Guangbin Luo - US grants

DESCRIPTION (Applicant's Description): The long-term objective of this grant is to study the underlying mechanisms that govern somatic recombination and somatic loss of heterozygosity (LOH) during cancer development and to develop mouse models with increased rates of somatic LOH as tools to identify novel tumor suppressor genes. Somatic LOH plays an important role in cancer development. It is largely responsible for the second event that is required for the complete loss of function of a tumor suppressor gene. Thus the understanding of these mechanisms will be important for the advancement of our knowledge on the initiation and/or progression of cancer and hence may lead to better diagnosis, prevention and treatments of this disease. In mammalian cells, there exist two major pathways that can lead to LOH: chromosome mis- segregation, and recombination-based (segmental) LOH, with the segmental type of LOH being the predominant type in human cancer. The human Bloom syndrome provides a unique model to study this important process. Bloom syndrome is a cancer-prone disorder due to loss of function mutations in a gene encoding a RecQ DNA helicase. Studies in E. coli and in yeast suggest that the RecQ helicase is an antagonist of recombination. Both Bloom syndrome patients and the corresponding yeast mutants display a hyper-recombination phenotype. It is therefore hypothesized that Bloom syndrome patients are predisposed to cancer due to an elevated segmental LOH process that accelerates the uncovering of phenotypic effects of otherwise silent single mutations in tumor suppressor genes. in addition, it has been reported very recently that the Rothmund- Thomson syndrome, another cancer prone syndrome, is caused by mutations in RecQ4, another gene coding for a RecQ DNA helicase. Thus, mouse models of these syndromes should provide excellent tools to study segmental LOH and may also be useful for gene identification in a genetic screen. The specific aims of this grant are: to generate mouse models of the human Bloom syndrome and Rothmund-Thomson syndrome; to study effects of mutations in Blm and RecQ4 on somatic recombination and LOH; to examine the effects of mutations in Blm and RecQ4 on tumor susceptibility in mice; and to evaluate the usefulness of these mouse models as tools to increase the ease of identifying novel tumor suppressor genes in the context of somatic insertion- tagged mutagenesis.

DESCRIPTION (provided by applicant): The long-term objective of this project is to define the roles and mechanisms by which the RecQ DNA helicases regulate mitotic recombination and suppress mitotic crossover. Homologous recombination can lead to crossover, which can lead to translocations, deletions, and loss of heterozygosity (LOH), all of which have been implicated as potential cancer causing or - promoting events. Thus, in mammals, mitotic recombination is highly regulated to prevent excessive crossover. Mutations in the RecQ family member BLM give rise to Bloom syndrome; a disease typified by increased mitotic crossovers and elevated risks to cancer, underscoring the importance of RecQ helicases in the suppression of mitotic crossover and carcinogenesis. Previously, we have shown that a mouse model of Bloom syndrome has increased tumor susceptibility as a result of diminished suppression of mitotic crossover. We have now found that a second RecQ family member, mouse RecqlS, is also involved in the suppression of mitotic crossover, and that it interacts genetically with Blm in this suppression. We hypothesize that in mammals, replication demise is the primary cause of spontaneous mitotic crossover. Two independent pathways: a Blm-specific pathway and a RecqlS-specific pathway, respectively, are responsible for the restoration of stalled forks via non-recombination-based mechanism to suppress mitotic crossover and reduce cancer risk. In this application, we propose to use the knockout mouse cell lines and knockout mouse models we have created to test this hypothesis and to study the relative contributions of these two proposed pathways in the suppression of mitotic crossover and carcinogenesis. These studies will provide significant insights into the regulation of mitotic recombination and the origins of the early oncogenic LOH events during carcinogenesis.

[unreadable] DESCRIPTION (provided by applicant): The long term goals of this proposal are to understand the molecular mechanism by which Recql4 affects chromosome segregation during mitosis and the role of Recql4 deficiency in carcinogenesis and premature aging in mice. Aneuploidy is a hallmark of human solid tumors. While it remains uncertain whether chromosome instability plays a contributing role in tumor initiation, it is clear that it provides cancerous cells with the great adaptability to their ever changing microenvironments, including those that are brought about by therapeutic interventions. On the other hand, chromosomal instability may represent an important link between cancer and aging. However, the mechanisms responsible for chromosomal instability have not been fully understood. We have created a Recql4 knockout mouse model for Type II Rothmund-Thomson syndrome (RTS), a cancer-prone genetic disorder caused by mutations in RECQL4. RECQL4 encodes one of the five homologues of the RecQ family of DNA helicases in humans. Recql4 knockout mice recapitulate all the major phenotypes of Type II RTS, including chromosome instability, cancer predisposition and premature aging. Our studies on this knockout mouse model have led to the discovery that premature centromere separation is the underlying cause of aneuploidy, cancer predisposition, and perhaps premature aging in these mice. Thus, these knockout mice provide a powerful tool to study a novel mechanism of chromosomal instability and a unique mechanism that links cancer and aging. Here we propose to further define the molecular mechanism by which Recql4 is involved in sister-chromatid cohesion and chromosome segregation; and the mechanism that links cancer and aging in Recql4 knockout mice. These studies will advance our understanding regarding the mechanisms that govern chromosome instability and how these mechanisms contribute to carcinogenesis and aging. Furthermore, it may also result in novel targets or strategies for treatments of cancer. [unreadable] [unreadable]