Homozygous deletion of HFE is the common cause of hemochromatosis in Sardinia

Abstract

We recently characterized an Alu-mediated recombination causing the loss of the complete HFE gene sequence. Here, we describe the case of a novel homozygous patient. We further show that HFE deletion results from a founder effect and that it represents the common cause of hemochromatosis in Sardinia. We recently reported the case of a 47-year old woman with a moderate iron overload due to an Alu-mediated recombination causing the loss of the complete HFE gene sequence.1 The same chromosomal alteration was identified by Pelucchi and co-workers in another woman. Despite a younger age at diagnosis (29 years), their patient showed a more impressive iron overload.2 A third case, a man, came to our attention. At the age of 44 he showed a transferrin saturation level of 80% and a serum ferritin level of 2080 μg/L. He was not genotyped for the p.C282Y and p.H63D variations because of an inability to amplify the HFE exons 2 and 4. We confirmed absence of the HFE gene in this patient but we were also interested in its origins. Indeed, the patient and both previously reported women were of Sardinian descent. The Sardinian population is genetically differentiated from the other Caucasian populations.3 It represents a genetic isolate where the p.C282Y mutation is considered as rare or even absent.4 This led us to assume that the HFE deleted allele was present at the population level and, related to a founder effect, was the common cause of hemochromatosis in this Mediterranean island. To characterize the contribution of the HFE deletion in the Sardinian population, we first established the frequency of this mutation in a sample set of 198 controls who originated from different districts of the island (90% of them were from South Sardinia, while the others originated from central and Northern parts of the island). Genotype analysis was performed using a rearrangement specific PCR, as previously described.1 The HFE deletion was detected at the heterozygous state in 4 subjects, giving a carrier frequency of 2.02% and an estimated homozygous frequency of 0.01% (one person in 10,000). We also looked for the p.C282Y mutation. It was identified at the heterozygous state in one of 193 subjects (0.52%). We next proceeded to analyze polymorphic microsatellite repeat markers flanking HFE. Based on results from the consanguineous family reported in our initial study,1 we selected a set of informative markers and defined a first chromosomal region of 12.049 megabases (Mb). This chromosomal region encompassed two HLA-class I loci which were added to the haplotypes analysis. Final results are summarized in Table 1 and Figure 1. A conserved haplotype of 3.150 Mb was deduced from the consideration of alleles shared by the 2 homozygous cases identified by us (P1, P2), and the 4 mutation carriers (MC 1–4). However, a recombination event could account for differences in HLA-A, HLA-B and D6S273 alleles of one of the study subjects (MC 3 in Table 1). This allowed us to consider a common haplotype of 7,359 Mb. Figure 1. Physical map of the chromosome 6 region flanking HFE. Distance between the D6S1621 and D6S2414 marker is indicated in grey; the arrow marks a possible ancestral haplotype. Distance between the D6S1621 and D6S1022 markers is indicated in black; the arrow ... Table 1. Allelic distribution of markers surrounding HFE. *The parentheses mark differences between the 6 individuals; these differences are probably related to a recombination event on chromosome 6 of the mutation carrier number 3. By their position on the same maritime routes, the histories of the West Mediterranean islands are very similar. Studies based on HLA polymorphisms have confirmed singularity of these populations, and have further revealed a close relationship between the Sardinian and Corsican populations. However, it must be pointed out that the HLA-A*2-B*58 haplotype is frequent in Sardinia (4.9%) and almost absent in the other West Mediterranean islands (Corsica and Balearic Islands). In fact, this haplotype could be of African origin.5 If so, comparing frequency of the HFE deleted allele (2%) to that of the HLA-A*2-B*58 haplotype (4.9%), one may assume that deletion of the HFE gene arose in the Sardinian population after contacts with African populations. But an African ancestry for the HFE deletion cannot be completely excluded. To conclude, we show that HFE deletion results from a founder effect rather than that of a mutational hotspot. We also demonstrate that, with an estimated homozygous frequency of one person in 10,000, HFE deletion is the common cause of hemochromatosis in Sardinia.