Huifang Dai

Fen1 mutations result in autoimmunity, chronic inflammation and cancers

Functional deficiency of the FEN1 gene has been suggested to cause genomic instability and cancer predisposition. We have identified a group of FEN1 mutations in human cancer specimens. Most of these mutations abrogated two of three nuclease activities of flap endonuclease 1 (FEN1). To demonstrate the etiological significance of these somatic mutations, we inbred a mouse line harboring the E160D mutation representing mutations identified in human cancers. Selective elimination of nuclease activities led to frequent spontaneous mutations and accumulation of incompletely digested DNA fragments in apoptotic cells. The mutant mice were predisposed to autoimmunity, chronic inflammation and cancers. The mutator phenotype results in the initiation of cancer, whereas chronic inflammation promotes the cancer progression. The current work exemplifies the approach of studying the mechanisms of individual polymorphisms and somatic mutations in cancer development, and may serve as a reference in developing new therapeutic regimens through the suppression of inflammatory responses.

Overexpression and Hypomethylation of Flap Endonuclease 1 Gene in Breast and Other Cancers

Flap endonuclease 1 (FEN1) is a structure-specific nuclease best known for its critical roles in Okazaki fragment maturation, DNA repair, and apoptosis-induced DNA fragmentation. Functional deficiencies in FEN1, in the forms of somatic mutations and polymorphisms, have recently been shown to lead to autoimmunity, chronic inflammation, and predisposition to and progression of cancer. To explore how FEN1 contributes to cancer progression, we examined FEN1 expression using 241 matched pairs of cancer and corresponding normal tissues on a gene expression profiling array and validated differential expression by quantitative real-time PCR and immunohistochemistry. Furthermore, we defined the minimum promoter of human FEN1 and examined the methylation statuses of the 5′ region of the gene in paired breast cancer tissues. We show that FEN1 is significantly up-regulated in multiple cancers and the aberrant expression of FEN1 is associated with hypomethylation of the CpG island within the FEN1 promoter in tumor cells. The overexpression and promoter hypomethylation of FEN1 may serve as biomarkers for monitoring the progression of cancers. (Mol Cancer Res 2008;6(11):1710–7)

Methylation of FEN1 suppresses nearby phosphorylation and facilitates PCNA binding

Flap endonuclease 1 (FEN1), a structure-specific endo- and exo- nuclease, exhibits multiple functions that determine essential biological processes, such as cell proliferation and cell death. As such, the enzyme must be precisely regulated in order to execute each of its functions with the right timing and in a specific subcellular location. Here, we report that FEN1 is methylated at arginine residues, primarily at R192. The methylation suppresses FEN1 phosphorylation at S187. The methylated form, but not the phosphorylated form of FEN1, strongly interacts with Proliferating Cell Nuclear Antigen (PCNA), ensuring the on and off timing of its reaction. Mutations of FEN1 disrupting arginine methylation and PCNA interaction result in unscheduled phosphorylation and cause failure of its localization to DNA replication or repair foci. This consequently leads to a defect in Okazaki fragment maturation, a delay of cell cycle progression, impairment of DNA repair, and high frequency of genome-wide mutations.

Mammalian DNA2 helicase/nuclease cleaves G‐quadruplex DNA and is required for telomere integrity

Efficient and faithful replication of telomeric DNA is critical for maintaining genome integrity. The G‐quadruplex (G4) structure arising in the repetitive TTAGGG sequence is thought to stall replication forks, impairing efficient telomere replication and leading to telomere instabilities. However, pathways modulating telomeric G4 are poorly understood, and it is unclear whether defects in these pathways contribute to genome instabilities in vivo. Here, we report that mammalian DNA2 helicase/nuclease recognizes and cleaves telomeric G4 in vitro. Consistent with DNA2s role in removing G4, DNA2 deficiency in mouse cells leads to telomere replication defects, elevating the levels of fragile telomeres (FTs) and sister telomere associations (STAs). Such telomere defects are enhanced by stabilizers of G4. Moreover, DNA2 deficiency induces telomere DNA damage and chromosome segregation errors, resulting in tetraploidy and aneuploidy. Consequently, DNA2‐deficient mice develop aneuploidy‐associated cancers containing dysfunctional telomeres. Collectively, our genetic, cytological, and biochemical results suggest that mammalian DNA2 reduces replication stress at telomeres, thereby preserving genome stability and suppressing cancer development, and that this may involve, at least in part, nucleolytic processing of telomeric G4.

Disruption of the FEN-1/PCNA Interaction Results in DNA Replication Defects, Pulmonary Hypoplasia, Pancytopenia, and Newborn Lethality in Mice

ABSTRACT The interaction between flap endonuclease 1 (FEN-1) and proliferation cell nuclear antigen (PCNA) is critical for faithful and efficient Okazaki fragment maturation. In a living cell, this interaction is probably important for PCNA to load FEN-1 to the replication fork, to coordinate the sequential functions of FEN-1 and other enzymes, and to stimulate its enzyme activity. The FEN-1/PCNA interaction is mediated by the motif 337QGRLDDFFK345 of FEN-1, such that an F343AF344A (FFAA) mutant cannot bind to PCNA but retains its nuclease activities. To determine the physiological roles of the FEN-1/PCNA interaction in a mammalian system, we knocked the FFAA Fen1 mutation into the Fen1 gene locus of mice. FFAA/FFAA mouse embryo fibroblasts underwent DNA replication and division at a slower pace, and FFAA/FFAA mutant embryos displayed significant defects in growth and development, particularly in the lung and blood systems. All newborn FFAA mutant pups died at birth, likely due to pulmonary hypoplasia and pancytopenia. Collectively, our data demonstrate the importance of the FEN-1/PCNA complex in DNA replication and in the embryonic development of mice.

Nucleolar Localization and Dynamic Roles of Flap Endonuclease 1 in Ribosomal DNA Replication and Damage Repair

ABSTRACT Despite the wealth of information available on the biochemical functions and our recent findings of its roles in genome stability and cancer avoidance of the structure-specific flap endonuclease 1 (FEN1), its cellular compartmentalization and dynamics corresponding to its involvement in various DNA metabolic pathways are not yet elucidated. Several years ago, we demonstrated that FEN1 migrates into the nucleus in response to DNA damage and under certain cell cycle conditions. In the current paper, we found that FEN1 is superaccumulated in the nucleolus and plays a role in the resolution of stalled DNA replication forks formed at the sites of natural replication fork barriers. In response to UV irradiation and upon phosphorylation, FEN1 migrates to nuclear plasma to participate in the resolution of UV cross-links on DNA, most likely employing its concerted action of exonuclease and gap-dependent endonuclease activities. Based on yeast complementation experiments, the mutation of Ser187Asp, mimicking constant phosphorylation, excludes FEN1 from nucleolar accumulation. The replacement of Ser187 by Ala, eliminating the only phosphorylation site, retains FEN1 in nucleoli. Both of the mutations cause UV sensitivity, impair cellular UV damage repair capacity, and decline overall cellular survivorship.

Human DNA polymerase β polymorphism, Arg137Gln, impairs its polymerase activity and interaction with PCNA and the cellular base excision repair capacity

DNA polymerase β (Pol β) is a key enzyme in DNA base excision repair, and an important factor for maintaining genome integrity and stability. More than 30% of human tumors characterized to date express DNA Pol β variants, many of which result from a single nucleotide residue substitution. However, in most cases, their precise functional deficiency and relationship to cancer susceptibility are still unknown. In the current work, we show that a polymorphism encoding an arginine to glutamine substitution, R137Q, has lower polymerase activity. The substitution also affects the interaction between Pol β and proliferating cell nuclear antigen (PCNA). These defects impair the DNA repair capacity of Pol β in reconstitution assays, as well as in cellular extracts. Expression of wild-type Pol β in pol β−/− mouse embryonic fibroblast (MEF) cells restored cellular resistance to DNA damaging reagents such as methyl methanesulfonate (MMS) and N-methyl-N-nitrosourea (MNU), while expression of R137Q in pol β−/− MEF cells failed to do so. These data indicate that polymorphisms in base excision repair genes may contribute to the onset and development of cancers.

The FEN1 E359K germline mutation disrupts the FEN1–WRN interaction and FEN1 GEN activity, causing aneuploidy-associated cancers

Polymorphisms and somatic mutations in Flap Endonuclease 1 (FEN1), an essential enzyme involved in DNA replication and repair, can lead to functional deficiencies of the FEN1 protein and a predisposition to cancer. We identified a FEN1 germline mutation that changed residue E359 to K in a patient whose family had a history of breast cancer. We determined that the E359K mutation, which is in the protein–protein domain of FEN1, abolished the interaction of FEN1 with Werner syndrome protein (WRN), an interaction that is critical for resolving stalled DNA replication forks. Furthermore, although the flap endonuclease activity of FEN1 E359K was unaffected, it failed to resolve bubble structures, which require the FEN1 gap-dependent endonuclease activity. To determine the etiological significance of E359K, we established a mouse model containing this mutation. E359K mouse embryonic fibroblasts (MEF) were more sensitive to DNA crosslinking agents that cause replication forks to stall. Cytological analysis suggested that the FEN1–WRN interaction was also required for telomere stability; mutant cell lines had fragile telomeres, increased numbers of spontaneous chromosomal anomalies and higher frequencies of transformation. Moreover, the incidence of cancer was significantly higher in mice homozygous for FEN1 E359K than in wild-type mice, suggesting that the FEN1 E359K mutation is oncogenic.

Okazaki fragment maturation involves α-segment error editing by the mammalian FEN1/MutSα functional complex

During nuclear DNA replication, proofreading‐deficient DNA polymerase α (Pol α) initiates Okazaki fragment synthesis with lower fidelity than bulk replication by proofreading‐proficient Pol δ or Pol ε. Here, we provide evidence that the exonuclease activity of mammalian flap endonuclease (FEN1) excises Pol α replication errors in a MutSα‐dependent, MutLα‐independent mismatch repair process we call Pol α‐segment error editing (AEE). We show that MSH2 interacts with FEN1 and facilitates its nuclease activity to remove mismatches near the 5′ ends of DNA substrates. Mouse cells and mice encoding FEN1 mutations display AEE deficiency, a strong mutator phenotype, enhanced cellular transformation, and increased cancer susceptibility. The results identify a novel role for FEN1 in a specialized mismatch repair pathway and a new cancer etiological mechanism.

Triptolide-assisted phosphorylation of p53 suppresses inflammation-induced NF-κB survival pathways in cancer cells

ABSTRACT Chronic inflammation plays important roles in cancer initiation and progression. Resolving chronic inflammation or blocking inflammatory signal transduction may prevent cancer development. Here, we report that the combined low-dose use of two anti-inflammatory drugs, aspirin and triptolide, reduces spontaneous lung cancer incidence from 70% to 10% in a mouse model. Subsequent studies reveal that such treatment has little effect on resolving chronic inflammatory conditions in the lung, but it significantly blocks the NF-κB-mediated expression of proliferation and survival genes in cancer cells. Furthermore, triptolide and aspirin induce distinct mechanisms to potentiate each other to block NF-κB nuclear localization stimulated by inflammatory cytokines. While aspirin directly inhibits IκB kinases (IKKs) to phosphorylate IκBα for NF-κB activation, triptolide does not directly target IKKs or other factors that mediate IKK activation. Instead, it requires p53 to inhibit IκBα phosphorylation and degradation. Triptolide binds to and activates p38α and extracellular signal-regulated kinase 1/2 (ERK1/2), which phosphorylate and stabilize p53. Subsequently, p53 competes with IκBα for substrate binding to IKKβ and thereby blocks IκBα phosphorylation and NF-κB nuclear translocation. Inhibition of p38α and ERK1/2 or p53 mutations could abolish the inhibitory effects of triptolide on NF-κB. Our study defines a new p53-dependent mechanism for blocking NF-κB survival pathways in cancer cells.