Featured Scientist: Dr. Xiongbin Lu

OneArray^® Research In Focus: MicroRNA in DNA Damage Response

Genomic mutations play a key role in cancer initiation and progression. When cells lose their intrinsic ability for DNA repair, they become susceptible to uncontrollable growth when exposed to environmental genotoxic stresssors. Activating appropriate DNA damage responses is critical for genome integrity and proper function and survival for all organisms, which is why these pathways are conserved in many aspects from humans to yeast. The occurrence of DNA damage is detected by sensors that initiate a cascade of events including replication stalling, cell cycle checkpoints, cell cycle arrest or apoptosis if damage is beyond salvageable. Ataxia Telegiectasia Mutation (ATM) kinase is a key sensor for DNA double stranded breaks (DSBs). Having over 700 targets at its disposal, ATM initiates and propagates appropriate DNA damage response signals using phosphorylation as its preferred mode of action. A well known ATM target is a cell cycle regulator called p53. This is an important tumor suppressor that is often disregulated in a wide array of cancers.

Observations made in recent studies suggest that a group of evolutionarily conserved small non-coding RNA molecules called microRNA plays a part in DNA damage responses. MicroRNAs are 18-25 nucleotides in length and have the ability to regulate gene expression by binding to perfect or imperfect base pairing at the 3' UTR or target mRNA transcripts. microRNAs are transcribed by RNA polymerase II or III first into primary microRNA (pri-microRNA). While still in the nucleus, pri-microRNAs are cleaved by Drosha-DGCR8 microprocessors to generate a hairpin-shaped precursor pre-microRNAs. Once transported to the cytosol, pre-microRNAs are cleaved by Drosha and Dicer endoribonucleases into mature functional microRNAs. (Diagram depicting microRNA maturation process is adapted from Trends Biochem Sci. 2011 Jul 7.) In 2010, Dr. Xiongbin Lu from MD Anderson Cancer Center identified a microRNA, miR-16, as a key inhibitor for the Wip1 phosphatase (Cancer Res 70(18):7176-86). "It is an exciting discovery because Wip1 is a master inhibitor in the ATM-p53 DNA damage pathway and the Wip1 gene is amplified in many types of human cancer. When we were studying the expression of miR-16 in the DNA damage response, we found that it was induced rapidly after DNA damage, which really stirred up my interest to answer one important question: how do microRNAs respond to DNA damage?" - Dr. Xiongbin Lu

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In his most recent publication in Molecular Cell (Mol Cell 41(4):371-383), Dr. Lu provided a key piece of evidence describing a mechanism of how DNA damage response can affect microRNAs. In this study, Dr. Lu first conducted a microarray-based microRNA profiling study (PhalanxBio) in mouse embryonic fibroblasts with DNA DSBs caused by a radiomimetic drug, neocarzinostatin. "microRNA expression profiling played a key role in this project, from which we identified DNA damage-induced microRNAs and found that KSRP-associated microRNAs are induced in an ATM-dependent manner" says Dr. Lu. This confirms microRNAs as part of the downstream events of DNA damage pathway, and also implicates microRNA-processing as part of the mechanism. KH-type splicing regulatory protein (KSRP) is a key component in both Drosha and Dicer microRNA-processing complexes, and has been shown to regulate the maturation of a subset of microRNAs. DNA damage activated ATM phosphorylates KSRP at three residues Ser132, Ser274 and Ser670, which contributes to increased binding of KSRP to pri-microRNAs. This brings pri-microRNAs close to Drosha microprocessors, hence facilitates microRNA maturation. This is the first reported mechanism of DNA damage-induced microRNAs expression. It is of great interest to determine whether other mechanisms such as the activities of Dicer or Drosha complexes and microRNA nuclear export are involved in the regulation of microRNAs during DNA damage response.

Accumulating evidence suggest that DNA damage signaling serves as an anticancer barrier in tumorigenesis, underpinning a critical role of ATM in tumor suppression. Dr. Lu's study showed that loss of ATM abrogated DNA damage-induced microRNA biogenesis in both human and mouse cells. Some of these microRNAs have been reported to be tumor suppressor candidates, one of which is miR-16. Knockout of miR-16 leads to lymphoid leukemias in mice that are often observed in the Ataxia Teligiectasia (AT) patients. There is a growing volume of reports identifying microRNAs as biomarkers in a variety of human cancers. Deficiencies in microRNA expression may ultimately contribute to the initiation and progression of tumors. Dr. Lu believes microRNAs soon will be used for prognosis purposes. MicroRNA inhibitors such as antagomirs are already being tested to inhibit oncogenic microRNAs in vitro and in vivo. One notable advantage for microRNA-targeted therapeutics is that microRNA-based therapeutics can potentially target not just a single gene but a group of genes while having minimal off-target side effects. Elucidating the transcriptional regulation and function of microRNAs will have a wide impact on our understanding of various diseases and moves us forward with therapeutics development.