Genetic and Epigenetic Crosstalk Define TP53-mediated Human Cancer Susceptibility

Deregulation of epigenetic programming is crucial to tumorigenesis, yet the extent to which epigenetics plays a role in cancer susceptibility remains unclear. This work details systematic molecular investigations of the familial cancer predisposition syndrome, Li-Fraumeni Syndrome (LFS), to garner a better understanding of the role of epigenetics in cancer susceptibility in humans. LFS is a prototypical cancer susceptibility syndrome characterized by a 73-100% lifetime incidence of developing at least one malignancy in affected individuals. The majority of LFS cases are associated with inherited or de novo mutations in the TP53 gene, encoding the tumor suppressor protein, p53. This work reports the first genome-wide survey of DNA methylation in peripheral blood leukocytes from carriers of germline TP53 mutations, and reveals that TP53 mutation carriers harbor a distinct DNA methylation profile relative to non-carriers overall, and in subgroup analyses stratified by cancer diagnosis. Many TP53 mutation-associated differential methylation marks occur at previously characterized p53 transcription factor binding sites. In particular, striking hypomethylation of the miR-34A promoter, a known tumor suppressor in the p53 regulatory network, is detected in leukocytes from LFS patients in two independent cohorts. By contrast, miR-34A promoter hypermethylation is a recurrent epigenetic event in primary LFS tumors of various histologies and correlates with poor clinical outcome in patients with choroid plexus carcinomas. These findings suggest a role for miR-34A as an important tumor suppressor in LFS, and demonstrate that loss of miR-34A by somatic epimutation may comprise a step in neoplastic progression in this clinical context. Detailed molecular investigations to better understand the functional role of miR-34A in the background of mutant TP53 reveal that miR-34A is essential for cell viability and regulates a network of noncoding RNAs, including components of the minor spliceosome, as well as various long non-coding RNAs. The function of miR-34A in cell cycle states may be mediated by these transcriptional changes. Collectively, this work provides the first insights into the mechanisms by which germline TP53 mutations may confer cancer predisposition and implicates a novel gene, miR-34A, in the pathogenesis of TP53-associated human cancer susceptibility.Ph.D