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EGFR Inhibits DNA Mismatch Repair

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EGFR Inhibits DNA Mismatch Repair COMMENTARY EGFR inhibits DNA mismatch repair Peggy Hsieh1 and Alexander H. Pearlman IDLs of 1–2 nucleotides, whereas MutSβ pref- Genetics & Biochemistry Branch, National Institute of Diabetes and Digestive and Kidney erentially targets IDLs. MutSα and MutSβ Diseases, National Institutes of Health, Bethesda, MD 20892 scan the helix until a mismatch is encoun- tered. This search occurs in the context of the Safeguarding the integrity of the genome andtranscriptionandislinkedtopoorprog- advancing replication fork and PCNA, using the PCNA-interacting protein boxes of MSH6 should not be left to chance. Indeed, all or- noses in several malignancies, but molecular α β ganismshavehighlyeffectivemechanisms mechanisms are largely unknown (7, 8). In the and MSH3. MutS and MutS bind to mis- matches and bend the DNA using distinct to detect and remove errors and lesions in nucleus, EGFR phosphorylates PCNA, increas- molecular interactions. Differential binding DNA. DNA mismatch repair (MMR) serves ing its stability (9). This would be expected to and hydrolysis at two nonequivalent nucle- as the final safeguard in assuring the fidelity have important consequences, but the func- otide binding sites license conformational of DNA replication from bacteria to humans tional significance was unclear until now. In changes that turn MutS proteins into slid- and backstops the nucleotide selection and PNAS, Ortega et al. reveal that phosphoryla- ing clamps that diffuse along the DNA. exonuclease proofreading activities of repli- tion of PCNA by EGFR inhibits MMR in cells Nucleotide binding also mediates interaction cative polymerases (Fig. 1) (1–3). The loss of and switches DNA synthesis in vitro from high- with a second conserved MMR protein, MutLα, MMR results in greatly elevated rates of spon- fidelity to error-prone (Fig. 2) (10). This inter- a heterodimer of two MutL homologs, MLH1 taneous mutation, is the underlying cause of section between MMR and EGFR underscores the importance of understanding the nuclear and PMS2, and the formation of a poorly de- Lynch syndrome colon cancer, and is impli- α β roles of EGFR and other RTKs, and highlights fined ternary complex of MutS or MutS , cated in a subset of sporadic tumors. In its MutLα,andDNA. postreplication capacity, MMR functions dur- the relationship between the control of cell pro- liferation and the maintenance of genome sta- Structural studies reveal that MLH1 and ing S phase in close association with the rep- PMS2 dimerize through their C termini and lication fork. This process is mediated by bility, two central aspects of tumorigenesis. Although replication is remarkably faith- upon ATP binding at an N-terminal ATP- proliferating cell nuclear antigen protein binding domain form a DNA clamp (13). (PCNA), a lynchpin in regulating access and ful, replicative polymerases incorporate an incorrect nucleotide leading to premutagenic, Notably, PMS2 harbors a PCNA-activated activity of DNA polymerases and other pro- non-Watson–Crick base pairs, such as G:T, endonuclease activity that contributes to teins involved in replication, repair, and re- about once every 104 to 105 times. In addi- DNA excision by providing additional sites combination (4). EGFR, a member of the ′ ′ tion, during replication through mono- and of entry for EXO1, a strict 5 to 3 exonuclease HER family of receptor tyrosine kinases dinucleotide repeats, the template strand can (14, 15). In this way MMR is bidirectional and (RTKs) that span the plasma membrane, misalign, forming insertion/deletion loops can be initiated from a nick on either side of regulates cell proliferation, differentiation, (IDLs) that yield insertion or deletion muta- the mismatch. The net result of excision is a and motility in response to extracellular tions. How do cells mitigate replication er- single-strand gap that is filled in with the aid ligands(e.g.,EGF)(5,6).Ligandbinding rors? A proofreading exonuclease activity of PCNA by a high-fidelity replicative poly- δ e promotes dimerization and allosteric activa- that is an integral component of replica- merase, Pol or Pol ,andligatedtorestorean tion of cytoplasmic kinase domains. Auto- tive polymerases removes misincorporated error-free duplex. phosphorylation initiates signaling cascades nucleotides, but it’s not ironclad. More prob- Essential to MMR is excision targeted to the involving RAS/MAPK, PI(3)K/Akt, Jak/STAT, lematic are IDLs that are likely to escape new strand containing the error and not the and others. EGFR also translocates to the nu- proofreading altogether. MMR serves as the template strand. PCNA has a critical role. cleus where it influences replication, repair, final gatekeeper, excising errors in new daugh- PCNA is a trimer of identical subunits arranged ter strands and giving replicative polymerases head-to-tail in a ring that encircles DNA (4). It a second chance in a gap-filling step. The loss loads near single-strand and double-strand of MMR by mutation or epigenetic silencing DNA junctions with the help of replication increases spontaneous mutation by 50- to factor C. PCNA is always oriented on DNA 1,000-fold and is a driver of malignancy in such that the same side of the trimer faces Lynch syndrome colon cancer and a subset the 3′ terminus. Thus, interactions of MMR of sporadic cancers (11, 12). proteins with PCNA fix their orientation on PCNA is a processivity factor for poly- DNA, and the catalytic activity of PMS2 is merases and likely functions in MMR at directed to the newly synthesized strand. mismatch recognition, excision on the newly Ortega et al. use in vitro assays to dissect synthesized strand, and DNA synthesis by the effects of PCNA-pY211 on MMR in replicative polymerases (1, 3). In eukaryotes, mismatch repair is initiated by conserved Author contributions: P.H. and A.H.P. wrote the paper. Fig. 1. Scheme for repair of replication errors. (A)Mis- mismatch binding proteins related to bacte- The authors declare no conflict of interest. incorporation of dT opposite dG by DNA polymerase leads to α rial MutS: MutS ,aheterodimerofMSH2- See companion article on page 5667. G:T mispair. (B) Template slippage at A5 repeat leads to +2 β 1 IDL (T7). MMR targets the newly synthesized strand (blue) to MSH6, and MutS ,aheterodimerofMSH2- To whom correspondence should be addressed. Email: peggyh@ restore the parental sequence. MSH3. MutSα targets base-base mispairs and niddk.nih.gov. 5556–5557 | PNAS | May 5, 2015 | vol. 112 | no. 18 www.pnas.org/cgi/doi/10.1073/pnas.1505168112 Downloaded by guest on September 28, 2021 by EGFR modulates DNA synthesis and COMMENTARY repairandiscorrelatedwithincreasedlevels K of i-67 proliferation marker in human breast tumors (16). DNA-PK and TIP60, an acety- lase and coactivator of ataxia telangiectasia mutated (ATM), may also be targets (17). Be- cause EGFR-directed signaling pathways are broad in scope and ubiquitous, differentiating direct vs. indirect responses is challenging; the identification of specific residues phosphory- lated by EGFR is critical. What triggers nuclear transport of EGFR and related T-cell receptors? One model features retrograde trafficking of EGFR bound to importin-β in endocytic vesi- cles, transit through the Golgi apparatus and endoplasmic reticulum, and nuclear entry via the nuclear pore complex and Sec61 β (8). How does ionizing radiation signal nuclear transport? Is it via chemical/ physical modification at the cell membrane: proteins or lipids? Is it a DNA damage sig- nal from the nucleus? And, what is the re- Fig. 2. A model for nuclear transport of ligand-bound EGFR and phosphorylation of PCNA. Retrograde trafficking of lationship between nuclear transport and EGFR delivers EGFR to the nucleus where it phosphorylates PCNA at Tyr211. Ortega et al. reveal that PCNA-pY211 inhibits endocytosis-mediated recycling and deg- MMR in colorectal cancer cell lines and promotes mutagenic DNA synthesis in vitro (10). ER, endoplasmic reticulum; INM, radation of RTKs vital to their regulation inner nuclear membrane; NPC, nuclear pore complex; ONM, outer nuclear membrane; RFC, replication factor C. (8, 18)? Understanding the intersection between cell proliferation and genome sta- several MMR-deficient colorectal cancer geometry, threatening many PCNA in- bility will reveal much about what diverts cell lines with elevated levels of EGFR, and teractions (10). Is DNA synthesis affected EGFR from normal paths to more reck- observe a correlation between levels of EGFR, in replication, translesion synthesis repair, less ones. PCNA-pY211, and MMR loss (10). Restora- or recombination? Additional in vitro and tion of MMR in these colorectal cancer cells in vivo studies are required. ACKNOWLEDGMENTS. This work was supported by the Division of Intramural Research, National Institute of requires the addition of MMR proteins What is the spectrum of nuclear EGFR Diabetes and Digestive and Kidney Diseases, National and unphosphorylated PCNA. PCNA-pY211 targets? Histone H4 phosphorylation at Tyr72 Institutes of Health. binds poorly to MutSα and MutSβ,sug- gesting that their recruitment to DNA by 1 Kunkel TA, Erie DA (2005) DNA mismatch repair. Annu Rev 10 Ortega J, et al. (2015) Phosphorylation of PCNA by EGFR inhibits PCNA might be compromised. PCNA iso- Biochem 74:681–710. mismatch repair and promotes misincorporation during DNA forms mimicking constitutively phosphorylated 2 Hsieh P, Yamane K (2008) DNA mismatch repair: Molecular synthesis. Proc Natl Acad Sci USA 112:5667–5672. PCNA-Y211, -Y211D and -Y211E, inhibit mechanism, cancer, and ageing. Mech Ageing Dev 129(7-8): 11 Hewish M, Lord CJ, Martin SA, Cunningham D, Ashworth A 391–407. MutLα endonuclease activity on mismatched (2010) Mismatch repair deficient colorectal cancer in the era of 3 Jiricny J (2013) Postreplicative mismatch repair. Cold Spring Harb personalized treatment. Nat Rev Clin Oncol 7(4):197–208. DNA, and PCNA-pY211 appears to direct Perspect Biol 5(4):a012633. 12 Peltomäki P (2014) Epigenetic mechanisms in the pathogenesis error-prone synthesis in vitro. These find- 4 Mailand N, Gibbs-Seymour I, Bekker-Jensen S (2013) Regulation of of Lynch syndrome.
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