Expression of the Multiple Sclerosis-Associated MHC Class II Allele HLA-DRB1*1501 Is Regulated by Vitamin D

Abstract

Multiple sclerosis (MS) is a complex trait in which allelic variation in the MHC class II region exerts the single strongest effect on genetic risk. Epidemiological data in MS provide strong evidence that environmental factors act at a population level to influence the unusual geographical distribution of this disease. Growing evidence implicates sunlight or vitamin D as a key environmental factor in aetiology. We hypothesised that this environmental candidate might interact with inherited factors and sought responsive regulatory elements in the MHC class II region. Sequence analysis localised a single MHC vitamin D response element (VDRE) to the promoter region of HLA-DRB1. Sequencing of this promoter in greater than 1,000 chromosomes from HLA-DRB1 homozygotes showed absolute conservation of this putative VDRE on HLA-DRB1*15 haplotypes. In contrast, there was striking variation among non–MS-associated haplotypes. Electrophoretic mobility shift assays showed specific recruitment of vitamin D receptor to the VDRE in the HLA-DRB1*15 promoter, confirmed by chromatin immunoprecipitation experiments using lymphoblastoid cells homozygous for HLA-DRB1*15. Transient transfection using a luciferase reporter assay showed a functional role for this VDRE. B cells transiently transfected with the HLA-DRB1*15 gene promoter showed increased expression on stimulation with 1,25-dihydroxyvitamin D3 ( P = 0.002) that was lost both on deletion of the VDRE or with the homologous “VDRE” sequence found in non–MS-associated HLA-DRB1 haplotypes. Flow cytometric analysis showed a specific increase in the cell surface expression of HLA-DRB1 upon addition of vitamin D only in HLA-DRB1*15 bearing lymphoblastoid cells. This study further implicates vitamin D as a strong environmental candidate in MS by demonstrating direct functional interaction with the major locus determining genetic susceptibility. These findings support a connection between the main epidemiological and genetic features of this disease with major practical implications for studies of disease mechanism and prevention.

Author Summary

Multiple Sclerosis (MS) is a complex neurological disease with a strong genetic component. The Major Histocompatibility Complex (MHC) on chromosome 6 exerts the strongest genetic effect on disease risk. A region at or near the HLA-DRB1 locus in the MHC influences the risk of MS. HLA-DRB1 has over 400 different alleles. The dominant haplotype of Northern Europe, marked by the presence of DRB1*1501, increases risk of MS by 3-fold. The environment also plays a key role in MS. The most striking illustration of this is the geographical distribution of the disease in populations matched for ethnicity. This has led to the proposal that sunshine, and in particular, vitamin D, is an environmental factor influencing the risk of MS. Circumstantial evidence supporting this comes from studies showing the involvement of vitamin D in immune and nervous system function. The current investigation sought to uncover any relationship between vitamin D and HLA-DRB1. It was found that vitamin D specifically interacts with HLA-DRB1*1501 to influence its expression. This study therefore provides more direct support for the already strong epidemiological evidence implicating sunlight and vitamin D in the determination of MS risk, and implies that vitamin D supplementation at critical time periods may be key to disease prevention.