Low-level constitutional mosaicism of a de novoBRCA1 gene mutation.

Background: Pathogenic BRCA1 mutations are usually inherited. Constitutional low-level BRCA1 mosaicism has never been reported.

Methods: Next-generation sequencing (NGS) of cancer gene panel of germline and tumour DNA in a patient with early onset, triple-negative breast cancer.

Results: Constitutional de novo mosaicism (5%) for a pathogenic (c.1953dupG; p.Lys652Glufs*21) BRCA1mutation was detected in leukocytes, buccal tissue and normal breast tissue DNA, with ∼50% mutation in tumorous breast tissue.

Conclusions: This is the first reported case of low-level, multiple tissue, constitutional mosaicism in BRCA1, and highlights the need to consider deep sequencing in affected individuals clinically suspected of having cancer predisposition whose tumours display a BRCA mutation.