Importance of TEX19.1 in chromosome segregation in murine ES cells
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Cells possessing the incorrect number of chromosomes (referred to as aneuploid) can arise as a result of chromosome mis-segregation. Prevention of aneuploidy is especially important in germ cells, as these cells pass genetic information to the next generation, but also in pluripotent cells as these give rise to all tissues and cells of the offspring, including germ cells. Aneuploid conceptions have a detrimental effect on pregnancy outcomes, are surprisingly common in humans (estimated 10-30%), and are a leading cause of miscarriage and developmental disorders. In both meiosis and mitosis, accurate chromosome segregation relies on the correct orientation of sister chromatids during metaphase, which ensures bipolar spindle attachment. Newly replicated sister chromatids are able to align properly on the spindle due to cohesion holding them together. The protein complex responsible for sister chromatid cohesion (SCC) is called cohesin, and has specific subunits depending on its particular role. The exact mechanism of SCC regulation and maintenance is not yet fully understood, however evidence suggests involvement of TEX19.1. Previous studies showed that TEX19.1 is crucial for maintenance of SCC and prevention of aneuploidy in both meiosis and mitotic ES cells. Moreover, Tex19.1-/- oocytes express decreased levels of acetylated SMC3 (AcSMC3), a subunit of the active (cohesive) form of cohesin. Importantly, it is currently unknown whether the defects in SCC in Tex19.1-/- ES cells are also caused by changes in AcSMC3, and if increased aneuploidy is caused by chromosome mis-segregation. In this project I explored the changes in chromatin associated cohesin in Tex19.1- /- mouse ES cells, by biochemically isolating chromatin from cell cycle synchronised ES cells. I found that levels of AcSMC3 in Tex19.1-/- ES cells are decreased in the G2 phase preceding cell divison, indicating that decreased SCC is the result of reduced ‘cohesive’ cohesin in ES cells, as observed previously in oocytes. I also explored the mechanisms driving aneuploidy in Tex19.1-/- ES cells by imaging cells during mitosis. I compared the incidence of errors commonly associated with chromosome mis-segregation between Tex19.1-/- and control cells, and found an increase in micronuclei in interphase Tex19.1-/- ES cells. My data supports the model that loss of Tex19.1 in ES cells causes changes in the amount of AcSMC3 associated with chromatin, and errors in chromosome segregation. Overall, my results suggest that TEX19.1 has a role in the maintenance of SCC and cohesive cohesin in the murine mitotic cell model, ensuring appropriate chromosome segregation. My findings also indicate that ES cells may be a more tractable cellular model than mouse oocytes to analyse the role of TEX19.1 and AcSMC3 on chromosome segregation.