A candidate gene for hemochromatosis: frequency of the C282Y and H63D mutations.

The gene whose alteration causes hereditary hemochromatosis (HFE according to the international nomenclature) was, more than 20 years ago, shown to map to 6p21.3. It has since escaped all efforts to identify it by positional cloning strategies. Quite recently, a gene named HLA-H was reported as being responsible for the disease. Two missense mutations, Cys282Tyr (C282Y) and His63Asp (H63D), were observed, but no proof was produced that the gene described is the hemochromatosis gene. To validate this gene as the actual site of the alteration causing hemochromatosis, we decided to look for the two mutations in 132 unrelated patients from Brittany. Our results indicate that more than 92% of these patients are homozygous for the C282Y mutation, and that all 264 chromosomes but 5 carry either mutation. These findings confirm the direct implication of HLA-H in hemochromatosis.

Clinical and family studies in genetic hemochromatosis: microsatellite and HFE studies in five atypical families.

A candidate gene (HFE) has been described for hereditary hemochromatosis on chromosome 6. The study of well-defined atypical hemochromatosis families using genetic markers may increase our understanding of the sensitivity and the specificity of genotyping in hemochromatosis. One hundred and thirteen Canadian families with genetic hemochromatosis were surveyed to find atypical families as possible examples of people with genetic recombinations. All families underwent clinical investigations including iron studies and HLA typing. Each individual was typed at three polymorphic microsatellite loci (D6S105, D6S1260, and D6S299) on chromosome 6. Sixteen subjects were studied for the two missense mutations described for the candidate gene for hemochromatosis (C282Y, H63D). There were eight HLA-identical siblings found in four different families (five men, three women; age range 30-72) with normal transferrin saturation and ferritin levels. There were two patients identified who were homozygous for the C282Y mutation without biochemical evidence of iron overload, and two patients with no evidence of the mutation with significant iron overload. Our conclusions are as follows: 1) finding HLA-identical siblings without iron overload does not confirm a genetic recombination, 2) difficulties in phenotypic definition of disease and the description of new iron overload syndromes that may differ from classical genetic HC cause complicated genetic studies, and 3) finding iron-loaded patients without a C282Y mutation and patients that are homozygous for the C282Y mutation without evidence of iron overload may limit the use of genotyping in population screening for hemochromatosis.

Linkage disequilibrium and extended haplotypes in the HLA-A to D6S105 region: implications for mapping the hemochromatosis gene (HFE).

The hemochromatosis gene (HFE) maps to 6p21.3, in close linkage with the HLA Class I genes. Linkage disequilibrium (LD) studies were designed to narrow down the most likely candidate region for HFE, as an alternative to traditional linkage analysis. However, both the HLA-A and D6S105 subregions, which are situated 2-3 cM and approximately 3 Mb apart, have been suggested to contain HFE. The present report extends our previous study based upon the analysis of a large number of HFE and normal chromosomes from 66 families of Breton ancestry. In addition to the previously used RFLP markers spanning the 400-kb surrounding HLA-A, we examined three microsatellites: D6S510, HLA-F, and D6S105. Our combined data not only confirm a peak of LD at D6S105, but also reveal a complex pattern of LD over the i82 to D6S105 interval. Within our ethnically well-defined population of Brittany, the association of HFE with D6S105 is as great as that with HLA-A, while the internal markers display a lower LD. Fine haplotype analysis enabled us to identify two categories of haplotypes segregating with HFE. In contrast to the vast majority of normal haplotypes, 50% of HFE haplotypes are completely conserved over the HLA-A to D6S105 interval. These haplotypes could have been conserved through recombination suppression, selective forces and/or other evolutionary factors. This particular haplotypic configuration might account for the apparent inconsistencies between genetic linkage and LD data, and additionally greatly complicates positional cloning of HFE through disequilibrium mapping.

Prevalence of the C282Y mutation in Brittany: penetrance of genetic hemochromatosis?

Hemochromatosis (GH) is an inborn error of iron metabolism, characterized by progressive iron loading that, if untreated, causes high morbidity and death. The gene responsible for the disease (HFE), located 4.5 megabases telomeric to the HLA-A locus, encodes a protein homologous to class I MHC molecules. A main mutation, C282Y, has been identified within the gene. Although hemochromatosis is considered as the most frequent inherited disease in the populations of Northern European origin, its prevalence in Brittany had not been evaluated yet. In this issue we report the C282Y mutation frequency in a cohort of 1000 newborns from maternity hospitals of the four breton départements. The homozygote frequency was 5/1000 and heterozygote frequency was 12%; such high frequencies raise the question of the penetrance of the disease and the relevance of systematic genotypic screening for hemochromatosis.