Revisiting multifocal breast cancer: a clonality study of ductal carcinoma using whole exome sequencing

Multifocal breast cancer (MFBC), ductal type, has been hypothesized to arise by one of two mechanisms: either through intramammary/intralymphatic spread from a single index tumor (MBC-1), or as multiple independent tumors with each focus carrying its corresponding ductal carcinoma in-situ (MBC-2). In order to improve our understanding of MFBC pathogenesis, we employed laser capture microdissection coupled with whole-exome sequencing to study clonal origin in MFBC. We selected three cases of MBC-1 (C1 to C3) and MBC-2 (C4 to C6) and analyzed three foci from each case. MBC-1 cases were histologically similar and showed a strong predilection for satellite foci, vascular invasion and nodal metastasis when compared to MBC-2. Our bioinformatics approach provided strong evidence for clonal relationships in MBC-1, as demonstrated by distinct clusters of genes conserved across all tumor foci. Conversely, no gene clusters were shared across all the foci in MBC-2, suggesting multiple independent tumors. These findings provide further support for the two distinct pathogenetic mechanisms in MFBC.