Molecular basis of the apolipoprotein H (beta 2-glycoprotein I) protein polymorphism.

Apolipoprotein H (apoH, protein; APOH, gene) is considered to be an essential cofactor for the binding of certain antiphospholipid autoantibodies to anionic phospholipids. APOH exhibits a genetically determined structural polymorphism due to the presence of three common alleles (APOH1, APOH2 and APOH3) detectable by isoelectric focusing (IEF) and immunoblotting. The APOH3 allele can be further characterized into two subtypes, APOH3w and APOH3B, based upon its reactivity with monoclonal antibody 3D11. In this study we have determined the molecular basis of the APOH protein polymorphism and its distribution in three large U.S. population samples comprising 661 non-Hispanic whites, 444 Hispanics and 422 blacks. By direct DNA sequencing of PCR amplified fragments corresponding to the eight APOH exons, we identified two missense mutations that correspond to the APOH1 and APOH3w alleles. A missense mutation (G-->A) in exon 3, which alters amino acid Ser to Asn at codon 88 and creates a restriction site for TSP509 I, was present in all APOH1 allele carriers. A second missense mutation (G-->C) at codon 316 in exon 8, which replaces amino acid Trp with Ser and creates a restriction site for BSTBI, was present in all APOH3w carriers. The distribution of the Ser 88 Asn and Trp 316 Ser mutations was significantly different between the three racial groups. The frequency of the Asn-88 allele was 0.011, 0.043, and 0.056 in blacks. Hispanics and non-Hispanic whites, respectively. While the Ser-316 allele was observed sporadically in blacks (0.008), it was present at a polymorphic frequency in Hispanics (0.027) and non-Hispanic whites (0.059). The identification of the molecular basis of the APOH protein polymorphism will help to elucidate the structural-functional relationship of apoH in the production of antiphospholipid autoantibodies.