Novel mutations in TUBB8 expand the mutational and phenotypic spectrum of patients with zygotes containing multiple pronuclei

Abstract

After fertilization, parental chromosomes decondense and form pronuclei. During these processes, germ cell genomes merge and give rise to the zygotic genome. Multiple pronuclei (MPN) formation is usually caused by polyspermic fertilization or oocyte-derived meiotic failure, and account for 15-18% of cytogenetically abnormal cases among spontaneous abortions. However, pathogenic gene mutations responsible for human MPN formation still need to be identified. Tubulin β eight class VIII (TUBB8) is the major β-tubulin isotype that assembles the human oocyte spindle. In this study, we identified 3 novel heterozygous missense mutations (c.524 T > C [p.V175A], c.10_12delins CTT [p.I4L], and c.1045 G > A [p.V349I]) in TUBB8 that were associated with a new phenotype: MPN in zygotes after in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI). These mutations were found in 3 independent female patients with infertility, and had experienced 2-3 failed IVF/ICSI attempts due to zygotic developmental arrest. These sites are evolutionarily conserved in primate TUBB8 genes as well as in other human β-tubulin isotypes, suggesting that they have important biochemical functions. This finding reveals previously unreported phenotypes caused by TUBB8 mutations and will be helpful for future genetic counseling of infertile patients with MPN.