Replication stress limits developmental potential of human preimplantation embryos

Abstract

Human preimplantation embryos show abnormal nucleation and DNA damage, compromising cell cycle progression and developmental potential. The molecular mechanisms and timing of these abnormalities are unknown. We identified endogenous DNA damage and repair mechanisms in human embryos and developed a murine model to study developmental consequences of replication stress in preimplantation embryos. Prospective laboratory study of human and mouse embryos Endogenous DNA damage and repair pathways were evaluated in humans using donated oocytes (n=25) and embryos (n=20). Immunofluorescent staining detected DNA damage (γH2AX, RPA), repair (RPAS33, RPA S4/S8, Rad51, 53BP1), and micronucleation. Consequences of genomic instability were studied in a murine model using aphidicolin, a DNA polymerase inhibitor that increases endogenous chromosome fragility1. Mouse zygotes (n=500) were briefly exposed to aphidicolin in the first cell cycle, then evaluated at 1-cell, 2-cell, 4-8-cell and blastocyst stage. Immunofluorescent staining detected DNA damage and repair. Cleavage progression, blastulation and embryo quality were assessed versus controls. Human and mouse karyotypes were determined with NGS. Human preimplantation embryos show endogenous DNA damage, demonstrated by γH2AX, RPA and abnormal nucleation. Cleavage embryos had significantly greater foci and micronucleation vs blastocysts (γH2AX cleavage mean 2.3 vs blastocyst 1.0, p <0.0001; RPA cleavage mean 1.7 vs 0.3, p<0.0001; abnormal nucleation cleavage mean 15.9% vs blastocyst 4.2%, p<0.0001). DNA damage foci coincided with RPAS33, indicating RPA phosphorylation by G2 checkpoint kinase ATR, Rad51, indicating repair by homologous recombination, and 53BP1, indicating unrepaired DNA is passed to daughter cells. Aphidicolin-induced replication delay resulted in DNA damage (γH2AX and RPA), and RPAS33, indicating an ATR-dependent G2 checkpoint. Additional DNA repair mechanisms included Rad51 and 53BP1, similar to human embryos. Though some unreplicated DNA is tolerated in mitosis and compatible with euploidy, aphidicolin-induced under replication in the first cell cycle precipitated instability in later cell cycles, leading to decreased blastulation (45% after 8h aphidicolin vs 91.8% control, p<0.0001), and poor quality embryos as evidenced by significantly fewer total cells and inner cell mass with significantly greater DNA damage and micronucleation with increasing duration of aphidicolin exposure compared to controls. DNA damage responses to incomplete replication in G2 (ATR and Rad51), and the G1 response to unreplicated DNA (53BP1) mirror endogenous repair activity in human preimplantation embryos. Developmental consequences of replication stress likely persist beyond the preimplantation stage and may contribute to failed implantation or miscarriage. The murine model of genomic instability enables further study of these processes and the development of targeted therapeutics.