This study identified the homozygous variant RRM2:c.262C > T:p.His88Tyr in two infertile patients that might be the potential cause of early embryonic development stagnation in their IVF-ET treatment, which resulted in depletion of all available embryos for transplantation. Bioinformatics analysis predicted this variant might affect RRM2 protein function and cause disease. According to the American College of Medical Genetics (ACMG) guidelines for classification of variants (6), this was classified as a “variant of uncertain significance”.
Ribonucleotide reductase (RR) consists of two subunits (RRM1 and RRM2), and is a rate-limiting enzyme in deoxynucleotide production for DNA synthesis (7). As optimal cell concentration of deoxyribonucleotides is essential for DNA synthesis, replication, and repair (8), RR plays an important role in cell proliferation. RR defects often lead to cell cycle arrest, developmental delay, and abnormally increased RR activities led to high frequency of gene mutation (9). Functionally, RRM1 controls substrate specificity, and RRM2 regulates the overall catalytic activity (10). RRM2 is widely expressed in human organs, with highest expression level in secondary oocyte (data from Bgee data Base for Gene Expression Evolution).
RRM2 could regulate zygotic genome activation (ZGA) through the yes-associated protein (YAP). In early mammalian embryos, genomic transcription is quiescent until ZGA occurs 2–3 days after fertilization. The coding gene of YAP, Yap1, was found to be highly expressed in human and mouse oocytes and early embryos (11, 12), and maternally accumulated YAP in oocyte is essential for ZGA (1). The embryos of Yap1 knockout female mice presented prolonged 2-cell stage and slower development into the 4-cell stage. Rrm2, the mouse homologous gene of human RRM2, and Rpl13 were found to be the target genes of YAP in early blastomeres, which were required to mediate maternal YAP's effect in conferring developmental competence on preimplantation embryos (13). This was in accordance with our observation that the embryonic development stagnation caused by RRM2 variation exhibited maternal effect recessive inheritance. That is, female homozygotes for the RRM2 mutation appeared phenotypically normal, whereas their offspring shown the mutant phenotype of lethality during early embryonic development.
RRM2 has also been reported to play an important role in inducing cell proliferation and decidualization in mouse uterus, suggesting its likely involvement in early embryo development and embryo implantation. Embryo implantation into the maternal uterus is a crucial step in the successful pregnancy of mammals. Currently, implantation and trophoblastic infiltration defects are major obstacles to successful pregnancy (14). Embryo implantation is a complex developmental process, including the process of embryo attachment, followed by invasion of the matrix, and proliferation and differentiation of endometrial stromal cells (15). It has been shown that Rrm2 is strongly expressed in decidual tissues and is up-regulated by progesterone and DNA damage in mouse (7). RRM2-specific inhibitors effectively reduced the weight of implantation sites and deciduoma (7). In the future, functional study by generating genetically modified mouse in the Rrm2 gene would help supporting the pathogenicity of this variant.
In summary, this study identified a homozygous variation of RRM2:exon3:c.262C > T:p.His88Tyr, which might alter RRM2 protein conformation and attenuate its function, and might be a potential cause of early embryonic development stagnation in two sisters who had multiple failed assisted reproduction attempts. This study also suggested that RRM2 might be a maternal effect gene. Further animal study is still required to confirm the role of RRM2 in early embryo development.