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

Abstract

AbstractMicrohomology-mediated short tandem duplication (TD) is among specific mutational signatures associated withBRCA1-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 inBrca1-deficient cells. We found that short TDs induced by DNA nicks are significantly stimulated inBrca1-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 inBrca1-deficient cells, thus promoting TD. Neither depletion ofRad51orBrca2nor 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 underlyingBRCA1-linked TD formation in cancer.