Maternal messages to live by: a personal historical perspective

Maternal Effect Genes as Risk Factors for Congenital Heart Defects

Maternal effect genes (MEGs) encode factors (e.g., RNA) in the oocyte that control embryonic development prior to activation of the embryonic genome. Over 80 mammalian MEGs have been identified, including several that have been associated with phenotypes in humans. Maternal variation in MEGs is associated with a range of adverse outcomes, which, in humans, include hydatidiform moles, zygotic cleavage failure, and offspring with multi-locus imprinting disorders. In addition, data from both animal models and humans suggest that the MEGs may be associated with structural birth defects such as congenital heart defects (CHDs). To further investigate the association between MEGs and CHDs, we conducted gene-level and gene-set analyses of known mammalian MEGs (n = 82) and two common groups of CHDs: conotruncal heart defects and left ventricular outflow tract defects. We identified 14 candidate CHD-related MEGs. These 14 MEGs include three (CDC20, KHDC3L, and TRIP13) of the 11 known human MEGs, as well as one (DNMT3A) of the eight MEGs that have been associated with structural birth defects in animal models. Our analyses add to the growing evidence that MEGs are associated with structural birth defects, in particular CHDs. Given the large proportion of individuals with structural birth defects for whom etiology of their condition is unknown, further investigations of MEGs as potential risk factors for structural birth defects are strongly warranted.

Maternal effect genes: Update and review of evidence for a link with birth defects

Maternal effect genes (MEGs) encode factors (e.g., RNA) that are present in the oocyte and required for early embryonic development. Hence, while these genes and gene products are of maternal origin, their phenotypic consequences result from effects on the embryo. The first mammalian MEGs were identified in the mouse in 2000 and were associated with early embryonic loss in the offspring of homozygous null females. In humans, the first MEG was identified in 2006, in women who had experienced a range of adverse reproductive outcomes, including hydatidiform moles, spontaneous abortions, and stillbirths. Over 80 mammalian MEGs have subsequently been identified, including several that have been associated with phenotypes in humans. In general, pathogenic variants in MEGs or the absence of MEG products are associated with a spectrum of adverse outcomes, which in humans range from zygotic cleavage failure to offspring with multi-locus imprinting disorders. Although less established, there is also evidence that MEGs are associated with structural birth defects (e.g., craniofacial malformations, congenital heart defects). This review provides an updated summary of mammalian MEGs reported in the literature through early 2021, as well as an overview of the evidence for a link between MEGs and structural birth defects.

ZAR1 and ZAR2 are required for oocyte meiotic maturation by regulating the maternal transcriptome and mRNA translational activation

Zar1 was one of the earliest mammalian maternal-effect genes to be identified. Embryos derived from Zar1-null female mice are blocked before zygotic genome activation; however, the underlying mechanism remains unclear. By knocking out Zar1 and its homolog Zar2 in mice, we revealed a novel function of these genes in oocyte meiotic maturation. Zar1/2-deleted oocytes displayed delayed meiotic resumption and polar body-1 emission and a higher incidence of abnormal meiotic spindle formation and chromosome aneuploidy. The grown oocytes of Zar1/2-null mice contained decreased levels of many maternal mRNAs and displayed a reduced level of protein synthesis. Key maturation-associated changes failed to occur in the Zar1/2-null oocytes, including the translational activation of maternal mRNAs encoding the cell-cycle proteins cyclin B1 and WEE2, as well as maternal-to-zygotic transition (MZT) licensing factor BTG4. Consequently, maternal mRNA decay was impaired and MZT was abolished. ZAR1/2 bound mRNAs to regulate the translational activity of their 3′-UTRs and interacted with other oocyte proteins, including mRNA-stabilizing protein MSY2 and cytoplasmic lattice components. These results countered the traditional view that ZAR1 only functions after fertilization and highlight a previously unrecognized role of ZAR1/2 in regulating the maternal transcriptome and translational activation in maturing oocytes.