Note for: DNA double-strand break repair in a cellular context

Note for: DNA double-strand break repair in a cellular context

(doi: 10.1016/j.clon.2014.02.004)

Tumour cells not only gain unlimited proliferative capacity, but the ability to adapt to a constantly changing microenvironment. DDR is modified to serve the cancerous phenotype, and if we understand the reasons behind we can get the right target to against the cancer.

Non-homologous end-joining (NHEJ) represents the major DNA double-strand break (DSB) repair pathway in mammalian cells. DNA-PKcs undergoes autophosphorylation at clustered sites. DNA-PKcs undergoes autophosphorylation at clustered sites. End-processing can involve the Artemis nuclease, polynucleotide kinase 3' phosphatase and polymerases, including pol-lamda or pol-mu.

Whereas NHEJ has beauty in its simplicity, homologous recombination’s elegance lies in its complexity and its exploitation of an undamaged homologous template to restore any lost sequence information.

One such process is alternative NHEJ (Alt-NHEJ), which occurs predominantly when NHEJ is compromised by the loss of Ku or LigIV. Alt-NHEJ requires CtIP-dependent resection, DNA ligase I or III, XRCC1 and PARP1. represents two ssDNA rejoining events using microhomology to tether the ends.

Homologous recombination only functions after replication in late S/G2 phase. homologous recombination’s major role is to promote recovery from replication fork stalling/collapse when the lesion may be a one-ended DSB or a region of ssDNA. The DNA damage complexity and the highly compacted (heterochromatic) chromatin status are two factors that can promote a switch from NHEJ to homologous recombination.

The phosphatidylinositol 3- kinase-related kinase, ataxia telangiectasia mutated (ATM) lies at the core of DDR signalling. NHEJ predominantly occurs independently of DDR signalling and vice versa. RNF168 is critical for the formation of 53BP1 foci, although the process is not fully understood.

The G1/S checkpoint involves p53-dependent transcriptional upregulation of the CDK inhibitor, p21, which prevents retinablastoma protein (Rb) phosphorylation. ATM signalling is the activation of apoptosis. Only certain tissues activate apoptosis after DNA damage, which are predominantly those that exploit apoptosis to remove unwanted cells during development, such as haematopoietic cells. Nonetheless, the downregulation of apoptosis is a significant step promoting carcinogenesis and is a component of DDR signalling that it frequently downregulated in tumour cells.

A major outcome of DDR signalling is a change to the chromatin in the DSB vicinity. most DSBs are repaired efficiently in G1 and G2 in the absence of ATM. Enumeration of gH2AX foci in G0/G1 primary fibroblasts, a sensitive assay for DSB repair, revealed a 10e15% subset of DSBs, whose repair requires ATM and DDR signalling proteins. Subsequently, ATM was shown to phosphorylate the heterochromatic factor, KRAB-ZFPassociated protein-1, causing release of the larger isoform of CHD3/Mi2a, a component of the NURD remodelling complex, from chromatin and hence localized heterochromatic relaxation. Furthermore, fibroblasts derived from patients with defects in heterochromatic proteins (disordered chromatin disorders) have a diminished requirement for ATM for DSB repair.

Homologous recombination exerts its major role in promoting recovery from replication stalling/collapse, where it is predominantly regulated by the phosphatidylinositol 3-kinase-related kinase, ATR. Resection proceeds, the ssDNA generated promotes a switch to ATR activation.

A-T cells not only harbour persistent unrepaired DSBs, but also increased levels of mis-rejoined DSBs, detectable as translocations. ATMplays an important role in promoting accurate repair. Thus, ATM and DDR signalling probably serve to enhance the usage of non-resection-mediated repair but allow resection to take place in a controlled manner when required. required. Further studies are required to elucidate exactly how ATM promotes accurate repair.

Exploitation of a system to induce a site specific DSB close to a transcriptionally active gene has shown that transcription is inhibited via an ATM and RNF8-dependent process.

Cancer cells elevated genome instability but they must compromise with proliferative capacity. a few pertinent comments will be made. First, there is well-documented evidence that ATM and other DDR signalling proteins are downregulated in tumour cells to preclude apoptosis or diminish cell cycle checkpoint arrest, changes that affect genomic instability with a minimal (and potentially enhanced) impact on survival. NHEJ has a major role in promoting recovery to DSBs, tumour cells rarely completely inactivate NHEJ. As discussed above, we have proposed that cells initially attempt to repair DSBs by accurate, resection-independent NHEJ, but that a switch to resection-mediated processes occurs if NHEJ does not rapidly ensue. We have also observed that DSBs more readily undergo resection in G2 phase tumour cells compared with primary cells. Moreover, some tumours have been reported to display upregulated Alt-NHEJ (and downregulated NHEJ). tumour cells modify DSB rejoining processes to promote genomic instability. While the downregulation of apoptosis and

checkpoint arrest is one aspect, there is increasing evidence that the nature of the DSB rejoining process can become altered, most particularly to promote resection and translocation formation. Understand how these processes are regulated and, importantly, the effect of factors such as chromatin, transcription and replication.