MRG was initially isolated as a member of gene family

MRG15 was initially isolated as a member of gene family that is related to cellular senescence and cell growth control and is evolutionarily highly conserved (). MRG15 has a chromodomain in the N-terminus () and can directly bind to di- or tri-methylated histone H3 at lysine 36 (H3K36me) through this domain (). It has been shown that MRG15 forms complexes with both histone acetyltransferases (HATs) and histone deacetylases (HDACs) (). Although a role of MRG15-containing mSin3/HDAC complex in mammals still remains unknown, similar complex in smad pathway yeast, named Rpd3S, is important for deacetylation of coding regions to suppress spurious intragenic transcription through H3K36me recognition (). Defect of a homologous complex in fission yeast causes increased accessibility of DNA to genotoxic agents and widespread antisense transcripts that are processed at coding regions and abrogates global protective functions of chromatin (). MRG15 is also a stable component of Tip60 HAT complex, which is implicated in transcriptional control as well as DNA repair and apoptosis (). Purification of the Tip60 complex demonstrated that the components of the fly complex were very similar to those in the mammalian complex (). Moreover, it is evident that Tip60 and MRG15 are essential components of this complex, as knockdown or deletion of either gene results in an inability to repair DNA double strand breaks (DSBs) as well as embryonic lethality. We, and others, have shown that MRG15 and Tip60 are both important for DNA DSB repair (). We have previously published that embryonic null NSCs exhibit defects in growth as well as neuronal lineage differentiation (). In this study, we have examined the molecular mechanism(s) of the growth defect underlying deficiency in mouse embryonic NSCs by comparing neurosphere cultures obtained from null and wild-type embryonic brains. We have found increased expression of p21Sdi1/Cip1/Waf1 (p21) in null NSC cultures that most likely leads to the growth defect observed in deficient cells, and provide evidence that inefficient DNA damage repair in null NSCs may contribute to this growth defect. Results
Discussion Chromatin regulation is involved in DNA replication, transcriptional regulation and DNA damage repair and is a crucial step for stem cell self-renewal and function. We had previously shown that inactivation of the chromatin regulator MRG15 impairs proliferation of embryonic NSCs. In this report, we demonstrate that this occurs through activation of p53 and resulting increased expression of the cdk inhibitor p21. We have found increased expression of p21 and activated p53 in primary cell cultures of Mrg15 null NSCs but not wild-type. Focus formation of 53BP1, which indicates the presence of DNA damage, also co-localizes with p21 in Mrg15 null NSCs in growing culture condition, without any extrinsic insults. Focus formation of 53BP1 after γ-irradiation is delayed in Mrg15 null NSCs compared with wild-type NSCs. Our observations suggest that chromatin regulation and DNA damage repair through MRG15 complex(es) are essential to establish and maintain a functional NSC pool in mouse brain during development. Maintenance of genomic integrity is important for stem cell function in various stem cells including NSCs (Nijnik et al., 2007; Rossi et al., 2007; McKinnon, 2009). Perturbations in genes involved in DNA damage response signaling pathways and/or DNA repair are associated with neurological disorders such as neurodegeneration, microcephaly and brain tumors, suggesting that the inability to respond to DNA damage interferes with normal tissue homeostasis (McKinnon, 2009; Lee and Mckinnon, 2007; Frappart and McKinnon, 2008). DNA damage response and repair are critical for stem/progenitor cell amplification and ensure the establishment of a functional nervous system. Mice deficient for anyone of the many genes that play a role in the cellular response to DNA damage (Atm, Mre11, Nbs1) (Stewart et al., 1999; Allen et al., 2001; Frappart et al., 2005; Shull et al., 2009) or genes actively involved in DNA repair (BRCA2 and Lig4) (Lee et al., 2000; O\'Driscoll et al., 2001; Frappart et al., 2007) all share a phenotype of neurological failure due to defective DNA damage repair. These deletions affect NSC self-renewal as well as neuronal function.