Genetically dissecting the meiotic checkpoint active during prophase I in female mice

Abstract

Females have a non-renewable number of gametes at birth. These oocytes are extremely sensitive to environmental factors that generate DNA damage. Oocyte death due to DNA damage can result in infertility and ovarian failure. In contrast to postnatal oocytes, at earlier stages of gametogenesis these cells withstand hundreds of developmentally programmed DNA breaks (DSBs). During the first meiotic division these DSBs promote synapsis (homologous chromosomes pairing), and recombination, which are both essential for sexual reproduction and environmental fitness. However, DSB repair and synapsis need to occur in a timely manner, or the quality of the gametes becomes compromised. The mechanisms that guarantee oocyte quality were hypothesized to operate through two independent pathways: one that surveys DNA integrity, and the other synapsis. However, I present experimental evidence that oocytes defective for either DNA repair or synapsis are eliminated by the same DNA damage response. Furthermore, through the detailed analysis of DNA repair dynamics, I provide evidence that the protein HORMAD2, which localizes to unsynapsed chromosomes, regulates DSB-repair. I hypothesize that HORMAD2 interferes with repair by preventing broken DNA from using the sister chromatid as a repair template. This “block to sister-chromatid repair” (BSCR) assures that the homologous chromosome is the substrate of choice. Whereas BSCR guarantees homologous recombination, it also prevents unsynapsed chromosomes from fixing DSBs. Thus, failure to synapse will result in persistent DSBs. Since DNA damage causes oocyte death postnatally, unsynapsed chromosome will trigger the DNA damage checkpoint. Through the understanding of this checkpoint, I was able to test if the transient inhibition of the DNA damage checkpoint protein (CHK2) prevents oocyte death. My finding that oocyte death was prevented, and fertility was preserved, provides evidence that chemically protecting oocyte from DNA damaging agents is a viable clinical approach. This result will hopefully translate into a treatment to delay ovarian failure. Taken together these results have implications on our current understanding of the prophase I checkpoint. TEACHING AS RESEARCH My interest in improving teaching strategies led me to research the qualitative outcome of using a novel teaching tool during the laboratory section of a histology course. I tested an interactive response system (IRS) as formative assessment tool. I found that IRS results in a positive experience, however my study was not able to detect quantitative difference on students’ grades was detected.