Evidence of Genetic Susceptibility in Disease Pathogenesis
There are considerable epidemiological data that implicate genetic susceptibility in the pathogenesis of CD and UC. Most notably, these include the familial prevalence of IBD, concordance rates in twin pairs and ethnic differences in disease susceptibility. In fact, the studies of twin pairs have provided the strongest impetus towards further investigation of genetic susceptibility in IBD. Three studies have been carried out in Europe, including Tysk’s important review of the Swedish Twin Registry in 1988.^1–3 The data from these studies, in Sweden, Denmark and the UK, combine to provide powerful evidence for the role of both genetic and environmental factors in disease susceptibility. The concordance rates for CD in monozygotic and dizygotic twin pairs from these studies are estimated to be 37 and 7%, respectively; in UC, the equivalent results are 10 and 3%, respectively. From these data, the co-efficient of heritability (an estimate of the proportion of susceptibility attributable to inherited factors alone) may be calculated. The derived co-efficient of heritability in CD is similar to the value calculated from twin studies carried out in insulin-dependent diabetes and multiple sclerosis, and in fact is stronger than in many other common complex diseases. The genetic contribution is clearly less strong in UC, but the concordance between monozygotic and dizygotic prevalence rates again argues for a genetic and perhaps a stronger relative environmental contribution to disease susceptibility in UC.

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