Jingjing Chai, Ph.D.

Dissertation research performed under the direction of Bernard E. Weissman

ABSTRACT
    The precise regulation of the accessibility of genomic DNA to transcription factors is essential to cellular proliferation, differentiation and death. SWI/SNF chromatin remodeling complexes play an important role in manipulating nucleosome structure. hSNF5/INI1, the smallest member of SWI/SNF complex, plays a critical role in human rhabdoid tumor development. However, the mechanism of hSNF5's anti-tumor function is still unknown. Previously, our lab and others found that expression of hSNF5 introduces replicative senescence and activates p16 INK4a expression in hSNF5 deficient rhabdoid tumor cell lines. We hypothesized that hSNF5 induced senescence is similar to normal human fibroblast senescence. We found that hSNF5 activated both p16 INK4a and p21 CIP1/WAF1 , but only p16 INK4a was required for the senescence caused by hSNF5. Interestingly, we also found that hSNF5 activated p18 INK4c and p21 CIP1/WAF1 to cause growth arrest in the absence of p16 INK4a upregulation. Additionally, we demonstrated that lack of hSNF5 didn't abrogate cellular responsiveness to DNA damage or growth inhibitory factors. Furthermore, we investigated hSNF5's role in choroid plexus carcinoma (CPC) progression by crossing TgT121 transgenic mice to Snf5+/- mice. We found that there was no acceleration of CPC, but there was a dramatic increase in rhabdoid tumor growth in TgT121, Snf5+/- mice. This acceleration was partially due to increased proliferation in rhabdoid tumors. To further investigate the mechanism, we found that in TgT121, Snf5+/- mice only rhabdoid tumors lost the wild type Snf5 allele, while both rhabdoid tumors and CPCs expressed the T121 transgene. These results suggest that loss of wild type Snf5 does not select for a more aggressive tumor phenotype in CPC, while inactivation of Rb family members accelerates rhabdoid tumor progression. Additionally, to further understand hSNF5's function, we tested various hSNF5 mutants in a rhabdoid tumor cell line. We found that the cooperation between the required regions (Rpt1, Rpt2 and the linker region) and the N-terminal non-conserved sequence or the coiled-coil domain is critical for hSNF5's growth arrest function. Further characterization of hSNF5 using mutation analysis in cell culture and animal studies will help us elucidate its role in suppressing rhabdoid tumors and sustaining normal embryo development.