Screening for Genetic Disease

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CHAPTER 20 Screening for Genetic Disease

Genetic disease affects individuals and their families dramatically but every person, and every couple having children, is at some risk of seeing a disorder with a genetic component suddenly appear. Our concepts and approaches to screening reflect the different burdens that these two realities impose. First, there is screening of individuals and couples known to be at significant or high risk because of a positive family history—sometimes referred to as targeted, or family, screening. This includes carrier, or heterozygote, screening, as well as presymptomatic testing. Second, there is the screening offered to the general population, who are at low risk—sometimes referred to as community genetics. Population screening involves the offer of genetic testing on an equitable basis to all relevant individuals in a defined population. Its primary objective is to enhance autonomy by enabling individuals to be better informed about genetic risks and reproductive options. A secondary goal is the prevention of morbidity resulting from genetic disease and alleviation of the suffering this would impose.

Screening Those at High Risk

Here we focus on the very wide range of general genetic disease as opposed to screening in the field of cancer genetics, which is addressed in Chapter 14. Prenatal screening is also covered in more detail in the next chapter. If it were easy to recognize carriers of autosomal and X-linked recessive disorders and persons who are heterozygous for autosomal dominant disorders that show reduced penetrance or a late age of onset, much doubt and uncertainty would be removed when providing information in genetic counseling. Increasingly, mutation analysis in genes that cause these disorders is indeed making the task easier. Where this is not possible, either because no gene test is available or the molecular pathology cannot be easily detected in a gene known to be associated with the disorder in question, a number of strategies and types of analysis is available to detect carriers for autosomal and X-linked recessive disorders, and for presymptomatic diagnosis of heterozygotes for autosomal dominant disorders.

Carrier Testing for Autosomal Recessive and X-Linked Disorders

In a number of autosomal recessive disorders, such as some of the inborn errors of metabolism (e.g., Tay-Sachs disease; p. 178) and the hemoglobinopathies (e.g., sickle cell disease; p. 159), carriers can be recognized with a high degree of certainty using biochemical or hematological techniques such that DNA analysis is not necessary. In other single-gene disorders, it is possible to detect or confirm carrier status by biochemical means in only a proportion of carriers; for example, mildly abnormal coagulation study results in a woman at risk of being a carrier for hemophilia (p. 309). A significant proportion of obligate carriers of hemophilia will have normal coagulation, however, so a normal result does not exclude a woman at risk from being a carrier.

There are several possible ways in which carriers of genetic diseases can be recognized.

Clinical Manifestations in Carriers

Occasionally, carriers for certain disorders can have mild clinical manifestations of the disease (Table 20.1), particularly with some of the X-linked disorders. These manifestations are usually so slight that they are apparent only on careful clinical examination. Such manifestations, even though minimal, are unmistakably pathological; for instance, the mosaic pattern of retinal pigmentation seen in manifesting female carriers of X-linked ocular albinism. Unfortunately, in most autosomal and X-linked recessive disorders there are either no manifestations at all in carriers, or they overlap with the variation seen in the general population. An example would be female carriers of hemophilia, who have a tendency to bruise easily—this is not a reliable sign of carrier status, as this is seen in a significant proportion of the general population, for different reasons. In X-linked adrenoleukodystrophy, a proportion of carrier females manifest neurological features, sometimes relatively late in life when the signs might easily be confused with the problems of aging. Thus clinical manifestations are helpful in detecting carriers only when they are unmistakably pathological, which is the exception rather than the rule with most single-gene disorders.

Table 20.1 Clinical and Biochemical Abnormalities Used in Carrier Detection of X-Linked Disorders

Disorder Abnormality
Clinical
Ocular albinism Mosaic retinal pigmentary pattern
Retinitis pigmentosa Mosaic retinal pigmentation, abnormal electroretinographic findings
Anhidrotic ectodermal dysplasia Sweat pore counts reduced, dental anomalies
Lowe syndrome Lens opacities
Alport syndrome Hematuria
Biochemical
Hemophilia A Reduced factor VIII activity : antigen ratio
Hemophilia B Reduced levels of factor IX
G6PD deficiency Erythrocyte G6PD activity reduced
Lesch-Nyhan syndrome fibroblasts Reduced hypoxanthine-guanine phosphoribosyl transferase activity in skin
Hunter syndrome Reduced sulfoiduronate sulfatase activity in skin fibroblasts
Vitamin D–resistant rickets Serum phosphate level reduced
Duchenne muscular dystrophy Raised serum creatine kinase level
Becker muscular dystrophy Raised serum creatine kinase level
Fabry disease Reduced α-galactosidase activity in hair root follicles

G6PD, Glucose 6-phosphate dehydrogenase.

Biochemical Abnormalities in Carriers

By far the most important approach to determining the carrier status for autosomal recessive and X-linked disorders has been the demonstration of detectable biochemical abnormalities in carriers of certain diseases. In some disorders, the biochemical abnormality seen is a direct product of the gene and the carrier status can be tested for with confidence. For example, in carriers of Tay-Sachs disease the range of enzyme activity (hexosaminidase) is intermediate between levels found in normal and affected people. In many inborn errors of metabolism, however, the enzyme activity levels in carriers overlap with those in the normal range, so that it is not possible to distinguish reliably between heterozygote carriers and those who are homozygous normal.

Carrier testing for Tay-Sachs disease in many orthodox Jewish communities, which are at significantly increased risk of the disorder, is highly developed. Because of faith-based objections to termination of pregnancy, carrier testing may be crucial in the selection of life partners. A couple considering betrothal will first see their rabbi. In addition to receiving spiritual advice, they will undergo carrier testing for Tay-Sachs disease. If both prove to be carriers, the proposed engagement will be called off, leaving them free to look for a new partner. If only one proves to be a carrier the engagement can proceed, although the rabbi does not disclose which one is the carrier. Although such a strategy to prevent genetic disease may be possible in many communities where inbreeding is the norm, and their ‘private’ diseases have been well characterized either biochemically or by molecular genetics, in practice this is very rare.

In many single-gene disorders, the biochemical abnormality used in the diagnosis of the disorder in the affected individual is not a direct result of action of the gene product but the consequence of a secondary or downstream process. Such abnormalities are further from the primary action of the gene and, consequently, are usually even less likely to be useful in identifying carriers. For example, in Duchenne muscular dystrophy (DMD) there is an increased permeability of the muscle membrane, resulting in the escape of muscle enzymes into the blood. A grossly raised serum creatine kinase (CK) level often confirms the diagnosis of DMD in a boy presenting with features of the disorder (p. 307). Obligate female carriers of DMD have, on average, serum CK levels that are increased compared with those of the general female population (Figure 20.1). There is, however, a substantial overlap of CK values between normal and obligate carrier females. Nevertheless, this information can be used in conjunction with pedigree risk information (p. 344) and the results of linked DNA markers (p. 345) to help calculate the likelihood of a woman being a carrier for this disorder.

image

FIGURE 20.1 Creatine kinase (CK) levels in obligate carrier females of Duchenne muscular dystrophy and women from the general population.

(Adapted from Tippett PA, Dennis NR, Machin D, et al 1982 Creatine kinase activity in the detection of carriers of Duchenne muscular dystrophy: comparison of two methods. Clin Chim Acta 121:345–359.)

There is another reason for difficulty with carrier testing in the case of X-linked recessive disorders. Random inactivation of the X chromosome in females (p. 103) means that many, often the majority, of female carriers of X-linked disorders cannot be detected reliably by biochemical methods. An exception to this involves analysis of individual clones to look for evidence of two populations of cells, as with peripheral blood lymphocytes in female carriers of some of the X-linked immunodeficiency syndromes (p. 204). In a clinical setting, this is referred to as ‘X-inactivation studies’.

Linkage between a Disease Locus and a Polymorphic Marker

DNA Polymorphic Markers

The advent of recombinant DNA technology revolutionized the approach to carrier detection. Relatively seldom, nowadays, do conventional biochemical and blood group polymorphisms have a role because there are few that are sufficiently informative to be of practical clinical value. The large number of different types of DNA sequence variants (p. 17) in the human genome means that, if sufficient numbers of families are available, linkage of any disease with a polymorphic DNA marker is possible, providing the disease is not genetically heterogeneous (see locus heterogeneity below). After this is achieved, the information can be applied to smaller families. The demonstration of linkage between a DNA sequence variant and a disease locus overcomes the need to identify a biochemical defect or protein marker and the necessity for it to be expressed in accessible tissues. In addition, use of markers at the DNA level also overcomes the difficulties that occur in carrier detection due to X-inactivation for women at risk for X-linked disorders (p. 103).

Linked polymorphic DNA markers were frequently used in determining the carrier status of females in families where DMD has occurred; nowadays, direct gene sequencing is more likely to be used. An example is shown in Figure 20.2; individual III3 wants to know whether she is a carrier and therefore at risk for having sons affected with DMD. Analysis of the pedigree reveals that her mother, II4, along with her sister, II1, and their mother, I2, are all obligate carriers of DMD. The family is informative for a polymorphic CA dinucleotide repeat (p. 69) in the closely flanking region 5′ to the dystrophin gene known as Dys 5′ II, which can be demonstrated by polymerase chain reaction (PCR) (p. 56). The mutation in the dystrophin gene in the family is segregating with allele 1 and, because individual III3 has inherited this allele from her mother, she is likely to be a carrier. Linked polymorphic DNA markers can be used for prenatal diagnosis to predict whether a male fetus is likely to be affected with DMD, even without knowing the specific dystrophin gene mutation in the affected male(s) (p. 308).

image

FIGURE 20.2 Family with Duchenne muscular dystrophy showing segregation of the CA repeat 5′ to the dystrophin gene known as Dys 5′ II.

(Courtesy J. Rowland, Yorkshire Regional DNA Laboratory, St. James’s Hospital, Leeds, UK.)

Potential Pitfalls with Linked Polymorphic DNA Markers

A number of potential pitfalls should be kept in mind with the use of linked polymorphic DNA markers.

Recombination

The first potential pitfall is the chance of recombination occurring between the polymorphic DNA marker and the disease locus to which linkage has been shown. The risk of a recombination can be minimized, in most instances, by the identification of either intragenic or closely linked markers on either side of the disease locus—termed flanking markers. In some instances, for example the dystrophin locus, there is a ‘hotspot’ for recombination (p. 308). Even with closely flanking or intragenic markers there appears to be a minimal chance of approximately 12% that recombination will occur in any meiosis in a female. The uncertainty introduced by this possibility needs to be accounted for when combining the results of linked polymorphic DNA markers with pedigree risks and the results of CK testing for women at risk of being carriers of DMD (p. 308).

Presymptomatic Diagnosis of Autosomal Dominant Disorders

Many autosomal dominant single-gene disorders either have a delayed age of onset (p. 341) or exhibit reduced penetrance (p. 340). The results of clinical examination, specialist investigations, biochemical studies, and family DNA studies can enable the genetic status of the person at risk to be determined before the onset of symptoms or signs. This is known as presymptomatic, or predictive, testing.

Clinical Examination

In some dominantly inherited disorders, simple clinical means can be used for presymptomatic diagnosis, taking into account possible pleiotropic effects of a gene (p. 109). For example, individuals with neurofibromatosis type I (NF1) can have a variety of clinical features (p. 298). It is not unusual to examine an apparently unaffected relative of someone with NF1, who has had no medical problems, only to discover that they have sufficient numbers of café-au-lait spots or cutaneous neurofibromas to confirm that they are affected. However, NF1 is a relatively rare example of a dominantly inherited disorder that is virtually 100% penetrant by the age of 5 or 6 years, with visible external features. With many other disorders, clinical examination is less helpful.

In tuberous sclerosis (TSC) a number of body systems may be involved and the external manifestations, such as the facial rash of angiokeratoma (Chapter 7; see Figure 7.5, A, p. 111) may not be present. Similarly, seizures and learning difficulties are not inevitable. In autosomal dominant polycystic kidney disease, which is extremely variable and may have a delayed age of onset, there may be no suspicion of the condition from routine examination, and hypertension may be borderline without raising suspicions of an underlying problem. Reaching a diagnosis in Marfan syndrome (p. 300) can be notoriously difficult because of the variable features and overlap with other joint hypermobility disorders, even though very detailed diagnostic criteria have been established.