Ding and Shen Human RPA is a heterotrimeric, single stranded DNA- binding protein required for DNA replication, recombina- tion, and repair (Wold 1997; Brosh et al 1999, 2002). RPA directly interacts with to WRN, markedly increasing the DNA helicase activity of WRN (Brosh et al 1999; Shen et al 2003) and its ability to unwind forked telomeric DNA structures (Ohsugi et al 2000; Opresko et al 2001). After DNA damage, WRN can colocalize with BLM and RPA (Constantinou et al 2000; Bischof et al 2001; Sakamoto et al 2001). RPA binds to WRN and BLM to stimulate their unwinding of long DNA duplexes (Bachrati and Hickson 2003). Another interesting replication protein, FEN-1, interacts with the 144-amino acid RQC domain on the C-terminal region of WRN (Brosh et al 2001c). FEN-1, a DNA structure-speciﬁc nuclease, participates in pathways of DNA metabolism that are important for genomic stability (Brosh et al 2002) and is involved in the maturation of Okazaki fragments during lagging strand DNA replication (Bambara et al 1997; Merrill and Holm 1998). WRN stimulates FEN-1-mediated cleavage activity of displaced ﬂaps that occur during lagging strand DNA synthesis at Okazaki fragments (Brosh et al 2001c; Sharma et al 2004). WRN-FEN-1 complex colocalizes in foci associated with arrested replication forks, and this complex plays a role in the unwinding and degradation of Holliday junction struc- tures associated with regressed replication forks (Sharma et al 2004). Defective Okazaki fragment processing causes DSBs, which may lead to the genomic instability in WS (Brosh et al 2002). WRN colocalizes with, and directly interacts with, human topo I (Laine et al 2003). WRN stimulates the ability of topo I to relax negatively supercoiled DNA and speciﬁcally stimulates the religation step of the relaxation reaction, and cell extracts from WS ﬁbroblasts exhibit a decreased ability to unwind negatively supercoiled DNA (Laine et al 2003). These ﬁndings provide the interrelationship between WRN helicases and topoisomerases in the maintenance of genomic integrity. DNA repair WS cells display sensitivity to 4-nitroquinoline 1-oxide (Ogburn et al 1997; Poot et al 2002), a carcinogen which causes the formation of DNA strand breaks and bulky DNA adducts (Nagao and Sugimura 1976), and are also hypersen- sitive to O6-methylguanine, a site-speciﬁc alkylating agent that can block DNA replication (Blank et al 2004). WS cells are sensitive to DNA crosslinking drugs (Poot et al 2001, 434 2002) and, since DNA crosslinks are normally repaired by homologous repair (HR), this suggests a defect in this repair pathway. WS cells display extensive deletions at nonhomolo- gous joined ends (Oshima et al 2002; Chen et al 2003), while expression of wild-type WRN prevents excessive DNA dele- tions (Oshima et al 2002; Chen et al 2003). These ﬁndings suggest that lack of WRN may increase DNA damage and disrupt the regulatory processes controlling DNA repair, for example, nonhomologous end joining (NHEJ), HR, and base excision repair (BER). Double-strand break repair The ﬁrst evidence for a link between WRN and double-strand break repair was the discovery that WRN interacts with both Ku and DNA-dependent protein kinase catalytic subunits (DNA-PKcs) (Cooper et al 2000; Li and Comai 2000; Cheng et al 2003; Opresko et al 2003) to participate in NHEJ (Li and Comai 2002). Assembly of DNA-PK and WRN at DNA ends allows DNA-PKcs to phosphorylate WRN, thus stimulating WRN enzymatic activity and facilitating efﬁcient processing of double-strand breaks (DSBs) prior to ligation (Yannone et al 2001). Deﬁciencies in either Ku or DNA-PKcs result in sensitivity to ionizing radiation due to defects in DSB repair. Complementation with the exonuclease/helicase double mutant or wild-type WRN restores NHEJ activity, suggesting that WRN is necessary for normal repair of DSBs (Chen et al 2003). Chromatographic studies showed that WRN is bound to PARP-1 in a complex that contains Ku70/80 (Li et al 2004). PARP-1 can induce apoptosis or necrosis in cells with extensive DNA damage. Absence of functional WRN prevents activation of PARP-1 in response to DNA damage caused by oxidative stress and alkylating agents (von Kobbe et al 2003b). Some proteins that participate in the recombinational repair pathway have been found to functionally interact with WRN. WRN is involved in resolving recombination intermediates in RAD51-dependent HR (Prince et al 2001; Saintigny et al 2002) and forms distinct nuclear foci that partially overlap with the RAD51 nuclear foci formed in response to DNA damage (including DSBs) (Sakamoto et al 2001). In vivo data show that WRN interacts function- ally with NBS1 (Cheng et al 2004), which is thought to act downstream of RAD51 (Saintigny et al 2002; Tauchi et al 2002; Monnat and Saintigny 2004). WRN colocalizes with, and interacts with, RAD52 (Baynton et al 2003), NBS1 in the MRN complex (Cheng et al 2004), and ﬂap endonuclease 1 (FEN-1) (Brosh et al 2001c; Sharma et al 2004). In addition, it Clinical Interventions in Aging 2008:3(3)