Trends in Biochemical Sciences
ReviewTidying up loose ends: the role of polynucleotide kinase/phosphatase in DNA strand break repair
Section snippets
Polynucleotide kinase/phosphatase: an essential enzyme for the repair of damaged DNA termini
Damage to cellular DNA is considered to be a significant factor in aging [1], cancer etiology and treatment 2, 3, 4, and neurological disorders [5]. DNA damage, in the form of base modification, base loss and strand breaks, can be triggered by intracellular agents, primarily reactive oxygen species (ROS), as well as by exogenous agents. As a result, cells have evolved a battery of repair pathways to counter the mutational and cytotoxic consequences of DNA damage [6]. Strand breaks can be
Molecular architecture of PNKP
PNKP is a multidomain enzyme that consists of an N-terminal forkhead-associated (FHA) domain and a C-terminal catalytic domain composed of fused phosphatase and kinase subdomains. The FHA domain is linked to the catalytic domain through a flexible polypeptide segment and acts to selectively bind acidic casein kinase 2 (CK2)-phosphorylated regions in XRCC1 [14] and XRCC4 [15], which are key scaffolding proteins in the repair of DNA SSBs and DSBs, respectively. The DNA repair factors aprataxin
PNKP and single-strand break repair (SSBR)
SSBR is a multienzyme pathway that employs different players depending on the causative agent. In the case of IR-induced strand breaks, which typically involve the loss of at least one nucleotide, the initial process of damage recognition and correction of the strand break termini is primarily carried out by poly(ADP-ribose) polymerase (PARP), XRCC1, AP endonuclease I (APE1) and/or PNKP, although other proteins, such as tyrosyl-DNA phosphodiesterase (TDP1) and aprataxin might provide backup
PNKP and base excision repair (BER)
BER is responsible for the repair of most minor base modifications induced by IR, ROS and alkylating agents. The first step is removal of the modified base by DNA glycosylase followed by cleavage of the DNA at the newly formed apurinic/apyrimidinic (AP) site by APE1 [44]. Alternatively, the glycosylase might have AP lyase activity that hydrolyzes the AP site [44]. The involvement of PNKP in the BER pathway became evident after the discovery of the nei endonuclease VIII-like-1 (NEIL1) and NEIL2
PNKP and double-strand break repair (DSBR)
Of the two major DSBR pathways, there is clear evidence for PNKP participation in nonhomologous end joining (NHEJ) 15, 47, 48, but the failure to influence IR-induced sister chromatid exchange by PNKP depletion suggests that PNKP is not involved in homologous recombination [48]. PNKP also plays a role in a back-up, XRCC1-dependent, DSB repair pathway [49]. Evidence for PNKP participation in NHEJ came initially from experiments using human cell-free extracts, showing that the PNKP kinase
Physiological roles and clinical potential of PNKP
The increased interest in PNKP reflects its involvement in several DNA repair pathways that protect cells from endogenous and exogenous genotoxic agents. Disruption of NHEJ genes and SSBR/BER genes are known to cause neurological disorders with various symptoms; for example, microcephaly is seen in individuals with mutations in LIG4 (which encodes DNA ligase IV) [53], whereas deletion of Xrcc1 in mice causes seizures [54]. A recent report established that a severe neurological autosomal
Concluding remarks
PNKP is a key enzyme in the cellular processing of stand break termini and not surprisingly participates in several DNA repair pathways (summarized in Figure 3). Further progress is required to clarify its regulation, interactions with other repair enzymes and physiological role in neurons and other tissues. Because of its involvement in a variety of repair pathways, PNKP is now regarded as a therapeutic target in the treatment of cancer and thus new inhibitory compounds will need to be
Acknowledgments
We thank Dr Susan Lees-Miller (University of Calgary) for critically reading the manuscript. This work was supported by grants from the Canadian Institutes of Health Research, the Canadian Cancer Society, the Alberta Cancer Foundation, and the National Institutes of Health. J.N.M.G. acknowledges the support of the Howard Hughes International Scholar program.
References (74)
DNA repair of oxidative DNA damage in human carcinogenesis: potential application for cancer risk assessment and prevention
Cancer Lett.
(2008)- et al.
The emerging role of DNA repair proteins as predictive, prognostic and therapeutic targets in cancer
Cancer Treat. Rev.
(2005) - et al.
3′-Phosphatase activity of the DNA kinase from rat liver
Biochem. Biophys. Res. Commun.
(1982) Molecular cloning of the human gene, PNKP, encoding a polynucleotide kinase 3′-phosphatase and evidence for its role in repair of DNA strand breaks caused by oxidative damage
J. Biol. Chem.
(1999)Molecular characterization of a human DNA kinase
J. Biol. Chem.
(1999)Targeting DNA repair pathways: a novel approach to reduce cancer therapeutic resistance
Cancer Treat. Rev.
(2009)The protein kinase CK2 facilitates repair of chromosomal DNA single-strand breaks
Cell
(2004)The ataxia-oculomotor apraxia 1 gene product has a role distinct from ATM and interacts with the DNA strand break repair proteins XRCC1 and XRCC4
DNA Repair (Amst.)
(2004)Structure of a tRNA repair enzyme and molecular biology workhorse: T4 polynucleotide kinase
Structure
(2002)The molecular architecture of the mammalian DNA repair enzyme, polynucleotide kinase
Mol. Cell
(2005)