A novel MOS variant causes large polar body formation and parthenogenetic activation; case report of an unusual phenomenon leading to female infertility.

Oocyte maturation is regulated by well-orchestrated molecular mechanisms and any abnormalities during these events could result in an oocyte that cannot be fertilized or develop into normal embryos. Recently, MOS gene variants were reported to result in oocytes with large polar body (PB) or early embryonic arrest in humans leading to infertility. A thirty-year-old woman with 12 years of infertility underwent two in vitro fertilization cycles, and in both she had oocytes with large PBs and parthenogenetic activation. Clinical exome sequence was performed and revealed a novel homozygous variant in MOS (NM₀05372. 1): c. 573C > A;p. (Phe191Leu). These findings might indicate that MOS mutation results in oocytes with a large PB and parthenogenesis in this patient. In conclusion, our data further confirms that MOS is important in proper oocyte maturation, and variant c 573 C > A results in the formation of a large PB and parthenogenetic activation as reported in animal models earlier. Patients with MOS variants need to be counselled about their fertility status.