**9. Functions of miRNAs in homologous recombination repair (HRR)**

HRR is the most widely used repair mechanism which can accurately repair DNA double strand breaks. HRR reconstitutes the genetic information using the sister chromatid as a template. Several proteins are involved in the HRR process. Rad 52 protein recognizes double-strand breaks and adheres to the free ends of the break while the Rad51 protein, together with tumor-suppressor protein BRCA1, searches the undamaged sister chromatid for homologous pairing (Haber, 2000; Orelli and Bishop, 2001).

Both miR-210 and miR-373 were up-regulated in hypoxic cells. Up-regulation of miR-210 significantly suppressed the expression level of RAD51, while up-regulation of miR-373 inhibited the expression of RAD52. The modulation of miR-210 to RAD51 and miR-373 to RAD52 were verified by microarray analysis and luciferase reporter gene assay. Both of the miRNAs can bind to the binding sites in the 3' UTRs of their respective target mRNAs (Crosby, et al., 2009). Thus, hypoxia-inducible miR-210 and miR-373 regulate HRR via targeting RAD51 and RAD52.

BRCA1 is a constituent of several different protein complexes and is a key protein for HRR. Expression of BRCA1 is commonly decreased in sporadic breast tumors, and this correlates with poor prognosis of breast cancer patients (Mueller and Roskelley, 2003). It was recently reported that miR-182 down-regulated BRCA1 expression. As a result, the HRR efficiency for DNA double strand break repair was impaired (Moskwa et al., 2011; Yao and Ventura, 2011). Antagonizing miR-182 enhanced BRCA1 protein level, which, in turn, protected cells from irradiation exposure. Over-expressing of miR-182 reduced BRCA1 protein level, which impaired HRR efficiency and rendered cells hypersensitive to irradiation. The impaired HRR phenotype due to miR-182 over-expression was able to be fully rescued by overexpressing of BRCA1. Thus, these data demonstrate miR-182-mediated down-regulation of BRCA1 suppresses HRR.

### **10. Conclusion**

miRNAs appear to be involved in DNA damage and repair in many ways. miRNA biogenesis, including miRNA gene transcription and miRNA maturation processes, is readily altered in response to DNA damage. miRNAs regulate the ATM and P53 that are the regulators of the global induction of miRNA biogenesis upon DNA damage. miRNAs are also involved in signal transduction processes that leads to cell cycle arrest, apoptosis or DNA repair upon DNA damage. miR-100 and miR-421 can regulate expression of ATM, a

Roles of MicroRNA in DNA Damage and Repair 351

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11.

critical protein in DNA damage signalling. miR-24 suppresses gene expression of H2AX, an initial sensor protein for DNA damage response. miR-16 down-regulates the expression level of Wip1, an inhibitor of ATM/ATR-p53 DNA damage signalling pathway. miRNAs can mediate the activity of P53, a core component of the DNA damage response. miR-504 and miR-125b negatively regulate p53 expression. miR-34a, miR-29 and miR-122 can indirectly modify P53 activity by regulating the P53-related factors. miRNAs play important roles in different types of DNA repair. miR-21 down-regulates MMR proteins, MSH2 and MSH6, while miR-155 reduced the expression of the MMS genes MLH1, MSH2 and MSH6. miR-373 suppresses expression of RAD23B, a key component of the NER. miR-101 downregulates the protein level of DNA-PKcs, an essential factor for NHEJ. miR-210, miR-373 and miR-182 down-regulate the expression of RAD51, RAD52 and BRCA1, respectively. RAD51, RAD52 and BRCA1 are all key components of HRR. With increased studies of miRNAs' roles in DNA damage and repair, more miRNAs will be discovered to involve in the DNA damage and repair pathways.

#### **11. Acknowledgements**

The views presented in this article do not necessarily reflect those of the Food and Drug Administration. We would like to thank Dr. Barbara Parsons and Mr. Jian Yan for their review of this manuscript.

#### **12. References**


critical protein in DNA damage signalling. miR-24 suppresses gene expression of H2AX, an initial sensor protein for DNA damage response. miR-16 down-regulates the expression level of Wip1, an inhibitor of ATM/ATR-p53 DNA damage signalling pathway. miRNAs can mediate the activity of P53, a core component of the DNA damage response. miR-504 and miR-125b negatively regulate p53 expression. miR-34a, miR-29 and miR-122 can indirectly modify P53 activity by regulating the P53-related factors. miRNAs play important roles in different types of DNA repair. miR-21 down-regulates MMR proteins, MSH2 and MSH6, while miR-155 reduced the expression of the MMS genes MLH1, MSH2 and MSH6. miR-373 suppresses expression of RAD23B, a key component of the NER. miR-101 downregulates the protein level of DNA-PKcs, an essential factor for NHEJ. miR-210, miR-373 and miR-182 down-regulate the expression of RAD51, RAD52 and BRCA1, respectively. RAD51, RAD52 and BRCA1 are all key components of HRR. With increased studies of miRNAs' roles in DNA damage and repair, more miRNAs will be discovered to involve in the DNA

The views presented in this article do not necessarily reflect those of the Food and Drug Administration. We would like to thank Dr. Barbara Parsons and Mr. Jian Yan for their

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**Part 2** 

**Evolution of DNA Repair** 


**Part 2** 

**Evolution of DNA Repair** 

354 DNA Repair

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**19** 

*USA* 

**Meiosis as an Evolutionary** 

**Adaptation for DNA Repair** 

Harris Bernstein1, Carol Bernstein1 and Richard E. Michod2 *1Department of Cellular and Molecular Medicine, University of Arizona 2Department of Ecology and Evolutionary Biology, University of Arizona* 

The adaptive function of sex remains, today, one of the major unsolved problems in biology. Fundamental to achieving a resolution of this problem is gaining an understanding of the function of meiosis. The sexual cycle in eukaryotes has two key stages, meiosis and syngamy. In meiosis, typically a diploid cell gives rise to haploid cells. In syngamy (fertilization), typically two haploid gametes from different individuals fuse to generate a new diploid individual. A unique feature of meiosis, compared to mitosis, is recombination between non-sister homologous chromosomes. Usually these homologous chromosomes are derived from different individuals. In mitosis, recombination can occur, but it is ordinarily between sister homologs, the two products of a round of chromosome replication. Birdsell & Wills (2003) have reviewed the various hypotheses for the origin and maintenance of sex and meiotic recombination, including the hypothesis that sex is an adaptation for the repair of DNA damage and the masking of deleterious recessive alleles. Recently, we presented evidence that among microbial pathogens, sexual processes promote repair of DNA damage, especially when challenged by the oxidative defenses of their biologic hosts (Michod et al., 2008). Here, we present evidence that meiosis is primarily an evolutionary adaptation for DNA repair. Since our previous review of this topic (Bernstein et al., 1988), there has been a considerable increase in relevant information at the molecular level on the DNA repair functions of meiotic recombination, and this new information is emphasized in

**2. Meiosis in protists and simple multicellular eukaryotes is induced in** 

Eukaryotes appeared in evolution more than 1.5 billion years ago (Javaux et al., 2001). Among extant eukaryotes, meiosis and sexual reproduction are ubiquitous and appear to have been present early in eukaryote evolution. Malik et al. (2008) found that 27 of 29 tested meiotic genes were present in *Trichomonas vaginalis*, and 21 of these 29 genes were also present in *Giardia intestinalis*, indicating that most meiotic genes were present in a common ancestor of these species. Since these lineages are highly divergent among eukaryotes, these authors concluded that each of these meiotic genes were likely present in the common ancestor of all eukaryotes. Dacks and Roger (1999) also proposed that sex has a single

**response to stressful conditions that likely cause DNA damage** 

**1. Introduction** 

the present chapter.
