Single strand DNA binding proteins 1 and 2 protect newly replicated telomeres

Abstract

Human single-strand (ss) DNA binding proteins 1 and 2 (hSSB1 and 2) are components of the hSSB1/2-INTS3-C9orf80 heterotrimeric protein complex shown to participate in DNA damage response and maintenance of genome stability. However, their roles at telomeres remain unknown. Here, we generated murine SSB1 conditional knockout mice and cells and found that mSSB1 plays a critical role in telomere end protection. Both mSSB1 and mSSB2 localize to a subset of telomeres and are required to repair TRF2-deficient telomeres. Deletion of mSSB1 resulted in increased chromatid-type fusions involving both leading-and lagging-strand telomeric DNA, suggesting that it is required for the protection of G-overhangs. mSSB1's interaction with INTS3 is required for its localization to damaged DNA. mSSB1 interacts with Pot1a, but not Pot1b, and its association with telomeric ssDNA requires Pot1a. mSSB1 ∆/∆ mice die at birth with developmental abnormalities, while mice with the hypomorphic mSSB1 F/F allele are born alive and display increased sensitivity to ionizing radiation (IR). Our results suggest that mSSB1 is required to maintain genome stability, and document a previously unrecognized role for mSSB1/2 in the protection of newly replicated leading-and lagging-strand telomeres.

Mouse models uncap novel roles of SSBs

“…Strikingly, all three studies reported pronounced upregulation of Ssb2 in response to Ssb1 deletion in both Ssb1 −/− tissues and MEFs [6][7][8]. Given that Ssb2 can substitute for Ssb1 in INTS3-SSB-C9Orf80 complexes, these results suggest that Ssb2 may compensate for some roles of Ssb1.…”

“…Given that Ssb2 can substitute for Ssb1 in INTS3-SSB-C9Orf80 complexes, these results suggest that Ssb2 may compensate for some roles of Ssb1. However, attempts to deplete Ssb2 even in wild-type MEFs led to rapid cell death, indicating that Ssb2 may play additional roles critical for cell viability [6,7]. The study by Gu et al [7] elegantly circumvented these effects through reconstitution of Ssb1 −/− MEFs with a mutant Ssb1 construct that does not result in concomitant upregulation of Ssb2.…”

“…Although the roles of Ssb1 in ATM and ATR activation are in agreement with previously described functions of Ssb1 in human cells [2][3][4], they contradict the recent reports by Feldhahn et al [6] and Shi et al [8] utilizing Ssb1 −/− npg MEFs. However, the fact that Gu et al [7] used SV40T immortalized MEFs and the other two studies [6,8] used primary MEFs may provide a context in which these phenotypes could be more readily observed.…”

“…In addition, the shelterin components POT1a/b are ssDNA binding proteins that protect the G-rich overhang in telomeres. Gu et al [7] investigated the putative role of Ssb1 and Ssb2 in mediating telomere homeostasis utilizing MEFs. Both Ssb1 and Ssb2 localized to a subset of telomeres and a number of telomere-related aberrations were observed in Ssb1 −/− MEFs.…”

“…Sandy Chang's laboratory assigned a new function to Ssb1 and Ssb2 in mediating telomere homeostasis [7]. Telomeres consist of TTAGGG repeats terminating in a G-rich 3′ single-stranded DNA (ssDNA) overhang, which is usually folded into the duplex DNA to protect it from activating the DNA damage response (DDR).…”