Additional file 2: Figure S1. The singular value decomposition (SVD) plots showing the correlation between covariates and primary components (PC). Figure S2. Box plots showing the methylation level of differentially methylated CpGs (DMPs) in the discovery cohort. Figure S3. Characteristics of the differential DNA methylation positions (DMPs) associated with AS in the validation cohort. Figure S4. Venn diagram depicting the overlaps between the discovery and validation cohorts. Figure S5. Box plots showing the methylation level of differentially methylated CpGs (DMPs) in the validation cohort.