Vitamin D and Genetic Susceptibility to Multiple Sclerosis: Understanding a Complex Biological Connection

Multiple sclerosis (MS) is a chronic autoimmune and inflammatory disorder of the central nervous system characterized by progressive damage to myelin and disruption of neurological function. Rather than arising from a single cause, MS develops through a complex interaction among genetic susceptibility, epigenetic regulation, and environmental exposures. The review article Vitamin D and Genetic Susceptibility to Multiple Sclerosis by Concetta Scazzone and colleagues examines one particularly important intersection within this multifactorial model: the relationship between vitamin D biology and inherited genetic variation. The authors review single-nucleotide polymorphisms (SNPs) in genes involved in vitamin D synthesis, metabolism, transport, and biological activity, asking whether these variants might modify an individual's susceptibility to MS. Their analysis emphasizes that vitamin D-related genetics should not be considered an isolated explanation for the disease, but rather one component of a broader polygenic and environmental network. Vitamin D Metabolism Provides a Biological Framework
Understanding the proposed connection requires consideration of vitamin D metabolism. Vitamin D exists principally as vitamin D₂ and vitamin D₃, with vitamin D₃ being produced largely in the skin following ultraviolet B exposure. The molecule must then undergo two hydroxylation reactions before becoming biologically active: the first occurs primarily in the liver and produces 25-hydroxyvitamin D [25(OH)D], the principal circulating indicator of vitamin D status, while the second occurs mainly in the kidney and generates the active hormone 1,25-dihydroxyvitamin D [1,25(OH)₂D]. Most circulating vitamin D metabolites are transported by vitamin D-binding protein (VDBP). The active hormone subsequently exerts genomic effects through the vitamin D receptor (VDR), which forms a complex with retinoid-X receptor-α (RXR-α), whereas membrane-associated rapid response steroid-binding proteins participate in non-genomic signaling. The metabolic diagram presented on page 3 of the review illustrates how enzymes, transport proteins, receptors, and regulatory molecules form an interconnected pathway, making genetic variation at several points biologically capable of altering vitamin D availability or activity.

Why Vitamin D Is Relevant to Multiple Sclerosis
The scientific interest in vitamin D and MS is supported by epidemiological, genetic, immunological, and experimental observations. Numerous studies reviewed by the authors have reported an inverse association between circulating vitamin D concentrations and MS risk, while Mendelian-randomization investigations have also suggested that genetically influenced reductions in vitamin D may contribute to disease susceptibility. Several biological mechanisms make such an association plausible. Vitamin D can influence CD4-positive T-cell differentiation by reducing pro-inflammatory Th1 and Th17 responses while favoring Th2 and regulatory T-cell activity. This is particularly relevant because myelin-reactive Th1 and Th17 cells contribute to inflammatory damage within the central nervous system. Vitamin D has additionally been implicated in myelination and remyelination, including the regulation of oligodendrocyte precursor-cell differentiation and microglial clearance of damaged myelin. Importantly, the review reports that more than 80% of MS-associated genes are enriched for vitamin D response elements in their promoter regions, suggesting a possible molecular route through which vitamin D status could influence the expression of genes already associated with MS susceptibility.

Genetic Variants Affecting Vitamin D Synthesis and Metabolism
Several of the most biologically compelling candidate genes encode enzymes controlling vitamin D production and activation. The DHCR7/NADSYN1 locus is associated with circulating 25(OH)D concentrations because DHCR7 helps determine whether the precursor 7-dehydrocholesterol is directed toward cholesterol or vitamin D synthesis; nevertheless, studies assessing this locus in MS populations have produced inconsistent associations. Similar complexity characterizes CYP2R1, the major hepatic 25-hydroxylase. Certain CYP2R1 polymorphisms have been associated with altered vitamin D concentrations, and the low-frequency rs117913124 variant was reported to approximately double the risk of vitamin D insufficiency and increase the odds of developing MS by about 40%, although this observation had not been independently replicated at the time of the review. CYP27B1, which produces active 1,25(OH)₂D, has attracted particularly strong interest because genome-wide association and family studies identified several potentially relevant variants, including rare loss-of-function alleles; collectively, the review considers CYP27B1 a plausible contributor to MS susceptibility. In contrast, findings concerning CYP24A1, the enzyme responsible for vitamin D catabolism, remain controversial: some investigations identified associations with MS, whereas studies in Canadian, Chinese, Danish, and Tasmanian populations did not reproduce them.

Transport Proteins and the Central Role of the Vitamin D Receptor
Genetic variation can also influence vitamin D transport and cellular signaling rather than its synthesis alone. The GC gene encodes VDBP, and variants such as rs7041, rs4588, and rs2282679 can alter circulating vitamin D concentrations or the binding characteristics of the protein. Nevertheless, investigations reviewed in the article generally failed to demonstrate a consistent direct association between VDBP polymorphisms and MS susceptibility, raising the possibility that their contribution, if present, occurs indirectly through effects on vitamin D status. The megalin-DAB2-cubilin system also participates in cellular uptake of the vitamin D–VDBP complex, although genetic evidence linking this pathway to MS remained limited. By far the most extensively investigated signaling gene is VDR. Four polymorphisms—ApaI (rs7975232), BsmI (rs1544410), TaqI (rs731236), and FokI (rs2228570)—have received particular attention. ApaI, BsmI, and TaqI primarily affect regulation of VDR expression, whereas FokI can influence receptor structure and transcriptional activity. Despite decades of investigation, however, studies conducted in different populations have generated contradictory results, meaning that VDR remains biologically important but genetically unresolved as a determinant of MS risk.

Why the Evidence Remains Inconclusive
One of the principal messages of the review is that biological plausibility does not automatically translate into consistent genetic association. Differences in ethnicity, allele frequencies, environmental exposure, vitamin D status, disease phenotype, sample size, statistical power, and interactions among multiple genes may all contribute to discrepancies among studies. The authors specifically note that many VDR studies contained relatively small samples and could therefore have been vulnerable to limited statistical power, while genetic effects may also depend on environmental factors and clinical heterogeneity. Interpretation of vitamin D itself introduces additional uncertainty because observational studies cannot always exclude reverse causality, and differences in laboratory measurement of serum 25(OH)D have historically complicated comparisons across populations. Several biologically relevant pathways also remain insufficiently studied: for example, FGF-23 and Klotho regulate vitamin D homeostasis through effects on CYP27B1 and CYP24A1, yet the review reports that genetic alterations in these molecules had not been directly investigated for their influence on MS risk.

Toward a More Integrated Model of MS Susceptibility
The review ultimately portrays MS as what previous researchers have termed a genetic and environmental “perfect storm.” Its inheritance is polygenic, meaning that individual common genetic variants generally contribute only modestly to overall susceptibility, while their combined effects may become important in particular environmental and biological contexts. Vitamin D-related genes therefore represent potentially meaningful pieces of a much larger genetic architecture rather than deterministic causes of MS. The strongest body of research concerns VDR, while evidence for many other vitamin D pathway genes remains limited or contradictory. Consequently, the review does not establish that vitamin D-related polymorphisms independently cause MS; instead, it identifies a biologically credible gene–environment framework requiring larger, well-powered, ethnically diverse, and functionally informed investigations. A clearer understanding of genetic determinants of vitamin D status may eventually contribute to improved risk stratification and more individualized clinical management, but such applications require substantially stronger evidence before they can be incorporated into routine care.

Disclaimer: This blog post is based on the provided research article and is intended for informational purposes only. It is not intended to provide medical advice. Please consult with a healthcare professional for any health concerns.

References:
Scazzone, C., Agnello, L., Bivona, G. et al. Vitamin D and Genetic Susceptibility to Multiple Sclerosis. Biochem Genet 59, 1–30 (2021). https://doi.org/10.1007/s10528-020-10010-1