The DNA damage response (DDR) is a major mechanism that antagonizes DNA damage and involves the cell cycle checkpoints and DNA damage repair. The focus of the Zhang lab is to study the DDR and investigate how it can be exploited for personalized medicine.
DNA double-strand breaks (DSBs) are the most dangerous type of DNA damage because incorrectly repaired DSBs can result in genomic instability, a major driving force in the development of cancer. DSBs can be caused directly or indirectly by ionizing radiation (IR), replication stress-generating chemotherapeutic drugs and targeted therapeutic agents. In addition, oncogene expression/action is a major source of replication stress and DSBs. Cells that are deficient in their response to DSBs are sensitive to the treatment modalities mentioned above. Therefore, perturbations in the DSB response that promote carcinogenesis through genomic instability are also the Achilles' heel of cancer. Understanding the molecular mechanisms controlling the DSB response has become a central topic in cancer biology and therapy, and has been the key to developing personalized, molecularly-driven strategies for cancer treatment. The goal of my research is to explore novel strategies for specifically targeting cancer cells by studying the response to DNA DSBs.
There are four major ongoing research projects in the Zhang laboratory:
1. Delineation of the role of RNF126 in the DNA DSB response and its implications for cancer therapy.
RNF126 is an E3 ubiquitin ligase. We have reported that RNF126 promotes DSB repair by facilitating homologous recombination (HR) and contributes to resistance to radiotherapy and chemotherapy (Oncogene, 2016). In addition, our recent publication demonstrated that RNF126 expression is an independent predictor of poor prognosis in breast cancer. Breast cancers expressing RNF126 are sensitive to inhibitors that target the cell cycle checkpoint protein ATR and its main downstream protein CHK1 (Clinical Cancer Research, 2018). Currently, we are testing whether RNF126 has a broad role in the DDR and if it is an efficacy biomarker for CHK1 and ATR inhibitors in combination with radiotherapy or chemotherapy.
2. Identification of novel genes that sensitize cancer cells to IR and inhibitors targeting ATR and CHK1 using Decode Pooled lentiviral shRNA screening libraries.
Synthetic lethality arises when a combination of deficiencies in two genes leads to cell death, while a deficiency in only one of these genes does not. Thirteen years ago, the first molecularly targeted therapeutic (PARP inhibitor) exploiting a synthetic lethality was discovered. This inhibitor specifically targets cancer with inactivated BRCA1 or BRCA2 tumor suppressor genes and was approved by the FDA in 2016 for the treatment of metastatic ovarian cancer. Our recent novel synthetic lethality screen discovered a variety of novel genes that determine ATR or CHK1 inhibitor sensitivity. This work was done using a genome-wide Decode Pooled shRNA Library that encompasses 95,700 lentiviral shRNA expression constructs targeting 18,205 human protein-coding genes. Using a similar approach and shRNA library, we also identified novel genes that contribute to IR resistance. We are currently investigating the mechanisms of these novel synthetic interactions using both in vitro and in vivo models of breast, lung and ovarian cancer.
3. Determination of the mechanisms that control HR-mediated DSB repair in response to replication stress, especially at Common Fragile Sites (CFSs).
CFSs are the genomic regions prone to form breaks during replication stress and represent frequent genomic alterations in both tumors and precancerous lesions. Crossover HR is the predominant pathway to repair DSBs caused by collapsed replication forks. We have reported that ATR inhibition induces a compensatory increase in crossover HR (Nucleic Acids Research, 2012), presumably as a survival mechanism. An invited review based on this finding was published in Cell & Bioscience 2013 and is the most highly accessed article from that journal since its publication. Identifying the mechanisms controlling crossover HR will lead to the discovery of new targets for cancer therapy and identify new approaches to offset the resistance to ATR and CHK1 inhibitors and chemotherapeutic drugs that cause replication stress.
4. Identification of novel genes that regulates alternative non-homologous end joining (alt-NHEJ).
Alt-NHEJ has been implicated in radioresistance. We discovered that 53BP1 promotes DSB repair not only in classical NHEJ-proficient but also NHEJ-deficient cells (Nucleic Acids Research, 2015) by promoting alt-NHEJ. Most importantly, the promotion of alt-NHEJ by 53BP1 is observed only in the presence of functional BRCA1. In contrast, 53BP1 limits alt-NHEJ usage in cells that are deficient in BRCA1. We wrote a review on this topic that was published in the International Journal of Radiation Oncology*Biology*Physics, 2015. ASTRO issued a press release to highlight this review to the national media in March 2016. Currently, we are interested in exploring the new mechanisms and genes controlling alt-NHEJ.
Junran Zhang, MD,PhD - Principal Investigator