How Genes, Lifestyle, and Environment Shape Multiple Sclerosis Risk

Multiple sclerosis (MS) is an immune-mediated neurodegenerative disease of the central nervous system in which inflammatory activity can begin years before the first recognizable neurological episode. The review by Tomas Olsson, Lisa F. Barcellos, and Lars Alfredsson, published in Nature Reviews Neurology, emphasizes that this long preclinical period is central to understanding disease causation. Magnetic resonance imaging can already reveal multiple lesions at clinical onset, suggesting that pathogenic processes may have been operating silently for a considerable period. Consequently, exposures recorded close to diagnosis may not necessarily represent the biologically relevant period of exposure. The authors therefore frame MS not as the result of a single genetic defect or environmental insult, but as the product of interactions among genetic susceptibility, immune regulation, and environmental or lifestyle exposures. Genetic predisposition explains only part of overall susceptibility, whereas smoking, Epstein–Barr virus (EBV) infection, insufficient vitamin D or sunlight exposure, adolescent obesity, and several other factors contribute to risk. Importantly, many of these non-genetic influences are potentially modifiable, making their study relevant not only to disease mechanisms but also to prevention.

Genetic Susceptibility and the Central Role of HLA
The strongest genetic associations discussed in the review occur within the human leukocyte antigen (HLA) region, which encodes molecules responsible for presenting antigens to T lymphocytes. The class II allele HLA-DRB1*15:01 is associated with approximately a threefold increase in MS risk, whereas the class I allele HLA-A*02 is associated with protection, with an odds ratio of approximately 0.6. Individuals who carry HLA-DRB115:01 while lacking HLA-A02 have a combined odds ratio of roughly five. Beyond the HLA region, genome-wide association studies available at the time of the review had identified approximately 110 additional susceptibility variants, most exerting relatively modest individual effects. Strikingly, many of these variants occur near genes involved in adaptive or innate immune function, reinforcing the interpretation of MS as fundamentally immune mediated. The importance of HLA is therefore not simply statistical. Because HLA molecules determine which peptide antigens are presented to CD4+ and CD8+ T cells, these genetic variants provide a biologically plausible bridge between inherited susceptibility and environmental exposures capable of altering antigen presentation, immune activation, or tolerance.

Gene–Environment Interaction: When Risks Become Greater Together
One of the most significant themes of the article is that genetic and environmental risk factors cannot always be understood independently. The authors define biological interaction using departure from additivity: interaction is suggested when the disease risk among individuals exposed to two causal factors exceeds the risk expected from simply adding their separate effects. Smoking provides a particularly powerful example. Smoking alone is associated with a moderate increase in MS risk, but among smokers who carry HLA-DRB115:01 and lack protective HLA-A02, the reported combined odds ratio rises to approximately 14, compared with roughly five for the same genetic background among non-smokers. Similar interactions are described for EBV-related measures and adolescent obesity. These findings are mechanistically important because HLA molecules regulate adaptive immune recognition. Thus, when an environmental exposure shows a substantial interaction with HLA genotype, the observation supports the hypothesis that both factors may influence a shared immunological pathway. The review consequently presents gene–environment interaction not merely as an epidemiological curiosity, but as a method for identifying biological mechanisms that could connect external exposures with the autoimmune response ultimately directed against the central nervous system.

Smoking, EBV, and Obesity as Major Modifiable or Environmental Influences
Among the exposures examined, smoking, EBV infection, and adolescent obesity receive particular attention because each has substantial epidemiological support and evidence of interaction with genetic susceptibility. Smoking shows a dose–response relationship with MS risk, and passive smoke exposure is also associated with increased risk, supporting the possibility that pulmonary irritation and inflammation contribute to systemic immune activation. The authors propose that inflammatory processes in the lung may facilitate activation or migration of autoreactive T cells, a hypothesis supported by experimental autoimmune encephalomyelitis models. EBV represents a different but equally important pathway: previous infectious mononucleosis is associated with more than a twofold increase in MS risk, and elevated antibody responses to EBV nuclear antigen 1 are strongly associated with subsequent disease. Yet the review carefully notes that EBV causality was not definitively established within the evidence available at that time. Adolescent obesity is likewise associated with approximately doubled MS risk, and individuals with high BMI who also possess the HLA-DRB115:01 risk allele while lacking HLA-A02 were reported to have an approximately 14-fold elevation in risk. Proposed mechanisms include chronic low-grade inflammation, increased leptin signalling, altered regulatory T-cell activity, and reduced vitamin D bioavailability.

Vitamin D, Sunlight, Shift Work, and the Importance of Adolescence
A recurring conclusion of the review is that timing matters, with adolescence emerging as a particularly sensitive developmental period. Lower vitamin D concentrations and limited ultraviolet radiation exposure are associated with increased MS risk, although separating the biological effects of vitamin D from those of ultraviolet radiation is difficult because sunlight contributes directly to vitamin D metabolism while also exerting independent immunological effects. Evidence cited in the review indicates that higher vitamin D concentrations, especially before approximately 20 years of age, are associated with lower subsequent MS risk. Adolescent obesity similarly appears more relevant than BMI measured later in adulthood, and migration studies suggest that moving between low- and high-risk geographical regions before adolescence can alter an individual's later risk profile. Night-shift work before age 20 was also associated with an approximately 1.7-fold increase in MS risk, potentially implicating disturbed circadian rhythms and melatonin-dependent immune regulation. Collectively, these observations support the concept of a developmental “window of susceptibility,” during which environmental exposures may exert disproportionate effects on immune maturation. The review therefore shifts attention away from exposures occurring immediately before diagnosis and toward biological events many years earlier.

Emerging Factors, the Microbiome, and Epigenetic Mechanisms
The authors also examine associations that were considerably less established than smoking, EBV, obesity, or vitamin D deficiency. Organic solvent exposure and night work were associated with increased risk, whereas oral tobacco or nicotine exposure, cytomegalovirus seropositivity, alcohol consumption, and high coffee intake showed inverse associations in some studies. These findings require considerable caution: an observed reduction in risk does not establish that an exposure is protective or suitable for preventive use. Likewise, experimental work suggested that very high salt exposure could promote pathogenic TH17-cell differentiation, but the authors stressed that the quantities used in experimental models were far beyond typical human consumption and that stronger human evidence was required. The gut microbiome was presented as a particularly promising research frontier. Germ-free experimental animals showed markedly altered susceptibility to autoimmune neuroinflammation, suggesting that intestinal microorganisms can influence activation of adaptive immune cells, yet human data were still limited. Epigenetics provides another potential bridge between genes and environment: DNA methylation and histone modifications can respond to exposures such as smoking while also being influenced by genotype. The review therefore proposes epigenomic research as a means of understanding how environmental signals become biologically embedded in immune cells and potentially alter MS susceptibility.

From Epidemiological Association to Prevention and Mechanistic Understanding
The principal contribution of this review is its integration of genetics, epidemiology, and immunology into a unified model of MS susceptibility. The authors argue that many apparently diverse risk factors ultimately converge on the immune system, especially adaptive immune pathways. Figure 4 of the article illustrates this convergence by linking exposures such as smoking, EBV infection, obesity, night-shift work, and organic solvents—as well as factors associated with reduced risk—to B cells, T cells, natural killer cells, antigen-presenting cells, inflammatory mediators, and ultimately CNS injury. Nevertheless, the review repeatedly acknowledges the limitations of observational epidemiology, including reverse causation, residual confounding, recall bias, exposure misclassification, and selection bias; association alone must therefore not be equated with causation. Within the evidence assessed by the authors, the strongest gene–environment interactions involving smoking, EBV, and adolescent obesity support a model in which peripheral immune dysregulation precedes and helps drive CNS autoimmunity. The final implication is clinically significant: because several environmental determinants are modifiable, understanding them could eventually permit risk-reduction strategies, particularly for individuals with a family history of MS. At the same time, the authors emphasize that mechanistic understanding remains incomplete and that larger epidemiological, microbiome, and epigenetic studies are required before many associations can be translated confidently into preventive medicine.

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:
Zhang, Y., Zhou, Y., van der Mei, I. A. F., Simpson, S., Ponsonby, A. L., Lucas, R. M., Tettey, P., Charlesworth, J., Kostner, K., Taylor, B. V., & Ausimmune/AusLong Investigators Group (2019). Lipid-related genetic polymorphisms significantly modulate the association between lipids and disability progression in multiple sclerosis. Journal of neurology, neurosurgery, and psychiatry, 90(6), 636–641. https://doi.org/10.1136/jnnp-2018-319870