Biology and Biomedical Sciences: Molecular Genetics and Genomics
Date of Award
Doctor of Philosophy (PhD)
Chair and Committee
53BP1 is a tumor suppressor protein that is important in the non-homologous end joining: NHEJ) pathway for DNA double strand break repair. Mice lacking 53BP1 present with increased radiosensitivity and genomic instability and are cancer prone. Loss of 53BP1 has been implicated by our laboratory in causing genomic instability in cells that lack A-type lamins and work by others has shown that loss of 53BP1 is associated with breast cancers of the poorest prognosis - BRCA1-mutated and Triple Negative Breast Cancers: TNBC) - and contributes to their resistance to current therapies, such as PARP inhibitors: PARPi). Work in our laboratory had previously identified a novel mechanism for regulation of 53BP1 protein levels - degradation by the cysteine protease cathepsin L: CTSL). The main questions addressed in this dissertation are whether CTSL-mediated degradation of 53BP1 is a mechanism contributing to BRCA1-mutated and TNBC tumor progression and resistance to therapy and if activation of this pathway is responsible for genomic instability in progeria and other laminopathies.
We found that CTSL-mediated degradation of 53BP1 is activated upon loss of BRCA1, rescuing homologous recombination: HR) and proliferation defects. Inhibiting CTSL using CTSL inhibitors or vitamin D treatment stabilizes levels of 53BP1 protein, leading to an increase in NHEJ and defects in HR. Stabilization of 53BP1 using CTSL inhibitors or vitamin D leads to increased genomic instability and compromises proliferation following ionizing radiation or treatment with PARPi, which could represent a novel therapeutic strategy for breast cancers with the poorest prognosis. Furthermore, we identified nuclear CTSL as a novel biomarker for TNBC, which correlated inversely with 53BP1. We also identified the signature of low nuclear vitamin D receptor: VDR) with low 53BP1 and high nuclear CTSL in TNBC and BRCA1-mutated tumors, revealing a novel triple biomarker for stratification of patients with these cancers. We hypothesize that these patients can be treated with vitamin D or cathepsin inhibitors to stabilize 53BP1 levels to render tumor cells susceptible to PARPi treatment or other DNA damaging strategies.
We also show that the Δexon9Lmna mouse model of progeria exhibits telomere shortening and defects of heterochromatin, but not an increase in telomere deletions or an increase in genomic instability as seen upon loss of A-type lamins. Interestingly, the levels of 53BP1 are maintained in cells from these mice. These results demonstrate that different lamins mutations present with varying phenotypes and that 53BP1 status could be an important determinant for maintenance of genomic instability in some lamins-related diseases.
Grotsky, David Alan, "A New Pathway Responsible For 53BP1 Loss in Breast Cancer and Laminopathies" (2012). All Theses and Dissertations (ETDs). 1004.