Linkage analysis of 6p21 polymorphic markers and the hereditary hemochromatosis: localization of the gene centromeric to HLA-F

HFE gene: Structure, function, mutations, and associated iron abnormalities

The hemochromatosis gene HFE was discovered in 1996, more than a century after clinical and pathologic manifestations of hemochromatosis were reported. Linked to the major histocompatibility complex (MHC) on chromosome 6p, HFE encodes the MHC class I-like protein HFE that binds beta-2 microglobulin. HFE influences iron absorption by modulating the expression of hepcidin, the main controller of iron metabolism. Common HFE mutations account for ~90% of hemochromatosis phenotypes in whites of western European descent. We review HFE mapping and cloning, structure, promoters and controllers, and coding region mutations, HFE protein structure, cell and tissue expression and function, mouse Hfe knockouts and knockins, and HFE mutations in other mammals with iron overload. We describe the pertinence of HFE and HFE to mechanisms of iron homeostasis, the origin and fixation of HFE polymorphisms in European and other populations, and the genetic and biochemical basis of HFE hemochromatosis and iron overload.

Mucosal immunity and gastrointestinal antigen processing

The intestine is the largest lymphoid organ in the body by virtue of lymphocyte numbers and quantity of immunoglobulin produced. This is largely related to the enormous antigen load to which these cells are exposed on a daily basis. However, despite this, the mucosa-associated lymphoid tissue appears to be regulated by unique mechanisms, and this is reflected in specific phenomena (oral tolerance, controlled or physiologic inflammation) as well as unusual lymphoid populations (intraepithelial lymphocytes) that respond to alternative pathways of activation. This, coupled with the existence of novel antigen-presenting cells (intestinal epithelial cells) sets the scene for distinct immune responses. It is these distinct regulatory factors that support immunosuppression or tolerance rather than active immunity at a site juxtaposed to the external environment. This review defines these novel interactions and suggests how alteration in normal function may result in allergic or inflammatory responses. A clearer understanding of mucosal immunoregulation may lead to new therapeutic approaches for these diseases.

Linkage disequilibrium analysis in Australian haemochromatosis patients indicates bipartite association with clinical expression

BACKGROUND/AIMS

Hereditary haemochromatosis shows a wide variation in phenotypic expression, which is thought to be due, in part, to genetic factors. A single missense mutation in HFE, leading to an amino acid substitution (C282Y) has been shown to be the causative mutation, clearly responsible for clinical expression of the disorder. Since homozygosity for the C282Y mutation can give rise to a disorder which shows wide variation in clinical expression, we investigated the possibility that genetic modifiers of HFE may exist.

METHODS

Linkage disequilibrium analysis was performed on chromosome 6p21.3 in 74 patients homozygous for the C282Y mutation using microsatellite markers spanning the haemochromatosis gene region. Phenotypic expression was evaluated based on transferrin saturation, serum ferritin, hepatic iron concentration and index, and iron grade.

RESULTS

Linkage disequilibrium (LD) analysis showed a predominant ancestral haplotype from D6S265 to D6S2236 covering a region of approximately 5 Mb. The overall LD distribution in this region showed two peaks of highly significant association at D6S105 (2 Mb proximal to HFE) and at D6S2239 approximately 50 kb distal to HFE. Male patients homozygous for D6S105 allele 8, had significantly higher hepatic iron indices than patients heterozygous or nullizygous for D6S105-8 (p<0.038).

CONCLUSION

This analysis indicates that modifying gene(s) or another mutation affecting HHC clinical expression may be located in the region of D6S105.

Determination of gene frequencies for two common haemochromatosis mutations in the Danish population by a novel polymerase chain reaction with sequence-specific primers

Hereditary haemochromatosis (HH), a condition of abnormal iron metabolism which leads to iron overload and organ damage, previously known as bronze diabetes or idiopathic haemochromatosis, is the most common disease-producing genetic disorder among Europeans. Two mutations, C282Y and H63D, are described for the candidate gene, HFE, reported as being responsible for the disease. Since molecular testing of these mutations will be of value in early diagnosis of haemochromatosis, the aim of this study was to develop a simple, fast and inexpensive technique for the determination of the polymorphism in the HFE gene on a large scale. We designed sequence-specific primers for polymerase chain reaction (PCR-SSP) and tested 200 randomly selected healthy Danes and found the result completely comparable to results obtained by a previously described method, PCR-RFLP. The gene frequencies in the Danish population are similar to reported results for the White population, with a frequency of 0.068 for the C282Y mutation and a frequency of 0.128 for the H63D mutation.

Haemochromatosis: a gene at last?

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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.

A novel MHC class I-like gene is mutated in patients with hereditary haemochromatosis

Hereditary haemochromatosis (HH), which affects some 1 in 400 and has an estimated carrier frequency of 1 in 10 individuals of Northern European descent, results in multi-organ dysfunction caused by increased iron deposition, and is treatable if detected early. Using linkage-disequilibrium and full haplotype analysis, we have identified a 250-kilobase region more than 3 megabases telomeric of the major histocompatibility complex (MHC) that is identical-by-descent in 85% of patient chromosomes. Within this region, we have identified a gene related to the MHC class I family, termed HLA-H, containing two missense alterations. One of these is predicted to inactivate this class of proteins and was found homozygous in 83% of 178 patients. A role of this gene in haemochromatosis is supported by the frequency and nature of the major mutation and prior studies implicating MHC class I-like proteins in iron metabolism.

A new non-HLA multigene family associated with the PERB11 family within the MHC class I region

In an effort to initiate steps designed to characterize the idiopathic hemochromatosis disease gene, the HLA-A/HLA-F region where this gene is in disequilibrium linkage with some polymorphic markers has been overlapped by a yeast artificial chromosome (YAC) contig. In order to achieve the physical mapping of these YACs and of the corresponding genomic region, we subcloned one of the YACs involved. A computer-assisted analysis of the sequence of one subclone led to the isolation of a potential exon that proved to belong to a new expressed messenger named HCGIX. After Southern blot analysis, the corresponding cDNA clone was found to belong to a new multigene family whose members are dispersed throughout the HLA class I region and are closely associated with members of another recently described multigene family designated PERB11. The data reported here suggest that these two multigene families form a cluster that have been dispersed together throughout the telomeric part of the major histocompatibility complex and have been involved in the genesis of this human class I region.

Allelic association of microsatellites of 6p in Italian hemochromatosis patients

Hemochromatosis (HC) is an inherited disorder of iron metabolism and is frequently seen in Caucasians. The biochemical defect and the responsible gene are unknown, but the HC locus is closely linked to HLA-A on human chromosome 6 in the region 6p21.3. Although extensive studies have been performed in several populations, the precise location of the gene is still undefined. Linkage disequilibrium with HC has been detected for loci that are 3 cM apart: HLA class I and D6S105, which is located on the telomeric side of HLA-A. We have analyzed the inheritance of several multi-allele polymorphisms that map to 6p (D6S265, Y52, HLA-F, D6S306, D6S105, D6S464, D6S299) in 34 Italian HC families and in 17 unrelated patients. Significant association with HC was shown for alleles of multiple markers in the HLA-A region, for the distant marker D6S105, but not for the D6S299 marker at 4 cM from HLA-A on the telomeric side. HC status was unambiguously assigned to 70 affected and 63 unaffected chromosomes from family studies. Thirty five different haplotypes were found in 70 HC chromosomes when considering four markers most tightly associated with the disease. A predominant haplotype comprising alleles 1-3-1-8 (marker order D6S265, HLA-A, Y52, D6S105) accounted for 30% of the HC chromosomes and was absent in normals. A minority of other HC haplotypes could be related to the major haplotype by assuming single crossover events. Results of haplotype studies suggest a founder effect in the Italian population, as previously shown in Australian patients, and a possible common mutation shared with affected individuals of Celtic origin.

A new highly polymorphic marker in the 5' untranslated region of HLA-F shows strong allelic association with haemochromatosis

The 5' untranslated region of HLA-F contains a polypurine tract comprising repeats of tri- and hexa-nucleotide motifs. We have recently demonstrated that this polypurine tract is highly polymorphic by using the polymerase chain reaction. Here, we demonstrate that some of the alleles can be explained by a deletion of approximately 100 bp DNA and show that alleles of this novel, highly polymorphic locus are as strongly associated with haemochromatosis as HLA-A3 or D6S105-8. The observed frequency of heterozygosity at HLA-RF is extremely high (95%) and this locus has been found to be informative in pedigrees that are non-informative at HLA-A and D6S105. We also show an example of replication slippage at HLA-F in one pedigree.

Hemochromatosis: association of severity of iron overload with genetic markers

We postulated that the severity of iron overload in homozygous hemochromatosis probands is related to the expression of HLA-A3 or D6S105 allele 8. Therefore, we used these markers to characterize Alabama hemochromatosis probands and normal control subjects. We then quantified the blood removed by phlebotomy to exhaust body iron stores and maintain normal serum ferritin concentrations in our hemochromatosis probands. Induction and maintenance phlebotomy requirements were significantly greater in presumed HLA-A3 homozygotes or in D6S105 allele 8 homozygotes than in homozygous probands lacking these markers. Intermediate values were observed in probands who were HLA-A3 or allele 8 heterozygotes, respectively. We also analyzed data from males and females separately. Among subjects of the same sex, the induction and maintenance phlebotomy requirements in subjects presumed to be HLA-A3 homozygotes or in allele 8 homozygotes were greater than those of other groups. Our results support the hypothesis that the severity of iron overload in hemochromatosis is determined predominantly by genetic factors, and provide evidence that two or more mutations for hemochromatosis exist. However, the design of our study does not permit a distinction to be made between allelic and locus heterogeneity for the hemochromatosis gene(s).

Genetics of haemochromatosis

This review is largely concerned with the frequency of genetic haemochromatosis (GH) and attempts to find the gene responsible. Studies of disease prevalence are reviewed along with the association of GH with other inherited disorders. The high prevalence of the disorder found in a number of surveys of populations of European origin along with the relatively few patients presenting with the clinical features of the advanced disease remains a paradox. The tight linkage between HLA-A and GH has been known since 1975 but it has not been possible to distinguish between a telomeric or centromeric location for the gene (HFE) relative to HLA-A. The recent explosion in detailed knowledge of the genetic map of the region telomeric of HLA-A on chromosome 6p has made it possible to examine new genetic markers. The very strong association between GH and D6S105-8 suggests a gene location telomeric to HLA-A. The lack of a precise location, and uncertainty about either the primary biochemical abnormality or the tissues involved has delayed the identification of the gene but expressed genes in the region around HLA-A are now being isolated and tested.

Hereditary hemochromatosis: a prevalent disorder of iron metabolism with an elusive etiology

Hereditary hemochromatosis is a prevalent inherited disorder with an estimated frequency of homozygosity of 0.2 to 0.45% in Caucasians. The disease is characterized by progressive iron overload until a massive accumulation of body iron occurs. Undetected, the disorder eventually can produce either cirrhosis, diabetes mellitus, cardiac disease, arthritis, or hepatocellular carcinoma or a combination of these manifestations. Early diagnosis and treatment prevents organ damage and normalizes life expectancy. Screening studies to detect hemochromatosis are most effectively accomplished by measurement of the serum iron and total iron binding capacity. Treatment is most effectively performed by frequent phlebotomy until body stores are empty and then 3 to 4 times yearly for life. The basic defect of hemochromatosis appears to increase iron absorption, decrease iron excretion, and produce preferential deposit of iron in hepatic parenchymal cells rather than Kupffer cells. The genetic abnormality of hemochromatosis is located on chromosome 6 in close association with the gene for HLA antigens. Recent speculation postulates that tumor necrosis factor may be involved in the etiology of this disease because of its location on chromosome 6 and its effect upon iron transport.

Exclusion of ferritins and iron-responsive element (IRE)-binding proteins as candidates for the hemochromatosis gene

We have looked for genes for ferritin and its translational control protein that could account for anomalies in the expression of ferritin (FT) and the transferrin receptor in the duodenum of individuals with hemochromatosis (HC). We show that there are probably only two FTH-like sequences near the HC locus on the short arm of chromosome 6 and no FTL-like sequences. We report the cloning of the previously uncharacterized FTH sequence from 6p (FTHL15) and show that it is probably a processed pseudogene. This gene has been mapped with a panel of radiation hybrid cells to near 6p12. Additionally, we show that there are no sequences on chromosome 6p for a protein that coordinately regulates expression of ferritin and the transferrin receptor.

Alleles at D6S265 and D6S105 define a haemochromatosis-specific genotype

Hereditary haemochromatosis is an HLA-linked, recessive disorder with HLA-A3 a strong marker for the gene. We have identified molecular markers for two serologically indistinguishable subtypes of HLA-A3 and examined these in 42 patients with haemochromatosis. The common HLA-A3 subtype HLA-A*0301 (highly correlated with allele 1 of D6S265) was a slightly better marker for haemochromatosis (RR = 10.1, Chi2 = 30) than the serologically recognized A3 antigen (RR = 9.1; Chi2 = 27.3). Allele 8 of the more telomeric locus D6S105 was also strongly associated with haemochromatosis (RR = 13.0; Chi2 = 21.1) but alleles at this locus were not in strong linkage disequilibrium with HLA-A alleles in the control subjects. The co-occurrence of D6S265-1 and D6S105-8 alleles yielded a higher risk (RR = 16.9; Chi2 = 44). Homozygosity for the haplotype including these markers was specific for haemochromatosis, i.e. did not occur in 376 healthy subjects but was observed in 21.4% of patients. These results refine the HLA-A3 association with haemochromatosis, suggest that the haemochromatosis gene is located on the telomeric side of HLA-A and define a possible haplotype in which the first mutation may have occurred.

MHC genes in autoimmunity

In the last year progress has been made towards elucidating the roles of the MHC gene products in autoimmunity. A major advance has been the recent determination of the crystallographic structure of the human MHC class II molecule, which will be invaluable in delineating the minimum structural requirements for peptides that induce autoimmune disease. In addition, the use of animal models and transgenic mouse technology is continuing to increase our understanding of the involvement of the MHC gene products in immunopathogenesis.