Reduction of Werner Syndrome Protein Enhances G:C → A:T Transition byO6-Methylguanine in Human Cells

Easily-controllable, helper phage-free single-stranded phagemid production system

Background

Single-stranded (ss) DNAs are utilized in various molecular biological and biotechnological applications including the construction of double-stranded DNAs with a DNA lesion, and are commonly prepared by using chimeric phage-plasmids (phagemids) plus M13-derived helper phages. However, the yields of ss DNA with these methods are poorly reproducible, and multiple factors must be optimized.

Results

In this report, we describe a new arabinose-inducible ss phagemid production method without helper phage infection. The newly exploited DNA derived from VCSM13 expresses the pII protein, which initiates ss DNA synthesis, under the control of the araBAD promoter. In addition, the packaging signal is deleted in the DNA to reduce the contamination of the phage-derived ss DNA. The phagemid DNA of interest, carrying the M13 origin of replication and the packaging signal, was introduced into bacterial cells maintaining the modified VCSM13 DNA as a plasmid, and the ss phagemid DNA production was induced by arabinose. The DNA recovered from the phage particles had less contamination from VCSM13 DNA, as compared to the conventional method. Moreover, we extended the method to purify the ss DNAs by using an anion-exchange column, to avoid the use of hazardous chemicals.

Conclusion

Using this combination of methods, large quantities of phagemid ss DNAs of interest can be consistently obtained.

3D-printed antioxidant antibacterial carboxymethyl cellulose/ε-polylysine hydrogel promoted skin wound repair

Developing a wound dressing for the treatment of large and irregular-shaped wounds remains a great challenge. Herein we developed novel printable bionic hydrogels with antibacterial and antioxidant properties which could effectively overcome the challenge by inhibiting inflammation and accelerating wound healing. The CMC/PL (CP) hydrogels were customized with glycidyl methacrylate (GMA) modified carboxymethyl cellulose (CMC) and ε-polylysine (ε-PL) via ultraviolet (UV) light polymerization using a 3D printer. Except for the high compression modulus (238 kPa), stable rheological properties, and effective degradability, these CP hydrogels also had an excellent inhibitory effect (95%) on both Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus). Remarkably, CP hydrogels could remove the excessive reactive oxygen species (ROS) and protect the fibroblasts from damage. Compared with the commercial dressing (Tegaderm ™ film), CP hydrogels showed a better ability to increase the expression of VEGF and CD31, accelerate granulation tissue regeneration, and promote wound healing. This work provides a new strategy to fabricate on-demand multi-functional hydrogels in the field of skin tissue engineering.

Action-at-a-distance mutations at 5'-GpA-3' sites induced by oxidized guanine in WRN-knockdown cells

G:C sites distant from 8-oxo-7,8-dihydroguanine (G O, 8-hydroxyguanine) are frequently mutated when the lesion-bearing plasmid DNA is replicated in human cells with reduced Werner syndrome (WRN) protein. To detect the untargeted mutations preferentially, the oxidized guanine base was placed downstream of the reporter supF gene and the plasmid DNA was introduced into WRN-knockdown cells. The total mutant frequency seemed higher in the WRN-knockdown cells as compared to the control cells. Mutation analyses revealed that substitution mutations occurred at the G:C pairs of 5'-GpA-3'/5'-TpC-3' sites, the preferred sequence for the apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like 3 (APOBEC3)-family cytosine deaminases, in the supF gene in both control and knockdown cells. These mutations were observed more frequently at G sites than C sites on the DNA strand where the G O base was originally located. This tendency was promoted by the knockdown of the WRN protein. The present results imply the possible involvement of APOBEC3-family cytosine deaminases in the action-at-a-distance (untargeted) mutations at G:C (or G) sites induced by G O and in cancer initiation by oxidative stress.

New indicator Escherichia coli strain for rapid and accurate detection of supF mutations

Background

The supF gene of Escherichia coli is useful for forward mutation analysis in bacterial and mammalian cells used in mutagenesis and DNA repair studies. Indicator E. coli strains, such as KS40/pOF105, have been used to analyze supF mutations. However, KS40/pOF105 is not enough to select supF mutants on nutrient-rich agar plates. Therefore, in this study, a new indicator E. coli strain for rapid and accurate detection of supF mutations was developed.

Results

The gyrA and rpsL genes with an amber mutation were integrated into the chromosomal DNA of E. coli KS40 to produce a new indicator strain, RF01. RF01 cells transformed by the wild-type supF gene were sensitive to nalidixic acid and streptomycin on LB agar plates. supF mutant frequencies and mutation spectra in RF01 were similar to those in KS40/pOF105. In addition, some mutations in supF were only detected in RF01.

Conclusion

RF01 is a new and useful indicator E. coli strain for analyzing supF mutations.

Werner Syndrome Protein (WRN) Regulates Cell Proliferation and the Human Papillomavirus 16 Life Cycle during Epithelial Differentiation

Human papillomaviruses recruit a host of DNA damage response factors to their viral genome to facilitate homologous recombination replication in association with the viral replication factors E1 and E2. We previously demonstrated that SIRT1 deacetylation of WRN promotes recruitment of WRN to E1-E2 replicating DNA and that WRN regulates both the levels and fidelity of E1-E2 replication. The deacetylation of WRN by SIRT1 results in an active protein able to complex with replicating DNA, but a protein that is less stable. Here, we demonstrate an inverse correlation between SIRT1 and WRN in CIN cervical lesions compared to normal control tissue, supporting our model of SIRT1 deacetylation destabilizing WRN protein. We CRISPR/Cas9 edited N/Tert-1 and N/Tert-1+HPV16 cells to knock out WRN protein expression and subjected the cells to organotypic raft cultures. In N/Tert-1 cells without WRN expression, there was enhanced basal cell proliferation, DNA damage, and thickening of the differentiated epithelium. In N/Tert-1+HPV16 cells, there was enhanced basal cell proliferation, increased DNA damage throughout the epithelium, and increased viral DNA replication. Overall, the results demonstrate that the expression of WRN is required to control the proliferation of N/Tert-1 cells and controls the HPV16 life cycle in these cells. This complements our previous data demonstrating that WRN controls the levels and fidelity of HPV16 E1-E2 DNA replication. The results describe a new role for WRN, a tumor suppressor, in controlling keratinocyte differentiation and the HPV16 life cycle.IMPORTANCE HPV16 is the major human viral carcinogen, responsible for around 3 to 4% of all cancers worldwide. Our understanding of how the viral replication machinery interacts with host factors to control/activate the DNA damage response to promote the viral life cycle remains incomplete. Recently, we demonstrated a SIRT1-WRN axis that controls HPV16 replication, and here we demonstrate that this axis persists in clinical cervical lesions induced by HPV16. Here, we describe the effects of WRN depletion on cellular differentiation with or without HPV16; WRN depletion results in enhanced proliferation and DNA damage irrespective of HPV16 status. Also, WRN is a restriction factor for the viral life cycle since replication is disrupted in the absence of WRN. Future studies will focus on enhancing our understanding of how WRN regulates viral replication. Our goal is to ultimately identify cellular factors essential for HPV16 replication that can be targeted for therapeutic gain.

Large deletions and untargeted substitutions induced by abasic site analog on leading versus lagging strand templates in human cells

The tetrahydrofuran-type abasic site analog (THF) induces large deletion mutations in human cells. To compare the large deletions induced by THF on leading and lagging strand templates, plasmid DNAs bearing the analog at a specific position outside the supF gene were introduced into human U2OS cells. The replicated DNAs recovered from the transfected cells were electroporated into an Escherichia coli indicator strain. THF on the lagging strand template produced more supF mutants than THF on the leading strand template. This unequal mutagenicity was due to the higher frequencies of not only large deletions but also untargeted base substitutions induced in the gene. These results suggested that both types of mutations occur more frequently when abasic sites are formed on the lagging strand template.

Analysis of large deletion mutations induced by abasic site analog in human cells

Background

Abasic sites are formed spontaneously and by nucleobase chemical modifications and base excision repair. A chemically stable abasic site analog was site-specifically introduced into replicable plasmid DNAs, which were transfected into human U2OS cells. The amplified DNAs were recovered from the cells and used for the transformation of a bacterial indicator strain.

Results

Large deletion mutations were induced by the analog, in addition to point mutations at the modified site. No apparent sequence homology at the deletion junctions was found.

Conclusion

These results suggested that the large deletions induced by the abasic site analog are formed by homology-independent events.