image: A mechanism by which knockouts of maternal mRNA clearing factors lead to zygotic arrest in mice. Abnormal accumulation of maternal mRNAs in mouse zygotes leads to transcriptional leakage before the correct timing of ZGA, which is the primary cause of excessive R-loop formation and genome instability. Maternal mRNA excess-associated increases in chromatin accessibility leads to precocious transcription and R-loop formation in zygotes after pronuclear formation. Accumulated R-loops in the pronuclei impair zygotic DNA replication, increase genomic instability, and cause S-phase arrest of the first zygotic cell cycle.
Credit: ©Science China Press
The beginning of embryonic development relies on a precise genetic handover, known as the maternal-to-zygotic transition (MZT), where control shifts from the mother's instructions to the embryo's own genome. A new study published in Science Bulletin reveals that the timely cleanup of maternal messenger RNA (mRNA) molecules is critical to preventing genomic instability and ensuring this transition succeeds.
The study, led by researchers from the Life Sciences Institute of Zhejiang University, found that in mouse embryos lacking key maternal mRNA degradation factors like Pabpn1l, chromatin becomes abnormally accessible at the wrong genomic locations. This leads to precocious transcription from lingering maternal mRNA templates. The critical consequence is the formation of R-loops—three-stranded structures where newly synthesized RNA binds back to the DNA. These structures act like roadblocks to the DNA replication machinery, causing replication stress, DNA damage, and ultimately arresting embryonic development at the one-cell stage, as illustrated.
Key Findings
The study found that in embryos lacking factors such as Pabpn1l and Cnot6l that regulate maternal mRNA degradation, chromatin accessibility is abnormally increased in the promoter regions of maternal genes. This finding indicates that the degradation of maternal mRNA is crucial for maintaining the normal state of chromatin. When maternal mRNA degradation fails, the increased chromatin accessibility may lead to abnormal gene expression regulation. Further research revealed that this increased chromatin accessibility triggers premature and ectopic transcription. These abnormal transcription products form large amounts of R-loop triple-stranded structures with the DNA template. An R-loop is a three-stranded structure composed of an RNA-DNA hybrid and a single-stranded DNA. The formation of this structure can impede the progression of the DNA replication fork, causing replication stress and DNA damage. These DNA damages continuously activate the S-phase checkpoint, ultimately leading to embryonic development arrest in the zygotic stage.
To gain a deeper understanding of the impact of failed maternal mRNA degradation on embryonic development, the research team utilized ATAC-seq technology to detect chromatin accessibility and found that chromatin accessibility was significantly increased in embryos lacking Pabpn1l. They further employed S⁴U-seq technology to examine the synthesis of nascent RNA and discovered that nascent RNA synthesis was abnormally increased in the mutants. Additionally, the researchers used immunofluorescence techniques to detect transcriptional activity and DNA damage markers, revealing a direct link between R-loop formation and DNA damage.
Significance and Innovation of the Study
The innovation of this study lies in its being the first to link the clearance of maternal mRNA with three core biological processes of the zygotic genome: chromatin reprogramming, transcriptional regulation, and replication fidelity. The study not only reveals the mechanism by which insufficient maternal mRNA degradation leads to genomic instability through the induction of R-loops and consequently causes early embryonic development arrest but also provides a potential theoretical basis for the etiological analysis of related human reproductive disorders. This finding is of great significance for understanding the molecular mechanisms of embryonic development and offers new ideas for the future development of treatments for reproductive disorders.
Conclusions and Future Work
These findings uncover the molecular mechanisms underlying embryonic development arrest due to failed maternal mRNA degradation. The clearance of maternal mRNA is not only crucial for maintaining chromatin state but also affects embryonic development by influencing transcriptional regulation and genomic stability. Moreover, the study found that overexpressing RNase H1, an enzyme that specifically degrades R-loops, could significantly alleviate DNA damage in mutant zygotes, improve DNA replication efficiency, and partially rescue embryonic development. This discovery not only provides new insights into the molecular mechanisms of embryonic development but also offers a potential theoretical basis for the etiological analysis of related human reproductive disorders.
The study's results deepen our understanding of the mechanisms governing the beginning of life and provide a potential theoretical basis for analyzing the causes of human reproductive disorders. The team plans to further explore the regulatory mechanisms of maternal mRNA degradation and the roles of R-loops in other biological processes. These studies will help us gain a more comprehensive understanding of the molecular mechanisms of embryonic development and provide new strategies for treating related diseases.
Journal
Science Bulletin