Deletion of Nono, a Novel Double-Strand Break Repair Factor, Leads to Hematopoietic Stem Cell Defects

Three genes, SFPQ, NONO, and PSPC1 encode for a small family of tandem RRM domain-containing proteins with diverse functions in RNA processing, transcription, and DNA repair. Our previous work has shown that an SFPQ•NONO complex promotes a distinct sub-pathway of non-homologous end joining (NHEJ) in vitro, implicating it as a novel double-strand break (DSB) repair factor. Consistently, attenuation of NONO function in human fibroblasts leads to partial radiosensitivity, delayed resolution of DNA repair foci, and an increase in radiation-dependent chromosome aberrations. To investigate the in vivo function of this complex, we knocked out Nono, the mouse homolog of the human NONO gene. We hypothesized that a DSB repair deficiency would lead to stem cell exhaustion, sensitivity to DNA damaging agents, or both. First, we investigated hematopoietic stem cells (HSCs), which are known to be sensitive to impairment of DNA repair. Nono-deficient (gt/0) mice showed reduced bone marrow and spleen cellularity, potentially due to reduced body size. The proportion of HSCs (LSK-SLAM) in relation to total bone marrow cells was similar in Nono-deficient and wild type (WT) mice, and bone marrow cell cultures yielded similar numbers and types of colonies. However, HSCs from Nono-deficient mice showed severe impairment in competitive repopulation assays in primary and secondary recipients. HSCs from gt/0 mice also displayed significantly higher levels of ROS and proliferation (accessed via BrdU incorporation), as well as higher percentage of Ki-67 positivity when compared to HSCs from WT mice. Together, these results indicate that Nono-deficient HSCs exhibit impaired capacity for self-renewal that may be caused by excessive HSC proliferation, a primary defect in response to genotoxic stress or a combination of both. To further elucidate this phenomenon, we investigated potential differences in sensitivity to the DNA crosslinking agent mitomycin C (MMC) and radiation. As previously observed in other genomic instability models (e.g., Fanconi anemia), colony-forming-cells from Nono-deficient mice were highly sensitive to MMC and X-rays (2 Gγ). Utilizing FACS-sorted HSCs, we determined that NONO protein levels are induced after X-ray treatment, and that Nono-deficient HSCs show delayed resolution of γ-H2AX foci and increased apoptosis. We also evaluated changes in the testis, another organ frequently affected by DNA repair gene mutations. Accordingly, testes of Nono-deficient mice showed growth retardation that became apparent between 18 and 26 days of postnatal development. The co-occurrence of hematopoietic and germ cell defects is reminiscent of other DNA repair gene defects, including those seen in Fanconi anemia. Collectively, our data identify NONO as a novel and important regulator of both HSC maintenance and germ cell function. Its potential effect in other organ systems and mechanism of action are under investigation.