SMARCAL1 Maintains Telomere Integrity During DNA Replication
Poole, Lisa Ann
DNA replication is constantly challenged by several sources of replication stress that can affect the accuracy and timely completion of this essential biological process. The DNA damage response (DDR) exists to address these sources of stress and promote rapid and accurate duplication of nearly 7 billion base pairs of DNA with each round of replication in humans. The SNF2 family of DNA translocases contains multiple proteins within the DDR that are recruited to stalled replication forks to promote repair and replication restart. SMARCAL1 (SWI/SNF related, matrix-associated, actin-dependent, regulator of chromatin, subfamily A-like 1), ZRANB3 (zinc-finger RANBP2-type containing 3), and HLTF (helicase-like transcription factor) share similar replication fork remodeling capabilities, such as fork reversal. Despite their biochemical similarities, genetic studies indicate they must have unique cellular activities. In my dissertation work, I sought to differentiate the functions of these highly similar enzymes in cells. I used a combination of biochemical, molecular biological, and genomics approaches to investigate the cellular requirement for each of these proteins with a specific focus on SMARCAL1. Here, I present data showing that SMARCAL1 has an important function at telomeres, which present an endogenous source of replication stress. SMARCAL1-deficient cells accumulate telomere-associated DNA damage and have greatly elevated levels of extrachromosomal telomere DNA (C-circles). Although these telomere phenotypes are often found in tumor cells employing the alternative lengthening of telomeres (ALT) pathway for telomere elongation, SMARCAL1 deficiency does not yield other ALT phenotypes such as telomere recombination. The activity of SMARCAL1 at telomeres can be separated from its genome maintenance activity in bulk chromosomal replication since it does not require its interaction with replication protein A (RPA). Importantly, this telomere maintenance function is not shared by ZRANB3 or HLTF. These results provide the first identification of an endogenous replication stress source that SMARCAL1 is needed to resolve and defines differences between members of this class of replication fork repair enzymes.