Current understanding of genomic stability maintenancein pluripotent stem cells

Characterization of long-term ex vivo expansion of tree shrew spermatogonial stem cells

Tree shrews ( Tupaia belangeri chinensis) share a close relationship to primates and have been widely used in biomedical research. We previously established a spermatogonial stem cell (SSC)-based gene editing platform to generate transgenic tree shrews. However, the influences of long-term expansion on tree shrew SSC spermatogenesis potential remain unclear. Here, we examined the in vivo spermatogenesis potential of tree shrew SSCs cultured across different passages. We found that SSCs lost spermatogenesis ability after long-term expansion (>50 passages), as indicated by the failure to colonize the seminiferous epithelium and generate donor spermatogonia (SPG)-derived spermatocytes or spermatids marking spermatogenesis. RNA sequencing (RNA-seq) analysis of undifferentiated SPGs across different passages revealed significant gene expression changes after sub-culturing primary SPG lines for more than 40 passages on feeder layers. Specifically, DNA damage response and repair genes (e.g., MRE11, SMC3, BLM, and GEN1) were down-regulated, whereas genes associated with mitochondrial function (e.g., NDUFA9, NDUFA8, NDUFA13, and NDUFB8) were up-regulated after expansion. The DNA damage accumulation and mitochondrial dysfunction were experimentally validated in high-passage cells. Supplementation with nicotinamide adenine dinucleotide (NAD +) precursor nicotinamide riboside (NR) exhibited beneficial effects by reducing DNA damage accumulation and mitochondrial dysfunction in SPG elicited by long-term culture. Our research presents a comprehensive analysis of the genetic and physiological attributes critical for the sustained expansion of undifferentiated SSCs in tree shrews and proposes an effective strategy for extended in vitro maintenance.

Replication stress causes delayed mitotic entry and chromosome 12 fragility at the ANKS1B large neuronal gene in human induced pluripotent stem cells

Substantial background level of replication stress is a feature of embryonic and induced pluripotent stem cells (iPSCs), which can predispose to numerical and structural chromosomal instability, including recurrent aberrations of chromosome 12. In differentiated cells, replication stress-sensitive genomic regions, including common fragile sites, are widely mapped through mitotic chromosome break induction by mild aphidicolin treatment, an inhibitor of replicative polymerases. IPSCs exhibit lower apoptotic threshold and higher repair capacity hindering fragile site mapping. Caffeine potentiates genotoxic effects and abrogates G2/M checkpoint delay induced by chemical and physical mutagens. Using 5-ethynyl-2'-deoxyuridine (EdU) for replication labeling, we characterized the mitotic entry dynamics of asynchronous iPSCs exposed to aphidicolin and/or caffeine. Under the adjusted timing of replication stress exposure accounting revealed cell cycle delay, higher metaphase chromosome breakage rate was observed in iPSCs compared to primary lymphocytes. Using differential chromosome staining and subsequent locus-specific fluorescent in situ hybridization, we mapped the FRA12L fragile site spanning the large neuronal ANKS1B gene at 12q23.1, which may contribute to recurrent chromosome 12 missegregation and rearrangements in iPSCs. Publicly available data on the ANKS1B genetic alterations and their possible functional impact are reviewed. Our study provides the first evidence of common fragile site induction in iPSCs and reveals potential somatic instability of a clinically relevant gene during early human development and in vitro cell expansion.

DPPA5A suppresses the mutagenic TLS and MMEJ pathways by modulating the cryptic splicing of Rev1 and Polq in mouse embryonic stem cells

Genetic alterations are often acquired during prolonged propagation of pluripotent stem cells (PSCs). This ruins the stem cell quality and hampers their full applications. Understanding how PSCs maintain genomic integrity would provide the clues to overcome the hurdle. It has been known that embryonic stem cells (ESCs) utilize high-fidelity pathways to ensure genomic stability, but the underlying mechanisms remain largely elusive. Here, we show that many DNA damage response and repair genes display differential alternative splicing in mouse ESCs compared to differentiated cells. Particularly, Rev1 and Polq, two key genes for mutagenic translesion DNA synthesis (TLS) and microhomology-mediated end joining (MMEJ) repair pathways, respectively, display a significantly higher rate of cryptic exon (CE) inclusion in ESCs. The frequent CE inclusion disrupts the normal protein expressions of REV1 and POLθ, thereby suppressing the mutagenic TLS and MMEJ. Further, we identify an ESC-specific RNA binding protein DPPA5A which stimulates the CE inclusion in Rev1 and Polq. Depletion of DPPA5A in mouse ESCs decreased the CE inclusion of Rev1 and Polq, induced the protein expression, and stimulated the TLS and MMEJ activity. Enforced expression of DPPA5A in NIH3T3 cells displayed reverse effects. Mechanistically, we found that DPPA5A directly regulated CE splicing of Rev1. DPPA5A associates with U2 small nuclear ribonucleoprotein of the spliceosome and binds to the GA-rich motif in the CE of Rev1 to promote CE inclusion. Thus, our study uncovers a mechanism to suppress mutagenic TLS and MMEJ pathways in ESCs.