Faculty Bibliography

When linkage between a recessive Mendelian disease and specific candidate genes is investigated, identity by descent scores in affected sib pairs may be used for tests of linkage and heterogeneity. Statistical tests for performing this analysis are presented. The efficiency and statistical power of the method are also investigated using computer simulations.

Forty-one multiplex families, from published sources and new data from the National Cancer Institute, segregating for Hodgkin's disease and HLA, have been studied. A reanalysis of these data strongly suggests a recessive mode of inheritance for susceptibility to Hodgkin's disease. The HLA haplotype sharing data between affected relatives demonstrate that approximately 60% of cases in multiplex families are due to an HLA-linked susceptibility gene, the remaining 40% being due to other familial factors. The data clearly support the hypothesis of etiological heterogeneity for Hodgkin's disease, with both HLA-linked and HLA-unlinked factors being responsible. Finally, there is an increased concordance of histological types between affected relatives, but this concordance seems independent of HLA sharing.

Segregation of familial cutaneous melanoma has been shown to be compatible with autosomal dominant transmission with incomplete penetrance. However, the combined phenotype of melanoma and a known melanoma-precursor lesion, the dysplastic nevus (DN), has not previously been found to fit a Mendelian model of inheritance using complex segregation analysis. Employing a life-table and disease-free survival analysis approach, we estimated the lifetime incidence of melanoma in the sibs and offspring of DN-affected individuals to be 46%, consistent with a highly penetrant, autosomal dominant mode of inheritance. To further elucidate the relationship between the two traits, we conducted a linkage analysis between the melanoma locus and a hypothetical DN locus, and obtained a maximum lod score of 3.857 at theta = .08. Furthermore, all families giving evidence for linkage were in the coupling phase and the maximum likelihood estimate of theta was not significantly different from 0 (P = .1). This provides evidence that the DN and melanoma traits may represent pleiotropic effects of a single, highly penetrant gene behaving in an autosomal dominant manner.

Haplotypes for four new restriction site polymorphisms (detected by Rsa I, Taq I, HincII, and Sac I) and a previously identified DNA length polymorphism (5' FP), all at the insulin locus, have been studied in U.S. Blacks, African Blacks, Caucasians, and Pima Indians. Black populations are polymorphic for all five markers, whereas the other groups are polymorphic for Rsa I, Taq I, and 5' FP only. The data suggest that approximately equal to 1 in 550 base pairs is variant in this region. The polymorphisms, even though located within 20 kilobases, display low levels of nonrandom association. Population genetic analysis suggests that recombination within this 20-kilobase segment occurs 24 times more frequently than expected if crossing-over occurred uniformly throughout the human genome. These findings suggest that population associations between DNA polymorphisms and disease susceptibility genes near the insulin gene or structural mutations in the insulin gene will be weak. Thus, population studies would probably require large sample sizes to detect associations. However, the low levels of nonrandom association increase the information content of the locus for linkage studies, which is the best alternative for discovering disease susceptibility genes.