Elucidating the Role of the Smc5/6 Complex in Genome Organization and Repair

Abstract

Effective repair and maintenance of genetic material during cell proliferation is vital for cell survival. In this context, structural maintenance of chromosome (SMC) proteins play crucial and well-defined functions in the maintenance of genomic integrity. Specifically, the Smc5/6 complex has multifarious roles in folding and repairing chromosome segments to preserve genomic stability. How the Smc5/6 complex performs such roles has been at the forefront of research efforts for decades. In this thesis, I explored the molecular basis for the extensive roles of the Smc5/6 complex on damaged chromatin and its impact on cellular health and human diseases. An in-depth bioinformatic analysis of mutations affecting the human SMC5/6 complex in cancer genomics cohorts revealed that subunits of the SMC5/6 complex are frequently altered in several types of cancers. I introduced a large number of SMC5/6 complex mutations identified in human cancers into the budding yeast Saccharomyces cerevisiae and demonstrated that most of these mutations are associated with a loss of Smc5/6 complex activity. My results suggest a direct link between loss of SMC5/6 complex activity and cancer development. Next, I investigated the nature of endogenous substrates bound by the SMC5/6 complex on chromosomes. I found that R-loop structures, a common bi-product of chromosomal transactions, are recognized by the SMC5/6 complex in vivo. To identify whether Smc5/6 complex promotes genomic integrity by controlling R-loop formation, I explored the possibility that the Smc5/6 complex participates in the degradation of R-loop structures. Importantly, I demonstrated that the Smc5/6 complex promotes the degradation of R-loops by RNase H2 enzyme at highly transcribed genes and telomeres. Finally, I identified important interfaces within the subunits of the Smc5/6 complex that are responsible for its repair activity and showed that mutations in these regions may lead to chromosome breakage syndromes in humans. Taken together, this thesis reveals novel functions of the Smc5/6 complex and how misregulation of this enzyme impacts human health and disease states.