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SSB1 and SSB2 are newly identified single-stranded (ss) DNA binding proteins that play a crucial role in genome maintenance in humans. We recently generated a knockout mouse model of Ssb1 and revealed its essential role for neonatal survival. Notably, we found compensatory up-regulation of Ssb2 protein levels in multiple tissues of conditional Ssb1-/- mice, suggesting functional compensation between these 2 proteins. We report here the first description of Ssb2-/- knockout mice. Surprisingly, unlike Ssb1 knockout mice, Ssb2-/- mice are viable and fertile and do not exhibit marked phenotypic changes when compared with their Ssb2+/+ and Ssb2+/- littermates. Notably, we did not detect any pathologic changes in the thymus, spleen, or testes, tissues with the most abundant expression of Ssb2. Moreover, Ssb2-/- mouse embryonic fibroblasts (MEFs) did not show any sensitivity to DNA-damaging agents, or defects in DNA repair capacity. However, we observed modest up-regulation of Ssb1 levels in Ssb2-/- MEFs as well as in Ssb2-/- thymus and spleen, suggesting that Ssb1 is likely able to compensate for the loss of Ssb2 in mice. Altogether, our results show that Ssb2 is dispensable for embryogenesis and adult tissue homeostasis, including thymopoiesis, splenic development, male fertility, and DNA repair in mice. [ABSTRACT FROM AUTHOR]

Single-stranded DNA binding proteins (SSBs) regulate multiple DNA transactions, including replication, transcription, and repair. We recently identified SSB1 as a novel protein critical for the initiation of ATM signaling and DNA double-strand break repair by homologous recombination. Here we report that germline Ssb1-/- embryos die at birth from respiratory failure due to severe rib cage malformation and impaired alveolar development, coupled with additional skeletal defects. Unexpectedly, Ssb1-/- fibroblasts did not exhibit defects in Atm signaling or γ-H2ax focus kinetics in response to ionizing radiation (IR), and B-cell specific deletion of Ssb1 did not affect class-switch recombination in vitro. However, conditional deletion of Ssb1 in adult mice led to increased cancer susceptibility with broad tumour spectrum, impaired male fertility with testicular degeneration, and increased radiosensitivity and IR--induced chromosome breaks in vivo. Collectively, these results demonstrate essential roles of Ssb1 in embryogenesis, spermatogenesis, and genome stability in vivo. [ABSTRACT FROM AUTHOR]