The molecular mechanism by which the tumor suppressor PTEN and DAXX maintains centromeric heterochromatin structure and the contribution to tumor suppression.

The tumor suppressor PTEN is one of the most commonly mutated genes in cancer. PTEN’s lipid phosphatase activity antagonizes PI3K signaling and affects many aspects of cellular function, including regulating cell survival and growth. In addition to its cytoplasmic localization, PTEN also resides in the nucleus and functions independent of the PI3K pathway to maintain the structural integrity of chromosomes, promote DNA repair in response to genotoxic stress, DNA replication and chromosome segregation. The pleiotropic effects of PTEN loss have made it challenging to pin-point the molecular mechanism by which PTEN mutation drives carcinogenesis. The histone chaperone DAXX assembles the histone variant H3.3 into chromatin to promote gene repression and DAXX is highly upregulated in diverse cancer types and treatment resistant tumors. In this seed grant, and the ultimate multi-PI application, we will test the hypothesis that PTEN retains DAXX, and subsequently its cargo H3.3 on centromeric heterochromatin to suppress transcription of RNA from centromeric satellite repeats. We propose that it is the expression of these satellite RNAs in cells with PTEN mutants or overexpressed DAXX that causes replication stress, genome instability and tumorigenesis. We will use mouse cells deficient in PTEN, H3.3, and / or DAXX and PTEN mutant knock in mouse models of breast cancer to address this hypothesis, leveraging the unique strengths of our four labs. Our studies will reveal how PTEN maintains centromeric heterochromatin structure and will provide key insight into the tumor-promoting roles of PTEN mutation and DAXX overexpression.

This work will reveal the mechanistic basis for how PTEN mutations drive tumorigenesis, as well as revealing how overexpression of DAXX drives tumorigenesis, via a concerted mechanism to incorporate H3.3 into chromatin in order to repress expression of the satellite repeats.