When Genes Converge: Gene–Environment Interactions in Multiple Sclerosis

Jacobs and colleagues’ 2021 study, “Gene-Environment Interactions in Multiple Sclerosis: A UK Biobank Study,” investigates a central question in multiple sclerosis research: whether inherited genetic susceptibility modifies the effect of environmental exposures on disease risk. Multiple sclerosis is widely understood as a complex immune-mediated neurological disorder in which both genetic architecture and external exposures contribute to susceptibility. The authors used UK Biobank data to test whether polygenic risk scores for multiple sclerosis interact with early-life and adolescent environmental risk factors, particularly childhood body size, smoking, and pubertal timing. Their main contribution is the demonstration that childhood obesity-related risk may be amplified in individuals with a high burden of genetic risk, including risk outside the major histocompatibility complex region.

Study Design and Population
The study used a large case-control framework within UK Biobank, including 2,250 individuals with multiple sclerosis and 486,000 controls. Multiple sclerosis cases were identified through ICD-coded diagnoses, self-report, general practitioner records, and death registration data. The investigators excluded individuals diagnosed before age 20 to reduce ambiguity about whether environmental exposures occurred before disease onset. This design allowed the authors to evaluate exposures occurring in childhood or adolescence, thereby limiting reverse causation. The cohort’s scale was a major strength, although its demographic composition—predominantly White British, middle-aged, and relatively healthier or more affluent than the general population—also limits generalizability.

Environmental Exposures Examined
The authors evaluated ten early-life or adolescent exposures previously implicated in multiple sclerosis risk, including month of birth, breastfeeding, childhood body size at age 10, maternal smoking exposure, age at menarche, age at voice breaking, age at first sexual intercourse, smoking before age 20, birth weight, and infectious mononucleosis before age 20. Three exposures showed strong evidence of association with multiple sclerosis: larger childhood body size, smoking before age 20, and earlier age at menarche. In Figure 1, the forest plot visually summarizes these associations, showing that individuals who described themselves as “plumper” at age 10 had higher odds of multiple sclerosis than those who described themselves as thinner, while earlier menarche was associated with increased risk because each additional year of later menarche corresponded to lower odds.

Construction of Polygenic Risk Scores
To quantify genetic liability, the researchers developed polygenic risk scores using external weights from the largest available genome-wide association study of multiple sclerosis. They created two principal categories of scores: one including the major histocompatibility complex region, termed PRS-MHC, and one excluding it, termed PRS-non-MHC. This distinction is scientifically important because the HLA region within the MHC is the strongest known genetic contributor to multiple sclerosis, but the authors wanted to determine whether broader autosomal genetic risk also modifies environmental effects. Figure 2 shows that both PRS categories were associated with multiple sclerosis risk, with stronger predictive performance when the MHC region was included, while the non-MHC score still captured meaningful polygenic susceptibility.

Main Finding: Interaction Between Childhood Body Size and Genetic Risk
The most important result was the evidence of additive interaction between childhood body size and polygenic risk. In practical terms, the increased risk associated with being “plumper” at age 10 was greater among individuals with high genetic susceptibility to multiple sclerosis. This interaction was observed both for the PRS including the MHC and for the PRS excluding the MHC, suggesting that the effect is not driven solely by classical HLA risk alleles. The attributable proportion due to interaction was approximately 0.17 for both PRS models, indicating that a measurable proportion of disease risk among individuals with both risk factors may be attributable to their combined effect rather than to either factor independently.

Biological Interpretation and Scientific Significance
The findings support the broader hypothesis that multiple sclerosis risk emerges from biologically meaningful convergence between immune-related genetic susceptibility and environmental or developmental exposures. Childhood obesity may influence immune function through chronic low-grade inflammation, altered adipokine signaling, vitamin D metabolism, or changes in pubertal development, all of which could plausibly affect autoimmune risk. The observation that non-MHC genetic risk also interacts with childhood body size is particularly important because prior gene-environment studies in multiple sclerosis have focused heavily on HLA alleles such as HLA-DRB1*15:01 and HLA-A*02:01. This study therefore extends the field beyond single high-effect immune loci toward a more polygenic model of risk modulation.

Limitations and Implications for Prevention
The authors appropriately emphasize that statistical interaction does not prove direct biological interaction, and the findings require replication in independent longitudinal cohorts. Several exposures were self-reported retrospectively, including childhood body size and smoking, making recall bias a relevant concern. In addition, UK Biobank’s population structure limits applicability to non-European ancestry groups. Nevertheless, the study has important implications for prevention research. If childhood adiposity confers greater multiple sclerosis risk among genetically susceptible individuals, then future prevention trials may benefit from genetic risk stratification. The article ultimately argues for a more individualized model of multiple sclerosis prevention, in which environmental risk modification is interpreted in the context of inherited susceptibility.

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:
Jacobs, B. M., Noyce, A. J., Bestwick, J., Belete, D., Giovannoni, G., & Dobson, R. (2021). Gene-environment interactions in multiple sclerosis: a UK Biobank study. Neurology: Neuroimmunology & Neuroinflammation, 8(4), e1007.