The genome of all living organisms is subjected to various types of genotoxic stress from both environment, including ultraviolet or ionizing radiation, various chemicals, and reactive oxygen species (ROS), generated during normal cellular metabolism. In order to maintain the integrity of the genome, organisms have developed genome surveillance mechanisms to activate DNA damage response (DDR) network. This response activates cell cycle checkpoint to arrest the cell cycle progression transiently and ensure proper repair of the damaged DNA. The DDR pathway is activated by various types of DNA lesion and is executed by a complex network of signaling kinases. Among these, ataxia-telangiectasia mutated (ATM) and ATM and Rad3-related (ATR) kinases are crucial. Activated ATM or ATR kinase phosphorylate a number of substrates, including CHK1 and CHK2 kinases, which in turn target other proteins to induce cell cycle checkpoint and facilitate DNA repair process.
The DNA double-strand breaks (DSBs) and DNA daughter strand gaps (DSGs) that arise during normal replication are considered to be the primary cause for genomic instability. The natural inducers of DSBs are stalled replication forks caused by blocking lesions due to ROS generation in the cell, DNA bound proteins, unusual DNA structures and replication slow zones. There are two major mechanisms to repair DSBs; homologous recombination (HR) and non-homologous end joining (NHEJ). NHEJ mediated repair requires micro or no homology and is frequently associated with micro insertions or deletions, hence NHEJ is generally error-prone. Whereas HR mediated repair utilizes intact homologous DNA as a template and the copied information is accurate. HR is also essential for the repair of replication associated DNA lesions such as DSGs and facilitates restart of replication. Replication associated DSGs and the one ended DSBs that arise due to collapsed replication forks are repaired by HR using neighboring sister chromatid as a template and thereby protects the integrity of the genome.
In collaboration with nucleotide excision repair and translesion synthesis, HR is also involved in the repair of DNA interstrand cross-links (ICLs). Thus, HR is important for the maintenance of genome integrity and its dysfunction can lead to aberrant genetic rearrangements resulting in chromosomal translocations, deletions, amplifications or loss of heterozygosity (LOH), which are all hallmarks of cancer cells. Indeed, HR dysfunction is associated with several cancer susceptibility genetic diseases such as ataxia telangiectasia, ATR-seckel syndrome, Nijmegen breakage syndrome, Bloom syndrome and Fanconi anemia. Moreover, mutations in BRCA1 and BRCA2, that are known to regulate HR, cause hereditary breast and ovarian cancers, implying a crucial role of HR and the genes that regulate HR in safeguarding the genome and in cancer prevention. Our lab is interested in understanding the molecular mechanism(s) of DNA damage signaling, HR mediated DNA repair, genome instability and cancer, and various genes that regulate these processes.
Tuberculosis (TB) and AIDS are the major global health crisis, especially in developing countries. Infection with TB is a significant cause of AIDS associated mortality in developing countries. Mycobacterium tuberculosis, the causative agent of TB, can persist for decades in infected individuals in the latent state as an asymptomatic disease and can emerge to cause active disease at a later stage. There is evidence that M. tuberculosis cells are exposed to DNA damaging agents such as reactive oxygen intermediates (ROI) and reactive nitrogen intermediates (RNI) generated by host macrophages. Thus, DNA repair pathways and the mechanisms that are involved in the maintenance of genome integrity appear to be an important factor for M. tuberculosis pathogenesis and persistence in the host. Helicases are known to play an important role in various nucleic acid transactions, including DNA replication, repair and recombination. Thus, helicases could be a potential drug target against M. tuberculosis. We are interested to study various helicases and other DNA repair/recombination proteins from M. tuberculosis through genetic, biochemical and biophysical approach. Understanding the role of these proteins could provide new insights into pathogenesis of M. tuberculosis in humans.
Saxena, S., Somyajit. K., and Nagaraju, G. (2018). XRCC2 regulates replication fork progression during dNTP alterations. Cell Reports 25:3273-3282.
Mishra, A., Saxena, S., Kaushal, A., and Nagaraju, G. (2018). RAD51C/XRCC3 facilitates mitochondrial DNA replication and maintains integrity of the mitochondrial genome. Mol. Cell. Biol. 38:1-18.
Nath, S., Somyajit, K., Mishra, A., Scully, R., and Nagaraju, G. (2017). FANCJ helicase controls the balance between short- and long-tract gene conversions between sister chromatids. Nucleic Acids Res. 45:8886-00.
Somyajit, K., Saxena, S., Babu, S., Mishra, S., and Nagaraju, G. (2015). Mammalian RAD51 paralogs protect nascent DNA at stalled forks and mediate replication restart. Nucleic Acids Res. 43:9835-55.
Thakur, R.S., Basavaraju, S., Khanduja, J.S., Muniyappa, K., and Nagaraju, G. (2015). Mycobacterium tuberculosis RecG but not RuvAB or RecA is efficient at remodeling the stalled replication forks: Implications for multiple mechanisms of replication restart in mycobacteria. J. Biol. Chem. 290:24119-39.
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