Constitutive Promoter Hypermethylation Can Be a Predisposing Event in Isolated Early-Onset Breast Cancer.

Early age at onset of breast cancer (eoBC) is suggestive of an increased genetic risk. Although genetic testing is offered to all eoBC-affected women, in isolated cases the detection rate of pathogenic variants is <10%. This study aimed at assessing the role of constitutive promoter methylation at BC-associated loci as an underlying predisposing event in women with eoBC and negative family history. Promoter methylation at 12 loci was assessed by the MassARRAY technology in blood from 154 negative patients with eoBC and negative family history, and 60 healthy controls. Hypermethylation was determined, within each promoter, by comparing the patient's mean methylation value with thresholds based on one-sided 95% bootstrap confidence interval of the controls' mean. Three patients had hypermethylated results, two at and one at . Analyses on tumor tissue from the patient exceeding the highest threshold at revealed a mean methylation >60% and loss of heterozygosity at chromosome 17q. The patient hypermethylated at showed low methylation in the tumor sample, ruling out a role for methylation-induced silencing in tumor development. In isolated eoBC patients, constitutive promoter methylation may be a predisposing event. Further studies are required to define the impact of methylation changes occurring at BC-predisposing genes and their role in tumorigenesis.