A new polymorphic probe close to HLA-A

Cloning of a human homologue of the mouse Tctex-5 gene within the MHC class I region

Using a positional cloning strategy to identify the hemochromatosis gene (HFE), we isolated seven cDNAs by cDNA selection from a region of 400 kilobases (kb) located near the HLA-A and HLA-F loci. In this paper, we report the study of one of the corresponding genes, referred to as HCG V (hemochromatosis candidate gene), localized 150 kb centromeric to HLA-A. This gene was found to be expressed ubiquitously in the form of a 1.8 kb transcript, and to be apparently well conserved during evolution. The gene spanned 3.1 kb and is organized in three exons and two introns. The cDNA of 1620 base pairs (bp) showed an open reading frame of 378 bp, encoding for a 126 amino acid polypeptide which displayed a strong identity with the predicted product of a mouse Tctex-5 gene (t complex, testis expressed) localized in the t complex on chromosome 17. The HCG V gene was assessed as a potential candidate for hemochromatosis in regard to its localization in the linkage disequilibrium area between HFE and polymorphic markers. The study of deletions and point mutations in hemochromatosis patients revealed a single bp polymorphism within the coding region; however, no associated disease changes were found. Therefore we conclude that HCG V is unlikely to be involved in the pathogenesis of hemochromatosis.

Physical map of the HLA-A/HLA-F subregion and identification of two new coding sequences

As part of an effort to characterize the hemochromatosis gene, we selected three non-chimeric yeast artificial chromosomes (YACs) overlapping with the YAC B30 previously described and forming an 800 kilobase contig covering the HLA-A/HLA-F region. The precise physical map of these YACs and of the corresponding genomic region were established. Nine concentrated sites of CpG cutter elements, potentially HTF islands, were mapped. In addition, several probes have been generated as tools for mapping and examining transcripts produced in the region. This allowed for the characterization and localization of two new coding sequences, provisionally named HCG (for hemochromatosis candidate gene) and numbered VIII and IX.

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.

Allelic associations and homozygosity at loci from HLA-B to D6S299 in genetic haemochromatosis

Haemochromatosis (GH) is an autosomal recessive disorder in which increased iron absorption causes iron overload. The gene (HFE) is closely linked to HLA-A on chromosome 6 (6p21.3) but has not yet been identified. We have examined eight polymorphic loci, HLA-B (most centromeric), I82, D6S265, HLA-A, D6S128, HLA-F, D6S105, and D6S299 (most telomeric) in 37 unrelated patients and 60 control subjects. There are also significant positive associations between GH and alleles at all loci except D6S299. Analysis of 48 GH chromosomes in which haplotypes could be established showed that the most common haplotype was I82-2:D6S265-1:HLA-A3:D6S128-2:HLA-F1:D6S105-8. This was present in 28 of 48 chromosomes. In 14 the haplotype included HLA-B7 but only in seven did this extend beyond the telomere to D6S299-2 (the most common allele on GH chromosomes at this locus). In 36 out of 48 chromosomes the two locus haplotype, F1:D6S105-8 was present. Since haemochromatosis appears to originate from a founder mutation we have examined linkage disequilibrium between these various loci and GH using calculations of pexcess. The maximum value (0.72, 95% CI 0.55-0.85) is given by D6S105-8 but is not significantly different from values for HLA-A3 and HLA-F1 (0.50, 95% CI 0.34-0.61 and 0.49, 0.25-0.66 respectively). However, both HLA-A and D6S105 give a value for pexcess which is significantly higher than that for the most centromeric marker, HLA-B (0.17, 95% CI 0.02-0.30). We have counted the number of patients who are homozygous for the common allele at each locus. At D6S105, 22 patients are homozygous for allele 8, with 18 homozygous for HLA-F1 and 10 homozygous for A3. The pattern of cumulative homozygosity suggests a gene location closer to D6S105 than HLA-A. We have also analysed our data for divergence from the apparent founder haplotype (A3:F1:105-8) and have calculated the theoretical frequencies of crossovers between loci. These data suggest a location telomeric to D6S105. A more precise localisation of the gene may be possible with the identification of new markers around D6S105.

Analysis of HLA-A/B recombinant families with new polymorphic markers

The MHC in humans is a much studied region of the genome, the genes of which display a high rate of polymorphism and a high rate of linkage disequilibrium. Four families in which intra-class-I recombination has occurred have been analyzed with six polymorphic markers between HLA-A and -B in order to determine the full haplotypes of the whole families and to localize the points of crossover. The previously proposed order of the markers was confirmed by recombination mapping. In one family, the crossover was shown to have occurred in the 20-kb stretch of DNA bounded by the two markers (P3B and P5) between which no evidence of linkage disequilibrium was found, a region which constitutes of only about 1% of the distance between HLA-A and HLA-B. Although supportive of the suggestion of a hot spot of recombination in this region, based on the apparent lack of linkage disequilibrium between the markers P3B and P5, more such families need to be tested in order to confirm or refute this hypothesis.

Anonymous markers located on chromosome 6 in the HLA-A class I region: allelic distribution in genetic haemochromatosis

Two yeast artificial chromosomes of the HLA class I region were subcloned. Four of the subclones studied displayed restriction polymorphisms that corresponded to six bi-allelic series. Allelic distribution of the anonymous markers was then studied by comparing a control population with a group of patients with familial haemochromatosis. Only one marker presents an unequivocal association with the haemochromatosis gene and is 100 kb centromeric to HLA-A. This association however is not as strong as with HLA-A3. The results suggest two possible locations for the haemochromatosis gene: less than 100 kb centromeric to the HLA-A locus, or on the telomeric side.

Dinucleotide repeat polymorphism at the FTHPI locus of chromosome 6

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