Modifiable Lifestyle Factors and Non-Scarring Alopecia: A Two-Sample Mendelian Randomization Study - Materials and Methods

Materials and Methods:\n\nStudy Design:\nThis study utilized a Two-Sample Mendelian Randomization (MR) design to investigate the potential causal relationship between modifiable lifestyle factors and the risk of non-scarring alopecia.\n\nData Sources:\nTwo large-scale genome-wide association studies (GWAS) were used as the primary data sources for this analysis. The first dataset consisted of genetic variants associated with the modifiable lifestyle factors of interest, namely smoking status, alcohol consumption, body mass index (BMI), and physical activity. The second dataset included genetic variants associated with the outcome of non-scarring alopecia.\n\nGenetic Instrument Selection:\nTo select genetic instruments for the lifestyle factors, a systematic literature review and meta-analysis were conducted to identify single nucleotide polymorphisms (SNPs) that were significantly associated with each lifestyle factor. SNPs were selected based on their genome-wide significance (p < 5 x 10^-8) and lack of association with confounding factors. The selected SNPs were subsequently used as instrumental variables for the MR analysis.\n\nStatistical Analysis:\nThe Two-Sample MR analysis was performed using the inverse-variance weighted (IVW) method, which provides robust estimates of causal effects by combining the genetic associations of the lifestyle factors with the outcome. In addition to IVW, sensitivity analyses, including weighted median, MR-Egger regression, and Mendelian randomization pleiotropy residual sum and outlier (MR-PRESSO), were conducted to assess the robustness of the findings and detect potential pleiotropic effects.\n\nData Harmonization and Quality Control:\nTo ensure data quality and comparability across the two datasets, a rigorous data harmonization process was undertaken. This involved harmonizing the genetic variants, allele frequencies, and effect sizes across the lifestyle factors and non-scarring alopecia datasets. Quality control procedures were applied to exclude SNPs with low imputation quality, deviations from Hardy-Weinberg equilibrium, and potential population stratification.\n\nMultiple Testing Correction:\nTo address the issue of multiple testing, the significance threshold was adjusted using the Bonferroni correction method. The corrected p-value threshold (p < 0.05 / number of lifestyle factors tested) was applied to determine the statistical significance of the causal effects.\n\nEthical Considerations:\nThis study followed ethical guidelines and obtained necessary approvals from relevant research ethics committees. Consent and data-sharing agreements were obtained from participants in the original GWAS studies.\n\nLimitations:\nPotential limitations of the study include the assumption of no pleiotropy or violation of the exclusion-restriction assumption, which may affect the validity of the causal estimates. Additionally, the MR analysis relies on the assumption of a linear relationship between lifestyle factors and non-scarring alopecia, which may not capture potential non-linear effects.