DNAm age calculation

We applied Horvath’s method to calculate the DNAm age [6], which is currently the most robust predictor of chronological age [21]. Briefly, 353 dinucleotide markers were selected from 21,369 CpG probes on the Illumina 27 K and 450 K platforms with a penalized regression model in a large sample (n = 8000), including 51 healthy tissues and cell types and covering the entire adult life span. These markers were weighted to estimate the DNAm age (in units of years). It shows high age correlations (r = 0.96) and small mean deviation from calendar age (3.6 years) in its validation cohort. Mathematical details and software tutorials for DNAm age calculation can be found in the Additional files of Horvath [6]. An online age calculator (https://dnamage.genetics.ucla.edu) is available, by which the DNAm ages for the adjacent normal tissues and the cancerous tissues from the breast cancer patients in the dataset were obtained.