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UBC Theses and Dissertations

The role of tumor suppressor Inhibitor of Growth 1 in lesion bypass pathway and genomic stability Wong, Pak Cheung Ronald


The lesion bypass pathway, which is regulated by monoubiquitination of proliferating cell nuclear antigen (PCNA), is essential for resolving replication stalling due to DNA lesions. This process is important for preventing genomic instability and cancer development. In this thesis, I demonstrated a novel tumor suppressor function of p33ING1 (ING1b) in preserving genomic stability upon replication stress through regulating PCNA monoubiquitination. We found that ING1b knockdown cells are more sensitive to UV due to defects in recovering from UV-induced replication blockage, leading to enhanced genomic instability. We revealed that ING1b is required for the E3 ligase Rad18-mediated PCNA monoubiquitination in lesion bypass. Interestingly, ING1b-mediated PCNA monoubiquitination is associated with the regulation of histone H4 acetylation. For the first time, we have shown that chromatin remodeling contributes to the stabilization of stalled replication fork and to the regulation of PCNA monoubiquitination during lesion bypass. Previously, our group showed that ING1b is phosphorylated by the S phase checkpoint kinase Chk1 at S126 residue. We further showed that ING1b cooperated with Chk1 in maintaining genomic stability. We found that ING1b interacted with Chk1 after UV through ING1b S126 residue. Furthermore, we found that ING1b S126 residue was required for PCNA monoubiquitination, Pol-eta foci and therefore preventing chromosomal aberrations after UV. These data suggest that ING1b cooperates with Chk1 through the S126 residue in mediating PCNA monoubiquitination, lesion bypass and genomic stability. We further explored the role of the E3 ligase Rad18, a key regulator for the lesion bypass pathway, in melanoma using melanoma tissue microarray. We found that Rad18 expression was significantly upregulated in melanoma. Strong Rad18 expression correlated with worse 5-year patient survival in the sun-protected sites. Interestingly, we found an opposite role of Rad18 on patient survival in the sun-exposed sites. Furthermore, we showed that melanoma cell proliferation and the expression of pAkt and cyclin D1 were reduced upon Rad18 knockdown. The work presented in this thesis leads to a better understanding of the role of ING1b and lesion bypass pathway in genomic stability and melanoma development. It has implications on designing new strategies for cancer therapy.

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