N6-methyladenosine (m6A) methylated modification as the most prevalent and abundant epigenetic modification in eukaryote mRNA, has been a hot topic in scientific research. The biological function of m6A modification required the participation of specific molecules called methyltransferases (Writers), demethylases (Erasers), and methylation readers (Readers) respectively. M6A modification was reported to relate with the development of embryonic stem cells, DNA damage response, sex determination and cancer occurrence etc (Bi et al., 2019, Karthiya and Khandelia, 2020, Zhao et al., 2021, Tai et al., 2022). In recent years, recurrent spontaneous abortion (RSA) has become a difficult problem in gynecology and obstetrics disease due to its complex etiology. RSA not only effected women's reproductive health, but also increased their psychological and mental burdens. With the molecular researches on the mechanism of embryo implantation and migration and invasion of trophoblast cells at the maternal-fetal interface, studies demonstrated that m6A methylation was important in the establishment of implantation and maintenance of pregnancy and m6A methylation became a new research direction on infertility (Shi et al., 2017, Huang et al., 2022).

Heterogeneous nuclear ribonucleoprotein C (HNRNPC) belonged to the member of hnRNPs subfamily. The hnRNPs were RNA binding proteins and they complexed with heterogeneous nuclear RNA. As the “Readers” in m6A modification regulation, HNRNPC was proved to correlate with immune cells infiltration, proliferation, migration and invasion of cancer cells (Wang et al., 2020a, Wang et al., 2020b, Wang et al., 2021). 5-Methyltetrahydrofolate-homocysteine methyltransferase (MTR) catalyzed the remethylation of homocysteine to form methionine. Methyl vitamin B12 and reduction of 5-methyltetrahydrofolate-homocysteine methyltransferase reductase were required to maintain the activity of MTR. Down-regulated MTR activity resulted in hyperhomocysteinemia and folate deficiency, which were a risk factor for spina bifida, neural tube defects, recurrent pregnancy loss, and other diseases (Lin et al., 2019, Zhao et al., 2020). However, more and more studies showed that folate excess may also have adverse effects on growth and development.

Based on previous research, we found that m6A level of RSA patients decreased and m6A methyltransferase 3 (METTL3) regulated the stability and expression of Zinc finger and BTB domain containing 4 and related to RSA (Huang et al., 2022). In this study, we found that the mRNA expression level of HNRNPC and the mRNA m6A level of MTR significantly decreased in RSA patients, while the mRNA expression level of MTR increased. Through series of cell validation assays, we clarified that MTR might be one of targets of HNRNPC and HNRNPC regulated the invasion of trophoblast by altering the expression of MTR in RSA. Our research provided more evidence that HNRNPC regulated embryonic development by affecting m6A modification.