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Principal Investigator: Dr. Paula Cohen
Contact Information: E-mail: pc242@cornell.edu - Phone: 607-253-4301
Sponsor: March of Dimes Birth Defects Foundation
Title: Role of MMR Proteins in Ovarian Meiosis and Meiotic Nondisjuction
Annual Direct Cost: $28,528
Project Period: 01/01/05-11/30/05
Meiotic nondisjunction has a significant impact on human reproduction and fetal health. As many as 15-25% of all human conceptions are aneuploid, often as a result of nondisjunction during maternal meiosis. Most chromosomally abnormal conceptions result in miscarriage, but approximately 1/300 children are born with autosomal or sex chromosomal trisomies that result in multiple congenital malformation syndromes such as Down Syndrome. Molecular studies have demonstrated that the majority of trisomies involving chromosomes 14, 15, 16, 21 and 22 are attributed to nondisjunction errors in maternal meiosis I, suggesting that failed recombination in oocytes may play a significant role in the etiology of human aneuploidy. Moreover, the incidence of such trisomies rises sharply with maternal age. In women under the age of 25 years, approximately 2% of all clinically-recognized pregnancies are trisomic, while among women over 40 years of age, this number rises to approximately 35% irrespective of race, geography or socio-economic status. Thus the regulatory events surrounding meiotic recombination have become a serious focus for research in recent years. Such studies are the focus of the present proposal, with an aim to understanding more fully the complexities of meiotic recombination in mammals, and to understanding the reasons for the high rates of female nondisjunction seen in meiosis I in women, particularly those over the age of 35.
DNA mismatch repair (MMR) proteins play an essential and evolutionarily conserved role in meiotic recombination during prophase I. The MMR protein family consists of DNA recognition molecules that are homologous to the bacterial MutS protein. These MutS homologs form heterodimers that then recruit MutL homolog heterodimers that are responsible for the initiation of DNA repair in mitotic cells and for, as yet unidentified, recombination activities in meiotic cells. Of the MutS homologs in this family, MSH2, 3 and 6, are specific to somatic cell repair, while MSH4 and MSH5 are specific to meiosis. By contrast, the MutL homologs, which bind to MutS heterodimers, are shared between meiotic and mitotic cells. Mutations in the mouse genes encoding MSI-I4 and MSH5 result in almost complete inhibition of homologous pairing at zygonema of prophase I, while targeted mutation of the MutL homologs, MLH1 or PMS2, results in meiotic defects later in prophase I. These studies indicate that MMR proteins are essential regulators of meiotic progression throughout prophase I.
We hypothesize that MMR proteins mediate events at the onset and resolution of recombination at chiasmata sites via interactions with stage-specific protein complexes on meiotic chromosomes. Disruption of these interactions and activities is predicted to correlate with increased meiosis I nondisjunction and thus be a potential causative factor in human aneuploidy. The specific aims of the current proposal are (1) to examine the expression and localization of MMR proteins throughout meiosis I in oocytes from mice and humans; (2) to examine the consequences of temporally-restricted MutS homolog mutations on recombination in mouse oocytes during meiosis I and II, and (3) to examine the role of MMR proteins in meiosis II and in oocytes from older donors (both mice and human), and the relevance of such functions to aneuploidy rates in older women. Such studies will explore the function of MMR complexes throughout meiosis I and II and will provide novel data concerning the regulation of female meiosis and recombination. Furthermore, these studies will utilize mouse models that are deficient, or conditionally deficient, in one or more MMR genes to examine the consequence of MMR deficiencies on meiotic recombination and gamete viability. In summary, the current studies will provide novel and important information regarding the regulation of mammalian meiosis, will allow for a detailed comparison of regulatory events between, male and female meiosis, and will examine the role of key regulatory proteins, namely the MMR family, in the etiology of human aneuploidy and in the age-related increase in aneuploidy events.
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