RNA transcription termination factors and persistent R-loops: potential carcinogenic determinants after high or low LET IR.

Neurologic and carcinogenic effects caused by prolonged exposure to high linear energy transfer (LET) ionizing radiation (IR) represent major health-limiting obstacles during an estimated two-three year mission to Mars. Examination of DNA damage caused by high or low LET exposures has shown that high LET IR exposures cause far greater formation of multiply damaged sites, including complex DNA double strand breaks (DSBs) that are more difficult for cells to repair correctly. Presumably, these lesions lead to neurologic (cognitive and visual), and visual (cataracts) defects, as well as increased carcinogenic effects. This original research article focuses on potential health risks for individuals with defects in factors that function to accurately terminate RNA transcription, a process involving specific key steps in transcription, including: (i) accurate mRNA, miRNA, siRNA or lncRNA maturation, (ii) RNA polymerase II (RNA Pol II) stalling and dislodging; and (iii) subsequent resolution of DNA:RNA:DNA hybrids, known as ‘Rloops’. Defects in RNA Pol II processing, in general, lead to persistent R-loop formation and ultimately formation of complex DSBs caused by unstable RNA:DNA structures and collisions between persistent R-loops and DNA replication and/or RNA transcriptional processes. We offer original data as evidence that low LET IR exposures cause delayed formation of persistent R-loops in wild-type cells. Further, we show that these unique and understudied DNA lesions result in indirect complex DSBs, breaks not generated by original deposition of energy. We also show that cells deficient in one RNA termination factor, Kub5-Hera (K-H/RPRD1B) show major defective DSB repair kinetics after high LET IR treatments, consistent with far greater R-loop formation and delayed and slower repair of persistent R-loop-derived DSBs than after low LET IR doses. Thus, haploinsufficient loss of one essential RNA transcription termination scaffold factor, K-H (aka., RPRD1B), results in simultaneous defective repair of both R-loops and complex DSBs created by low or high LET IR. Whole body low LET IR-exposed haplo-insufficient K-H mice showed hypersensitive carcinogenesis in a dose-dependent manner. These mice also have neurological and visual defects, including loss of hind limb function and *Correspondence: Julio.Morales@UTSouthwestern.edu or David.Boothman@UTSouthwestern.edu ** This review summarizes, in part, a mini-symposium, entitled “Role of R-loop-related factors in DNA damage, genomic instability, and DNA repair” presented at the Annual Radiation Research Society meeting held in Las Vegas, NV in October, 2014. Please see the summary of that mini-symposium published through the Radiation Research Society (RRS). This review covers our current understanding of the roles of persistent R loops in carcinogenesis after high or low LET IR treatments. As such, published and non-published data are discussed with all unpublished data due to appear in 2015. a These authors contributed equally to this peer-reviewed paper. RNA transcription termination factors and persistent R-loops: potential carcinogenic determinants after high or low LET IR. Morales JC, Motea EA, Patidar PL, Fattah FJ, Ilcheva M, Burma S, Story MD, Boothman DA. https://three.jsc.nasa.gov/articles/Boothman_R-Loop_Factors.pdf. Date posted: 07/09/2015