RGS12 as a Novel Maternal-Effect Gene Causes Arrest at the Pronuclear Stage of Human Zygote

Background: Early embryonic arrest is one of the major causes of female infertility after in vitro fertilization (IVF), but the causal gene of arrest at the pronuclear (PN) zygote stage is largely unknown. Results: To understand this process, we recruited a family characterized by recurrent PN arrest during IVF cycles and performed whole-exome sequencing. The missense variant c.C1630T (p.R544W) in RGS12 was responsible for a phenotype characterized by paternal transmission. RGS12 controls Ca2+ oscillation, which is required for oocyte activation after fertilization. Single-cell transcriptome profiling of PN-arrest zygotes revealed defective established translation, RNA processing and cell cycle, which explained the failure of complete oocyte activation. Furthermore, we identified proximal genes involved in Ca2+ oscillation–cytostatic factor–anaphase-promoting complex (Ca2+ oscillation–CSF–APC) signaling, including upregulated CaMKII, ORAI1, CDC20, and CDH1 and downregulated EMI1 and BUB3. The findings indicated abnormal spontaneous Ca2+ oscillations leading to oocytes with prolonged low CSF and high APC level, which resulted in defective nuclear envelope breakdown and DNA replication. The changes in levels of critical genes were confirmed by examining other independent PN-arrest zygotes. However, the PN-arrest zygote phenotype was not consistent with that of RGS12-deficient mice, thereby indicating species-specific functions between human and mouse. Conclusion: Our findings expand our knowledge of the genetic determinants of human early embryonic arrest at the PN stage and provide guidance for selecting clinically infertile individuals with PN-arrest zygotes for Ca2+ intervention.

Cdc2 and Mos Regulate Emi2 Stability to Promote the Meiosis I–Meiosis II Transition

The transition of oocytes from meiosis I (MI) to meiosis II (MII) requires partial cyclin B degradation to allow MI exit without S phase entry. Rapid reaccumulation of cyclin B allows direct progression into MII, producing a cytostatic factor (CSF)-arrested egg. It has been reported that dampened translation of the anaphase-promoting complex (APC) inhibitor Emi2 at MI allows partial APC activation and MI exit. We have detected active Emi2 translation at MI and show that Emi2 levels in MI are mainly controlled by regulated degradation. Emi2 degradation in MI depends not on Ca2+/calmodulin-dependent protein kinase II (CaMKII), but on Cdc2-mediated phosphorylation of multiple sites within Emi2. As in MII, this phosphorylation is antagonized by Mos-mediated recruitment of PP2A to Emi2. Higher Cdc2 kinase activity in MI than MII allows sufficient Emi2 phosphorylation to destabilize Emi2 in MI. At MI anaphase, APC-mediated degradation of cyclin B decreases Cdc2 activity, enabling Cdc2-mediated Emi2 phosphorylation to be successfully antagonized by Mos-mediated PP2A recruitment. These data suggest a model of APC autoinhibition mediated by stabilization of Emi2; Emi2 proteins accumulate at MI exit and inhibit APC activity sufficiently to prevent complete degradation of cyclin B, allowing MI exit while preventing interphase before MII entry.