Can DNA Methylation in Peritumoral and Contralateral Breast Tissue Predict Recurrence or Second Breast Cancers?

Despite major advances in early breast cancer detection and therapeutic strategies, locoregional and distant recurrences, as well as the development of a second primary breast cancer, remain major clinical challenges. Current prognostic tools primarily rely on tumor-specific features, such as the histological grade, hormone receptor status, and proliferative index, and, more recently, on molecular signatures aimed at improving risk stratification and predicting recurrence. However, these approaches remain imperfect, and there is an urgent need to develop complementary strategies. Growing attention has been focused on the tumor microenvironment and the surrounding non-tumoral tissue, which may harbor clinically relevant molecular alterations. Emerging evidence suggests that DNA methylation changes can be detected in the adjacent and contralateral breast tissue and reflect early steps of carcinogenesis or predisposition to tumor development. This phenomenon, often referred to as field cancerization, raises new questions about the dynamics of cancer development. The aim of this work is to provide an integrative overview of DNA methylation alterations in normal breast tissue, including peritumoral and contralateral areas, and to examine their potential as predictive biomarkers of recurrence, based on the available data from tumoral tissue. In theory, these applications seem promising, but their role needs to be confirmed in large prospective trials, in order to overcome barriers to clinical implementation. The currently available evidence does not support a role for DNA methylation in the selection of locoregional and systemic treatment strategies, particularly with a view to reducing the rising number of uni- and bilateral mastectomies performed without any demonstrated survival benefit.