**6. Conclusion**

The chromatin remodeling in spermatids involves transient DNA-strand breaks. Our group has generated evidences that a significant number of double-strand breaks are generated. These DSBs seemingly trigger a damage response as H2AFX is phosphorylated and a DNA repair pathway yet to be identified. As a result, no such DSBs are found in the mature sperm unless a pathological condition prevails. The non-templated DNA repair of these transient DSBs are expected to introduce small mutations likely distributed randomly across the haploid genome although their distribution remains to be established. Meiosis is well known to produce new combination of alleles but is not a primary driver of sequence divergence (Noor, 2008). Potential new gene function must arise through point mutations or indels and the present review suggests chromatin remodeling of spermatids as an appropriate context for such induction of new polymorphism and possible translocations. Although the frequency of mutation may be lower in germ cells than in somatic cells (Walter et al., 1998), we hypothesize that most of the new mutations generated during spermatogenesis may be through the process of endogenous strand breaks and repair during spermiogenesis. Owing to the 1% chance for a random mutation to occur within genes due to exonic representation in the genome, most mutations are expected to be silent but, if within coding sequences, potential alteration of gene function may be transmitted. In summary, repair of the endogenous DSBs in spermatids may represent a new male-driven source of genetic variation.

### **7. References**

316 DNA Repair

Caddle et al., 2007; Manoj S Gandhi and Nikiforov, 2009). Thus, chromosomes with close chromosomal territories in spermatids could be more prone to interchromosomal translocation during chromatin remodeling. In addition, the potential for the spermatidal chromatin remodeling to produce non-B DNA structure may exacerbate the propensity for spermatids to produce translocation (Hidehito Inagaki and Kurahashi, 2009; Raghavan and Lieber, 2004). Further investigation is needed on the mechanism and potential involvement

As outlined above, NHEJ repair pathways are known to create insertions and deletions (indels) as they use microhomology to join the two DNA ends. This type of mutations may be particularly harmful in coding sequences as it may cause codon frameshifts or alteration of mRNA splicing. Moreover, Y chromosomes microdeletions, associated with increased male infertility, may exhibit the classical signature of micro-homology based DNA repair pathways such as SSA and B-NHEJ as the deletion occurs between repetitive, often palindromic sequences (Paulo Navarro-Costa and Plancha; Yen, 1998). Although SSA is available during most of spermatogenesis, the B-NHEJ signature on the highly repetitive Y chromosome may be indicative of a faulty DNA repair in spermatids as this pathway is

Several diseases with dynamic mutation, characterized by the expansion over generation of a trinucleotidic repeat (TNRs), are associated with a paternal bias of expansion, such as Huntington disease (HD), spinocerebellar ataxia type 2 and 7 (Cancel et al., 1997; Stevanin et al., 1998; Zühlke et al., 1993). TNRs are microsatellites sequences in coding or non-coding region of the genome. Their stability, which is relative to the chance of adopting a secondary structure, is dependent of the nature of the sequence and the length of the TNR (Kovtun et al.; Tóth et al., 2000). The exact mechanism of TNR expansion or contraction is still not clear. However, studies show strong evidences that TNR expansion in the huntingtin gene occurs during spermiogenesis; in a transgenic mouse model carrying the mutated human gene, an increased length of the CAG repetition was observed in mature spermatozoa but not in early haploid spermatids and other tissues. Kovtun and McMurray also demonstrated the involvement of MSH2, a protein involved in the gap repair and mismatch repair pathways, as this expansion was absent in HD mice MSH2-/- (Albin and Tagle, 1995; Kovtun and McMurray, 2001). The remodeling chromatin of spermatids may promote secondary

The chromatin remodeling in spermatids involves transient DNA-strand breaks. Our group has generated evidences that a significant number of double-strand breaks are generated. These DSBs seemingly trigger a damage response as H2AFX is phosphorylated and a DNA repair pathway yet to be identified. As a result, no such DSBs are found in the mature sperm unless a pathological condition prevails. The non-templated DNA repair of these transient DSBs are expected to introduce small mutations likely distributed randomly across the haploid genome although their distribution remains to be established. Meiosis is well known to produce new combination of alleles but is not a primary driver of sequence divergence (Noor, 2008). Potential new gene function must arise through point mutations or

structure formation at TNRs, providing an ideal context for such mutations.

of chromatin remodeling in such events.

**Insertions and deletions** 

inhibited during meiosis. **Dynamic mutations** 

**6. Conclusion** 


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**1. Introduction** 

interaction in the repair of DSBs.

transformation.

**17** 

Erika T. Brown

*United States* 

*BRCA2* **Mutations and** 

*Medical University of South Carolina* 

**Consequences for DNA Repair** 

The *BRCA2* gene was the second gene discovered to be associated with early-onset, familial breast cancer. The BRCA2 protein is expressed in breast, ovarian, prostate, and pancreatic tissues and is associated with cancer predisposition in all four, with breast cancer being the most predominant (Goggins, Schutte et al. 1996). BRCA2 is functionally defined as a tumor suppressor and is most critical in maintenance of genomic integrity and DNA repair fidelity. The importance of BRCA2 in maintaining genomic integrity is based on its function to specifically repair double-strand DNA breaks (DSBs) via the process of homologous recombination (HR). However, BRCA2 resolves genomic lesions in concert with a number of DNA repair proteins, the most significant being RAD51 (Sharan, Morimatsu et al. 1997),(Yuan, Lee et al. 1999). RAD51 is a recombinase that is highly conserved, having homologues in *E. coli* and yeast, as well as in mammals. BRCA2 modulates the activity of RAD51 during DNA repair, and they both are found in nuclear DNA damage-induced foci, which are complexes of DNA repair proteins bound to DNA during the process of repair (Roth, Porter et al. 1985; Roth and Wilson 1986; Derbyshire, Epstein et al. 1994; Jackson and Jeggo 1995; Takata, Sasaki et al. 1998; Johnson and Jasin 2000). The relationship between BRCA2 and RAD51 has been determined to be a fundamental

The role of BRCA2 as a tumor suppressor has been established by its importance in maintaining genomic integrity. The inability of the cell to repair DSBs can potentially cause small-scale lesions in regions of the DNA that encode single genes and incite large-scale lesions, such as chromosomal anomalies. The consequence of such damage can disrupt the normal expression of gene products that are required to regulate cell growth and arrest and induce apoptosis, thereby establishing a cellular environment that can foster malignant

Cancer cells that express mutated BRCA2 have been shown to have elevated sensitivity to the anti-cancer therapeutics called PARP (Poly [ADP-ribose] polymerase) inhibitors. PARP inhibitors prevent the binding of PARP to sites of damaged DNA, which serves as a signal to initiate DNA repair (Schreiber, Dantzer et al. 2006); (Ratnam and Low 2007). The effectiveness of PARP inhibitors in BRCA2-mutated cells is based on the premise of synthetic lethality, which is when two pathway defects alone are innocuous, but combined become lethal (Ratnam and Low 2007). The unresolved DSBs of BRCA2-mutated cells combined with the inhibition of PARP activity are effective in promoting DNA damage-

