Susceptibility Versus Causative Genes
As the study of common and genetically complex human diseases identifies the significant contribution of heredity in their development, it is likely that more genes or genetic risk factors will be found to affect susceptibility to disease rather than the more traditionally considered causative genes. Historical successes in the localization of genes have been with diseases whose mode of inheritance is known (as illustrated above). These disorders are often highly or completely penetrant and are due to a defect in a single gene, yet these Mendelian disorders are often relatively rare in the population. However, in recent years, genomic research has uncovered genetic risk factors for many diseases that were suspicious for genetic etiology but were unexplained by traditional Mendelian cause and effect. Some of the most common and deadly diseases of society, such as cardiovascular disease, cancer, and obesity, have significant genetic components that are evident from non‐random family clustering. These diseases are termed “complex” because they are likely due to the interaction of multiple factors, both environmental and genetic. Susceptibility genes for such complex disorders are substantially harder to identify than genes responsible for Mendelian disorders.
A well‐characterized example of a susceptibility locus is that of the apolipoprotein E (APOE) gene and Alzheimer disease (AD). The APOE gene on chromosome 19 has three different alleles, scored as 2, 3, and 4, which occur with frequencies 6%, 78%, and 16% in most European populations, respectively (e.g. Saunders et al. 1993). These alleles differ in their DNA sequence by only one base at codons 112 and/or 158 (Figure 2.11). The APOE 4 allele increases risk and decreases age of onset in familial and sporadic late‐onset AD and early‐onset sporadic AD. The 2 allele has been shown to be protective to some extent for risk to develop AD (Corder et al. 1994, 1995a,b; Farrer et al. 1997). It is important to note that for APOE and AD, the 4 allele is not by itself sufficient or necessary for the development of AD but has been shown to be associated with increased susceptibility to AD.
Table 2.4 Salient features of human repeat expansion diseases.
| Condition | Gene symbol | Repeat type | Repeat localization | Repeat number abnormal range | Inheritance pattern | Clinical features |
|---|---|---|---|---|---|---|
| Fragile X syndrome | FRAXA | CGG | 5’ Untranslated region | 200–1000 (premutation range of 52–200) | X‐linked | Moderate to severe mental retardation, behavioral abnormalities, macroorchidism, large ears, and prominent jaw |
| Huntington disease (HD) | HD | CAG | Open reading frame | >36 (premutation range 27–35) | Autosomal dominant; expansion more common in paternal allele | Choreiform movements, dystonia, psychiatric illness, cognitive decline, dementia |
| Myotonic dystrophy | DM1 | CTG | 3’ Untranslated region | Mild: 50–150 Classic: ~100–1000 Congenital: >2000 | Autosomal dominant; expansion more common in maternal allele | Weakness, myotonia, ptosis, cataracts, cardiac arrhythmia; endocrine abnormalities, frontal balding |
| Friedrich Ataxia | FXN | GAA | Intron | 66–1700 uninterrupted repeats (premutation range of 34–65 uninterrupted repeats) | Autosomal recessive | Ataxia, sensory loss, weakness, diabetes mellitus, cardiomyopathy |
Figure 2.11 Single base pair changes in exon 4 of APOE define the 2, 3, and 4 alleles at this locus.
(Source: Modified from Pericak‐Vance and Haines (1995).)
Summary
The study of genes, chromosomes, and patterns of transmission of human traits within families has led to remarkable discoveries that are useful in genetic counseling for recurrence risk, presymptomatic testing, prenatal diagnosis (see Chapter 5), and in the understanding of the pathogenesis of diseases. The genetic basis of Mendelian disease is relatively straightforward and is well understood in many cases. The situation in common, complex diseases is markedly different from the study of Mendelian disease, since more than one gene as well as various non‐genetic factors are typically associated with trait phenotype expression. Yet, many of the same principles hold true in complex disease: Mendel’s laws regarding the transmission of genes and alleles at loci are as important to the study of resemblance between relatives in genetically complex disease as in Mendelian disease; the same holds true for the extent and result of the differing types of mutation. The potential rewards of localizing disease susceptibility genes for common, complex diseases and their underlying pathways are abundant, especially with respect to the prospects of complex disease prevention and treatment, genetic counseling, and genomic medicine.
References
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