LAUSR

Activation Induced Cytidine Deaminase (AID) is a DNA-directed cytidine deaminase that plays a critical role in somatic hypermutation and immunoglobulin class switching in activated B- lymphocytes. AID is often ectopically expressed and promotes genomic hypermutation in a range of lymphoid malignancies. Deamination of genomic cytidines results in point mutations, single strand DNA breaks (SSB), and double strand DNA breaks (DSB). The resulting DNA damage stress leads to an obligate dependency on homologous recombination for cellular survival. RAD51 is a critical component of the homologous recombination pathway, forming nucleoprotein filaments at sites of DNA damage or replication fork stalls. RAD51 mediates homologous DNA strand exchange to promote recombinational repair of breaks and damaged replication forks. We have previously shown that RAD51 depletion leads to accumulation of DNA breaks, and ultimately cell death, in AID expressing cells. We now describe the mechanism of action for a novel small molecule, CYT01B, that targets RAD51 mediated recombination. Treatment of AID-expressing cells with CYT01B, attenuates RAD51 focus formation, reduces the nuclear concentration of RAD51, and promotes RAD51 protein degradation. CYT01B reduces homologous recombination activity in cells, culminating in cytologically visible chromosome fragmentation following DNA damage. Finally, CYT01B treated cells undergo replication catastrophe associated with caspase 3/7 activation and persistent PCNA nuclear signal. We conclude that CYT01B is a novel homologous recombination inhibitor, and propose a new synthetic lethality mechanism, induced by CYT01B, mediated by DNA replication stress and replication catastrophe. Citation Format: Melinda Day, Amber Cyr, Tyler Maclay, Kevin Mills. CYT01B suppresses RAD51 focus formation and induces replication catastrophe following AID induced DNA damage [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 2566.