Mammalian MutY homolog (MYH or MUTYH) protects cells from oxidative DNA damage

Genetic Complexity in Recurrent Basal Cell Carcinoma: A MUTYH Variant Case Report

The MYUTH gene plays a critical role in preserving the integrity of the human genome, with mutations being identified in several different cancer diagnoses. It serves its purpose by encoding a DNA glycosylase enzyme responsible for preventing oxidative damage through the excision of adenine that is incorrectly paired with guanine or cytosine. Mutations of the MUTYH gene have been most frequently associated with MUTYH-associated polyposis (MAP) and colorectal cancer. Biallelic mutations of the MUTYH gene are implicated in MAP, and carriers of this mutation have an increased lifetime risk of developing colorectal cancer of 43% to 100%, depending on the appropriate screening and surveillance steps taken. This case describes a patient with recurrent basal cell carcinomas (BCC) and subsequent genetic testing that revealed a pathogenic monoallelic mutation of the MUTYH gene, as well as the interventions that were subsequently performed. It highlights a potentially new patient population that would benefit from early screening to assess the risk of developing colorectal cancers as well as BCC.

MYH16 upregulation is associated with lung adenocarcinoma aggressiveness and immune infiltration

Myosin heavy chain 16 (MYH16) may significantly affect cell cycle progression. Nevertheless, there is a lack of evidence about the clinical relevance of MYH16 upregulation in pan cancers, including lung adenocarcinoma (LUAD). MYH16 expression patterns were evaluated in various bioinformatics databases using The Cancer Genome Atlas data set. Clinical and pathological factor data were employed to risk-stratify patients. The Kaplan-Meier plotter approach was used to estimate survival rates. Tumor immune infiltration was explored via the TIMER tool, and gene set enrichment analysis (GSEA) was used to identify the pathways involved in MYH16 upregulation. The results showed that MYH16 was abnormally upregulated in pan cancers, including LUAD. MYH16 expression induction in LUAD was found to be related to the tumor stage. Furthermore, MYH16 upregulation was correlated with LUAD development and worse overall survival, particularly in women. Notably, MYH16 overexpression in LUAD tissues corresponded to the amount of immune infiltration in the tumor. Additionally, univariate Cox hazard regression analysis revealed that MYH16 may be an independent prognostic indicator for LUAD. Furthermore, a nomogram was constructed according to MYH16 expression and clinical characteristics. BMP6 expression deficiency may be a key factor contributing to MYH16 upregulation in LUAD. Finally, GSEA demonstrated that MYH16 might mediate meiosis and gene silencing through RNA signaling pathways. This study, for the first time, showed that MYH16 upregulation in LUAD is associated with various risk factors, increased cancer aggressiveness, enhanced infiltration of tumor immune cells, and reduced survival rates.

Functionalization of Mesoporous Silica with a G-A-Mismatched dsDNA Chain for Efficient Identification and Selective Capturing of the MutY Protein

MUTYH adenine DNA glycosylase and its homologous protein (collectively MutY) are typical DNA glycosylases with a [4Fe4S] cluster and a helix-hairpin-helix (HhH) motif in its structure. In the present work, the binding behaviors of the MutY protein to dsDNA containing different base mismatches were investigated. The type and distribution of base mismatch in the dsDNA chain were found to influence the DNA-protein binding interaction greatly. The [4Fe4S] cluster of the MutY protein is able to identify a G-A mismatch in the dsDNA chain specifically by monitoring the anomalies of charge transport in the dsDNA chain, allowing the entrance of the identified dsDNA chain into the internal cavity of the MutY protein and the strong DNA-protein binding at the HhH motif of the protein through multiple H-bonds. The dsDNA chain with a centrally located G-A mismatch is thus functionalized on mesoporous silica (MSN) via amination reaction, and the obtained dsDNA(G-A)@MSN is used as a powerful sorbent for the selective capturing of the MutY protein from complex samples. By using 0.5% NH3·H2O (m/v) as a stripping reagent, efficient isolation of the MutY protein from different cell lines and bacteria is achieved and the recovered MutY protein is demonstrated to maintain favorable DNA adenine glycosylase activity.

DNA binding by the Rad9A subunit of the Rad9-Rad1-Hus1 complex

The Rad9-Rad1-Hus1 checkpoint clamp activates the DNA damage response and promotes DNA repair. DNA loading on the central channel of the Rad9-Rad1-Hus1 complex is required to execute its biological functions. Because Rad9A has the highest DNA affinity among the three subunits, we determined the domains and functional residues of human Rad9A that are critical for DNA interaction. The N-terminal globular domain (residues 1–133) had 3.7-fold better DNA binding affinity than the C-terminal globular domain (residues 134–266) of Rad9A^1-266. Rad9A^1-266 binds DNA 16-, 60-, and 30-fold better than Rad9A^1-133, Rad9A^134-266, and Rad9A^94-266, respectively, indicating that different regions cooperatively contribute to DNA binding. We show that basic residues including K11, K15, R22, K78, K220, and R223 are important for DNA binding. The reductions on DNA binding of Ala substituted mutants of these basic residues show synergistic effect and are dependent on their residential Rad9A deletion constructs. Interestingly, deletion of a loop (residues 160–163) of Rad9A^94-266 weakens DNA binding activity by 4.1-fold as compared to wild-type (WT) Rad9A^94-266. Cellular sensitivity to genotoxin of rad9A knockout cells is restored by expressing WT-Rad9A^full. However, rad9A knockout cells expressing Rad9A mutants defective in DNA binding are more sensitive to H₂O₂ as compared to cells expressing WT-Rad9A^full. Only the rad9A knockout cells expressing loop-deleted Rad9A mutant are more sensitive to hydroxyurea than cells expressing WT-Rad9A. In addition, Rad9A-DNA interaction is required for DNA damage signaling activation. Our results indicate that DNA association by Rad9A is critical for maintaining cell viability and checkpoint activation under stress.

Mammalian MutY Homolog (MYH or MUTYH) is Critical for Telomere Integrity under Oxidative Stress

Telomeres consist of special features and proteins to protect the ends of each chromosome from deterioration and fusion. The telomeric DNA repeats are highly susceptible to oxidative damage that can accelerate telomere shortening and affect telomere integrity. Several DNA repair factors including MYH/MUTYH DNA glycosylase, its interacting partners Rad9/Rad1/Hus1 checkpoint clamp, and SIRT6 aging regulator, are associated with the telomeres. MYH prevents C:G to A:T mutation by removing adenine mispaired with a frequent oxidative DNA lesion, 8-oxoguanine. Here, we show that hMYH knockout (KO) human HEK-293T cells are more sensitive to H₂O₂ treatment, have higher levels of DNA strand breaks and shorter telomeres than the control hMYH ^+/+ cells. SIRT6 foci increase at both the global genome and at telomeric regions in H₂O₂-treated hMYH ^+/+ cells. However, in untreated hMYH KO HEK-293T cells, SIRT6 foci only increase at the global genome, but not at the telomeric regions. In addition, the hMYH KO HEK-293T cells have increased extra-chromosomal and intra-chromosomal telomeres compared to the control cells, even in the absence of H₂O₂ treatment. After H₂O₂ treatment, the frequency of extra-chromosomal telomeres increased in control HEK-293T cells. Remarkably, in H₂O₂-treated hMYH KO cells, the frequencies of extra-chromosomal telomeres, intra-chromosomal telomeres, and telomere fusions are further increased. We further found that the sensitivity to H₂O₂ and shortened telomeres of hMYH KO cells, are restored by expressing wild-type hMYH, and partially rescued by expressing hMYH^Q324H mutant (defective in Hus1 interaction only), but not by expressing hMYH^V315A mutant (defective in both SIRT6 and Hus1 interactions). Thus, MYH interactions with SIRT6 and Hus1 are critical for maintaining cell viability and telomeric stability. Therefore, the failure to coordinate 8-oxoG repair is detrimental to telomere integrity.

Transcription coupled base excision repair in mammalian cells: So little is known and so much to uncover

Oxidized bases in the genome has been implicated in various human pathologies, including cancer, aging and neurological diseases. Their repair is initiated with excision by DNA glycosylases (DGs) in the base excision repair (BER) pathway. Among the five oxidized base-specific human DGs, OGG1 and NTH1 preferentially excise oxidized purines and pyrimidines, respectively, while NEILs remove both oxidized purines and pyrimidines. However, little is known about why cells possess multiple DGs with overlapping substrate specificities. Studies of the past decades revealed that some DGs are involved in repair of oxidized DNA base lesions in the actively transcribed regions. Preferential removal of lesions from the transcribed strands of active genes, called transcription-coupled repair (TCR), was discovered as a distinct sub-pathway of nucleotide excision repair; however, such repair of oxidized DNA bases had not been established until our recent demonstration of NEIL2's role in TC-BER of the nuclear genome. We have shown that NEIL2 forms a distinct transcriptionally active, repair proficient complex. More importantly, we for the first time reconstituted TC-BER using purified components. These studies are important for characterizing critical requirement for the process. However, because NEIL2 cannot remove all types of oxidized bases, it is unlikely to be the only DNA glycosylase involved in TC-BER. Hence, we postulate TC-BER process to be universally involved in maintaining the functional integrity of active genes, especially in post-mitotic, non-growing cells. We further postulate that abnormal bases (e.g., uracil), and alkylated and other small DNA base adducts are also repaired via TC-BER. In this review, we have provided an overview of the various aspects of TC-BER in mammalian cells with the hope of generating significant interest of many researchers in the field. Further studies aimed at better understanding the mechanistic aspects of TC-BER could help elucidate the linkage of TC-BER deficiency to various human pathologies.

An ordered assembly of MYH glycosylase, SIRT6 protein deacetylase, and Rad9-Rad1-Hus1 checkpoint clamp at oxidatively damaged telomeres

In the base excision repair pathway, MYH/MUTYH DNA glycosylase prevents mutations by removing adenine mispaired with 8-oxoG, a frequent oxidative lesion. MYH glycosylase activity is enhanced by Rad9-Rad1-Hus1 (9-1-1) checkpoint clamp and SIRT6 histone/protein deacetylase. Here, we show that MYH, SIRT6, and 9-1-1 are recruited to confined oxidatively damaged regions on telomeres in mammalian cells. Using different knockout cells, we show that SIRT6 responds to damaged telomeres very early, and then recruits MYH and Hus1 following oxidative stress. However, the recruitment of Hus1 to damaged telomeres is partially dependent on SIRT6. The catalytic activities of SIRT6 are not important for SIRT6 response but are essential for MYH recruitment to damaged telomeres. Compared to wild-type MYH, the recruitment of hMYH^V315A mutant (defective in both SIRT6 and Hus1 interactions), but not hMYH^Q324H mutant (defective in Hus1 interaction only), to damaged telomeres is severely reduced. The formation of MYH/SIRT6/9-1-1 complex is of biological significance as interrupting their interactions can increase cell's sensitivity to H₂O₂ and/or elevate cellular 8-oxoG levels after H₂O₂ treatment. Our results establish that SIRT6 acts as an early sensor of BER enzymes and both SIRT6 and 9-1-1 serve critical roles in DNA repair to maintain telomere integrity.

The Similarities between Human Mitochondria and Bacteria in the Context of Structure, Genome, and Base Excision Repair System

Mitochondria emerged from bacterial ancestors during endosymbiosis and are crucial for cellular processes such as energy production and homeostasis, stress responses, cell survival, and more. They are the site of aerobic respiration and adenosine triphosphate (ATP) production in eukaryotes. However, oxidative phosphorylation (OXPHOS) is also the source of reactive oxygen species (ROS), which are both important and dangerous for the cell. Human mitochondria contain mitochondrial DNA (mtDNA), and its integrity may be endangered by the action of ROS. Fortunately, human mitochondria have repair mechanisms that allow protecting mtDNA and repairing lesions that may contribute to the occurrence of mutations. Mutagenesis of the mitochondrial genome may manifest in the form of pathological states such as mitochondrial, neurodegenerative, and/or cardiovascular diseases, premature aging, and cancer. The review describes the mitochondrial structure, genome, and the main mitochondrial repair mechanism (base excision repair (BER)) of oxidative lesions in the context of common features between human mitochondria and bacteria. The authors present a holistic view of the similarities of mitochondria and bacteria to show that bacteria may be an interesting experimental model for studying mitochondrial diseases, especially those where the mechanism of DNA repair is impaired.

DNA lesions and repair in trypanosomatids infection

Pathological processes such as bacterial, viral and parasitic infections can generate a plethora of responses such as, but not restricted to, oxidative stress that can be harmful to the host and the pathogen. This stress occurs when there is an imbalance between reactive oxygen species produced and antioxidant factors produced in response to the infection. This imbalance can lead to DNA lesions in both infected cells as well as in the pathogen. The effects of the host response on the parasite lead to several kinds of DNA damage, causing alterations in the parasite's metabolism; the reaction and sensitivity of the parasite to these responses are related to the DNA metabolism and life cycle of each parasite. The present review will discuss the survival strategies developed by host cells and Trypanosoma cruzi, focusing on the DNA repair mechanisms of these organisms throughout infection including the relationship between DNA damage, stress response features, and the unique characteristics of these diseases.

The DNA repair enzyme MUTYH potentiates cytotoxicity of the alkylating agent MNNG by interacting with abasic sites

Higher expression of the human DNA repair enzyme MUTYH has previously been shown to be strongly associated with reduced survival in a panel of 24 human lymphoblastoid cell lines exposed to the alkylating agent N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). The molecular mechanism of MUTYH-enhanced MNNG cytotoxicity is unclear, because MUTYH has a well-established role in the repair of oxidative DNA lesions. Here, we show in mouse embryonic fibroblasts (MEFs) that this MNNG-dependent phenotype does not involve oxidative DNA damage and occurs independently of both O⁶-methyl guanine adduct cytotoxicity and MUTYH-dependent glycosylase activity. We found that blocking of abasic (AP) sites abolishes higher survival of Mutyh-deficient (Mutyh ^-/-) MEFs, but this blockade had no additive cytotoxicity in WT MEFs, suggesting the cytotoxicity is due to MUTYH interactions with MNNG-induced AP sites. We found that recombinant mouse MUTYH tightly binds AP sites opposite all four canonical undamaged bases and stimulated apurinic/apyrimidinic endonuclease 1 (APE1)-mediated DNA incision. Consistent with these observations, we found that stable expression of WT, but not catalytically-inactive MUTYH, enhances MNNG cytotoxicity in Mutyh ^-/- MEFs and that MUTYH expression enhances MNNG-induced genomic strand breaks. Taken together, these results suggest that MUTYH enhances the rapid accumulation of AP-site intermediates by interacting with APE1, implicating MUTYH as a factor that modulates the delicate process of base-excision repair independently of its glycosylase activity.

When you're strange: Unusual features of the MUTYH glycosylase and implications in cancer

MUTYH is a base-excision repair glycosylase that removes adenine opposite 8-oxoguanine (OG). Variants of MUTYH defective in functional activity lead to MUTYH-associated polyposis (MAP), which progresses to cancer with very high penetrance. Whole genome and whole exome sequencing studies have found MUTYH deficiencies in an increasing number of cancer types. While the canonical OG:A repair activity of MUTYH is well characterized and similar to bacterial MutY, here we review more recent evidence that MUTYH has activities independent of OG:A repair and appear centered on the interdomain connector (IDC) region of MUTYH. We summarize evidence that MUTYH is involved in rapid DNA damage response (DDR) signaling, including PARP activation, 9-1-1 and ATR signaling, and SIRT6 activity. MUTYH alters survival and DDR to a wide variety of DNA damaging agents in a time course that is not consistent with the formation of OG:A mispairs. Studies that suggest MUTYH inhibits the repair of alkyl-DNA damage and cyclopyrimidine dimers (CPDs) is reviewed, and evidence of a synthetic lethal interaction with mismatch repair (MMR) is summarized. Based on these studies we suggest that MUTYH has evolved from an OG:A mispair glycosylase to a multifunctional scaffold for DNA damage response signaling.

Analysis of MUTYH alternative transcript expression, promoter function, and the effect of human genetic variants

The human DNA repair gene MUTYH, whose mutational loss causes a colorectal polyposis and cancer predisposition, contains three alternative first exons. In order to analyze alternative transcription and the effect of genetic alterations found in humans, we established a cell-based minigene experimental model supporting transcription and splicing and thoroughly verified its functionality. We identified highly conserved promoter areas and inactivated them in the minigene, and also introduced six human variants. Moreover, the potential contribution of CpG island methylation and specific transcription factors on MUTYH transcription was addressed. The findings allowed to attribute regulatory roles to three conserved motifs in the promoter: an M4 motif, a transcription factor IIB recognition element, and a GC box. Moreover, the data showed that three patient variants compromised MUTYH expression and therefore have the potential to cause pathogenic effects. We did not find evidence for a biologically relevant contribution of CpG island methylation or a direct transcriptional activation by DNA damage. Besides insight into the regulation of MUTYH transcription, the work therefore provides a functional MUTYH minigene experimental system suitable as a diagnostic tool for analyzing patient variants, and a functional map of the promotor that also can facilitate pathogenicity classifications of human variants.

TISSUE EXPRESION OF THE GENES MUTYH AND OGG1 IN PATIENTS WITH SPORADIC COLORECTAL CANCER

BACKGROUND

MTUYH and OGG1 genes have importance in the base excision repair systems of oxidized DNA bases. Modification of the tissue expression of these genes is related to the increased risk of developing colorectal cancer.

AIM

To evaluate the tissue expression of MUTYH and OGG1 comparing normal and neoplastic tissues of patients with sporadic colorectal cancer and to correlate it with clinical and histopathological variables.

METHOD

MUTYH and OGG1 tissue expression was quantified by RT-PCR in patients with colorectal cancer and the values were compared in normal and neoplastic tissues. MUTYH and OGG1 expression was measured and normalized to the constitutive 18S gene. The level of expression of both genes was correlated with the variables: age, gender, tumor location, size of the tumor, histological type, degree of cell differentiation, invasion depth in the intestinal wall, angiolymphatic infiltration, lymph node involvement and TNM staging.

RESULTS

Was found downregulation of both genes in neoplastic when compared to normal tissue. There was downregulation of the MUTYH in larger tumors and in patients with angiolymphatic invasion. Tumors with more advanced TNM stages (III and IV) presented downregulation of both genes when compared to those with earlier stages (I and II).

CONCLUSION

The MUTYH and OGG1 genes present downregulation in the more advanced stages of colorectal cancer.

Chk1 inhibition as a novel therapeutic strategy in melanoma

Melanoma patients respond poorly to chemotherapies because they acquire drug resistance. Therapies that can overcome the resistance to inhibitors of the mutated BRAF protein kinase in melanoma are urgently needed. Chk1 protein kinase is a central component of the DNA damage response and plays a crucial role in controlling cell cycle progression. Analyses indicate that low mRNA expression of Chk1 is significantly associated with good overall survival of melanoma patients. To evaluate the effectiveness of Chk1 inhibitors in melanoma therapy, we have generated BRAF inhibitor (PLX4032 or vemurafenib) resistant melanoma cell lines (A375-PLX-R and WM9-PLX-R) from A375 and WM9, respectively. We observe that AKT (protein kinase B) is constitutively activated in A375-PLX-R, but not in WM9-PLX-R cells, suggesting that these cells develop resistance to PLX4032 through different mechanisms. We show that a potent and specific inhibitor of Chk1 (PF477736) is effective in reducing cell viability and colony formation of PLX4032-resistant cells. Even more impressively, PF477736 triggers PLX4032-resistant melanoma cells to regain sensitivity to the PLX4032. Mouse xenograft studies show that treating A375-PLX-R derived tumors with combined PLX4032 and PF477736 significantly reduce tumor growth. Combined treatments with PLX4032 and PF477736 reduce the levels of total Chk1 protein and alter Chk1 phosphorylation at several sites in both PLX4032 sensitive and resistant melanoma cells. Combinatorial treatments with PLX4032 and PF477736 to melanoma cells substantially induce DNA damage and cell death. Our results suggest that Chk1 inhibitors may provide new therapy options for melanoma patients.

atz-1 Influences meiosis to maintain germline chromosomal stability in Caenorhabditis elegans

Exchange of genetic information during meiosis occurs in all sexually reproducing species to produce haploid gametes from diploid cells. This process involves tight coordination of a meiotic specific cohesin complex, the synaptonemal complex, and DNA damage repair mechanisms. In this study, we describe a putative myosin heavy chain protein orthologous to human myosin 1, F28D1.2, which we named Abnormal Transition Zone (atz-1). Deletion of atz-1 results in embryonic lethality and a depleted transition zone, accompanied by reduced expression of the meiotic cohesin protein, REC-8. atz-1 mutants display disorganized and aggregated chromosomal bodies in diakinetic oocytes. In addition to this, atz-1 mutants are hypersensitive to mild inhibition of DNA damage repair, suggesting that DNA replication in atz-1 mutants is impaired. Moreover, the atz-1 mutant phenotype is germline specific and resupplying somatically expressed atz-1 does not rescue the reproductive defects associated with atz-1 mutants. Overall, our data suggest that atz-1 contributes to meiosis and maintains germline chromosomal stability.

Association of monoallelic MUTYH mutation among Egyptian patients with colorectal cancer

Colorectal cancer (CRC) is a heterogeneous triat that involves both environmental and genetic factors. Genetic mutations of MUTYH (p.Y179C and p.G396D) have been reported to be associated with increased risk of CRC among several ethnic populations. The aim of this work is to assess the association of the monoallelic MUTYH mutations (p.Y179C and p.G396D) with increased risk of CRC among Egyptian patients. This study included 120 unrelated CRC Egyptian patients who were compared with 100 healthy controls from the same locality. For all individuals, DNA was genotyped for MUTYH p.Y179C and MUTYH p.G396D mutations using the T-ARMS-PCR technique. The frequencies of monoallelic MUTYH mutations showed a strong association with the increased risk of CRC among Egyptian patients compared with controls (12.5 vs. 4.0 %, OR = 3.49, 95 % CI = 1.12-10.90, P = 0.03). Moreover, the frequency of MUTYH p.Y179C mutation was noted to be significantly higher among CRC patients compared to controls rather than MUTYH p.G396D mutation. Interestingly, CRC patients with tumors in the right side colon showed an evidence for association with the MUTYH p.Y179C mutation compared with tumors in the left side colon (p = 0.01). MUTYH p.Y179C mutation was associated with an increased risk of CRC among Egyptian patients rather than MUTYH p.G396D mutation.

MutY DNA Glycosylase Protects Cells From Tumor Necrosis Factor Alpha-Induced Necroptosis

Numerous studies have implied that mutY DNA glycosylase (MYH) is involved in the repair of post-replicative mispairs and plays a critical role in the base excision repair pathway. Recent in vitro studies have shown that MYH interacts with tumor necrosis factor receptor type 1-associated death domain (TRADD), a key effector protein of tumor necrosis factor receptor-1 (TNFR1) signaling. The association between MYH and TRADD is reversed during tumor necrosis factor alpha (TNF-α)- and camptothecin (CPT)-induced apoptosis, and enhanced during TNF-α-induced survival. After investigating the role of MYH interacts with various proteins following TNF-α stimulation, here, we focus on MYH and TRADD interaction functions in necroptosis and its effects to related proteins. We report that the level of the MYH and TRADD complex was also reduced during necroptosis induced by TNF-α and zVAD-fmk. In particular, we also found that MYH is a biologically important necrosis suppressor. Under combined TNF-α and zVAD-fmk treatment, MYH-deficient cells were induced to enter the necroptosis pathway but primary mouse embryonic fibroblasts (MEFs) were not. Necroptosis in the absence of MYH proceeds via the inactivation of caspase-8, followed by an increase in the formation of the kinase receptor- interacting protein 1 (RIP1)-RIP3 complex. Our results suggested that MYH, which interacts with TRADD, inhibits TNF-α necroptotic signaling. Therefore, MYH inactivation is essential for necroptosis via the downregulation of caspase-8. J. Cell. Biochem. 118: 1827-1838, 2017. © 2017 Wiley Periodicals, Inc.

MutY-Homolog (MYH) inhibition reduces pancreatic cancer cell growth and increases chemosensitivity

Patients with pancreatic ductal adenocarcinoma (PC) have a poor prognosis due to metastases and chemoresistance. PC is characterized by extensive fibrosis, which creates a hypoxic microenvironment, and leads to increased chemoresistance and intracellular oxidative stress. Thus, proteins that protect against oxidative stress are potential therapeutic targets for PC. A key protein that maintains genomic integrity against oxidative damage is MutY-Homolog (MYH). No prior studies have investigated the function of MYH in PC cells. Using siRNA, we showed that knockdown of MYH in PC cells 1) reduced PC cell proliferation and increased apoptosis; 2) further decreased PC cell growth in the presence of oxidative stress and chemotherapy agents (gemcitabine, paclitaxel and vincristine); 3) reduced PC cell metastatic potential; and 4) decreased PC tumor growth in a subcutaneous mouse model in vivo. The results from this study suggest MYH may be a novel therapeutic target for PC that could potentially improve patient outcome by reducing PC cell survival, increasing the efficacy of existing drugs and reducing metastatic spread.

Exome-wide single-base substitutions in tissues and derived cell lines of the constitutive Fhit knockout mouse

Loss of expression of Fhit, a tumor suppressor and genome caretaker, occurs in preneoplastic lesions during development of many human cancers. Furthermore, Fhit-deficient mouse models are exquisitely susceptible to carcinogen induction of cancers of the lung and forestomach. Due to absence of Fhit genome caretaker function, cultured cells and tissues of the constitutive Fhit knockout strain develop chromosome aneuploidy and allele copy number gains and losses and we hypothesized that Fhit-deficient cells would also develop point mutations. On analysis of whole exome sequences of Fhit-deficient tissues and cultured cells, we found 300 to >1000 single-base substitutions associated with Fhit loss in the 2% of the genome included in exomes, relative to the C57Bl6 reference genome. The mutation signature is characterized by increased C>T and T>C mutations, similar to the "age at diagnosis" signature identified in human cancers. The Fhit-deficiency mutation signature also resembles a C>T and T>C mutation signature reported for human papillary kidney cancers and a similar signature recently reported for esophageal and bladder cancers, cancers that are frequently Fhit deficient. The increase in T>C mutations in -/- exomes may be due to dNTP imbalance, particularly in thymidine triphosphate, resulting from decreased expression of thymidine kinase 1 in Fhit-deficient cells. Fhit-deficient kidney cells that survived in vitro dimethylbenz(a)anthracene treatment additionally showed increased T>A mutations, a signature generated by treatment with this carcinogen, suggesting that these T>A transversions may be evidence of carcinogen-induced preneoplastic changes.

SIRT6 protein deacetylase interacts with MYH DNA glycosylase, APE1 endonuclease, and Rad9-Rad1-Hus1 checkpoint clamp

Background

SIRT6, a member of the NAD⁺-dependent histone/protein deacetylase family, regulates genomic stability, metabolism, and lifespan. MYH glycosylase and APE1 are two base excision repair (BER) enzymes involved in mutation avoidance from oxidative DNA damage. Rad9–Rad1–Hus1 (9–1–1) checkpoint clamp promotes cell cycle checkpoint signaling and DNA repair. BER is coordinated with the checkpoint machinery and requires chromatin remodeling for efficient repair. SIRT6 is involved in DNA double-strand break repair and has been implicated in BER. Here we investigate the direct physical and functional interactions between SIRT6 and BER enzymes.

Results

We show that SIRT6 interacts with and stimulates MYH glycosylase and APE1. In addition, SIRT6 interacts with the 9-1-1 checkpoint clamp. These interactions are enhanced following oxidative stress. The interdomain connector of MYH is important for interactions with SIRT6, APE1, and 9–1–1. Mutagenesis studies indicate that SIRT6, APE1, and Hus1 bind overlapping but different sequence motifs on MYH. However, there is no competition of APE1, Hus1, or SIRT6 binding to MYH. Rather, one MYH partner enhances the association of the other two partners to MYH. Moreover, APE1 and Hus1 act together to stabilize the MYH/SIRT6 complex. Within human cells, MYH and SIRT6 are efficiently recruited to confined oxidative DNA damage sites within transcriptionally active chromatin, but not within repressive chromatin. In addition, Myh foci induced by oxidative stress and Sirt6 depletion are frequently localized on mouse telomeres.

Conclusions

Although SIRT6, APE1, and 9-1-1 bind to the interdomain connector of MYH, they do not compete for MYH association. Our findings indicate that SIRT6 forms a complex with MYH, APE1, and 9-1-1 to maintain genomic and telomeric integrity in mammalian cells.

Electronic supplementary material

The online version of this article (doi:10.1186/s12867-015-0041-9) contains supplementary material, which is available to authorized users.

Association of the Rad9-Rad1-Hus1 checkpoint clamp with MYH DNA glycosylase and DNA

Cell cycle checkpoints provide surveillance mechanisms to activate the DNA damage response, thus preserving genomic integrity. The heterotrimeric Rad9-Rad1-Hus1 (9-1-1) clamp is a DNA damage response sensor and can be loaded onto DNA. 9-1-1 is involved in base excision repair (BER) by interacting with nearly every enzyme in BER. Here, we show that individual 9-1-1 components play distinct roles in BER directed by MYH DNA glycosylase. Analyses of Hus1 deletion mutants revealed that the interdomain connecting loop (residues 134-155) is a key determinant of MYH binding. Both the N-(residues 1-146) and C-terminal (residues 147-280) halves of Hus1, which share structural similarity, can interact with and stimulate MYH. The Hus1(K136A) mutant retains physical interaction with MYH but cannot stimulate MYH glycosylase activity. The N-terminal domain, but not the C-terminal half of Hus1 can also bind DNA with moderate affinity. Intact Rad9 expressed in bacteria binds to and stimulates MYH weakly. However, Rad9(1-266) (C-terminal truncated Rad9) can stimulate MYH activity and bind DNA with high affinity, close to that displayed by heterotrimeric 9(1-266)-1-1 complexes. Conversely, Rad1 has minimal roles in stimulating MYH activity or binding to DNA. Finally, we show that preferential recruitment of 9(1-266)-1-1 to 5'-recessed DNA substrates is an intrinsic property of this complex and is dependent on complex formation. Together, our findings provide a mechanistic rationale for unique contributions by individual 9-1-1 subunits to MYH-directed BER based on subunit asymmetry in protein-protein interactions and DNA binding events.

MUTYH mediates the toxicity of combined DNA 6-thioguanine and UVA radiation

The therapeutic thiopurines, including the immunosuppressant azathioprine (Aza) cause the accumulation of the UVA photosensitizer 6-thioguanine (6-TG) in the DNA of the patients' cells. DNA 6-TG and UVA are synergistically cytotoxic and their interaction causes oxidative damage. The MUTYH DNA glycosylase participates in the base excision repair of oxidized DNA bases. Using Mutyh-nullmouse fibroblasts (MEFs) we examined whether MUTYH provides protection against the lethal effects of combined DNA 6-TG/UVA. Surprisingly, Mutyh-null MEFs were more resistant than wild-type MEFs, despite accumulating higher levels of DNA 8-oxo-7,8-dihydroguanine (8-oxoG).Their enhanced 6-TG/UVA resistance reflected the absence of the MUTYH protein and MEFs expressing enzymatically-dead human variants were as sensitive as wild-type cells. Consistent with their enhanced resistance, Mutyh-null cells sustained fewer DNA strand breaks and lower levels of chromosomal damage after 6-TG/UVA. Although 6-TG/UVA treatment caused early checkpoint activation irrespective of the MUTYH status, Mutyh-null cells failed to arrest in S-phase at late time points. MUTYH-dependent toxicity was also apparent in vivo. Mutyh-/- mice survived better than wild-type during a 12-month chronicexposure to Aza/UVA treatments that significantly increased levels of skin DNA 8-oxoG. Two squamous cell skin carcinomas arose in Aza/UVA treated Mutyh-/- mice whereas similarly treated wild-type animals remained tumor-free.

Histone/protein deacetylase SIRT1 is an anticancer therapeutic target

SIRT1, a member of the NAD(+)-dependent histone/protein deacetylase family, is involved in chromatin remodeling, DNA repair, and stress response and is a potential drug target. 5-fluorouracil (FU) and the SN1-type DNA methylating agent temozolomide (TMZ) are anticancer agents. In this study, we demonstrate that sirt1 knockout mouse embryonic fibroblast cells are more sensitive to FU and DNA methylating agents than normal cells. Based on these findings, the chemotherapy efficacy of SIRT1 inhibitors in combination with FU or TMZ were tested with human breast cancer cells. We found that treatments combining SIRT1 inhibitors with FU or TMZ show synergistic reduction of cell viability and colony formation of breast cancer cells. Thus, inhibition of SIRT1 activity provides a novel anticancer strategy.

Regulation of human MutYH DNA glycosylase by the E3 ubiquitin ligase mule

Oxidation of DNA is a frequent and constantly occurring event. One of the best characterized oxidative DNA lesions is 7,8-dihydro-8-oxoguanine (8-oxo-G). It instructs most DNA polymerases to preferentially insert an adenine (A) opposite 8-oxo-G instead of the appropriate cytosine (C) thus showing miscoding potential. The MutY DNA glycosylase homologue (MutYH) recognizes A:8-oxo-G mispairs and removes the mispaired A giving way to the canonical base excision repair that ultimately restores undamaged guanine (G). Here we characterize for the first time in detail a posttranslational modification of the human MutYH DNA glycosylase. We show that MutYH is ubiquitinated in vitro and in vivo by the E3 ligase Mule between amino acids 475 and 535. Mutation of five lysine residues in this region significantly stabilizes MutYH, suggesting that these are the target sites for ubiquitination. The endogenous MutYH protein levels depend on the amount of expressed Mule. Furthermore, MutYH and Mule physically interact. We found that a ubiquitination-deficient MutYH mutant shows enhanced binding to chromatin. The mutation frequency of the ovarian cancer cell line A2780, analyzed at the HPRT locus can be increased upon oxidative stress and depends on the MutYH levels that are regulated by Mule. This reflects the importance of tightly regulated MutYH levels in the cell. In summary our data show that ubiquitination is an important regulatory mechanism for the essential MutYH DNA glycosylase in human cells.

Metal-mediated DNA damage and cell death: mechanisms, detection methods, and cellular consequences

Metal ions cause various types of DNA damage by multiple mechanisms, and this damage is a primary cause of cell death and disease.