Identification of novel ubiquitin-driven pathways in the DNA damage response and their utilization for targeted therapy of chronic lymphocytic leukemia

• Applicant: Dr. Ron Jachimowicz
• Subject Area: Hematology, Oncology
• Term: from 2014 to 2017
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 254951892

Maintenance of genomic stability is essential for cellular homeostasis and prevention of undue cell death or neoplasia. The DNA damage response (DDR) is a major axis in maintenance of genomic stability. A critical DNA lesion that undermines genomic stability is the double-strand break (DSB), which leads to a cellular response with a broad signaling pathway. Identification of new branches in this network is a major aim in this field. The DSB response is mobilized by the protein kinase ATM, which phosphorylates key players in this network. The Shiloh lab identified the ATM gene and made significant contributions in the investigation of the ATM-mediated network. In addition to phosphorylation, an important protein post-translational modification in the DDR is ubiquitylation. Using meta-analysis of published data carried out in the Shiloh lab, the applicant identified three novel DDR players in putative ubiquitin-driven pathways in the DDR, which are probably ATM substrates: UBR1, UBE3A and UBQLN4. A streamlined experimental paradigm, which is well-established in the Shiloh lab, will be used to carry out in-depth investigation of these new DDR branches and understand their mechanistic aspects. The Shiloh lab has recently studied another ubiquitylation-driven DDR pathway, involving the monoubiquitylation of histone H2B by the RNF20-RNF40 heterodimer. This pathway is critical for timely DSB repair via its two main mechanisms - nonhomologous end joining (NHEJ) and homologous recombination repair (HRR). Disabling mutations in genes encoding for different components of the HRR-network occur frequently in various different cancer entities. For instance, ATM-defective cells and lymphomas are critically dependent on the NHEJ kinase DNA-PK for their survival even in the absence of exogenous DNA damage. Furthermore, BRCA1/2-defective cells rely on intact base excision repair (BER)-mediated DNA repair for their survival. Intriguingly, both the NHEJ and the BER pathway contain signaling components that might prove valuable drug targets for the treatment of HRR-deficient cancers. To address these issues, another part of this project will be focused upon chronic lymphocytic leukemia (CLL), which represents the most common leukemia entity in adults. Notably, loss of chromosome arm of 11q, which contains the ATM gene, is common in CLL, and the second ATM allele is frequently inactivated by mutations. These patients are often therapy-resistant and exhibit poor survival rates. We will examine the possibility that combined inhibition of base excision repair (using PARP-1 inhibitors) and DSB repair via NHEJ (using DNA-PK inhibitors) could be used for treating HRR-defective CLL, taking advantage of novel synthetic lethal interaction between ATM and the catalytic subunit of DNA-PK, DNA-PKcs, in combination with PARP1 inhibitors. Furthermore, we will employ an RNF20-depleted CLL mouse model, to validate and functionally characterize the role of RNF20 in the DDR.

• DFG Programme: Research Fellowships
• International Connection: Israel
• Host: Professor Dr. Yosef Shiloh