HLA disease associations: models for the study of complex human genetic disorders

The genes of the human leukocyte antigen (HLA) region, the major histocompatibility complex (MHC) of humans, control a variety of functions involved in immune response and influence susceptibility to over 40 diseases. Theoretical studies in the development of models to determine the modes of inheritance of the HLA-associated diseases have led to a better understanding of the inheritance patterns in insulin-dependent diabetes mellitus (IDDM), rheumatoid arthritis, multiple sclerosis, ankylosing spondylitis, hemochromatosis, celiac disease, and others. It is now clear that many of the HLA-associated diseases involve heterogeneity in their HLA components, as well as non-HLA genetic factors. This review is presented using HLA-associated diseases, and in particular IDDM, as the example of interest, but the observations and techniques presented have direct relevance to the study of all human diseases with a complex genetic component. Three methods for localizing disease-predisposing genes are presented: (1) association studies, including population, family, and relative predispositional effects, (2) affected sib pair and other affected-relative methods, and (3) lod score analysis. A variety of complementary methods for studying the mode(s) of inheritance of the alleles at the disease-predisposing locus and for identifying the alleles and amino acids directly involved in the disease process also are presented.