TUBB8 variants in infertile Japanese women, including a novel homozygous nonsense variant causing zygote multinucleation

Research question: What is the prevalence of tubulin beta 8 class VIII (TUBB8) variants in Japanese women with oocyte/zygote/embryo maturation arrest (OZEMA), and what are the phenotype–genotype correlations in this population? Design: This multicentre retrospective study analysed 39 Japanese women with primary infertility characterized by OZEMA. Whole-exome sequencing was performed to identify TUBB8 variants, followed by Sanger sequencing, in-silico analysis, and structural modelling. Clinical data were collected across multiple treatment cycles, including oocyte maturation, fertilization, and embryonic development. Time-lapse imaging observed abnormal fertilization processes. Results: Six TUBB8 variants were identified in 15.4% (6/39) of participants, including one novel homozygous nonsense variant (p.Thr136*) and two novel heterozygous missense variants (p.Phe294Leu and p.Trp344Arg). All six patients exhibited degrees of oocyte maturation arrest, predominantly at metaphase I. The novel homozygous nonsense variant was uniquely associated with multipronuclear (MPN) zygote formation. Both novel heterozygous missense variants, one confirmed de novo, caused complete metaphase I arrest. Time-lapse imaging in the patient with the homozygous nonsense variant demonstrated impaired polar body extrusion characterized by unusually broad cytoplasmic protrusions. These protrusions were reabsorbed into the oocyte, resulting in MPN zygotes. Conclusions: In this first study of TUBB8 in Japanese patients with OZEMA, three novel variants with unique phenotype–genotype correlations were identified. The phenotypes observed with the homozygous p.Thr136* variant suggest that complete TUBB8 loss disrupts not only meiotic progression but also post-fertilization events, including second polar body extrusion and pronuclear formation. These findings expand the spectrum of TUBB8-related reproductive phenotypes, and suggest that genetic testing can help avoid repeated unsuccessful treatments for primary infertility.