A breast tissue-specific epigenetic clock provides accurate chronological age predictions and reveals de-correlation of age and DNA methylation in tumor-adjacent and tumor samples.

Epigenetic clocks have been widely used to estimate biological age across various tissues, but their accuracy in breast tissue remains suboptimal. Classical models such as Horvath's and Hannum's clocks perform poorly in predicting chronological age in breast tissue, underscoring the need for a tissue-specific approach. In this study, we introduce a Breast Tissue-specific Epigenetic Clock (BTEC), developed using DNA methylation data from 553 healthy breast tissue samples across seven different studies. BTEC significantly outperformed existing clocks, demonstrating superior correlation with chronological age (r = 0.91) and lower prediction errors (MAE = 3.0 y) without requiring dataset-specific regressions adjustments. Notably, BTEC showed consistent performance across ancestry groups, in contrast to existing clocks which exhibited variable ancestry-related biases. We also found that residual-based epigenetic age acceleration, commonly used to correct for prediction bias, is highly dependent on the choice of reference dataset and does not provide a generalizable correction. BTEC's chronological age predictions for tumor-adjacent samples showed distortions, with an average deviation of -2.3 y, which was even more pronounced in tumor samples, where the average difference between predicted and chronological age was -14 to -15 y. When analyzed by molecular subtype, the distortion was greater in the more aggressive HER2+ and TNBC tumors compared to HR+ tumors. Overall, our findings indicate that breast tumors do not generally exhibit accelerated epigenetic aging. The probes used by BTEC were associated with known oncogenes such as JAK1, TP73, PDGFRA, CCND1 and STAT5B, as well as genes involved in cancer-related pathways, including Wnt, Hedgehog and ErbB signaling. Importantly, extreme deviations in epigenetic age, as measured by BTEC-derived epigenetic age acceleration (EAA), were associated with patient survival in HR+ breast cancer patients in an exploratory analysis, highlighting the prognostic value of tissue-specific epigenetic aging measures. These findings demonstrate that BTECs not only improve age prediction in breast tissue but also capture biologically meaningful alterations in tumor epigenetic aging with potential clinical implications.