DNA methylation accelerated age as captured by epigenetic clocks influences breast cancer risk

**Background:** Breast cancer remains the leading cancer among women in the U.S., with a 0.4% annual incidence increase since 2004 and rising rates of early onset breast cancer (diagnosis before age 50). Age is one of the strongest predictors of breast cancer, yet the mechanisms driving early onset disease remain unclear. Accelerated biological age, particularly as measured by DNA methylation (DNAm) using epigenetic clocks, may provide insights. DNAm is an established hallmark of oncogenesis and is reversible and modifiable through lifestyle and psychological interventions. This review synthesizes evidence on epigenetic clock measurement of DNAm accelerated age and breast cancer outcomes.

**Methods:** The review followed PROSPERO Scoping Review Protocol guidance. Seven PubMed searches were conducted from January to April 2022 using terms for breast neoplasms, epigenetics/DNA methylation, and risk. The searches yielded 2,908 articles. Three independent reviewers screened titles and abstracts, with conflicts resolved by consensus. Inclusion criteria were: (1) use of an epigenetic clock to assess accelerated age, (2) primary outcome of breast cancer risk, (3) U.S.-based sample, (4) peer-reviewed PubMed-indexed journal, (5) published by April 30, 2022. Five articles met inclusion criteria. Data extraction used a uniform tool covering research design, sample, specimen type, analysis, and results.

**Key Results:** Five studies were included, using a total of 10 different epigenetic clocks. The Horvath clock was most frequently used (3 of 5 studies). Statistically significant associations with breast cancer were found for the Hannum, Horvath, Levine, GrimAge, DNAmAge, and Yang clocks. DNAm age correlated strongly with chronological age: Ren et al. reported r=0.96 for the Horvath clock; Kresovich et al. (2019a) reported correlations of r=0.88 (Hannum), r=0.87 (Horvath), and r=0.83 (Levine). DNAm accelerated age varied by biosample type. Rozenblit et al. found higher age correlation in peripheral blood than breast tissue. Hofstatter et al. found that non-cancerous breast tissue from women with luminal breast cancer was approximately five years epigenetically older than tissue from unaffected women. Ren et al. found decelerated (younger) DNAm age in malignant vs. non-malignant breast tissue, with younger DNAm age predicting poorer prognosis. Kresovich et al. (2019a) identified age acceleration predating menopausal transition as an independent risk factor. The largest sample was from the Sister Study (N=2,764); the smallest was Hofstatter et al. (N=88). Non-Hispanic White women were the majority in all studies; the most diverse sample (Rozenblit et al.) was 75% Non-Hispanic White with 178 African American women. No studies considered social factors (e.g., socioeconomic position, adversity) or epidemiological risk factors.

**Clinical Implications:** DNAm accelerated age as measured by epigenetic clocks shows promise as a biomarker for breast cancer risk surveillance. The menopausal transition may be a critical window for risk assessment. However, the lack of diverse populations and social factor consideration limits generalizability and the ability to address breast cancer disparities, particularly for minoritized women who experience higher mortality. As DNAm is modifiable through lifestyle and psychological interventions, future research incorporating diverse samples and social determinants could lead to targeted prevention strategies. The field would benefit from standardized approaches to enable cross-study comparison.