Analysis of HLA-class-II-encoded antigen-processing genes TAP1 and TAP2 in primary biliary cirrhosis

The search for genes involved in the aetiology of primary biliary cirrhosis (PBC) has centred on the major histocompatibility complex (MHC) on chromosome 6. Genotyping studies have confirmed an association with HLA class II allele DR8. We investigated polymorphisms in two newly identified genes (TAP1 and TAP2) situated close to the DR locus and thought to encode membrane transporter molecules involved in endogenous antigen processing. Genomic DNA extracted from PBC patients was compared with local healthy controls. TAP1 was analysed by amplification refractory mutation system (ARMS) PCR, and two alleles (A and B) were identified. In 126 PBC patients and 116 controls, allele frequencies were (A:B) 81:19% and 79:21%, respectively (NS). TAP2 analysis was by PCR followed by Bfal restriction digest, and again two alleles (A and B) were identified. Their frequencies in 109 PBC patients and 96 controls were (A:B) 76:24% and 73:27%, respectively (NS). No TAP1-TAP2 haplotype was associated with PBC. TAP allele frequencies were estimated within the DR8 subgroups (22 PBC, 14 controls). B allele frequency for TAP1 was increased in both DR8-positive PBC patients and controls compared with DR8-negative patients and controls (41% vs. 14% in PBC; 43% vs. 18% in controls), but no disease association was found. However, the increased frequency of TAP1B in DR8-positive subjects (42% DR8-positive vs. 16% DR8-negative, p < 0.001) indicates linkage disequilibrium between these two loci.