Dissecting the genetics of type 1 diabetes: relevance for familial clustering and differences in incidence.

A combination of genetic and environmental factors is most likely the cause of Type 1 diabetes. Results from twin data, familial clustering of the disease and difference in incidence according to ethnicity infer the presence of specific disease genes. The genetic component of Type 1 diabetes cannot be classified according to a classical model of inheritance but is due to an interaction between different genes and environmental factors. The major genes are within the HLA region that are responsible for 40% of the genetic susceptibility, although other genes are important (non-HLA genes). To date, more than 10 specific loci have been localized on different chromosomes. The gene involved has been characterized only for two of such loci, IDDM1 and IDDM2, while in the other cases the presence of some susceptibility genes can be envisaged and their identification represents the goal of genetic research in coming years. Fine mapping of the loci will certainly increase our understanding of the genetics of Type 1 diabetes; the limitation in detecting some of the remaining genes by linkage studies can be overcome by association studies. That is possible via the collection of a large number of affected families (over 1000) in homogeneous populations.