Genomic, transcriptomic and epigenomic characterization of multifocal breast cancer

A significant proportion of breast cancer patients develop multiple synchronous unilateral breast tumors, also referred to as multifocal tumors, which represent a diagnostic and therapeutic challenge. Multifocality has been associated with a possible adverse patient outcome, propensity for axillary nodal involvement, and increased risk of local recurrence following breast conserving surgery when compared to unifocal tumors. Previous studies indicate that most lesions from multifocal tumors are concordant with regard to the currently used pathological parameters (histological subtype and grade, hormonal receptors and HER2 status). However, to our knowledge, no study has yet provided a detailed molecular analysis of multifocal breast cancer. Here, we aimed to better define the incidence of multifocal breast cancer using a systematic analysis of pathology reports of primary breast cancers and to compare different foci from 5 multifocal breast cancers in a global analysis using genomic, transcriptomic and epigenomic data. We demonstrate that multifocality is a frequent finding since it concerns 22% (945/4340) of breast cancer patients with primary ductal breast cancer. We find that different lesions from multifocal breast cancer can differ at the (epi)genomic and transcriptomic level even when the foci present similar histology, grading, hormonal and HER2 status despite having a common genetic background. Since the number of (epi)genetic alterations with potential clinical utility is rapidly growing due to increasing numbers of targeted therapies, these findings suggest that it might be necessary to interrogate the different lesions of multifocal breast cancers for adequate treatment management of these tumors.

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