Jiang Cheng Shen

The Processing of Holliday Junctions by BLM and WRN Helicases Is Regulated by p53

BLM, WRN, and p53 are involved in the homologous DNA recombination pathway. The DNA structure-specific helicases, BLM and WRN, unwind Holliday junctions (HJ), an activity that could suppress inappropriate homologous recombination during DNA replication. Here, we show that purified, recombinant p53 binds to BLM and WRN helicases and attenuates their ability to unwind synthetic HJ in vitro. The p53 248W mutant reduces abilities of both to bind HJ and inhibit helicase activities, whereas the p53 273H mutant loses these abilities. Moreover, full-length p53 and a C-terminal polypeptide (residues 373–383) inhibit the BLM and WRN helicase activities, but phosphorylation at Ser376 or Ser378 completely abolishes this inhibition. Following blockage of DNA replication, Ser15 phospho-p53, BLM, and RAD51 colocalize in nuclear foci at sites likely to contain DNA replication intermediates in cells. Our results are consistent with a novel mechanism for p53-mediated regulation of DNA recombinational repair that involves p53 post-translational modifications and functional protein-protein interactions with BLM and WRN DNA helicases.

Unwinding the molecular basis of the Werner syndrome

Werner syndrome (WS) is an autosomal recessive disease manifested by the premature onset of age-related phenotypes, including diseases such as atherosclerosis and cancer. This mimicry of normal aging with the possible exception of central nervous system manifestations has made it a focus of recent molecular studies on the pathophysiology of aging. In culture, cells obtained from patients with WS are genetically unstable, characterized by an increased frequency of nonclonal translocations and extensive DNA deletions. The WS gene product (WRN) is a DNA helicase belonging to the RecQ family, but is unique within this family in that it also contains an exonuclease activity. In addition to unwinding double-stranded DNA, WRN helicase is able to resolve aberrant DNA structures such as G4 tetraplexes, triplexes and 4-way junctions. Concordant with this structure-specificity, WRN exonuclease preferentially hydrolyzes alternative DNA that contains bubbles, extra-helical loops, 3-way junctions or 4-way junctions. WRN has been shown to bind to and/or functionally interact with other proteins, including replication protein A (RPA), proliferating cell nuclear antigen (PCNA), DNA topoisomerase I, Ku 86/70, DNA polymerase delta and p53. Each of these interacting proteins is involved in DNA transactions including those that resolve alternative DNA structures or repair DNA damage. The biochemical activities of WRN and the functions of WRN associated proteins suggest that in vivo WRN resolves DNA topological or structural aberrations that either occur during DNA metabolic processes such as recombination, replication and repair, or are the outcome of DNA damage.

The Werner Syndrome Exonuclease Facilitates DNA Degradation and High Fidelity DNA Polymerization by Human DNA Polymerase δ

Background: WRN and DNA polymerase δ are involved in DNA replication and repair. Results: WRN synergizes with Pol δ to degrade alternate DNA structures. WRN excises terminal mismatches to enable DNA chain extension by Pol δ. Conclusion: WRN and Pol δ together minimize the accumulation of aberrant DNA structures and ensure unhindered DNA replication. Significance: WRN contributes to the fidelity of DNA transactions, including replication. DNA Polymerase δ (Pol δ) and the Werner syndrome protein, WRN, are involved in maintaining cellular genomic stability. Pol δ synthesizes the lagging strand during replication of genomic DNA and also functions in the synthesis steps of DNA repair and recombination. WRN is a member of the RecQ helicase family, loss of which results in the premature aging and cancer-prone disorder, Werner syndrome. Both Pol δ and WRN encode 3′ → 5′ DNA exonuclease activities. Pol δ exonuclease removes 3′-terminal mismatched nucleotides incorporated during replication to ensure high fidelity DNA synthesis. WRN exonuclease degrades DNA containing alternate secondary structures to prevent formation and enable resolution of stalled replication forks. We now observe that similarly to WRN, Pol δ degrades alternate DNA structures including bubbles, four-way junctions, and D-loops. Moreover, WRN and Pol δ form a complex with enhanced ability to hydrolyze these structures. We also present evidence that WRN can proofread for Pol δ; WRN excises 3′-terminal mismatches to enable primer extension by Pol δ. Consistent with our in vitro observations, we show that WRN contributes to the maintenance of DNA synthesis fidelity in vivo. Cells expressing limiting amounts (∼10% of normal) of WRN have elevated mutation frequencies compared with wild-type cells. Together, our data highlight the importance of WRN exonuclease activity and its cooperativity with Pol δ in preserving genome stability, which is compromised by the loss of WRN in Werner syndrome.

A Rapid Assay for Measuring Nucleotide Excision Repair by Oligonucleotide Retrieval

Nucleotide excision repair (NER) excises bulky DNA lesions induced by mutagens and carcinogens. The repair process includes recognition of DNA damage, excision of a short patch of nucleotides containing the damaged base, re-synthesis of a new DNA strand and ligation of the nicks to restore the sequence integrity. Mutation or aberrant transcription of NER genes reduces repair efficiency and results in the accumulation of mutations that is associated with the development of cancer. Here we present a rapid, sensitive and quantitative assay to measure NER activity in human cells, which we term the Oligonucleotide Retrieval Assay (ORA). We used oligonucleotide constructs containing the UV-damaged adduct, cyclobutane pyrimidine dimer (CPD), to transfect human cells, and retrieved the oligonucleotides for quantification of the repaired, CPD-free DNA by real-time quantitative PCR. We demonstrate that ORA can quantify the extent of NER in diverse cell types, including immortalized, primary and stem-like cells.

Sphingosine, a Modulator of Human Translesion DNA Polymerase Activity

Background: DNA polymerase η is a specialized, error-prone DNA polymerase capable of synthesis past bulky DNA adducts. Results: Sphingosine and sphinganine stimulate the activity of Pol η. Conclusion: Sphingosine modulates DNA lesion bypass in addition to controlling cell proliferation following DNA damage. Significance: There are no known stimulators of DNA polymerases. Stimulation by sphingosine represents a novel mode of modulating Pol η activity. Translesion (TLS) DNA polymerases are specialized, error-prone enzymes that synthesize DNA across bulky, replication-stalling DNA adducts. In so doing, they facilitate the progression of DNA synthesis and promote cell proliferation. To potentiate the effect of cancer chemotherapeutic regimens, we sought to identify inhibitors of TLS DNA polymerases. We screened five libraries of ∼3000 small molecules, including one comprising ∼600 nucleoside analogs, for their effect on primer extension activity of DNA polymerase η (Pol η). We serendipitously identified sphingosine, a lipid-signaling molecule that robustly stimulates the activity of Pol η by ∼100-fold at low micromolar concentrations but inhibits it at higher concentrations. This effect is specific to the Y-family DNA polymerases, Pols η, κ, and ι. The addition of a single phosphate group on sphingosine completely abrogates this effect. Likewise, the inclusion of other sphingolipids, including ceramide and sphingomyelin to extension reactions does not elicit this response. Sphingosine increases the rate of correct and incorrect nucleotide incorporation while having no effect on polymerase processivity. Endogenous Pol η activity is modulated similarly as the recombinant enzyme. Importantly, sphingosine-treated cells exhibit increased lesion bypass activity, and sphingosine tethered to membrane lipids mimics the effects of free sphingosine. Our studies have uncovered sphingosine as a modulator of TLS DNA polymerase activity; this property of sphingosine may be associated with its known role as a signaling molecule in regulating cell proliferation in response to cellular stress.