Double-strand DNA breaks (DSBs) occur in all cells multiple times per day. Homologous recombination (HR) and nonhomologous DNA end-joining (NHEJ) are the two primary pathways for repairing DSBs. However, NHEJ is the more general pathway because it does not require a homologous donor, and NHEJ is sufficiently flexible that it can join almost any pair of incompatible DNA ends. The flexibility of NHEJ is essential because natural causes of DSBs (ionizing radiation, free radicals, enzyme malfunction) generate DNA ends with diverse chemical and structural configurations. Hence, that flexibility is well-suited for the task, but the price that we pay for that flexibility is that NHEJ causes DNA sequence changes at most sites where it functions (in humans and other vertebrates). Hence, NHEJ generates somatic cell mutations that cause cancer and likely contribute to aging. The flexibility of NHEJ represents one of the most sophisticated protein:DNA interaction pathways. Though we know most of the proteins that participate in NHEJ and know, in broad terms, what they do, we do not have a clear picture of how they function together, their spatial configuration, or the temporal relationships. With a clearer mechanistic and structural picture of human NHEJ, we will be in a position to develop small molecule inhibitors that may be useful for treating cancer (as chemotherapy or as a radiation sensitizer).
Aims 1 and 2 of this proposal focus precisely on the issues of structural and spatial relationships among the NHEJ proteins and with the DNA ends.
Aim 3 tests the hypothesis that the NHEJ ligase complex is important for making key end-to-end contacts during NHEJ.
Aim 4 describes development of a more robust NHEJ reconstitution.
In Aim 4, that system is then used to identify any remaining NHEJ factors by testing for enhancement of activity.
Aim 5 describes analysis of how histone octamers influence the nuclease, polymerase, and ligase activities of NHEJ. These studies will markedly improve our understanding of how NHEJ participates in processes such as chromosomal translocations that are of key importance in cancer etiology and how NHEJ might be made 'druggable'for therapeutic purposes.

Public Health Relevance

This project would clarify our insights into one of the key pathways for the repair of DNA damage. These insights will allow us to understand how some of the DNA damage that causes cancer and aging occurs. The benefits of some chemotherapy and therapeutic radiation work can be increased by this deeper level of understanding of this key DNA repair pathway.

National Institute of Health (NIH)
National Cancer Institute (NCI)
Research Project (R01)
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Study Section
Cancer Etiology Study Section (CE)
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Pelroy, Richard
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University of Southern California
Schools of Medicine
Los Angeles
United States
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Pannunzio, Nicholas R; Lieber, Michael R (2018) Concept of DNA Lesion Longevity and Chromosomal Translocations. Trends Biochem Sci 43:490-498
Reid, Dylan A; Conlin, Michael P; Yin, Yandong et al. (2017) Bridging of double-stranded breaks by the nonhomologous end-joining ligation complex is modulated by DNA end chemistry. Nucleic Acids Res 45:1872-1878
Pannunzio, Nicholas R; Lieber, Michael R (2017) AID and Reactive Oxygen Species Can Induce DNA Breaks within Human Chromosomal Translocation Fragile Zones. Mol Cell 68:901-912.e3
Conlin, Michael P; Reid, Dylan A; Small, George W et al. (2017) DNA Ligase IV Guides End-Processing Choice during Nonhomologous End Joining. Cell Rep 20:2810-2819
Pannunzio, Nicholas R; Lieber, Michael R (2016) RNA Polymerase Collision versus DNA Structural Distortion: Twists and Turns Can Cause Break Failure. Mol Cell 62:327-334
Chang, Howard H Y; Lieber, Michael R (2016) Structure-Specific nuclease activities of Artemis and the Artemis: DNA-PKcs complex. Nucleic Acids Res 44:4991-7
Pannunzio, Nicholas R; Lieber, Michael R (2016) Dissecting the Roles of Divergent and Convergent Transcription in Chromosome Instability. Cell Rep 14:1025-1031
Reid, Dylan A; Keegan, Sarah; Leo-Macias, Alejandra et al. (2015) Organization and dynamics of the nonhomologous end-joining machinery during DNA double-strand break repair. Proc Natl Acad Sci U S A 112:E2575-84
Lu, Zhengfei; Pannunzio, Nicholas R; Greisman, Harvey A et al. (2015) Convergent BCL6 and lncRNA promoters demarcate the major breakpoint region for BCL6 translocations. Blood 126:1730-1
Li, Sicong; Chang, Howard H; Niewolik, Doris et al. (2014) Evidence that the DNA endonuclease ARTEMIS also has intrinsic 5'-exonuclease activity. J Biol Chem 289:7825-34

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