The relative efficiency of homology‐directed repair has distinct effects on proper anaphase chromosome separation

Abstract

Since several tumors show deficiency in homologous recombination (HR), it is important to understand how HR deficiency can lead to cancer. Notably, cells deficient in HR show increased chromosomal instability (CIN), which is a hallmark of cancer. To examine the role of HR in limiting CIN, we considered that HR defects could cause at least two types of anaphase abnormalities: DNA linkages and loss of spindle bipolarity. DNA linkages can be visualized using DAPI, and/or by immunostaining of the helicase PICH, which can form ultrafine bridges (UFBs). We examined HEK293 cells with inducible expression of dominant‐negative alleles of the strand exchange factor RAD51: RAD51K133R and RAD51K133A, which cause a mild and severe HR defect, respectively. RAD51K133R causes a rapid increase in UFBs, but does not cause loss of spindle polarity. Similarly, BRCA1 or BRCA2 mutant cells displayed increased UFBs frequencies. In contrast, RAD51K133A expression causes UFBs, DAPI bridges, multipolar anaphases, and cell death. Based on these findings, we propose that mild HR defects allow proliferation of cells that are prone to CIN due to defective chromosome separation during anaphase, thus promoting tumor development. In contrast, severe HR defects lead to multipolar anaphases that result in cell death. We are currently examining anaphase abnormalities in cancer cells to develop biomarkers for HR deficiency.