Circular RNAs generated from the BRCA1 pseudogene regulates the DNA damage response through SERBP1 RNA-binding protein

The genomic region encompassing the BRCA1 gene includes the BRCA1P1 pseudogene (BRCA1P1) within ~170kb at chromosome 17q21. The homology and proximity of BRCA1 and BRCA1P1 create a hot spot for recombination, resulting in a large genomic rearrangement found in some families with breast and ovarian cancers. While the significant role of BRCA1 in regulating the DNA damage response (DDR) is well defined, little is known about the biological properties of the BRCA1P1 pseudogene. To functionally annotate BRCA1P1, we conducted a CRISPR-Cas 9 knockout of the pseudogene, as well as knockdown of the expression using antisense oligonucleotides. Both knockout and knockdown of the pseudogene result in increases in apoptosis and sensitivity to DNA damaging drugs. Depletion of BRCA1P1 accumulates spontaneous DNA damage foci and leads to replication fork stalling. Mechanical studies reveal that the BRCA1P1 generates circular RNAs, which are retained in nuclei. Mass spectrometry followed by chromatin isolation by RNA purification (ChIRP) identifies a specific association of the circular RNAs with SERBP1 RNA-binding protein, which was confirmed by ChIRP-Western. Consequently, BRCA1P1 knockout decreased SERBP1 protein expression in the nuclei. Based on the previous report that SERBP1 binds to mRNAs of DDR proteins and controls their translation, we propose a mechanism of BRCA1P1-driven regulation of DDR protein expression through SERBP1. Our data indicate that the BRCA1P1 pseudogene plays a role in regulating replication fork progression and genomic instability in breast cancer cells through an interaction with SERBP1 RNA-binding protein. Citation Format: Yoo J. Han, Jing Zhang, Jennifer M. Mason, Toshio F. Yoshimatsu, Xinxin Du, Ian Hurley, Danny E. Kim, Aparna Anantharaman, Laia P. Brunet, Aleix Prat, John Kwon, Kannanganattu V. Prasanth, Olufunmilayo I. Olopade. Circular RNAs generated from the BRCA1 pseudogene regulates the DNA damage response through SERBP1 RNA-binding protein [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1373.