PgmNr M268: SMC5/6 complex is required for the formation of bivalent chromosomes capable of segregation during meiosis I in oocytes.

Authors:
G. H. Hwang 1 ; M. O'Brien 2 ; F. Sun 2 ; J. Eppig 2 ; MA Handel 2 ; P. Jordan 1


Institutes
1) Johns Hopkins Bloomberg School of Public Health, Baltimore, MD; 2) The Jackson Laboratory, Bar Harbor, ME.


Abstract:

The frequency of chromosome segregation errors in oocytes increases as women age, especially after the age of ~35, and results in dramatically increased incidence of miscarriage and developmental abnormalities.  Despite the correlation between age and chromosome segregation errors, in most cases the causes remain unknown. Recent work using mouse oocytes has linked the structural maintenance of chromosomes (SMC) complexes to being important for accurate chromosome segregation following meiotic resumption. SMC complexes include three major classes: cohesin, condensin, and SMC5/6. During the first meiotic division cohesin is required to ensure sister chromatids remain associated with one another, and condensin is required for the structural integrity and resolution of bivalents. Until our recent work, the functions of the SMC5/6 complex during oogenesis had not been examined. We show that the SMC5/6 complex is enriched at the pericentromeric heterochromatin, and also localizes along chromosome arms during meiosis. To determine the role of the SMC5/6 complex during meiotic resumption, we constructed a conditional knockout (cKO) allele of mouse Smc5. Using the Tg(Zp3-cre)93Knw (Zp3-Cre) transgene we deleted Smc5 in growing Smc5-condition ready (Smc5tm1c(KOMP)Wtsi) oocytes prior to meiotic resumption. The Smc5 cKO mutant mice are infertile. After in vitro fertilization of MII oocytes obtained from 3 month-old mice, markedly reduced percentages of 2-cell embryos from Smc5 cKO were observed, and blastocysts were nearly absent. These Smc5 cKO mutant oocytes fail to accurately separate homologous chromosomes during meiosis I, and resulted in aneuploid MII oocytes. Despite what appears to be an inability to resolve concatenation between chromosomes during meiosis we did not observe topoisomerase II alpha mislocalization. However, we did observe defects in condensin localization along the chromosome axes. Remarkably, these phenotypes are less prominent when analyzing oocytes isolated from younger mice. We have obtained data to suggest that the SMC5/6 complex remains stable for weeks following ZP3-Cre-mediated deletion of Smc5, but levels present prior to deletion diminish with age. Taken together, our results demonstrate that the SMC5/6 complex is essential for the formation of bivalents that are capable of accurate segregation during meiosis I, and age-related meiotic aberrancies may be directly related to a gradual reduction in SMC5/6 protein levels.