The hemochromatosis founder mutation in HLA-H disrupts beta2-microglobulin interaction and cell surface expression

We recently reported the positional cloning of a candidate gene for hereditary hemochromatosis (HH), called HLA-H, which is a novel member of the major histocompatibility complex class I family. A mutation in this gene, cysteine 282 --> tyrosine (C282Y), was found to be present in 83% of HH patient DNAs, while a second variant, histidine 63 --> aspartate (H63D), was enriched in patients heterozygous for C282Y. The functional relevance of either mutation has not been described. Co-immunoprecipitation studies of cell lysates from human embryonic kidney cells transfected with wild-type or mutant HLA-H cDNA demonstrate that wild-type HLA-H binds beta2-microglobulin and that the C282Y mutation, but not the H63D mutation, completely abrogates this interaction. Immunofluorescence labeling and subcellular fractionations demonstrate that while the wild-type and H63D HLA-H proteins are expressed on the cell surface, the C282Y mutant protein is localized exclusively intracellularly. This report describes the first functional significance of the C282Y mutation by suggesting that an abnormality in protein trafficking and/or cell-surface expression of HLA-H leads to HH disease.

Effects of thiazolidinediones on growth and differentiation of human aorta and coronary myocytes

Thiazolidinediones, insulin-sensitizing agents that lower insulin and lipid levels in insulin-resistant states, block agonist-induced Ca2+ entry into vascular smooth muscle (VSM) cells in vitro and lower blood pressure in animals and humans in vivo. In this study, we investigated the effects of ciglitazone and troglitazone on cell growth and DNA synthesis (as thymidine incorporation), and differentiation in cultured human aorta (haVSM) and human coronary artery (hcaVSM) VSM cells. Mitotically quiescent haVSM cells were stimulated with serum or platelet-derived growth factor (PDGF). Ciglitazone (40 micromol/L) inhibited haVSM cell proliferation by 84 +/- 16% (mean +/- SEM) (P < .05, n = 3), and serum and PDGF stimulated [3H]-thymidine incorporation by 91 +/- 18% (P < .03, n = 3) and 73 +/- 14% (P < .03, n = 4), respectively. Troglitazone (5 micromol/L) inhibited proliferation of haVSM cells by 78 +/- 14% (P < .05, n = 3) and hcaVSM cells by 91 +/- 18% (P < .05, n = 3). Proliferating VSM cells (synthetic phenotype) expressed small amounts of alpha-actin, whereas nonproliferating VSM cells (contractile phenotype) exhibited abundant alpha-actin. Exposure of proliferating haVSM cells to 40 micromol/L ciglitazone induced a marked increase in alpha-actin staining, consistent with transition to the well differentiated, contractile phenotype. To the extent that thiazolidinediones similarly affect growth factor-induced proliferation and differentiation of arterial myocytes in vivo, these agents may be useful in treating atherosclerosis and in preventing restenosis after angioplasty.

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.