DNA-PK is activated by SIRT2 deacetylation to promote DNA double-strand break repair by non-homologous end joining.

DNA-dependent protein kinase (DNA-PK) plays a critical role in non-homologous end joining (NHEJ), the predominant pathway that repairs DNA double-strand breaks (DSB) in response to ionizing radiation (IR) to govern genome integrity. The interaction of the catalytic subunit of DNA-PK (DNA-PKcs) with the Ku70/Ku80 heterodimer on DSBs leads to DNA-PK activation; however, it is not known if upstream signaling events govern this activation. Here, we reveal a regulatory step governing DNA-PK activation by SIRT2 deacetylation, which facilitates DNA-PKcs localization to DSBs and interaction with Ku, thereby promoting DSB repair by NHEJ. SIRT2 deacetylase activity governs cellular resistance to DSB-inducing agents and promotes NHEJ. SIRT2 furthermore interacts with and deacetylates DNA-PKcs in response to IR. SIRT2 deacetylase activity facilitates DNA-PKcs interaction with Ku and localization to DSBs and promotes DNA-PK activation and phosphorylation of downstream NHEJ substrates. Moreover, targeting SIRT2 with AGK2, a SIRT2-specific inhibitor, augments the efficacy of IR in cancer cells and tumors. Our findings define a regulatory step for DNA-PK activation by SIRT2-mediated deacetylation, elucidating a critical upstream signaling event initiating the repair of DSBs by NHEJ. Furthermore, our data suggest that SIRT2 inhibition may be a promising rationale-driven therapeutic strategy for increasing the effectiveness of radiation therapy.

YB1 modulates the DNA damage response in medulloblastoma.

Y-box binding protein 1 (YBX1 or YB1) is a therapeutically relevant oncoprotein capable of RNA and DNA binding and mediating protein-protein interactions that drive proliferation, stemness, and resistance to platinum-based therapies. Given our previously published findings, the potential for YB1-driven cisplatin resistance in medulloblastoma (MB), and the limited studies exploring YB1-DNA repair protein interactions, we chose to investigate the role of YB1 in mediating radiation resistance in MB. MB, the most common pediatric malignant brain tumor, is treated with surgical resection, cranio-spinal radiation, and platinum-based chemotherapy, and could potentially benefit from YB1 inhibition. The role of YB1 in the response of MB to ionizing radiation (IR) has not yet been studied but remains relevant for determining potential anti-tumor synergy of YB1 inhibition with standard radiation therapy. We have previously shown that YB1 drives proliferation of cerebellar granular neural precursor cells (CGNPs) and murine Sonic Hedgehog (SHH) group MB cells. While others have demonstrated a link between YB1 and homologous recombination protein binding, functional and therapeutic implications remain unclear, particularly following IR-induced damage. Here we show that depleting YB1 in both SHH and Group 3 MB results not only in reduced proliferation but also synergizes with radiation due to differential response dynamics. YB1 silencing through shRNA followed by IR drives a predominantly NHEJ-dependent repair mechanism, leading to faster γH2AX resolution, premature cell cycle re-entry, checkpoint bypass, reduced proliferation, and increased senescence. These findings show that depleting YB1 in combination with radiation sensitizes SHH and Group 3 MB cells to radiation.

EZH2 has a non-catalytic and PRC2-independent role in stabilizing DDB2 to promote nucleotide excision repair.

Small cell lung cancer (SCLC) is a highly aggressive malignancy with poor outcomes associated with resistance to cisplatin-based chemotherapy. Enhancer of zeste homolog 2 (EZH2) is the catalytic subunit of polycomb repressive complex 2 (PRC2), which silences transcription through trimethylation of histone H3 lysine 27 (H3K27me3) and has emerged as an important therapeutic target with inhibitors targeting its methyltransferase activity under clinical investigation. Here, we show that EZH2 has a non-catalytic and PRC2-independent role in stabilizing DDB2 to promote nucleotide excision repair (NER) and govern cisplatin resistance in SCLC. Using a synthetic lethality screen, we identified important regulators of cisplatin resistance in SCLC cells, including EZH2. EZH2 depletion causes cellular cisplatin and UV hypersensitivity in an epistatic manner with DDB1-DDB2. EZH2 complexes with DDB1-DDB2 and promotes DDB2 stability by impairing its ubiquitination independent of methyltransferase activity or PRC2, thereby facilitating DDB2 localization to cyclobutane pyrimidine dimer crosslinks to govern their repair. Furthermore, targeting EZH2 for depletion with DZNep strongly sensitizes SCLC cells and tumors to cisplatin. Our findings reveal a non-catalytic and PRC2-independent function for EZH2 in promoting NER through DDB2 stabilization, suggesting a rationale for targeting EZH2 beyond its catalytic activity for overcoming cisplatin resistance in SCLC.

The p.Gly61Glu Mutation in CYP1B1 Affects the Extracellular Matrix in Glaucoma Patients.

PURPOSE: The aim of this work was to assess the possible effects of CYP1B1 mutations on the extracellular matrix (ECM) in glaucoma patients. CYP1B1 mutations are the cause of disease in a notable fraction of primary congenital glaucoma (PCG) patients and in a smaller fraction of primary open angle glaucoma (POAG) patients. METHODS: The study was performed on a glaucoma family with the common homozygous p.Gly61Glu CYP1B1 mutation. The father was affected with POAG and three siblings had PCG. Microscopy was performed on the skin of the father and one son, as well as controls. Immunohistochemical studies were done using anti-CYP1B1 and anti-fibrillin-1 antibodies. Fibrillin-1 served as a marker for the ECM, and electron microscopy was also performed. RESULTS: CYP1B1 expression patterns were the same in the patients and controls. However, microfibrils that are associated with fibrillin-1 were less abundant and more fragmented in both patients. Electron microscopy showed disturbed collagen fibers only in the PCG patient. CONCLUSIONS: The p.Gly61Glu mutation in CYP1B1 affects the ECM structure. This implies that the ECM of the trabecular meshwork may also be disrupted in a manner that affects aqueous humor flow resulting in increased intraocular pressure and contributing to the glaucoma phenotype.