Speaker: Lei Wang
Title: Mutations in TUBB8 Cause Human Oocyte Meiotic Arrest
Human reproduction depends on the fusion of a mature oocyte with a sperm cell to form a fertilized egg. The genetic events that lead to the arrest of human oocyte maturation are unknown. We sequenced the exomes of five members of a four-generation family, three of whom had infertility because of oocyte meiosis I arrest. We Sanger sequenced a candidate gene, TUBB8, in DNA samples from these members, additional members, and members of 23 other similarly affected families. We assayed the expression of TUBB8 and all other -tubulin isotypes in human oocytes, early embryos, sperm cells and several somatic tissues by qRT-PCR. We evaluated the effect of the TUBB8 mutations on / tubulin heterodimer assembly in vitro, on microtubule architecture in HeLa cells, on microtubule dynamics in yeast cells, and on spindle assembly in mouse and human oocytes. We identified seven mutations in the primate-specific gene TUBB8 that are responsible for human oocyte meiosis I arrest in 7 out of 24 families with this condition. TUBB8 expression is unique to oocytes and the early embryo, where this gene accounts for almost all of the expressed-tubulin. The mutations affect chaperone-dependent folding and assembly of the α/ß-tubulin heterodimer, induce microtubule chaos upon expression in cultured cells, alter microtubule dynamics in vivo, and cause catastrophic spindle assembly defects and maturation arrest upon expression in mouse and human oocytes. TUBB8 mutations have dominant-negative effects that disrupt microtubule behavior, oocyte meiotic spindle assembly and maturation. They thereby cause female infertility.