Analysis of Aneuploidy During Mouse Spermatogenesis

Successful transition through meiosis is required for production of chromosomally-balanced gametes. When chromosome segregation goes awry during meiosis, aneuploidy can occur. Unfortunately, the mechanisms behind this nondisjunction are not well understood. Therefore, this dissertation has focused on learning more about the causative factors associated with aneuploidy during spermatogenesis. Are there factors that are always associated with leading to production of aneuploid sperm? One of the main goals of this dissertation is to find mouse models to study what factors may be involved in chromosome malsegregation; such as pairing, recombination, and transition through the division phases of meiosis. The first part of the dissertation will be an introduction into what is known about gamete aneuploidy. This section will review what is known about how meiotic error may arise in both humans and the mouse. The introduction will discuss links between factors that are thought to be associated with aneuploidy, and this dissertation will extend this information into new directions in analysis of predisposing factors of gamete aneuploidy. Part II focuses on a novel mouse model for gamete aneuploidy. PL/J males were found to be an important mouse model for both gamete aneuploidy and abnormal spermhead morphology. In addition, it was found that PL/J males exhibit both genetic and phenotypic complexity in regard to the traits of aneuploidy and abnormal sperm-head morphology. Parts III-VII discuss other useful mouse models for study of gamete aneuploidy. Robertsonian heterozygous (Rb/+) translocation mice and Mlh1 -/mice were both used to v examine what happens when meiosis goes awry. For example, both Rb/+ and Mlh1 -/mice were found to have a checkpoint that most likely detects unaligned or abnormal chromosome configurations. High percentages of MI spermatocytes in these mice were found to be apoptotic. In Part V, Brca2 -/mice were rescued with the human BRCA2 transgene. These mice survive, but are sterile. Analysis was performed to determine the point of arrest in these mice and if they have features of a normal progression through meiosis. The last two chapters focus on different approaches for the study of aneuploidy. Part VI examines whether the topoisomerase-II inhibitor, etoposide, can induce meiotic nondisjunction. It was shown by sperm FISH that etoposide does induce meiotic nondisjunction, with the highest frequency of nondisjunction occurring at MII. The next part of this section discusses use of a novel screen for detection of new meiotic mutations. A sperm FISH screen was used in this study to detect dominant mutations. This study showed that although screening by sperm FISH is feasible, it is not a practical screen when large numbers of gametes need to be scored. The last section, Part VIII, is a summary of what we have learned and what directions should be taken to increase our understanding of how meiotic error arises leading to nondisjunction. This section will compare and contrast what we have learned from each mouse model and what factors may contribute to production of aneuploid sperm. The discovery of factors associated with aneuploidy will be essential in learning how to prevent the deleterious effects that occur as a result of malsegregation of chromosomes.