Tapasin (hereafter TAPBP) is a transmembrane glycoprotein mediating the binding of TAP and MHC-I within the peptide loading complex in the endoplasmic reticulum, the interaction of which is important for optimal peptide loading. Absence of TAPBP in mice is known to affect MHC-I expression and CD8+ T cell development, and viruses such as cytomegalovirus are known to inhibit TAPBP gene transcription to impair antigen presentation in infected cells. Due to the importance of TAPBP in antigen presentation and T cell development, we hypothesized that expression of TAPBP would also affect HIV pathogenesis, and the genetic markers of expression could potentially serve as indicators for risk in the context of HIV outcome. Thus, we decided to examine the TAPBP gene to identify any single nucleotide polymorphisms (SNPs) that affected expression and ascertain the underlying mechanisms.
After scanning TAPBP for common SNPs, rs111686073 (G/C) was identified as a variant found only in blacks that significantly correlated with expression via qPCR in two South African cohorts. To verify the effect on expression, constructs of different lengths were generated from the 5′ UTR of TAPBP and examined with luciferase assays in HeLa cells. Electrophoretic mobility shift assays were performed to determine if the transcription factors AP-2a or Sp1, predicted to bind by Alibaba2, mediated the change in expression.
The SNP rs111686073, located in the 5′ UTR, was found to cause significant changes in mRNA levels in two separate black cohorts (p = 0.002, 0.004; Figure 1A). Luciferase assays confirmed that the SNP altered expression with the G variant conferring higher expression than the C variant (p = 0.0004; Figure 1B). The SNP was found to be a binding site for the transcription factor AP-2a, which appears to increase expression (Figure 1C).
The SNP rs111686073 is significantly associated with TAPBP expression, and the G variant results in increased expression over the C variant in luciferase assays, although AP-2a binds both. EMSA analysis indicates that the binding affinity of AP-2a likely mediates the observed effect on TAPBP expression. We hypothesize that the lowered expression of TAPBP may alter TAPBP-dependent HLA-B expression and antigen presentation and thus affects CD8+ T cell development. These effects may be important to keep in mind for vaccine studies and further understanding of HIV pathogenesis.