How Polygenic Risk Shapes the Lifetime Probability of Multiple Sclerosis
Multiple sclerosis is a chronic immune-mediated neurological disease in which both environmental exposures and inherited genetic susceptibility contribute to disease development. The article by Loonstra and colleagues addresses a central unresolved question in MS genetics: how much does inherited polygenic risk alter the actual lifetime probability of developing MS? Although genome-wide association studies have identified more than 200 susceptibility variants, the clinical interpretation of these variants has remained difficult because each individual variant usually confers only a small increase in risk. This study is therefore important because it translates aggregated genetic susceptibility into an estimate of lifetime disease risk using a population-based birth-year cohort.
Study Design: A Population-Based Birth-Year Approach
The investigators used Project Y, a Dutch cohort designed to identify nearly all persons with MS born in the Netherlands in 1966. This design minimized several common limitations in MS epidemiology, including selection bias, variable age distributions, and heterogeneous environmental exposure windows. Persons with MS were compared with non-MS control participants from Project Y and age-matched controls from the Amsterdam Dementia Cohort. By focusing on individuals born within a narrow time frame, the study created a more robust framework for estimating how genetic risk stratification relates to lifetime disease probability.
Construction of the Polygenic Risk Score
The polygenic risk score was calculated using susceptibility variants from the largest MS genome-wide association study available to the authors. The score incorporated autosomal variants, including variants located in the major histocompatibility complex region, which is known to have a strong role in immune-related disease susceptibility. Each variant was weighted according to its effect size, and the resulting scores were standardized. Participants were then grouped into deciles based on the distribution of polygenic risk scores among control participants, allowing the researchers to compare disease frequency across increasing levels of inherited genetic susceptibility.
Genetic Risk Strongly Stratifies Lifetime MS Probability
The most striking result was the large difference in estimated lifetime MS risk between individuals at the lower and upper ends of the polygenic risk distribution. Among women in the highest 10% of genetic risk, the estimated lifetime risk of MS was approximately 1 in 92, whereas among women in the lowest 30% of genetic risk it was approximately 1 in 2,739. Among men, the corresponding estimates were approximately 1 in 293 for the highest 10% and 1 in 7,900 for the lowest 30%. These findings demonstrate that polygenic risk meaningfully stratifies MS susceptibility, even though MS remains a multifactorial disease rather than a purely genetic disorder.
Sex-Specific Differences in Absolute Risk
The study also highlights the importance of interpreting genetic risk within a sex-specific epidemiological context. Women and men with comparable polygenic risk profiles did not have the same absolute lifetime risk of MS. Women consistently showed higher absolute risk, reflecting the well-established female predominance in MS incidence. However, the relative increase between low and high genetic risk categories was broadly similar between sexes, indicating that polygenic burden substantially modifies disease probability in both men and women. This distinction between absolute and relative risk is clinically important when communicating genetic findings to patients.
No Clear Association With Disease Course
Despite its strong association with MS susceptibility, the polygenic risk score was not significantly associated with age at symptom onset, age at progression, or time to secondary progression after statistical correction. This suggests that the genetic architecture underlying MS risk may differ from the genetic architecture influencing disease severity or progression. In other words, the inherited variants that increase the likelihood of developing MS may not necessarily determine how aggressive the disease will be after onset. This finding aligns with the broader concept that susceptibility and progression can be biologically distinct processes.
Clinical Relevance and Future Implications
The authors propose that polygenic risk scores may eventually support diagnostic certainty in individuals with suspected MS. A high score could strengthen suspicion when clinical, radiological, and laboratory findings are compatible with MS, whereas a very low score could serve as a diagnostic warning sign and prompt careful evaluation for MS mimics. Nevertheless, the study has important limitations: the findings are most applicable to individuals of European ancestry and to the Dutch 1966 birth cohort, and they should not be generalized uncritically to other populations. Overall, the article provides a valuable step toward translating MS genetics into clinically interpretable lifetime risk estimates, while also emphasizing that genetic risk prediction should complement, not replace, established neurological diagnosis.
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:
Loonstra, F. C., Álvarez Sirvent, D., Tesi, N., Holstege, H., Strijbis, E. M., Salazar, A. N., ... & Uitdehaag, B. (2024). Association of polygenic risk score with lifetime risk of developing multiple sclerosis in a population-based birth-year cohort. Neurology, 103(7), e209663.
