Vitamin D receptor (VDR) gene SNPs influence VDR expression and modulate protection from multiple sclerosis in HLA-DRB1*15-positive individuals

Abstract

Multiple sclerosis (MS) is an autoimmune disease with a multifactorial etiology. The HLA-DRB1*15 allele, is the main genetic risk factor for MS in Caucasians; recent findings showed that the transcription of this molecule is regulated by the vitamin D/vitamin D receptor (VDR) complex. We analyzed SNPs within the VDR gene in association with the HLA-DRB1 locus in 641 MS patients diagnosed according to McDonald criteria and 558 age- and sex-matched healthy controls, to verify possible correlations between the vitamin D/VDR complex, HLA-DRB1, and susceptibility to MS.

Results confirmed that HLA-DRB1*15 is a strong predisposing allele (p < 1 × 10⁻⁷; OR: 3.04; 95% CI: 2.02–4.60) for MS. Cosegregation analyses of VDR SNPs with HLA-DRB1*15 indicated a reduction of risk for MS given by the presence of the -DRB1*15-rs731236 T VDR haplotype (p = 9.5 × 10⁻⁵; OR: 2.52; 95% CI: 1.56–4.06) and, conversely, an augmented risk for disease associated with the -DRB1*15-rs731236 C VDR haplotype. Analyses performed on HLA-DRB1*15-positive MS patients and HC alone confirmed the protective role of rs731236 TT VDR genotype (p_y = 0.004; OR: 0.53; 95% CI: 0.33–0.83); notably, FACS, PCR, and confocal microscopy analyses showed that rs731236 TT genotype is associated with an augmented VDR expression in MBP-stimulated PBMC from patients.

In conclusion, rs731236 TT VDR genotype modulates VDR expression and confers protection against MS in HLA-DRB1*15-positive individuals. Results herein offer a model justifying the interaction between the major genetic (HLA-DRB*15) and environmental (vitamin D) factors associated with MS onset.

Introduction

Multiple sclerosis (MS) is a demyelinating autoimmune disease of the central nervous system resulting from a complex interaction between genetic and environmental factors. The chromosomal region most strongly linked to MS is the major histocompatibility complex (MHC) locus (Jersild et al., 1972, GAMES, 2003) and, in Caucasians, the HLA-DRB1*1501, -DRB5*0101, -DQA1*0102, and -DQB1*0602 alleles are more frequently associated with MS (Olerup and Hillert, 1991, Haines et al., 1998). Among the environmental factors suggested to be involved in the pathogenesis of this disease, limited vitamin D exposure is believed to play a major role (Acheson et al., 1960), as suggested by a number of observations. To summarize: (1) MS patients are vitamin D-deficient (Nieves et al., 1994) and dietary intake of this vitamin reduces the risk of developing the disease (Munger et al., 2006), (2) MS is differentially distributed according to the latitude, as the disease is mostly observed in the latitudes where sun exposure is lower, (3) the incidence of MS in the coastal regions of Norway is lower than in the rest of the Norwegian population, possibly because of the higher levels of fish consumption, a food rich in vitamin D, in the regions overlooking the sea (Larsen et al., 1985), and (4) individuals who migrate from one area of the globe to another before puberty take on the incidence of the area to which they migrate, whereas people who move after puberty carry with them the incidence of the area from which they migrated (Acheson, 1977, Alter et al., 1978, Kurtzke et al., 1997, Dean and Elian, 1997). These observations suggest an environmental risk factor for MS that could act during the first years of life.

Vitamin D is a seco-steroid hormone ingested in the diet or synthesized in the skin when 7-dehydrocolesterol reacts with UVB ultraviolet light at wavelengths between 270 and 300 nm (Norman, 1998). The biologically inert vitamin D₃ is hydroxylated in the liver and in the kidney into (1,25(OH)₂D), this form becomes active upon binding to the Vitamin D receptor (VDR) (Marshall, 2006). In the presence of (1,25(OH)₂D), VDR translocates to the nucleus (Prüfer and Barsony, 2002) and generates heterodimers with the retinoid X receptor (RXR). The heterodimers can then bind, together with the transcription factor IIB, a specific sequence (GGGTGGAGGGGTTCA) called vitamin D response element (VDRE) on DNA, resulting in the activation of a plethora of genes (Towers et al., 1999, Wang et al., 2005).

A recent report demonstrated the presence of a VDRE element in the proximal promoter region of the HLA-DRB1 gene (Ramagopalan et al., 2009). VDREs present a multitude of sequence variations and a spectrum of binding affinities for VDR; this variability enables VDREs to respond optimally to different concentrations of the VDR/(1,25(OH)₂D) complex (Feldman et al., 2005). The lack of variants in the VDRE sequence of the HLA-DRB1*15 allele led to the speculation that the augmented risk of MS seen in HLA-DRB1*15 carriers could be secondary to its transcription regulation through the VDR/(1,25(OH)₂D) complex. This hypothesis, linking the major genetic (HLA-DRB1*15) and environmental (vitamin D) factors for MS, prompted us to initiate this study.