Whole‐exome analysis of metaplastic breast carcinomas with extensive osseous differentiation

Aims Metaplastic breast carcinoma (MBC) is a rare type of triple‐negative breast cancer that shows vast histological and genetic heterogeneity. Osseous differentiation can be found in different subtypes of MBC. Whether MBCs with osseous differentiation are underpinned by specific genetic alterations has yet to be defined. The aim of this study was to investigate the repertoire of somatic mutations and copy number alterations (CNAs) in three MBCs with extensive osseous differentiation. Methods and results Tumour and normal DNA samples from three MBCs with extensive osseous differentiation were subjected to whole‐exome sequencing. Somatic mutations, CNAs and mutational signatures were determined by use of a validated bioinformatics pipeline. Our analyses revealed clonal TP53 hotspot mutations associated with loss of heterozygosity of the wild‐type allele coupled with mutations affecting genes related to the Wnt and/or the phosphoinositide 3‐kinase–AKT–mammalian target of rapamycin pathways in all cases analysed. All cases showed a dominant mutational signature 1, with two cases showing a secondary signature 3 in addition to other features of homologous recombination DNA repair defects. The oncostatin M receptor gene, which plays a role in mesenchymal differentiation and bone formation, was found to be mutated in two MBCs with extensive osseous differentiation and in none of 35 previously published 35 MBCs. Conclusion Our findings suggest that MBCs with osseous differentiation have somatic mutations similar to those of other forms of MBC.