5mC-hydroxylase activity is influenced by the PARylation of TET1 enzyme

5-hydroxymethylcytosine is a new epigenetic modification deriving from the oxidation of 5-methylcytosine by the TET hydroxylase enzymes. DNA hydroxymethylation drives DNA demethylation events and is involved in the control of gene expression. Deregulation of TET enzymes causes developmental defects and is associated with pathological conditions such as cancer. Little information thus far is available on the regulation of TET activity by post-translational modifications. Here we show that TET1 protein is able to interact with PARP-1/ARTD1 enzyme and is target of both noncovalent and covalent PARylation. In particular, we have demonstrated that the noncovalent binding of ADP-ribose polymers with TET1 catalytic domain decreases TET1 hydroxylase activity while the covalent PARylation stimulates TET1 enzyme. In addition, TET1 activates PARP-1/ARTD1 independently of DNA breaks. Collectively, our results highlight a complex interplay between PARylation and TET1 which may be helpful in coordinating the multiple biological roles played by 5-hydroxymethylcytosine and TET proteins.     

PARP-1及其抑制剂与肿瘤发生、发展及放射敏感性关系研究进展

聚腺苷二磷酸核糖聚合酶-1(PARP-1)是一种由氨基酸残基组成的细胞核酶,在DNA损伤修复过程中发挥重要作用。近年来,研究证实PARP-1与肿瘤的发生、发展等生物学行为密切相关,并且可能影响肿瘤放射敏感性。文章就PARP-1及其抑制剂与肿瘤的发生、发展及放射敏感性的关系作一综述。     

PARP-1 and PARP-2: New players in tumour development

Poly(ADP-ribose) polymerase-1 (PARP-1) and PARP-2 belong to a family of enzymes that, using NAD(+) as a substrate, catalyze poly(ADP-ribosyl)ation of proteins. PARP-1 and PARP-2 catalytic activity is stimulated by DNA-strand breaks targeting mainly proteins involved in chromatin structure and DNA metabolism, providing strong support for a dual role of both PARP-1 and PARP-2 in the DNA damage response as DNA damage sensors and signal transducers to downstream effectors. The DNA damage response has important consequences for genomic stability and tumour development. In order to manipulate DNA damage responses to selectively induce tumour cell death, a considerable effort is centred on defining the molecular mechanisms that allow cells to detect, respond to, and repair DNA damage. PARP inhibitors that compete with NAD+ at the highly conserved enzyme active site are arisen as new potential therapeutic strategies as chemo- and radiopotentiation and for the treatment of cancers with specific DNA repair defects as single-agent therapies. In the present review, we highlight emerging information about the redundant and specific functions of PARP-1 and PARP-2 in genome surveillance and DNA repair pathways. Understanding these roles might provide invaluable clues to design new cancer therapeutic approaches. In addition, we provide an overview of ongoing clinical trials with PARP inhibitors and the value of PARP-1 and PARP-2 expression as prognostic biomarkers in cancer.     

Inhibition of poly (ADP-Ribose) polymerase: A promising strategy targeting pancreatic cancer with BRCAness phenotype

The use of chemotherapeutic regimens for the treatment of pancreatic cancer is still limited because pancreatic cancer is usually diagnosed at an advanced stage as a refractory disease in which symptoms are difficult to recognize in the early stages. Furthermore, at advanced stages, there are important challenges to achieve clinical benefit and symptom resolution, even with the use of an expanded spectrum of anticancer drugs. Recently, a point of reduced susceptibility to conventional chemotherapies by breast cancer susceptibility gene (BRCA) mutations led to a new perspective for overcoming the resistance of pancreatic cancer within the framework of increased genome instability. Poly (ADP-Ribose) polymerase (PARP) -1 is an enzyme that can regulate intrinsic functions, such as response to DNA damage. Therefore, in an environment where germline mutations in BRCAs (BRCAness) inhibit homologous recombination in DNA damage, resulting in a lack of DNA damage response, a key role of PARP-1 for the adaptation of the genome instability could be further emphasized. Here, we summarized the key functional role of PARP-1 in genomic instability of pancreatic cancer with the BRCAness phenotype and listed clinical applications and outcomes of PARP-1 inhibitors to highlight the importance of targeting PARP-1 activity.     

Epigenetic regulation of human retinoblastoma

Retinoblastoma is a rare type of eye cancer of the retina that commonly occurs in early childhood and mostly affects the children before the age of 5. It occurs due to the mutations in the retinoblastoma gene (RB1) which inactivates both alleles of the RB1. RB1 was first identified as a tumor suppressor gene, which regulates cell cycle components and associated with retinoblastoma. Previously, genetic alteration was known as the major cause of its occurrence, but later, it is revealed that besides genetic changes, epigenetic changes also play a significant role in the disease. Initiation and progression of retinoblastoma could be due to independent or combined genetic and epigenetic events. Remarkable work has been done in understanding retinoblastoma pathogenesis in terms of genetic alterations, but not much in the context of epigenetic modification. Epigenetic modifications that silence tumor suppressor genes and activate oncogenes include DNA methylation, chromatin remodeling, histone modification and noncoding RNA-mediated gene silencing. Epigenetic changes can lead to altered gene function and transform normal cell into tumor cells. This review focuses on important epigenetic alteration which occurs in retinoblastoma and its current state of knowledge. The critical role of epigenetic regulation in retinoblastoma is now an emerging area, and better understanding of epigenetic changes in retinoblastoma will open the door for future therapy and diagnosis.     

The role of epigenetic therapies in colorectal cancer

Although developments in the diagnosis and therapy of colorectal cancer (CRC) have been made in the last decade, much work remains to be done as it remains the second leading cause of cancer death. It is now well established that epigenetic events, together with genetic alterations, are key events in initiation and progression of CRC. Epigenetics refers to heritable alterations in gene expression that do not involve changes in the DNA sequence. These alterations include DNA methylation, histone alterations, chromatin remodelers, and noncoding RNAs. In CRC, aberrations in epigenome may also involve in the development of drug resistance to conventional drugs such as 5-fluorouracil, oxaliplatin, and irinotecan. Thus, it has been suggested that combined therapies with epigenetic agents may reverse drug resistance. In this regard, DNA methyltransferase inhibitors and histone deacetylase inhibitors have been extensively investigated in CRC. The aim of this review is to provide a brief overview of the preclinical data that represent a proof of principle for the employment of epigenetic agents in CRC with a focus on the advantages of combinatorial therapy over single-drug treatment. We will also critically discuss the results and limitations of initial clinical experiences of epigenetic-based therapy in CRC and summarize ongoing clinical trials. Nevertheless, since recent translational research suggest that epigenetic modulators play a key role in augmenting immunogenicity of the tumor microenvironment and in restoring immune recognition, we will also highlight the recent developments of combinations strategies of immunotherapies and epigenetic therapies in CRC, summarizing preclinical, and clinical data to signify this evolving and promising field for CRC treatment.     

表观遗传学在结直肠癌中的应用进展

结直肠癌（CRC）的发生、发展伴随着许多基因的表达变化,但其具体调控机制至今仍未完全阐明。近年来对CRC表观遗传学尤其是微小RNA（miRNA）、异常DNA甲基化及组蛋白修饰状态等方面的研究受到广泛关注。研究证实,CRC进展过程中均存在异常的甲基化基因和miRNA的表达变化。与癌症基因组的基因突变一样,这类遗传学的分子改变在CRC中扮演着重要角色。表观遗传学的特异性改变可作为CRC诊断、治疗和预后的临床生物学标记物,对表观遗传学进行深入研究对CRC的防治具有重要指导意义。     

TNKS1BP1 functions in DNA double-strand break repair though facilitating DNA-PKcs autophosphorylation dependent on PARP-1

TNKS1BP1 was originally identified as an interaction protein of tankyrase 1, which belongs to the poly(ADP-ribose) polymerase (PARP) superfamily. PARP members play important roles for example in DNA repair, telomere stability and mitosis regulation. Although the TNKS1BP1 protein was considered to be a poly(ADP-ribosyl)ation acceptor of tankyrase 1, its function is still unknown. Here we firstly identified that TNKS1BP1 was up-regulated by ionizing radiation (IR) and the depletion of TNKS1BP1 significantly sensitized cancer cells to IR. Neutral comet assay, pulsed-field gel electrophoresis, and γH2AX foci analysis indicated that TNKS1BP1 is required for the efficient repair of DNA double-strand breaks (DSB). The TNKS1BP1 protein was demonstrated to interact with DNA-dependent protein kinase (DNA-PKcs) and poly(ADP-ribose) polymerase 1 (PARP-1), by co-immunoprecipitation analysis. Moreover, TNKS1BP1 was shown to promote the association of PARP-1 and DNA-PKcs. Overexpression of TNKS1BP1 induced the autophosphorylation of DNA-PKcs/Ser2056 in a PARP-1 dependent manner, which contributed to an increased capability of DNA DSB repair. Inhibition of PARP-1 blocked the TNKS1BP1-mediated DNA-PKcs autophosphorylation and attenuated the PARylation of DNA-PKcs. TNKS1BP1 is a newly described component of the DNA DSB repair machinery, which provides much more mechanistic evidence for the rationale of developing effective anticancer measures by targeting PARP-1 and DNA-PKcs.     

Histone modifications and silencing prior to DNA methylation of a tumor suppressor gene

We attempted to answer two central questions about epigenetic silencing of the tumor suppressor gene p16(INK4a) in this study: (1) whether the maintenance of associated histone modifications is dependent on DNA methylation and (2) whether such histone modifications can occur prior to DNA methylation. By coupling chromatin immunoprecipitation with gene targeting and the analysis of specific alleles, we found that elimination of DNA methylation from a p16(INK4a) allele resulted in profound changes in surrounding histones. After continued passage of such cells, methylation of histone H3 lysine-9 occurred in conjunction with re-silencing in the absence of DNA methylation. These results have important implications for understanding the biochemical events underlying the silencing of tumor suppressor genes and the resultant growth suppression.     

RETRACTED ARTICLE: Poly(Adenosine Diphosphate-Ribose) Polymerase-1 Expression in Cutaneous Malignant Melanomas as a New Molecular Marker of Aggressive Tumor

Poly(adenosine diphosphate-ribose) polymerases (PARPs) are a family of enzymes, which catalyses poly (ADP-ribosyl)ation of DNA-binding proteins and directly involved in genomic stability, DNA repair, and apoptosis. In this study, we evaluated the immunomorphology of PARP-1 in melanoma and its prognostic importance. We studied PARP-1 expression by immunohistochemistry in a selected series of 54 primary cutaneous malignant melanoma (CMM). The findings of the present study suggest that the neoplastic progression toward the invasive (both horizontal and vertical) growth phase of CMM cells is characterized by the loss of cleavage of PARP-1, probably signaling an imbalance of the apoptotic process in these cells and leading to further gain to aggression. Over-expression of full-length PARP-1 was correlated with recurrence and/or progression of the disease and so act as a promising new biological marker of CMM. Our study represents the evidence of a direct correlation between the PARP-1-mediated apoptotic process and the biologic behavior of CMM.     

聚ADP核糖聚合酶-1及其抑制剂与肿瘤治疗

聚ADP核糖聚合酶-1(PARP-1)能修复DNA损伤,从而维持细胞内环境与基因组稳定.多数恶性肿瘤细胞中PARP-1活性增强,因此,敲除PARP-1基因或使用PARP-1抑制剂能降低肿瘤细胞的DNA修复功能,增强其对DNA损伤因子的敏感性,从而提高肿瘤放疗和化疗疗效.PARP-1有望成为恶性肿瘤治疗的一个新靶点.     

Higher order chromatin organization in cancer

In spite of our increased understanding of how genomes are dysregulated in cancer and a plethora of molecular diagnostic tools, the front line and ‘gold standard’ detection of cancer remains the pathologist's detection of gross changes in cellular and tissue structure, most strikingly nuclear dis-organization. In fact, for over 140 years it has been noted that nuclear morphology is often disrupted in cancer. Even today, nuclear morphology measures include nuclear size, shape, DNA content (ploidy) and ‘chromatin organization’. Given the importance of nuclear shape to diagnoses of cancer phenotypes, it is surprising and frustrating that we currently lack a detailed understanding to explain these changes and how they might arise and relate to molecular events in the cell. It is an implicit hypothesis that perturbation of chromatin and epigenetic signatures may lead to alterations in nuclear structure (or vice versa) and that these perturbations lie at the heart of cancer genesis. In this review, we attempt to synthesize research leading to our current understanding on how chromatin interactions at the nuclear lamina, epigenetic modulation and gene regulation may intersect in cancer and offer a perspective on critical experiments that would help clarify how nuclear architecture may contribute to the cancerous phenotype. We also discuss the historical understanding of nuclear structure in normal cells and as a diagnostic in cancer.     

Roles of Chk1 in cell biology and cancer therapy

The evolutionally conserved DNA damage response (DDR) and cell cycle checkpoints preserve genome integrity. Central to these genome surveillance pathways is a protein kinase, Chk1. DNA damage induces activation of Chk1, which then transduces the checkpoint signal and facilitates cell cycle arrest and DNA damage repair. Significant progress has been made recently toward our understanding of Chk1 regulation and its implications in cancer etiology and therapy. Specifically, a model that involves both spatiotemporal and conformational changes of proteins has been proposed for Chk1 activation. Further, emerging evidence suggests that Chk1 does not appear to be a tumor suppressor; instead, it promotes tumor growth and may contribute to anticancer therapy resistance. Recent data from our laboratory suggest that activating, but not inhibiting, Chk1 in the absence of chemotherapy might represent an innovative approach to suppress tumor growth. These findings suggest unique regulation of Chk1 in cell biology and cancer etiology, pointing to novel strategies for targeting Chk1 in cancer therapy.     

Epigenetic and chromatin modifiers as targeted therapy of hematologic malignancies.

Epigenetic regulation of gene expression is mediated through alterations in the DNA methylation status, covalent modifications of core nucleosomal histones, rearrangement of histones, and by RNA interference. It is now abundantly clear that deregulation of epigenetic mechanisms cooperates with genetic alterations in the development and progression of cancer and leukemia. Epigenetic deregulation affects several aspects of tumor cell biology, including cell growth, cell cycle control, differentiation, DNA repair, and cell death. This raises the strong possibility that reversing deregulated epigenetic mechanisms may be an effective treatment strategy for leukemia and cancer. This treatment strategy may either be designed to separately or collectively target the specific perturbations in the epigenetic mechanisms found in human hematologic malignancies. The following review describes our current understanding of the important deregulated epigenetic mechanisms and the preclinical and clinical development of epigenetic and chromatin modifiers in the therapy of these disorders.