Allelic DNA synthesis followed by template switching underlies BRCA1-linked tandem duplication

Microhomology-mediated short tandem duplication (TD) is among specific mutational signatures associated with BRCA1 -deficient tumors. Several mechanisms have been proposed for its generation, but may not be applicable in repeat-less regions of the human genome. We thus developed a repeat-less TD reporter and a PCR-based site-specific TD assay to analyze short TDs induced by one-ended DNA double strand breaks (DSBs) converted from DNA nicks in Brca1 -deficient cells. We found that short TDs induced by DNA nicks are significantly stimulated in Brca1 -deficient cells. Analysis of TD products revealed that the TD formation is partly mediated by template switching of displaced nascent strand after allelic DNA synthesis. This suggests either allelic DNA synthesis or the strand annealing step of allelic break-induced replication might be more easily aborted in Brca1- deficient cells, thus promoting TD. Neither depletion of Rad51 or Brca2 nor inactivation of the Brca1 coiled-coil domain stimulated nick-induced TD, indicating that RAD51 loading by BRCA1 is dispensable for BRCA1-mediated TD suppression. These results together provide novel insights into the mechanisms underlying BRCA1 -linked TD formation in cancer.