**Roles of MicroRNA in DNA Damage and Repair**

Xinrong Chen and Tao Chen

*National Center for Toxicological Research/US Food and Drug Administration U.S.A.* 

#### **1. Introduction**

340 DNA Repair

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DNA damage mainly results from either endogenous metabolic activity, such as oxidative stress, or environmental exposure, such as ionizing irradiation. In human cells, endogenous and exogenous genotoxic agents produce as many as 1 million molecular lesions per cell per day. If the unrepaired lesions occur in certain critical genes, they can cause mutations that can lead to tumors (Lodish H, 2004).

There are several different types of DNA damage, including DNA hydrolysis, DNA adduction, DNA crosslinking, and DNA strand breakage. DNA hydrolysis is the breaking of DNA through the addition of water. Hydrolysis of DNA bases consists of deamination, depurination, and depyrimidination. A DNA adduct is a piece of DNA covalently bonded to a chemical. DNA crosslinks are links formed within a single (intrastrand) or between strands of DNA (interstrand). There are two types of DNA strand breaks, single strand breaks and double strand breaks. DNA double strand breaks are particularly hazardous to the cells because they can lead to genome rearrangements. (Rich et al., 2000).

Cells respond to DNA damage through a variety of different mechanisms, such as apoptosis, senescence, and DNA repair. Excessive DNA damage induces apoptosis, or programmed cell death, that eliminates cells with heavily damaged DNA, thus protecting the organism from the mutations potentially induced by the damage. Unrepaired DNA damage is a driving force for senescence. Senescence serves as a functional alternative to apoptosis in cases where the physical presence of cells is required for spatial reasons. If DNA replication occurs before DNA damage is repaired, mutations can be formed in the cells. To prevent mutation formation, cells have developed DNA repair mechanisms to correct DNA.

There are several different types of DNA repair. They are direct reversal, base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), non-homologous end-joining (NHEJ), and homologous recombination repair (HRR). Direct reversal can remove DNA damage by chemically reversing it. Since the correction only occurs in one of the four bases and not the phosphodiester backbone, this type of repair does not need any DNA template. For example, methylation of guanine bases can be directly reversed by methyl guanine methyl transferase (MGMT) that removes the methyl group. BER amends damage to single nucleotides produced by oxidation, alkylation, or hydrolysis. NER corrects ethylation products, bulky DNA adducts, helix-distorting changes, such as thymine dimers, and single-strand breaks. MMR repairs mismatched bases in double-stranded DNA (e.g., A:C or G:T). HRR is a mechanism for DNA double-strand repair that reconstitutes the

Roles of MicroRNA in DNA Damage and Repair 343

Li et al., 2011; Zenz et al., 2009). miR-34c, another member of miR-34 family, is transcriptionally up-regulated by P53 following DNA damage (Cannell et al., 2010). In addition to miR-34a, P53 can also regulate the expression of miR-192, miR-194, and miR-215. These miRNAs are considered tumor suppressor miRNAs (Braun et al., 2008;

miRNA biogenesis is globally induced upon DNA damage in an ATM (ataxia telangiectasia mutated) dependent manner (Zhang et al., 2011). The ATM gene encodes a DNA damageinducible kinase. ATM controls cell grow rate by interacting with other proteins, for example BRCA1, following DNA damage. In response to strand breaks or other type of DNA damage, the ATM protein coordinates DNA repair by activating other proteins. Because of its central role in cell division and DNA repair, the ATM protein is important in carcinogenesis. More than one-fourth of miRNAs were significantly upregulated after DNA damage, while loss of ATM activity abolished their induction. Their results show that DNA damage activates the ATM kinase that directly binds to and phosphorylates KH-type splicing regulatory protein (KSRP), leading to enhanced interaction between KSRP and primiRNAs and increased KSRP activity in miRNA processing. The increased activity, in turn, results in more pre-miRNAs from pri-miRNAs, so that more miRNA products are produced

Other studies show a different mechanism by which DNA damage signaling is linked to the miRNA maturation processes. Several miRNAs with growth suppressive function, including miR-16-1, miR-143 and miR-145, were regulated at the post transcriptional level through a P53-mediated miRNA maturation process in response to DNA damage (Suzuki et al., 2009; Toledo and Bardot, 2009). The P53 tumor suppressor protein binds to Drosha to facilitate the processing of pri-miRNAs to pre-miRNAs. Mutation in the DNA-binding domain of P53 decreases processing of pri-miRNAs by Drosha, and reduces the expression of the related miRNAs. In silico analyses, all three component of the P53 tumor suppressor, P53, P63, and P73, can regulate the major components of miRNA processing, such as Drosha-DGCR8, Dicer-TRBP2, and Agronaute proteins. Thus, when DNA damage activates the P53 gene, the activated P53 gene can modulate miRNA expression by affecting the

miR-24 regulates the DNA damage response by down-regulation of H2AX, the initial sensor protein for the DNA damage response. miR-100, miR-101 and miR-421 suppress ATM, the chief transducer of the DNA damage response, by targeting the 3'-UTR of ATM. miR-16 can up-regulate ATM activity by suppressing levels of Wip1. DNA repair pathways are regulated by a number of miRNAs involved in different types of DNA damage correction. the NER protein RAD23B was down-regulated by miR-373. MMR protein MSH2 and MSH6 were down regulated by miR-21 and MLH1/MSH2 were suppressed by miR-155. The HRR protein BRCA1 was down-regulated by miR-182 and RAD52 was suppressed by miR-210 and miR-373. The NHEJ protein DNA-PKcs was suppressed by miR-101 (Yan, Ng. 2010).

miRNAs regulate multiple aspects of the DNA damage response pathway, including regulation of signal transduction of DNA damage, changing expression level of master regulatory proteins such as P53, modulating key protein expression in different types of DNA repair such as MMR, NER, NHEJ and HRR. Figure 2 and Table 1 summarize recently

**4. miRNA regulation of signal transduction for DNA damage** 

reported miRNAs associated with DNA damage and repair.

Georges et al., 2008).

to respond to the DNA damage.

miRNA biogenesis processes.

original sequence using the sister chromatid as a template. NHEJ is a relatively simple way for DNA double-strand repair and it just rejoins two broken ends without correcting any deletions or rearrangements of DNA.
