A promoter polymorphism in the central MHC gene, IKBL, influences the binding of transcription factors USF1 and E47 on disease-associated haplotypes

The human major histocompatibility complex (MHC) contains genes that affect susceptibility to numerous immunopatliological diseases. We propose that genes in the central MHC between TNFA and HLA-B explain associations between the 8.1 haplotype (HLA-A1, B8, DR3) and disease. IKBL encodes a protein resembling members of the IκB protein family that regulate bioavailability of NFκB. We have identified two polymorphisms in the 500 bp upstream of the transcription start site of IKBL that distinguish the 8.1 haplotype from the resistant 7.1 haplotype (HLA-A3, B7, DR15). A single nucleotide polymorphism at -62 disrupts a putative E-box binding sequence. To investigate binding of transcription factors in vitro, we exposed 32P-labeled DNA fragments carrying both alleles to nuclear extracts, showing allele-specific binding of nuclear proteins from Jurkat cells but not from other lineages. Supershift studies using Jurkat nuclear extract showed that the E-box protein, E47, and ubiquitously expressed transcription factor USF1 bind to the E-box element of the 7.1 haplotype. Transient transfections of luciferase reporter constructs carrying promoter alleles of IKBL into Jurkat cells showed an effect of IKBL-62 alleles. In contrast, alleles at -421 did not affect transcription factor binding or transcription. IKBL was expressed at low levels in Jurkat cells but not in blood mononuclear cells, and expression declined following mitogenic stimulation. The restriction of IKBL expression to Jurkat cells is consistent with evidence that E47 is expressed in thymocytes and suggests IKBL may affect autoimmunity through an effect on T-cell selection.