Mutagenic Effect during Combined Exposure to Ionizing and Non-Ionizing Electromagnetic Radiation.

Using the method of dominant lethal mutations, we assessed the frequency of the death of Drosophila melanogaster embryos under combined exposure to ionizing γ-radiation and non-ionizing pulsed magnetic field at various doses and modes of exposure. Mutagenic effect of combined exposure is antagonistic in nature. The antagonism is more pronounced when the following mode of exposure was used: exposure to non-ionizing pulsed magnetic field for 5 h followed by exposure to γ-radiation at doses of 3, 10, and 60 Gy. In case of reverse sequence of exposures, the antagonistic effect was statistically significant after exposure to γ-radiation at doses of 3 and 10 Gy, whereas at a dose of 20 Gy, a synergistic interaction was noted.

Destructive effects of UVC radiation on Drosophila melanogaster: Mortality, fertility, mutations, and molecular mechanisms.

The detrimental effects of ultraviolet C (UVC) radiation on living organisms, with a specific focus on the fruit fly Drosophila melanogaster, were examined. This study investigated the impact of heightened UVC radiation exposure on D. melanogaster by assessing mortality and fertility rates, studying phenotypic mutations, and investigating the associated molecular mechanisms. The findings of this study revealed that UVC radiation increases mortality rates and decreases fertility rates in D. melanogaster. Additionally, phenotypic wing mutations were observed in the exposed flies. Furthermore, the study demonstrated that UVC radiation downregulates the expression of antioxidant genes, including superoxide dismutase (SOD), manganese-dependent superoxide dismutase (Mn-SOD), zinc-dependent superoxide dismutase (Cu-Zn-SOD), and the G protein-coupled receptor methuselah (MTH) gene. These results suggest that UVC radiation exerts a destructive effect on D. melanogaster by inducing oxidative stress, which is marked by the overexpression of harmful oxidative processes and a simultaneous reduction in antioxidant gene expression. In conclusion, this study underscores the critical importance of comprehending the deleterious effects of UVC radiation, not only to safeguard human health on Earth, but also to address the potential risks associated with space missions, such as the ongoing Emirate astronaut program.

Ultraviolet radiation shapes dendritic cell leukaemia transformation in the skin.

Tumours most often arise from progression of precursor clones within a single anatomical niche. In the bone marrow, clonal progenitors can undergo malignant transformation to acute leukaemia, or differentiate into immune cells that contribute to disease pathology in peripheral tissues1-4. Outside the marrow, these clones are potentially exposed to a variety of tissue-specific mutational processes, although the consequences of this are unclear. Here we investigate the development of blastic plasmacytoid dendritic cell neoplasm (BPDCN)-an unusual form of acute leukaemia that often presents with malignant cells isolated to the skin5. Using tumour phylogenomics and single-cell transcriptomics with genotyping, we find that BPDCN arises from clonal (premalignant) haematopoietic precursors in the bone marrow. We observe that BPDCN skin tumours first develop at sun-exposed anatomical sites and are distinguished by clonally expanded mutations induced by ultraviolet (UV) radiation. A reconstruction of tumour phylogenies reveals that UV damage can precede the acquisition of alterations associated with malignant transformation, implicating sun exposure of plasmacytoid dendritic cells or committed precursors during BPDCN pathogenesis. Functionally, we find that loss-of-function mutations in Tet2, the most common premalignant alteration in BPDCN, confer resistance to UV-induced cell death in plasmacytoid, but not conventional, dendritic cells, suggesting a context-dependent tumour-suppressive role for TET2. These findings demonstrate how tissue-specific environmental exposures at distant anatomical sites can shape the evolution of premalignant clones to disseminated cancer.

DNA damage and somatic mutations in mammalian cells after irradiation with a nail polish dryer.

Ultraviolet A light is commonly emitted by UV-nail polish dryers with recent reports suggesting that long-term use may increase the risk for developing skin cancer. However, no experimental evaluation has been conducted to reveal the effect of radiation emitted by UV-nail polish dryers on mammalian cells. Here, we show that irradiation by a UV-nail polish dryer causes high levels of reactive oxygen species, consistent with 8-oxo-7,8-dihydroguanine damage and mitochondrial dysfunction. Analysis of somatic mutations reveals a dose-dependent increase of C:G>A:T substitutions in irradiated samples with mutagenic patterns similar to mutational signatures previously attributed to reactive oxygen species. In summary, this study demonstrates that radiation emitted by UV-nail polish dryers can both damage DNA and permanently engrave mutations on the genomes of primary mouse embryonic fibroblasts, human foreskin fibroblasts, and human epidermal keratinocytes.

Low-dose but not high-dose γ-irradiation elicits the dominant-negative effect of mutant p53 in vivo.

Contrary to high doses irradiation (HDR), the biological consequences of dose irradiation (LDR) in breast cancer remain unclear due to the complexity of human epidemiological studies. LDR induces DNA damage that activates p53-mediated tumor-suppressing pathways promoting DNA repair, cell death, and growth arrest. Monoallelic p53 mutations are one of the earliest and the most frequent genetic events in many subtypes of cancer including ErbB2 breast cancer. Using MMTV/ErbB2 mutant p53 (R172H) heterozygous mouse model we found differential p53 genotype-specific effect of LDR vs. HDR on mammary tumorigenesis. Following LDR, mutant p53 heterozygous tumor cells exhibit aberrant ATM/DNA-PK signaling with defects in sensing of double-strand DNA brakes and deficient DNA repair. In contrast, HDR-induced genotoxic stress is sufficient to reach the threshold of DNA damage that is necessary for wtp53 induced DNA repair and cell cycle arrest. As a result, mutant p53 endows dominant-negative effect promoting mammary tumorigenesis after low-impact DNA damage leading to the selection of a genetically unstable proliferative population, with negligible mutagenic effect on tumors carrying wtp53 allele.

Increased Radiosensitivity of Solid Tumors Harboring ATM and BRCA1/2 Mutations.

PURPOSE: Preclinical data indicate that response to radiotherapy (RT) depends on DNA damage repair. In this study, we investigated the role of mutations in genes related to DNA damage repair in treatment outcome after RT. MATERIALS AND METHODS: Patients with solid tumor who participated in next generation sequencing panel screening using biopsied tumor tissue between October 2013 and February 2019 were reviewed and 97 patients that received RT were included in this study. Best response to RT and the cumulative local recurrence rate (LRR) were compared according to absence or presence of missense, nonsense, and frameshift mutations in ATM and/or BRCA1/2. RESULTS: Of the 97 patients, five patients harbored mutation only in ATM, 22 in only BRCA1/2, and six in both ATM and BRCA1/2 (ATMmtBRCAmt). Propensity score matching was performed to select the control group without mutations (ATMwtBRCAwt, n=33). In total, 90 RT-treated target lesions were evaluated in 66 patients. Highest objective response rate of 80% was observed in ATMmtBRCAmt lesions (p=0.007), which was mostly durable. Furthermore, the cumulative 1-year LRR was the lowest in ATMmtBRCAmt lesions and the highest in ATMwtBRCAwt lesions (0% vs. 47.9%, p=0.008). RT-associated toxicities were observed in 10 treatments with no significant difference among the subgroups (p=0.680). CONCLUSION: Tumors with ATM and BRCA1/2 mutations exhibited superior tumor response and local control after RT compared to tumors without these mutations. The results are hypothesis generating and suggest the need for integrating the tumor mutation profile of DNA repair genes during treatment planning.

Genetic and physical interactions between Polη and Rev1 in response to UV-induced DNA damage in mammalian cells.

In response to UV irradiation, translesion DNA synthesis (TLS) utilizes specialized DNA polymerases to bypass replication-blocking lesions. In a well-established polymerase switch model, Polη is thought to be a preferred TLS polymerase to insert correct nucleotides across from the thymine dimer, and Rev1 plays a scaffold role through physical interaction with Polη and the Rev7 subunit of Polζ for continual DNA synthesis. Defective Polη causes a variant form of xeroderma pigmentosum (XPV), a disease with predisposition to sunlight-induced skin cancer. Previous studies revealed that expression of Rev1 alone is sufficient to confer enhanced UV damage tolerance in mammalian cells, which depends on its physical interaction with Polζ but is independent of Polη, a conclusion that appears to contradict current literature on the critical roles of Polη in TLS. To test a hypothesis that the Rev1 catalytic activity is required to backup Polη in TLS, we found that the Rev1 polymerase-dead mutation is synergistic with either Polη mutation or the Polη-interaction mutation in response to UV-induced DNA damage. On the other hand, functional complementation of polH cells by Polη relies on its physical interaction with Rev1. Hence, our studies reveal critical interactions between Rev1 and Polη in response to UV damage.

Biochemical and photochemical mechanisms that produce different UV-induced mutation spectra.

Although UV-induced mutagenesis has been studied extensively, the precise mechanisms that convert UV-induced DNA damage into mutations remain elusive. One well-studied mechanism involves DNA polymerase (Pol) η and ζ, which produces C > T transitions during translesion synthesis (TLS) across pyrimidine dimers. We previously proposed another biochemical mechanism that involves multiple UV-irradiations with incubation in the dark in between. The incubation facilitates spontaneous deamination of cytosine in a pyrimidine dimer, and the subsequent UV irradiation induces photolyase-independent (direct) photoreversal that converts cytosine into monomeric uracil residue. In this paper, we first demonstrate that natural sunlight can induce both mutational processes in vitro. The direct photoreversal was also reproduced by monochromatic UVB at 300 nm. We also demonstrate that post-irradiation incubation in the dark is required for both mutational processes, suggesting that cytosine deamination is required for both the Pol η/ζ-dependent and the photoreversal-dependent mechanisms. Another Y-family polymerase Pol ι also mediated a mutagenic TLS on UV-damaged templates when combined with Pol ζ. The Pol ι-dependent mutations were largely independent of post-irradiation incubation, indicating that cytosine deamination was not essential for this mutational process. Sunlight-exposure also induced C > A transversions which were likely caused by oxidation of guanine residues. Finally, we constructed in vitro mutation spectra in a comparable format to cancer mutation signatures. While both Pol η-dependent and photoreversal-dependent spectra showed high similarities to a cancer signature (SBS7a), Pol ι-dependent mutation spectrum has distinct T > A/C substitutions, which are found in another cancer signature (SBS7d). The Pol ι-dependent T > A/C substitutions were resistant to T4 pyrimidine dimer glycosylase-treatment, suggesting that this mutational process is independent of cis-syn pyrimidine dimers. An updated model about multiple mechanisms of UV-induced mutagenesis is discussed.

Applied mutagenesis could improve economically important traits in bitter gourd (Momordica charantia L.).

Mutants with unique characters have played a key role in discovery of gene, mapping, functional genomics and breeding in many vegetable crops, but information on bitter gourd is lacking. Induction of mutation by gamma rays (Co60 source) at five different doses (50 Gy, 100 Gy, 150 Gy, 200 Gy and 250 Gy) was studied in four widely divergent bitter gourd genotypes BG-1346501, Meghna-2, Special Boulder and Selection-1 in M1 generation. Reduction in seed germination percentage, vine length and pollen fertility occurred in M1 generation with the increasing doses of mutagens. LD50 dose for BG-1346501, Meghna-2, Special Boulder and Selection-1 corresponded to 290.76 Gy, 206.12 Gy, 212.81 Gy and 213.49 Gy ᵞ radiation, respectively suggested low to medium doses (200-250 Gy) of gamma rays would be helpful in producing useful and exploitable mutants for further breeding. No remarkable effect of ᵞ radiation on fruit physicochemical characters in M1 generation were observed. M2 generation, raised from two widely divergent genotypes, BG-1346501 and Meghna-2, were screened critically and observed no significant reduction in seed germination and pollen viability, however little damage occurred particularly in vine length. There is possibility of isolating segregates in M2 generation with enhanced nutrient contents at low radiation dose. Highest mutation frequency resulted by treating Meghna-2 at 200 Gy and BG-1346501 at 100 Gy. Both genotype and mutagenic doses influenced mutagenic effectiveness. Spectrum of mutation was very low; number of putative mutants isolated from M2 generation was five in Meghna-2 and three in BG-1346501. Among six putative macro mutants isolated from M3 generation, we could identify two putative mutants, namely Meghna-2 with gynoecious sex form and BG-1346501 with high charantin, appreciable ß-carotene and high ascorbic acid contents having ample promise for further utilization in bitter gourd breeding after critical testing in subsequent generations for estimation of genetic gain and trait heritability to confirm the mutant stability.

UVB mutagenesis differs in Nras- and Braf-mutant mouse models of melanoma.

BRAF-mutant melanomas are more likely than NRAS-mutant melanomas to arise in anatomical locations protected from chronic sun damage. We hypothesized that this discrepancy in tumor location is a consequence of the differential sensitivity of BRAF and NRAS-mutant melanocytes to ultraviolet light (UV)-mediated carcinogenesis. We tested this hypothesis by comparing the mutagenic consequences of a single neonatal, ultraviolet-AI (UVA; 340-400 nm) or ultraviolet-B (UVB; 280-390 nm) exposure in mouse models heterozygous for mutant Braf or homozygous for mutant Nras Tumor onset was accelerated by UVB, but not UVA, and the resulting melanomas contained recurrent mutations affecting the RING domain of MAP3K1 and Actin-binding domain of Filamin A. Melanomas from UVB-irradiated, Braf-mutant mice averaged twice as many single-nucleotide variants and five times as many dipyrimidine variants than tumors from similarly irradiated Nras-mutant mice. A mutational signature discovered in UVB-accelerated tumors mirrored COSMIC signatures associated with human skin cancer and was more prominent in Braf- than Nras-mutant murine melanomas. These data show that a single UVB exposure yields a greater burden of mutations in murine tumors driven by oncogenic Braf.

Induced mutation in Agaricus bisporus by gamma ray to improve genetic variability, degradation enzyme activity, and yield.

PURPOSE: Genetic variability in white button mushroom cultivars is very low due to the life cycle. Induction mutations using gamma irradiation is a useful way to generate diversity in white button mushrooms to obtain genotype(s) with desirable traits. METHODS: Gamma irradiation Cobalt-60 was used for inducting genetic diversity in white button mushroom to obtain genotype(s) with desirable traits. Gamma irradiation with doses of 0-500 Gy was conducted on spores on Potato Dextrose Agar medium. RESULTS: The results showed significant differences in days to pin production and harvest, fruit body number, fresh and dry weight, yield, laccase, and manganese peroxidase enzyme activity. After isolating variants, 15 variants were selected on the base of their high yield and enzyme degradation activity. Their genetic variation was confirmed by Sequence Related Amplified Polymorphism (SRAP) markers, and then incubated on three types of substrates (50:50, 75:25, and 100:0 % compost: raw straw). The results showed that all variants, except GR18, colonized in 75:25, and GR3, GR4, GR9, GR61, GR72, and GR74 variants colonized in 50:50. In 100:0 substrate, GR55 and GR63 were the earliest variants, and GR9 produced the highest fruit body number. In 75:25 substrate, GR9, GR3, GR61, GR4, GR74, GR4, GR61, and GR72 showed higher yields. The highest laccase and manganese peroxidase activity were recorded in GR3, GR4, GR9, GR72, and GR61. The isolated 15 variants were clustered into two main groups by cluster analysis and genetic variation was confirmed by SRAP markers. CONCLUSION: The results showed that the diversity in the white button mushroom could be improved using gamma rays, and the variation would be useful for the development of future breeding programs.

Ppp6c haploinsufficiency accelerates UV-induced BRAF(V600E)-initiated melanomagenesis.

According to TCGA database, mutations in PPP6C (encoding phosphatase PP6) are found in c. 10% of tumors from melanoma patients, in which they coexist with BRAF and NRAS mutations. To assess PP6 function in melanoma carcinogenesis, we generated mice in which we could specifically induce BRAF(V600E) expression and delete Ppp6c in melanocytes. In these mice, melanoma susceptibility following UVB irradiation exhibited the following pattern: Ppp6c semi-deficient (heterozygous) > Ppp6c wild-type > Ppp6c-deficient (homozygous) tumor types. Next-generation sequencing of Ppp6c heterozygous and wild-type melanoma tumors revealed that all harbored Trp53 mutations. However, Ppp6c heterozygous tumors showed a higher Signature 1 (mitotic/mitotic clock) mutation index compared with Ppp6c wild-type tumors, suggesting increased cell division. Analysis of cell lines derived from either Ppp6c heterozygous or wild-type melanoma tissues showed that both formed tumors in nude mice, but Ppp6c heterozygous tumors grew faster compared with those from the wild-type line. Ppp6c knockdown via siRNA in the Ppp6c heterozygous line promoted the accumulation of genomic damage and enhanced apoptosis relative to siRNA controls. We conclude that in the presence of BRAF(V600E) expression and UV-induced Trp53 mutation, Ppp6c haploinsufficiency promotes tumorigenesis.

Targeting the DNA replication stress phenotype of KRAS mutant cancer cells.

Mutant KRAS is a common tumor driver and frequently confers resistance to anti-cancer treatments such as radiation. DNA replication stress in these tumors may constitute a therapeutic liability but is poorly understood. Here, using single-molecule DNA fiber analysis, we first characterized baseline replication stress in a panel of unperturbed isogenic and non-isogenic cancer cell lines. Correlating with the observed enhanced replication stress we found increased levels of cytosolic double-stranded DNA in KRAS mutant compared to wild-type cells. Yet, despite this phenotype replication stress-inducing agents failed to selectively impact KRAS mutant cells, which were protected by CHK1. Similarly, most exogenous stressors studied did not differentially augment cytosolic DNA accumulation in KRAS mutant compared to wild-type cells. However, we found that proton radiation was able to slow fork progression and preferentially induce fork stalling in KRAS mutant cells. Proton treatment also partly reversed the radioresistance associated with mutant KRAS. The cellular effects of protons in the presence of KRAS mutation clearly contrasted that of other drugs affecting replication, highlighting the unique nature of the underlying DNA damage caused by protons. Taken together, our findings provide insight into the replication stress response associated with mutated KRAS, which may ultimately yield novel therapeutic opportunities.

Exome of Radiation-induced Rat Mammary Carcinoma Shows Copy-number Losses and Mutations in Human-relevant Cancer Genes.

BACKGROUND/AIM: Our understanding of cancer risk from neutron exposure is limited. We aimed to reveal the characteristics of mammary carcinomas induced by neutrons. MATERIALS AND METHODS: Mammary carcinomas obtained from female Sprague-Dawley rats irradiated at 7 weeks of age with 0.97 Gy neutrons or 4 Gy γ-rays and from non-irradiated rats were classified into luminal and non-luminal subtypes by immunohistochemistry. Their mutational landscapes were determined by whole-exome sequencing. RESULTS: Neutrons significantly raised the incidence of luminal mammary carcinomas over the non-luminal subtype. Somatic mutations were identified in cancer genes involved in several signalling pathways, including Keap1/Nrf2, Pi3k/Akt and Wnt/ß-catenin. Focal copy-number losses involving cancer genes were observed mainly in carcinomas from the irradiated rats. CONCLUSION: Neutrons increase the incidence of luminal mammary carcinomas, probably through gene mutations similar to those found in human breast cancers, and focal copy-number losses including cancer genes that are characteristics of radiation-induced mammary carcinomas.

Actinic Keratoses: Reconciling the Biology of Field Cancerization with Treatment Paradigms.

This Perspective briefly reviews the relationship between UV-induced mutations in habitually sun-exposed human skin and subsequent development of actinic keratoses (AKs) and skin cancers. It argues that field therapy rather than AK-selective therapy is the more logical approach to cancer prevention and hypothesizes that treatment early in the process of field cancerization, even prior to the appearance of AKs, may be more effective in preventing cancer as well as more beneficial for and better tolerated by at-risk individuals. Finally, the Perspective encourages use of rapidly advancing DNA analysis techniques to quantify mutational burden in sun-damaged skin and its reduction by various therapies.

TGFß Signaling in Photoaging and UV-Induced Skin Cancer.

UVR is a major etiology for premature skin aging that leads to photoaging and UV-induced skin cancers. In the skin, TGFß signaling is a growth inhibitor for keratinocytes and a profibrotic factor in the dermis. It exerts context-dependent effects on tumor progression. Chronic UV exposure likely causes TGFß1/SMAD3 signaling activation and contributes to metalloproteinase-induced collagen degradation and photoinflammation in photoaging. UV irradiation also causes gene mutations in key elements of the TGFß pathway, including TGFßRI, TGFßRII, SMAD2, and SMAD4. These mutations enable tumor cells to escape from TGFß-induced growth inhibition and induce genomic instability and cancer stem cells, leading to the initiation, progression, invasion, and metastasis of cutaneous squamous cell carcinoma (cSCC). Furthermore, UV-induced mutations cause TGFß overexpression in the tumor microenvironment (TME) of cSCC, basal cell carcinoma (BCC), and cutaneous melanoma, resulting in inflammation, angiogenesis, cancer-associated fibroblasts, and immune inhibition, supporting cancer survival, immune evasion, and metastasis. The pleiotropic effects of TGFß provide possible treatment options for photoaging and skin cancer. Given the high UV-induced mutational burden and immune-repressive TME seen in cSCC, BCC, and cutaneous melanoma, treatment with the combination of a TGFß signaling inhibitor and immune checkpoint blockade could reverse immune evasion to reduce tumor growth.

Transgenerational effects in offspring of chronically irradiated populations of Drosophila melanogaster after the Chernobyl accident.

The zone of the Chernobyl nuclear disaster represents the largest area of chronic low-intensity radioactive impact on the natural ecosystems. The effects of chronic low-dose irradiation for natural populations of organisms and their offspring are unknown. The natural populations of Drosophila melanogaster sampled in 2007 in Chernobyl sites with different levels of radiation contamination were investigated. The offspring of specimens from these populations were studied under laboratory conditions to assess the effects of parental irradiation on the mutation process and survival of the offspring. Transgenerational effects of radioactive contamination were observed at the level of gross chromosomal rearrangements (dominant lethal mutations). The frequency of point/gene mutations (recessive sex-linked lethal mutations) of the offspring of the irradiated parents corresponded to the actual level of spontaneous mutations. The survival rate of offspring decreased over 160 generations and significantly correlated with the dominant lethal mutation levels. Our results provide a compelling evidence that other factors (distance from the Chernobyl Nuclear Power Plant, time after the initial exposure, selection site and origin of population) can affect the changes in the levels of the studied parameters along with the parental radiation exposure. They can also make a significant contribution to the health of the offspring of animals exposed to radioactive contamination. These data should be useful for future radioecological studies which will clarify the true mechanisms of transgenerational inheritance and generation of mutations to the offspring of chronically irradiated animals and their reactions to the interaction of various environmental factors.

Atypical UV Photoproducts Induce Non-canonical Mutation Classes Associated with Driver Mutations in Melanoma.

Somatic mutations in skin cancers and other ultraviolet (UV)-exposed cells are typified by C>T and CC>TT substitutions at dipyrimidine sequences; however, many oncogenic "driver" mutations in melanoma do not fit this UV signature. Here, we use genome sequencing to characterize mutations in yeast repeatedly irradiated with UV light. Analysis of ~50,000 UV-induced mutations reveals abundant non-canonical mutations, including T>C, T>A, and AC>TT substitutions. These mutations display transcriptional asymmetry that is modulated by nucleotide excision repair (NER), indicating that they are caused by UV photoproducts. Using a sequencing method called UV DNA endonuclease sequencing (UVDE-seq), we confirm the existence of an atypical thymine-adenine photoproduct likely responsible for UV-induced T>A substitutions. Similar non-canonical mutations are present in skin cancers, which also display transcriptional asymmetry and dependence on NER. These include multiple driver mutations, most prominently the recurrent BRAF V600E and V600K substitutions, suggesting that mutations arising from rare, atypical UV photoproducts may play a role in melanomagenesis.

An antiphage Escherichia coli mutant for higher production of L-threonine obtained by atmospheric and room temperature plasma mutagenesis.

Phage infection is common during the production of L-threonine by E. coli, and low L-threonine production and glucose conversion percentage are bottlenecks for the efficient commercial production of L-threonine. In this study, 20 antiphage mutants producing high concentration of L-threonine were obtained by atmospheric and room temperature plasma (ARTP) mutagenesis, and an antiphage E. coli variant was characterized that exhibited the highest production of L-threonine Escherichia coli ([E. coli] TRFC-AP). The elimination of fhuA expression in E. coli TRFC-AP was responsible for phage resistance. The biomass and cell growth of E. coli TRFC-AP showed no significant differences from those of the parent strain (E. coli TRFC), and the production of L-threonine (159.3 g L-1 ) and glucose conversion percentage (51.4%) were increased by 10.9% and 9.1%, respectively, compared with those of E. coli TRFC. During threonine production (culture time of 20 h), E. coli TRFC-AP exhibited higher activities of key enzymes for glucose utilization (hexokinase, glucose phosphate dehydrogenase, phosphofructokinase, phosphoenolpyruvate carboxylase, and PYK) and threonine synthesis (glutamate synthase, aspartokinase, homoserine dehydrogenase, homoserine kinase and threonine synthase) compared to those of E. coli TRFC. The analysis of metabolic flux distribution indicated that the flux of threonine with E. coli TRFC-AP reached 69.8%, an increase of 16.0% compared with that of E. coli TRFC. Overall, higher L-threonine production and glucose conversion percentage were obtained with E. coli TRFC-AP due to increased activities of key enzymes and improved carbon flux for threonine synthesis.

The deacetylase SIRT6 promotes the repair of UV-induced DNA damage by targeting DDB2.

The NAD+-dependent deacetylase and mono-ADP-ribosyl transferase SIRT6 stabilizes the genome by promoting DNA double strand break repair, thereby acting as a tumor suppressor. However, whether SIRT6 regulates nucleotide excision repair (NER) remains unknown. Here, we showed that SIRT6 was recruited to sites of UV-induced DNA damage and stimulated the repair of UV-induced DNA damage. Mechanistic studies further indicated that SIRT6 interacted with DDB2, the major sensor initiating global genome NER (GG-NER), and that the interaction was enhanced upon UV irradiation. SIRT6 deacetylated DDB2 at two lysine residues, K35 and K77, upon UV stress and then promoted DDB2 ubiquitination and segregation from chromatin, thereby facilitating downstream signaling. In addition, we characterized several SIRT6 mutations derived from melanoma patients. These SIRT6 mutants ablated the stimulatory effect of SIRT6 on NER and destabilized the genome due to (i) partial loss of enzymatic activity (P27S or H50Y), (ii) a nonsense mutation (R150*) or (iii) high turnover rates (G134W). Overall, we demonstrate that SIRT6 promotes NER by deacetylating DDB2, thereby preventing the onset of melanomagenesis.