Characterization of novel components of the synaptonemal complex a meiosis-specific chromosome structure important for meiotic recombination

Sexually reproducing organisms, including humans, need to accurately pass genetic material to the next generation. The majority of cells in our bodies retain two copies of the complete set of the genetic blueprint of life (the genome), one inherited from our mother and another from our father. Our bodies develop from a single fertilized egg, which is the outcome of a fusion of a sperm and an egg. Unlike the majority of cells, sperm and eggs cells are specialized for passing genetic material onto the next generation, and contain only a single full set of the genetic blue print. A fertilized egg thus contains two copies of the genetic material upon starting a new life. A specialized mechanism is required to produce sperm and egg cells, and this mechanism must half the amount of the genetic material in each cell. This mechanism, known as meiosis, involves a number of very elaborate and sophisticated processes that reduce the amount of the genetic material to exactly half as much as that of parental cells. If something goes wrong during the process, it can lead to spontaneous abortion and various genetic disorders such as Down's syndrome. We use budding yeast as a model organism to study this process. Budding yeast is one of the simplest organisms to undergo sexual reproduction using the same basic mechanism as humans. It thus serves as a great resource to investigate the mechanisms operating in meiosis. We have recently identified new components of the budding yeast synaptonemal complex, a conserved machine playing a crucial role in halving the amount of genetic material. The focus of this work is to understand what roles these newly discovered components play in the context of this elaborate machinery.

Meiosis plays a central role in the life cycle of sexually reproducing organisms. During meiosis, chromosomes are exactly halved and distributed into gametes (e.g., sperm and eggs) that are eventually used to form a new individual. Homologous recombination plays an essential role in the accurate segregation of homologous chromosomes at meiosis I. When homologous recombination is highly induced in prophase I, chromosomes undergo dynamic morphological changes and form a proteinaceous structure called the synaptonemal complex (SC) in which the homologous chromosomes are closely paired. The SC is important for various aspects of meiotic recombination, but the mechanism at the molecular level still remains elusive. In budding yeast, the Zip1 protein is a major component of the SC. Zip1 is involved in many of the key steps that ultimately lead to the accurate segregation of homologous chromosomes. These include homologous chromosome pairing and meiotic recombination. Through cytological screening, we have identified two novel meiosis-specific proteins which we have named Zil1 and Zil2 (Zip1-like Localization), that show a similar localization pattern to Zip1 on meiotic chromosomes. Zip1 localization was greatly disturbed by the absence of these proteins, indicating their important role in the function of SC. We also found that one of the proteins is subject to post-translational modification. In this proposal, we will first focus on identifying the defects caused by the loss of these newly identified proteins, by using the combination of genetics, molecular and cell biological approaches. Second, we will investigate how these defects are caused, with the emphasis on understanding how these new proteins are mechanistically related to Zip1 and other SC/recombination proteins.

The primary beneficiary of the proposed research is the medical community. Regardless of organism, fundamental mechanisms such as those required for repairing damaged DNA and segregating chromosomes are essentially the same. This proposal centers around understanding the regulation of homologous recombination during meiosis and its involvement in chromosome segregation. We propose to use a tractable model organism (budding yeast). This proposal will thus provide a framework for understanding the molecular mechanism of chromosome segregation during meiosis in humans. Pharmaceutical community is a future potential beneficiary. It is known that the risk of natural abortion and specific genetic disorders (such as Down's syndrome) caused by inaccurate chromosome segregation are dramatically increased in woman with age. Understanding the mechanisms underlying meiotic chromosome segregation will lead to a better understanding the causative mechanisms of such disorders. A mechanistic understanding is key to the identification of molecules/proteins critical for the fidelity of the process. Potentially these can serve as molecular markers to evaluate the functionality of the mechanism. Prospective applications include the development of diagnostic/risk assessment tools for natural abortion and genetic disorders associated with aneuploidy. Furthermore, it is theoretically possible to identify compounds to improve the accuracy of meiotic chromosome segregation. Papers written based on the result of this proposal will be deposited in an on-line publically available repository established and maintained by the University of Sussex. Additional engagement with the public will be planned in association with the GDSC which operates a public engagement policy. In parallel, to further publicize the results obtained through this proposal, the PI and PDRA will prepare a website designed to explain the research conducted in our group to lay-audiences as well as specialists. To exploit our results in order to obtain insight into the mechanism in humans, we will actively seek collaborations with researchers specialized in mouse and human meiosis (including clinicians). We will do so by attending meetings (such as Gordon Conference) to interact with researchers studying human and mammalian models. In order to communicate the progression of the research programme and the outcomes to a broader industrial audience, the University of Sussex, currently has two LTN business fellows, as well as a programme known as Curious. Both schemes are designed to communicate academic research to the industrial sector. Curious was designed to communicate the research knowledge from within the University to the wider business sector and was developed in collaboration with South East England Development agency. The aim is to forge partnerships with external organisations, leading to knowledge exchange and collaborations. Our group will take advantage of these programmes to investigate the possibility of our knowledge/discovery being used to develop new biomedical tools.