Haplotype analysis of hemochromatosis: Evaluation of different linkage- disequilibrium approaches and evolution of disease chromosomes

We applied several types of linkage-disequilibrium calculations to analyze the hereditary hemochromatosis (hh) locus. Twenty-four polymorphic markers in the major histocompatibility complex (MHC) class I region were used to haplotype hh and normal chromosomes. A total of 169 hh and 161 normal chromosomes were analyzed. Disequilibrium values were found to be high over an unusually large region beginning 150 kb centromeric of HLA-A and extending nearly 5 Mb telomeric of it. Recombination in this region was ~28% of the expected value. This low level of recombination contributes to the unusually broad region of linkage disequilibrium found with hh. The strongest disequilibrium was found at locus HLA-H (δ = .84) and at locus D6S2239 (δ = .85), a marker ~10 kb telomeric to HLA-H. All disequilibrium methods employed in this study found peak disequilibrium at HLA-H or D6S2239. The cys282tyr mutation in HLA-H, a candidate gene for hh, was found in 85% of disease chromosomes. A haplotype phylogeny for hh chromosomes was constructed and suggests that the mutation associated with the most common haplotype occurred relatively recently. The age of the hh mutation was estimated to be ~60-70 generations. Disequilibrium was maintained over a greater distance for hh-carrying chromosomes, consistent with a recent mutation for hh. Our data provide a reasonable explanation for previous difficulties in localizing the hh locus and provide an evolutionary history for disease chromosomes.