPE

The precise pathophysiological mechanism of most neurometabolic diseases remains unknown, despite the accelerating pace of gene discovery. Among the mechanisms of metabolic diseases, accumulation of metabolic products, deprivation of substrates downstream of a metabolic blockade, negative-feedback regulation of enzymes, and induction or repression of genes by excess metabolite are a few logical, and not mutually exclusive, possibilities, although they do not fully explain many of these diseases. Additional consideration must be given to intragenomic signaling effects, such that a point mutation in one gene may result in upregulation or downregulation of other, unrelated genes. Such gene expression changes are sometimes viewed as “compensatory,” but they may well be deleterious, contributing to pathogenesis. Last, extra diagnostic and pathophysiological challenges are posed by the abundance of genetic polymorphisms of uncertain function present in all individuals, together with difficulties in separating true phenotypical characteristics from mere epiphenomena that are only tangentially related to the fundamental disease mechanism. In fact, many diseases are diagnosed by assaying metabolites generated by processes far removed from the original enzyme defect, which are only empirically found to associate with the classic form of that particular disease. The risk of errors inherent to this diagnostic approach cannot be overemphasized, particularly when confronted with unusual or partial forms of otherwise frequent diseases or with diseases so rare that the collective experience available is too sparse or insufficiently documented.

Genotyping is also subject to special considerations when applied to neurometabolic diseases. Several well-known clinical entities can be confused with phenocopies (i.e., conditions that manifest similar phenotypes but are due to mutations of different, unrelated genes). For example, mutation of the SCO2 gene can result in a phenotype that resembles spinal muscular atrophy, which is typically due to SMN1 gene mutations. In this instance, the correct diagnosis is reached by paying attention to a clinical feature (cardiomyopathy) and a biochemical marker (lactic acidosis) that would be atypical in spinal muscular atrophy, again illustrating the value of the initial clinical and analytical assessment of the patient. Genotyping is additionally dependent on the abundance of a particular mutation in the patient’s tissues. Thus, mosaicism for MECP2 mutations in the male and skewed X chromosome inactivation in the female may account for the disparate phenotypes of Rett syndrome, encompassing male neonatal death, male mental retardation, asymptomatic female carrier status, or classic female Rett syndrome. The abundance of mitochondrial DNA mutations also varies depending on the tissue chosen for genotyping (a phenomenon known as heteroplasmy; see Chapter 88) and, in some cases, several easily accessible tissues (blood, urinary sediment, buccal smear) must be examined to detect or to exclude a low-abundance mutation. Last, carrier status should be investigated in all genetically susceptible individuals related to a patient with a low (incomplete) penetrance disease. For example, asymptomatic mothers of hypotonic infants affected by myotonic dystrophy should be examined for subtle signs of myotonia and, if appropriate, offered genotyping.

A PLEOMORPHIC PHENOMENOLOGY

The phenotypes of neurometabolic diseases are often, and sometimes predominantly, nonneurological. Thus, abnormal urine odor, hepatomegaly, cardiomyopathy, cardiac arrhythmia, facial and skeletal malformations, neutropenia or disordered coagulation, and hair and skin abnormalities, among others, are characteristic features of some diseases (Table 107-1).

TABLE 107-1 Extraneurological Manifestations of Metabolic Encephalopathies

Organs and Systems	Manifestations	Examples
Somatic dysmorphism	Coarse facies	Mucopolysaccharidoses
		Mucolipidoses
		GM1 gangliosidosis
	Characteristic facies	Zellweger disease
		Pyruvate dehydrogenase deficiency
		Sulfite oxidase deficiency
	Progeric appearance	Cockayne disease
Cerebral dysgenesis	Abnormal neuronal migration	Zellweger disease
	Corpus callosum agenesis	Pyruvate dehydrogenase deficiency
	Perisylvian hypotrophy	Glutaric aciduria type I
Ocular abnormalities	Nuclear cataracts	Galactosemia
Lens dislocation	Sulfite oxidase deficiency
	Cataracts	Cockayne disease
	Corneal opacification	Mucopolysaccharidoses
	Corneal opacification	Mucolipidoses
Hair abnormalities	Several abnormalities	Menkes disease
Hair abnormalities	Alopecia	Multiple carboxylase deficiency
Skin abnormalities	Rash	Biotinidase deficiency
Cardiopathy	Cardiomyopathy	Respiratory chain disorders
		Fatty acid oxidation disorders
		Pompe’s disease
		Glucogenoses III and IV
	Arrhythmia	Kearns-Sayre syndrome
	Arrhythmia	MELAS
Hepatopathy	Cholestasis	Niemann-Pick disease type C
	Cholestasis	Smith-Lemli-Opitz syndrome
	Hepatomegaly	Mucopolysaccharidoses
	Hepatomegaly	Mucolipidoses
	Cirrhosis	Zellweger disease
	Liver failure	Alpers disease
Intestinal abnormalities	Abdominal pain	Acute intermittent porphyria
Intestinal abnormalities	Pseudo-obstruction	MELAS
Nephropathy	Fanconi’s syndrome	Galactosemia
	Fanconi’s syndrome	Mitochondrial DNA deletion
	Renal tubular acidosis	Respiratory chain disorders
	Nephrotic syndrome	Respiratory chain disorders
	Nephrotic syndrome	Congenital glycosylation defects
Skeletal abnormalities	Dysostosis	Mucopolysaccharidoses
		Mucolipidoses
		GM1 gangliosidosis
	Patellar calcifications	Zellweger disease
Hematological disturbances	Acanthocytosis	Abetalipoproteinemia
	Anemia	Respiratory chain disorders
	Anemia	Glycolytic defects
	Pancytopenia	Organic acidurias
	Vacuolated lymphocytes	Pompe disease
		Mucolipidosis
		Sialidosis
Psychiatric disturbances	Various	Urea cycle defects
		Porphyrias
		Metachromatic leukodystrophy
		Krabbe disease
		Sanfilippo disease
		Wilson disease
		MELAS
Abnormal urine odor	‘Sweaty feet’	Glutaric aciduria type II
	Maple syrup	Maple syrup urine disease
	Musty	Phenylketonuria

Neurometabolic diseases may mimic other disorders. In the presence of a seemingly nonspecific constellation of nonprogressive abnormalities, neurometabolic diseases can be misdiagnosed for other, more common entities (Table 107-2). Misconceptions to be avoided include that metabolic diseases necessarily have a progressive clinical course and that terms such as “cerebral palsy” identify diseases, rather than heterogeneous syndromes defined by relatively loose criteria. In these cases, the index of clinical suspicion should remain high and metabolic screening should be applied, as some of these covert metabolic disorders are treatable.

TABLE 107-2 Manifestations of Occult Neurometabolic Disease

DIAGNOSIS AND ITS VALUE

The possibility of prenatal diagnosis through sampling of fetal tissue obtained by chorionic biopsy is a reality for numerous metabolic diseases but is limited to instances where a particular disease has been identified in the family or a suspicious malformation detected in the fetus. Biochemical or molecular genetic assays of amniocytes are available for an increasing number of conditions. Yet, the most effective mode of detection is by voluntary screening of certain populations at risk. When both members of a clinically normal couple are carriers of a recessive trait or when a dominant disease afflicts just one member, or even when the mother carries a pathogenic mitochondrial DNA mutation, several reproductive options are available. Among these, testing of an early embryo after in vitro fertilization and before implantation, in vitro fertilization by a healthy donor, nuclear transfer, abortion, or, if available, early initiation of therapy of an affected newborn are all preventive interventions. All of them are variably applied depending on technological and cultural factors.

Newborn screening is still an underdeveloped and underused methodology with the potential to detect many, if not most, metabolic disorders. Testing is usually performed between the first 24 and 48 hours of life and uses dry bloodspots obtained from the heel and placed onto a filter paper card that is sent to a referral laboratory.³ Examples of conditions universally screened for are phenylketonuria and congenital hypothyroidism. At the present time, over 50 diseases, including specific disorders of amino acid, organic acid, and fatty acid metabolism, can be commercially screened for by tandem mass spectroscopy alone. However, in the United States, for example, some states test for fewer than 10 disorders, whereas others test for more than 30.⁴ Diverse efforts are under way to make this testing uniformly regulated and available.

It is also possible to diagnose a neurometabolic disease postmortem, and every effort should be made to offer an exhaustive biochemical investigation to each family in whom an unexpected neonatal or an infantile sudden death occurs.⁵ At a minimum, dry blood cards and skin punch biopsies can be obtained; the latter can be deferred for up to 18 hours after death and are used to establish live fibroblast cultures for use in biochemical and genetic assays. Other tissues may also be harvested under the guidance of the appropriate metabolic consultant. Photographs and a radiographic skeletal survey are important additional investigational tools.

GROUNDWORK FOR FUTURE THERAPIES

The principles of metabolic therapy (Table 107-3) have changed little in recent years, but their mode of application is being improved continuously. In the emergency setting, the decompensation or first manifestation of a neurometabolic disease may be accompanied by poor feeding, tachypnea, and acidosis due to accumulation of organic acids. This situation is associated with intracellular dehydration and, if too rapidly corrected by administration of fluids and/or bicarbonate, may lead to cerebral edema. Thus, gradual replacement of estimated losses is mandatory. Exchange transfusion is effective for the transient removal of soluble toxic metabolites. Peritoneal dialysis is simple but not as effective as exchange transfusion or hemodialysis in the emergent clearance of organic compounds. Hemofiltration uses an extracorporeal membrane to replace a plasmatic ultrafiltrate with electrolytes and nutrients. Hemodialysis is the most effective and rapid method for the removal of small soluble compounds but requires a significant commitment of resources and is thus not routinely performed in the emergency setting. Diets that diminish the use of a deficient metabolic pathway can be administered enterally or infused parenterally. Diets containing low protein, low carbohydrate, or high glucose sometimes with extra fat supplementation, all meeting minimum caloric, protein, and essential amino acid requirements, are available for specific diseases. Cofactors and vitamins should be administered at high doses when suspecting a potential vitamin-responsive disorder and they may later be maintained at a lower dose if the clinical response is inconclusive. Parenteral enzyme infusions are used with some success in lysosomal storage diseases such as Fabry disease, Gaucher disease, mucopolysaccharidosis type I, and Pompe disease, for lack of better methods to deliver the missing enzymes to the most affected tissues. Bone marrow transplantation corrects the enzymatic deficiencies of cells of hematopoietic origin in some mucopolysaccharidoses, and in some cases, the enzyme is partially restored in the brain. Early transplantation may prevent progression of neurological disease, but its longterm benefits are obscured by residual problems such as progression of skeletal and joint disability. Maximum safety and effectiveness are realized when the disease is in the preclinical stage and an HLA-matched sibling donor is available. Hepatic and liver/kidney transplantation has been considered in a variety of disorders with mostly anecdotal, mixed success. Liver transplantation appears to benefit patients afflicted by Wilson’s disease (see Chapter 108). Gene therapy remains an elusive ideal, as difficulties relative to targeting, maintenance, and expression of the corrected gene construct are being solved. Approaches that are under active investigation include pharmacological stimulation of residual alleles or unrelated genes using histone deacetylation inhibitors and loosening of translational fidelity by aminoglycoside antibiotic derivatives applied to mutations that result in the generation of premature DNA termination codons.

TABLE 107-3 Modalities of Metabolic Therapy

CLASSIFICATION OF THE METABOLIC ENCEPHALOPATHIES

The metabolic disorders of the nervous system can be classified according to several criteria such as age at onset (birth, infancy, childhood, adolescence, and adulthood), size of the predominant abnormal metabolite (large polypeptides or carbohydrates or small intermediary metabolism compounds), mechanism of inheritance (mendelian or mitochondrial, including the special varieties of imprinted, anticipated, polygenetic, and intergenomic signaling diseases), and loss or gain of protein function or, as preferred here, from a cellular perspective (Table 107-4). Such a classification, based on cellular structure, reflects the increased understanding of disease as a perturbation of cellular function, as well as the improved comprehension of the relationships that exist between individual cellular functions and structures. Thus, just as the cell is the reference framework in which genome, proteome, molecular function, regulation, and phenotype converge constituting the fundamental living entity, diseases and their symptoms and treatments are better understood at the level of complexity provided by a cellular point of view.

TABLE 107-4 Metabolic Encephalopathies

I. Disorders of the cell membrane

Transport disorders

Disorders of glucose transport

Glucose transporter type 1 deficiency

Disorders of metal transport

Menkes disease

Wilson disease ^*

Excitability disorders

Neurotransmitter deficiencies

Segawa disease

Other disorders of amine synthesis

II. Disorders of intracellular organelles

Glycosylation disorders

Defects of oligosaccharide synthesis

Defects of oligosaccharide processing

Disorders of vesicular transport ^*

Disorders of filaments

Alexander disease ^*

III. Enzyme disorders

Urea cycle disorders

Galactosemia

Phenylketonuria

Nonketotic hyperglycemia

Glycogen storage disorders ^*

Lesch-Nyhan disease

The porphirias

Maple syrup urine disease

Glutaric acidurias I and II

Other organic acidurias

Sulfite oxidase deficiency

Canavan disease

Pantothenate kinase deficiency

Cofactor and vitamin disorders ^*

Thiamin deficiency

Pyridoxine dependency

Cobalamine disorders

Folate disorders

Biotin disorders

Biotinidase deficiency

Lipid and lipoprotein diseases

Abetalipoproteinemia

Tangier disease

Smith-Lemli-Opitz syndrome

Wolman disease

Cerebrotendinous xanthomatosis

Pelizaeus-Merzbacher disease ^*

IV. Disorders of the cell nucleus and protein synthesis

Disorders of DNA repair

Ataxiatelangiectasia

Disorders of RNA

Myotonic dystrophy ^*

Friedreich ataxia ^*

Disorders of protein synthesis

Vanishing white matter disease ^*

* Indicates diseases covered in other sections of the text.

Disorders of cell membranes primarily impact cell communication and the exchange of substances with the environment by disrupting membrane proteins or their ligands. Transport disorders, caused by primary deficiency of proteins responsible for selective permeability, exert particularly widespread cellular abnormalities derived from secondary intracellular substrate or cofactor deficiency. Disorders that cause abnormal neurotransmission include, apart from specific membrane receptor diseases, neurotransmitter deficiencies that render cell membranes inexcitable or abnormally modulated. Disorders of organelles predominantly include abnormalities in organelle production, the function of their membranes or their contents, or their abnormal movement, leading to their accumulation or malformation and to the buildup of nonmetabolized compounds or, in the case of mitochondrial diseases, to the deficit of energy production. Enzymatic disorders are due to mutation of soluble enzymes or to cofactor deficiencies caused by inadequate absorption, processing or binding affinity, all resulting in abnormal catalysis. The cellular abnormalities brought about by soluble enzyme deficiencies tend to be morphologically modest but functionally widespread, as the consequence of cellular substrate diffusion and the release (or deprivation) of circulating plasmatic compounds, including lipoproteins. Disorders of nuclear function (DNA and RNA synthesis, processing, and maintenance) and of the cellular protein synthesis machinery are associated with broad cellular abnormalities by virtue of the central mission for which the cell nucleus is responsible.

In general, membrane disorders tend to be associated with milder disease phenotypes, organelle disorders with slowly accumulating abnormalities, enzyme disorders with marked biochemical abnormalities detectable in tissues and fluids, and nuclear disorders with a bewildering array of cellular alterations and phenotypes, providing a sort of apparent, gross correlation between cellular function and disease severity. Nevertheless, disease is not a good indicator when cataloging the importance of the various cellular functions: the more central a process or reaction, the less probable it is that its disease is compatible with life. Thus, human diseases are most likely to affect the least important functions: those that exhibit redundancy, can be, at least, partially compensated for and, ultimately, allow for the survival of the organism. When mutation affects essential, highly conserved genes, the minimal form of genetic alteration, a polymorphism, may serve to drive evolution but not disease. Any higher-impact mutations, such as those causing sequence variation in an important protein area, are probably not tolerable and incompatible with life.

Glucose Transporter Type 1 Deficiency

Mutations in the GLUT1 gene, responsible for glucose transport through the blood-brain barrier and then through a series of further barriers provided by the membranes of the astrocytes in the cerebral neuropil, cause a variety of manifestations dominated by refractory infantile epilepsy and spastic ataxia in its more typical and better defined, classic form.⁶ The disease can be inherited as an autosomal dominant trait from oligosymptomatic adults, who sometimes experience only dyslexia or infrequent seizures, and can lead to particularly severe neurological disability when GLUT1 mutations (located in chromosome 1) compound in both alleles, a rare occurrence. Newly recognized phenotypes such as isolated ataxia or dystonia, both responsive to carbohydrate load or to ketogenic diet, are receiving increasing attention, as the full phenotypical spectrum of the disease remains unknown. The hallmark feature and main diagnostic parameter associated with the disease is hypoglychorrhachia (cerebrospinal fluid glucose usually below 40 mg/dL or 2.2 mM). Supportive evidence comes from a characteristic positron emission tomography pattern of globally diminished uptake of fluorodeoxyglucose with thalamocortical depression accentuating a relatively increased basal ganglia uptake. The disease may be further confirmed by assaying GLUT1-mediated glucose uptake directly in patient erythrocytes, followed by sequence analysis of GLUT1. New mutations continue to be identified at a constant pace and some mutational hotspots have been discovered; yet, genotype/phenotype correlations remain elusive.⁷ Treatment generally involves discontinuation of anticonvulsants, which are often either ineffective or detrimental, and supplementation with carbohydrate-rich compounds, lipoic acid (known to enhance the expression of GLUT1 in vitro) or, alternatively, the strict administration of a ketogenic diet to provide alternative substrates. It appears that the neurological abnormalities set in very early in infancy and, despite the responsiveness of epilepsy to the ketogenic diet, significant cognitive and motor disabilities persist as an invariant disabling feature.⁸

Menkes Disease

Mutation of the copper ATPase ATP7A, located in the trans-Golgi network and encoded by the X chromosome, causes the progressive copper deficiency disorder Menkes disease.⁹ Also known as kinky hair disease, Menkes disease is associated with copper deficiency, in contrast with Wilson disease, which is characterized by copper excess (see Chapter 108). ATP7A allows cellular copper to cross intracellular membranes and to be translocated from the trans-Golgi network to the plasma membrane in the presence of extracellular copper. The fundamental abnormality in this disease is thus the maldistribution of copper, which is unavailable as a cofactor of several enzymes including mitochondrial cytochrome c oxidase (see Chapter 88), lysyl oxidase, superoxide dismutase, dopamine beta-hydroxylase, and tyrosinase. Thus, the main features of the disease include mitochondrial respiratory chain dysfunction (complex IV deficiency); deficiency of collagen cross-links resulting in hair (pili torti and trichorrhexis nodosa) and vascular abnormalities (elongated cerebral vessels and subdural effusions); neuronal degeneration (markedly affecting Purkinje cells); and deficient melanin production.¹⁰ Affected neonates present with hypothermia, feeding difficulties, and seizures. The infants are pale and exhibit kinky hair. Serum copper concentration is low, and the ratio of urinary homovanillic acid/vanillylmandelic acid is elevated.¹¹ A variety of minimally symptomatic phenotypes, including ataxia or mental retardation, have been recognized. Intramuscular or subcutaneous administration of copper-histidine affords protection against intellectual deterioration but is less effective in preventing other somatic complications.¹²

Segawa Disease (Dopa-Responsive Dystonia)

Autosomal dominant mutations of the guanosine triphosphate cyclohydrolase I (GCH-I) gene, located in chromosome 14, cause the treatable dystonic syndrome known as Segawa disease. The fundamental biochemical abnormality is a decrease of tetrahydrobiopterin (BH4) associated with reduced tyrosine hydroxylase activity, leading to deficient dopaminergic transmission and extrapyramidal dysfunction.¹³ Tyrosine hydroxylase synaptic activity fluctuates throughout the day and decreases after the third decade of life: this probably accounts for the marked diurnal progression of symptoms and for the clinical stabilization observed after the fourth decade. Initial symptoms are often gait difficulties due to foot equinovarus posturing. Postural dystonia and tremor and small stature dominate the clinical symptomatology and can be prevented by administration of levodopa. Marked intrafamilial symptom severity variability exists, and nondystonic family members may suffer from major depressive disorder or obsessive-compulsive disorder responsive to enhancers of serotonergic neurotransmission and to levodopa administration. Sleep disorders, including difficulty falling asleep, excessive sleepiness, and frequent disturbing nightmares, are also features of this patient population.¹⁴ Autosomal recessive Segawa syndrome is due to mutations in the TH gene and causes early-onset parkinsonism responsive to levodopa, a more severe phenotype.¹⁵ Measurement of both total biopterin (most of which exists as BH4) and neopterin (the byproduct of the GTPCH1 reaction) in cerebrospinal fluid reveals that both compounds are decreased, a useful diagnostic clue to GCH-I deficiency.¹⁶ Decreased activity of GCH-I in stimulated mononuclear blood cells and fibroblasts further supports the diagnosis. Oral phenylalanine load can reveal a subclinical defect in phenylalanine metabolism due to liver BH4 deficiency in patients with Segawa disease.

Pyruvate Dehydrogenase Deficiency

Defects in the pyruvate dehydrogenase (PDH) complex are an important cause of lactic acidosis. PDH is a large mitochondrial matrix enzyme complex that catalyzes the oxidative decarboxylation of pyruvate to form acetyl-coenzyme A (CoA), nicotinamide adenine dinucleotide (NADH), and CO₂. Symptoms vary considerably in patients with PDH complex deficiencies, and almost equal numbers of affected males and females have been identified, despite the location of the PDH E1 alpha subunit gene (PDHA1) in the X chromosome, owing to selective female X-inactivation.¹⁷ Thus, the mechanisms for the clinical variation observed in E1 alpha deficiency patients and its resemblance to a recessive disease are mutation severity in males and the pattern of X-inactivation in females.¹⁸ Several dozen PDHA1 mutations have been identified. Patients harboring mutations in the E1 beta subunit, the E2 dihydrolipoyl transacetylase segment of the complex, the E3-binding protein, the lipoyl-containing protein X, and the PDH phosphatase have been reported.¹⁹ Neurodevelopmental abnormalities, microcephaly, epilepsy, and agenesis of the corpus callosum are characteristic features.²⁰ Infants may exhibit facial features of fetal alcoholic syndrome, and older children can present with intermittent weakness or alternating hemiplegia. Diagnosis of these disorders requires measurements of lactate and pyruvate in plasma and cerebrospinal fluid, analyses of amino acids in plasma and organic acids in urine, as well as neuroradiological investigations, including magnetic resonance spectroscopy to detect lactate. Enzymatic analysis of fibroblast PDH activity can be performed and molecular diagnosis is available. A ketogenical diet is recommended together with thiamine supplementation, which can afford a substantial response in responsive cases.²¹

Pyruvate Carboxylase Deficiency

Pyruvate carboxylase is an autosomal recessive disease due to mutation of the PC gene, located in chromosome 11. Pyruvate carboxylase catalizes the conversion of pyruvate to oxaloacetate in the presence of abundant acetyl-CoA, replenishing Krebs cycle intermediates in the mitochondrial matrix. The enzyme is bound to biotin. PC is involved in gluconeogenesis, lipogenesis and neurotransmitter synthesis.²² PC deficiency can manifest with three degrees of phenotypical severity: an infantile form (A) with infantile moderate lactic acidosis, mental and motor deficiencies, hypotonia, pyramidal tract dysfunction, ataxia, and seizures leading to death in infancy. Episodes of vomiting, acidosis and tachypnea can be triggered by metabolic inbalance or infection. A severe neonatal form (B) manifests with severe lactic acidosis, hypoglycemia, hepatomegaly, depressed consciousness, and severely abnormal development. Abnormal limb and ocular movements are common findings. Brain magnetic resonance imaging reveals cystic periventricular leukomalacia. Hyperammonemia and depletion of intracellular aspartate and oxaloacetate are profound. Early death is common. A rare benign form (C) causes episodic acidosis and moderate mental impairment compatible with survival and near normal neurological performance. A variety of mutations have been identified, with mosaicism probably accounting for the less severe phenotypes.²³ Enzymatic analysis of fibroblast PC activity can be performed, but molecular diagnosis can be complicated by mosaicism. Dietary modification with triheptanoin (a triglyceride) supplementation has been attempted as a means to increase acetyl-CoA and anaplerotic propionyl-CoA.²⁴ Liver transplantation has also been performed.²⁵

Glycosylation Disorders

Congenital disorders of glycosylation (CDG) are a group of autosomal recessive diseases defined by abnormal glycosylation of N-linked oligosaccharides.²⁶ Well over a dozen enzymes involved in the N-linked oligosaccharide synthetic pathway can be mutated, causing a variety of manifestations. In some cases, the phenotypes are incompletely known, as only a small number of patients have been studied whereas, in others, novel enzyme deficiencies are periodically reported.²⁷ Thus, genotype: phenotype correlations are still preliminary. CDG-Ia, the most common type of CDG, is due to phosphomannomutase 2 deficiency. Salient manifestations include inverted nipples, abnormal subcutaneous fat distribution and cerebellar hypoplasia. The clinical course has been divided into an infantile multisystem stage in which all somatic organs can be affected, a late-infantile and childhood ataxia/mental retardation stage, during which neuropathy, retinitis pigmentosa, and stroke-like episodes can manifest, and an adult stable disability stage. CDG-Ib is caused by mannose phosphate isomerase deficiency. Salient features include cyclical vomiting, hypoglycemia, hepatic fibrosis, and protein-losing enteropathy, occasionally associated with coagulation disturbances without neurological involvement. CDG-Ic is due to deficiency of man(9)GlcNAc(2)-PP-dolichyl-alpha-1,3-glucosyltransferase and is associated with hypotonia, intellectual deficits, ataxia, strabismus, and epilepsy.²⁸ The diagnosis of all types of CDG can be reached by analyzing serum transferrin glycoforms by isoelectric focusing to determine the number of sialylated N-linked oligosaccharide residues linked to the protein.²⁹ In select cases, molecular genetic analysis is feasible, including prenatal diagnosis. CDG-Ib is the only treatable type of CDG: mannose supplementation normalizes hypoproteinemia and coagulation defects and reverses both protein-losing enteropathy and hypoglycemia.

Organic Acidurias

The organic acidemias (or organic acidurias) are disorders characterized by the urinary excretion of nonamino organic acids, which result from the abnormal amino acid catabolism of branched chain amino acids or lysine. These disorders include, but are not limited to, maple syrup urine disease (MSUD), propionic acidemia, methylmalonic acidemia, isovaleric acidemia, 3-methylcrotonyl-CoA carboxylase deficiency, 3-hydroxy-3-methylglutaryl-CoA lyase deficiency, ketothiolase deficiency, glutaric aciduria type I, and succinic semialdehyde dehydrogenase deficiency, among other less well understood types.^30,³¹ Specific enzymatic defects are responsible for each disorder, but several acidurias are caused by more than one enzyme deficiency. They are all inherited in an autosomal recessive fashion and, not uncommonly, the first affected family member remains undiagnosed until a sibling experiences the same clinical symptoms. The most severe and common presentation is a toxic neonatal encephalopathy that necessitates prompt recognition and treatment. Newborns present with vomiting, poor feeding, and progressive neurological symptoms such as seizures, abnormal tone, and depressed consciousness, often leading to coma. Cerebral edema, leukoencephalopathy, perisylvian (opercular) hypotrophy, or basal ganglia necrosis are features frequently detectable in neuroimaging studies and may provide important diagnostic clues. Unrecognized children and adolescents can exhibit episodic ataxia, intellectual deficits, Reye syndrome, or psychiatric disturbances. Laboratory abnormalities include acidosis, ketosis, hyperammonemia, abnormal serum hepatic enzyme levels, hypoglycemia, and neutropenia. Secondary carnitine deficiency due to excessive excretion of acylcarnitine is common. The diagnosis is made by urine organic acid analysis, a method that is particularly sensitive when it is performed during clinical decompensation, as the pattern of urinary excretion may be normal during symptom-free intervening periods. Analysis of plasma amino acids may also help to distinguish among specific disorders, and direct enzyme activity measurements in lymphocytes or fibroblasts confirm the diagnosis. DNA sequence analysis is available for the most common disorders. Prenatal diagnosis relies on the analysis of amniotic fluid metabolites and it is simplified by DNA analysis in the context of a family in which a child has been previously diagnosed. Treatment relies on the replacement of enzyme substrates and precursors while meeting essential amino acid and caloric needs. Several special infant formulas are commercially available. Thiamine is used to treat thiamine-responsive MSUD and hydroxocobalamin to treat methylmalonic acidemia. Carnitine supplementation is used to correct secondary deficiency. In disorders of propionic acid metabolism, the periodic administration of antibiotics can reduce the production of propionate by intestinal flora.³² Hepatic or combined liver-renal transplantation has been attempted with moderate success in some of these disorders.

Urea Cycle Disorders

The urea cycle disorders result from defects in the metabolism of nitrogen, which is predominantly produced during the breakdown of proteins and other nitrogen-containing molecules. The urea cycle is the only source of endogenous arginine and it is the main clearance mechanism for waste nitrogen. Hyperammonemia is the defining feature of these disorders, that include deficiencies in the urea cycle enzymes carbamyl phosphate synthase I, ornithine transcarbamylase, argininosuccinic acid synthetase, argininosuccinic acid lyase and arginase, and the cofactor producer N-acetyl glutamate synthetase. With the exception of X-linked ornithine transcarboxylase deficiency, urea cycle disorders are inherited in an autosomal recessive fashion.³³ These disorders manifest in the neonatal period with cerebral edema, lethargy, anorexia, hyper- or hypoventilation, hypothermia, seizures, abnormal tone, respiratory alkalosis, and coma. In milder (or partial) urea cycle defects, ammonia accumulation may be triggered by illness, protein load, fasting, valproate administration, or stress at any later stage of life, resulting in mild elevations of plasma ammonia accompanying cyclical vomiting, lethargy, sleep disturbances, delusions, hallucinations, and psychosis. Slowly progressive spastic paraparesis and growth retardation can be manifestations of arginase deficiency.³⁴ A subset of carrier females manifest ornithine transcarboxylase deficiency owing to skewed X-inactivation, a state that may also lead to hyperammonemic crises during pregnancy or in the postpartum. A specific pattern of plasma amino acid abnormalities helps to arrive at the specific diagnosis. For example, glutamine, alanine, and asparagine are commonly elevated, whereas arginine may be reduced in all urea cycle disorders except in arginase deficiency, in which it is markedly elevated. Plasmatic citrulline and urinary orotic acid excretion also assist in dissecting the affected enzymatic pathway.³⁵ Enzyme activity assays, usually performed in liver tissue, are reserved for confirmatory diagnosis, whereas DNA sequencing analysis is available for most of these disorders. The treatment during a crisis involves dialysis or other forms of plasma filtration aimed at reducing plasma ammonia concentration. Intravenous administration of arginine chloride and of the nitrogen scavengers sodium phenylacetate and sodium benzoate blocks the production of ammonia. Longterm administration of oral sodium phenylbutyrate and arginine increase the excretion of nitrogen by providing an alternative pathway.³⁶ Nevertheless, dietary protein restriction constitutes the mainstay of maintenance therapy.

Galactosemia

Galactosemia is caused by deficiency of the enzyme galactose-phosphate uridyltransferase (GALT), which catalyzes the production of glucose-1-phosphate and uridyldiphosphate (UDP)-galactose from galactose-1-phosphate and UPD-glucose. The disorder is inherited in an autosomal recessive fashion and is always attributable to mutations in the GALT gene in chromosome 9. Within days of starting to feed milk or lactose-containing formulas, affected infants experience feeding difficulties, hypoglycemia, hepatic dysfunction, bleeding diathesis, jaundice, and hyperammonemia. When untreated, sepsis and death may occur. Those infants who survive but continue to ingest galactose develop intellectual deficits and cortical and cerebellar tract signs. Despite early initiation of dietary therapy, the longterm outcome can include cataracts, poor growth, language dysfunction, extrapyramidal signs and ataxia, and ovarian failure.³⁷ The diagnosis is established by measuring erythrocyte GALT activity and by isoelectric focusing of the enzyme. All newborn screening programs typically include galactosemia and, thus, the disease should be readily identified before becoming symptomatic.³⁸ Biochemical and molecular genetic tests are widely used for heterozygote detection and prenatal diagnosis.³⁹ Assay of erythrocyte galactose-1-phosphate concentration, measurement of urinary galactitol, and estimation of total body oxidation of ¹³C-galactose to ¹³CO₂ are used to quantify residual enzyme function and to monitor the response to dietary adjustments over time. The mainstay of therapy is lactose restriction, which rapidly reverses liver disease in newborns. On diagnosis, infants are immediately offered a lactose-free, soy-based formula that contains sucrose, fructose, and other nongalactose complex carbohydrates.

Phenylketonuria

Classic phenylketonuria (PKU) is caused by near-complete deficiency of phenylalanine hydroxylase activity leading to hyperphenylalaninemia. The phenylalanine hydroxylase gene, PAH, is located in chromosome 12 and mutations in PAH are inherited in an autosomal recessive fashion. PKU was the first metabolic cause of mental retardation to be identified and is routinely screened for in all newborns.^40,⁴¹ It is also an example of a disorder fully treatable by dietary restriction. A small proportion of infants with hyperphenylalaninemia have an underlying impaired synthesis or recycling of tetrahydrobiopterin (BH4) in the presence of a normal PAH gene, a condition that is independently treatable.⁴² Classic untreated PKU leads to microcephaly, epilepsy, and severe intellectual and behavioral disabilities. The excretion of excessive phenylalanine and its metabolites can confer a musty odor to the skin, and the associated inhibition of tyrosinase causes decreased skin and hair pigmentation. Patients also exhibit decreased myelin formation and deficient production of dopamine, norepinephrine, and serotonin. Motor disability can be prominent later in life. Untreated maternal PKU can produce congenital heart disease, intrauterine and postnatal growth retardation, microcephaly, and mental retardation in the offspring. The diagnosis is based on plasma phenylalanine measurement and DNA sequence analysis. Prenatal diagnosis using amniocytes is available. PKU treatment consists of dietary restriction of phenylalanine.⁴³ A fraction of patients with primary phenylalanine hydroxylase deficiency respond to the 6R-BH4 isomer, which may act by enhancing residual enzyme function.⁴⁴

Lesch-Nyhan Disease

Among the inherited disorders of purine and pyrimidine metabolism, Lesch-Nyhan disease, caused by hypoxanthine-guanine phosphoribosyltransferase (HPRT) deficiency is the most common. The enzyme, encoded by the HPRT1 gene in the X chromosome, catalyzes the conversion of hypoxanthine to inosinic acid (IMP) and of guanine to guanylic acid (GMP) in the presence of phosphoribosylpyrophosphate, recycling purines derived from DNA and RNA.⁴⁵ HPRT1 mutations diminish enzyme function or abundance and lead to uric acid overproduction. In addition to having hyperuricemia, hyperuricuria, and renal stones, male patients manifest abnormal neurological development during infancy. Hypotonia and failure to accomplish early motor milestones such as sitting, crawling, or walking can be prominent features. Later in childhood, other symptoms emerge, including abnormal involuntary movements such as dystonia, choreoathetosis, opisthotonus, and ballismus. Pyramidal tract dysfunction includes spasticity, hyperreflexia, and Babinski signs. Profound intellectual deficits and self-injurious behavior can be prominent as are other motor compulsions. Females are carriers of HPRT1 mutations and can manifest increased uric acid excretion. They may show symptoms of the disease when nonrandom X-chromosome inactivation or skewed inactivation of the normal HPRT1 allele occur.⁴⁶ A urinary urate-to-creatinine ratio above 2 is characteristic of the disease, as is an excessive urinary excretion of urate. Defective HPRT enzyme activity can be measured in blood cells, fibroblasts, or lymphoblasts. DNA sequencing detects mutations in virtually all cases. Treatment aims to restrain uric acid overproduction with allopurinol, which inhibits the conversion of hypoxanthine and xanthine to uric acid mediated by xanthine oxidase. Bone marrow transplantation seem to be of only limited value in correcting hyperuricemia and improving neurobehavioral symptoms.⁴⁷

Pantothenate Kinase Deficiency

Also known as pantothenate kinase-associated neurodegeneration (PKAN) and formerly called Hallervorden-Spatz disease, pantothenate kinase deficiency causes neuronal degeneration associated with cerebral iron accumulation. This disorder is caused by the absence of pantothenate kinase 2, which is encoded by the PANK2 gene located in chromosome 20, and participates in CoA biosynthesis, catalyzing the phosphorylation of pantothenate (vitamin B₅), N-pantothenoyl-cysteine, and pantetheine.⁴⁸ Accumulation of N-pantothenoyl-cysteine and pantetheine may induce cell toxicity directly or via free radical damage by chelating iron. Deficient pantothenate kinase 2 may also be predicted to result in CoA depletion and defective membrane biosynthesis in vulnerable cells such as rod photoreceptors. Accumulation of iron is specific to the globus pallidus and substantia nigra. Axonal spheroids, believed to represent swollen axons secondary to defective axonal transport, appear in the pallidonigral system, in the subthalamic nucleus, and in peripheral nerves.⁴⁹ Patients first present in early childhood with dystonia that interferes with ambulation, associated with dysarthria, rigidity, pigmentary retinopathy and pyramidal tract dysfunction with spasticity and Babinski signs. Intellectual development may be variably affected. Psychiatric symptoms, including personality changes with impulsivity, depression, and emotional lability, are common. A specific brain magnetic resonance imaging abnormality, the eye-of-the-tiger sign, is characteristic of the disease, with rare exceptions.⁵⁰ Hypoprebetalipoproteinemia and acanthocytosis may be additional manifestations of PANK2 mutations.⁵¹ The diagnosis relies on clinical and magnetic resonance imaging features. When both are consistent with PKAN, there is a high likelihood of identifying a pathogenic mutation in PANK2 by DNA sequencing, although large chromosomal deletions affecting one allele are likely to remain undetected by this method. Treatment strategies, including pantothenate (or phosphopantothenate) administration, have been advanced but not tested.

Smith-Lemli-Opitz Syndrome

Smith-Lemli-Opitz syndrome is a malformative autosomal recessive disorder caused by abnormal cholesterol metabolism resulting from deficiency of the enzyme 7-dehydrocholesterol reductase, which, in turn, is due to mutations of the DHCR7 gene, located in chromosome 11. Decreased activity of 7-dehydrocholesterol reductase results in failure to convert 7-DHC to cholesterol, elevated serum concentration of 7-dehydrocholesterol or elevated 7-dehydrocholesterol/cholesterol ratio. Patients show hypotonia and prenatal and postnatal growth retardation, microcephaly with intellectual deficiency and multiple malformations, including a characteristic facies (temporal narrowing, downslanting palpebral fissures, epicanthal folds, blepharoptosis, anteverted nares, cleft palate, and micrognathia), cardiac defects, underdeveloped external genitalia (hypospadias, bilateral cryptorchidism and undermasculinization resulting in female external genitalia), postaxial polydactyly, and two- or three-toe syndactyly. Holoprosencephaly can be an associated manifestation.^52,⁵³ Tandem mass spectrometry of dried blood card samples readily identifies patients and may be used for newborn screening. Direct analysis of the DHCR7 gene by DNA sequencing confirms the presence of a mutation in most cases.⁵⁴ Indicative prenatal clues on ultrasound examination include cardiac defects, cleft palate, genital abnormalities, growth retardation, or apparent female phenotype with a known 46,XY karyotype.^55,⁵⁶ The combination of low concentrations of unconjugated estriol, HCG, and α-fetoprotein on routine maternal serum testing at 16 to 18 weeks’ gestation is also suggestive of maternal carrier status and thus places the fetus at risk for Smith-Lemli-Opitz syndrome. Measurement of 7-dehydrocholesterol levels in amniotic fluid is available for prenatal diagnosis. Treatment with cholesterol supplementation and bile acids improves growth. The addition of the HMG-CoA reductase inhibitor simvastatin helps reduce serum 7-dehydrocholesterol.

Ataxia-Teleangiectasia

Ataxia-teleangiectasia (A-T) is due to mutation of the ATM (ataxiatelangiectasia mutated) gene located in chromosome 11 and is inherited in an autosomal recessive fashion. The ATM protein is a serine-protein kinase that is activated by double-stranded DNA breaks and coordinates cell cycle checkpoints prior to repair.⁵⁷ Hundreds of unique (private) mutations have been identified, which diminish ATM RNA abundance and impart a dominant negative potential to ATM containing missense mutations. The manifestations are dominated by progressive cerebellar ataxia beginning between 1 and 4 years of age (A-T is the most common cause of progressive cerebellar ataxia in childhood), oculomotor apraxia, frequent infections, choreoathetosis, telangiectasias of the conjunctivae, immunodeficiency, and increased risk for malignancy, particularly leukemia and lymphoma. Individuals with A-T are unusually sensitive to ionizing radiation. Children present with signs of cerebellar dysfunction shortly after learning to walk, including slurred speech and oculomotor apraxia. During early childhood, these deficits stabilize or improve for several years, until cerebellar degeneration occurs. Loss of Purkinje cells and depletion of granule cells are prominent, as is the enlargement of all cellular nuclei. Patients are typically confined to a wheelchair before the second decade of life, when tremor, chorea, myoclonus, and neuropathy with absent reflexes become apparent. Intelligence is generally preserved. The risk of malignancy is 38% and immune deficiency is common. Immunoglobulin subclass deficiency and thymic hypoplasia are also common, but the opportunistic infections typical of other immune deficiencies are rare.⁵⁸ The diagnosis of A-T relies on several tests: (1) elevation of serum α-fetoprotein (of predominantly hepatic origin), (2) immunoassay of ATM abundance, (3) radiosensitivity assay of colony-forming lymphoblastoid cells obtained from blood, (4) measurement of ATM kinase activity, and (5) ATM DNA sequence analysis.⁵⁹ Iron chelators and aminoglycoside antibiotics, which reduce translational fidelity in cells harboring missense ATM mutations, are therapies under investigation.⁶⁰

Friedreich Ataxia

Friedreich ataxia is characterized by slowly progressive ataxia with onset before the third decade of life, associated with depressed tendon reflexes, dysarthria, Babinski signs, and loss of propioception and vibration senses. Alternative manifestations include later onset or preserved tendon reflexes. Optic atrophy, cardiomyopathy and diabetes mellitus or glucose intolerance are associated manifestations. Loss of ambulation and severe disability occur before the third decade.^61,⁶² Most patients harbor mutations in the FRDA gene, usually in the form of a GAA triplet repeat expansion in intron 1, although compound heterozygosity with other FRDA mutations may also occur. The disease is inherited in an autosomal recessive fashion and is caused by the expansion of the triplet repeat normally present in chromosome 9. Normal FRDA alleles contain 5 to 33 GAA repeats, whereas premutation alleles (mutable normal alleles) contain 34 to 65 repeats: these may expand during parental transmission, resulting in disease-causing alleles that span 66 to 1700 repeats. FRDA encodes frataxin, a participant in iron-sulfur cluster biogenesis, and therefore in the synthesis of enzymes such as respiratory chain complexes I to III and aconitases.⁶³ Frataxin also regulates mitochondrial iron, perhaps by mediating mitochondrial iron efflux, leading to abnormal iron deposition in Friedreich ataxia. Patients have low blood levels of antioxidant enzymes, exhibiting a redox shift from the free form of glutathione to the protein-bound form. They also excrete large amounts of urinary 8-hydroxy-2′-deoxyguanosine and have elevated serum malondialdehyde, which are indirect indicators of oxidative DNA damage and lipid peroxidation, respectively.^64,⁶⁵ Myocardial energy production is also defective and associates with excessive ventricular wall thickness, a phenomenon that can be ameliorated with the administration of the antioxidant idebenone.⁶⁶ The diagnosis relies on the identification of excessive repeats by polymerase chain reaction or Southern blot. DNA sequencing is reserved for other compounded FRDA mutations.