Luis F.Z. Batista

Apoptosis in malignant glioma cells triggered by the temozolomide-induced DNA lesion O6-methylguanine

Methylating drugs such as temozolomide (TMZ) are widely used in the treatment of brain tumours (malignant gliomas). The mechanism of TMZ-induced glioma cell death is unknown. Here, we show that malignant glioma cells undergo apoptosis following treatment with the methylating agents N-methyl-N′-nitro-N-nitrosoguanidine (MNNG) and TMZ. Cell death determined by colony formation and apoptosis following methylation is greatly stimulated by p53. Transfection experiments with O6-methylguanine-DNA methyltransferase (MGMT) and depletion of MGMT by O6-benzylguanine showed that, in gliomas, the apoptotic signal originates from O6-methylguanine (O6MeG) and that repair of O6MeG by MGMT prevents apoptosis. We further demonstrate that O6MeG-triggered apoptosis requires Fas/CD95/Apo-1 receptor activation in p53 non-mutated glioma cells, whereas in p53 mutated gliomas the same DNA lesion triggers the mitochondrial apoptotic pathway. This occurs less effectively via Bcl-2 degradation and caspase-9, -2, -7 and -3 activation. O6MeG-triggered apoptosis in gliomas is a late response (occurring >120 h after treatment) that requires extensive cell proliferation. Stimulation of cell cycle progression by the Pasteurella multocida toxin promoted apoptosis whereas serum starvation attenuated it. O6MeG-induced apoptosis in glioma cells was preceded by the formation of DNA double-strand breaks (DSBs), as measured by γH2AX formation. Glioma cells mutated in DNA-PKcs, which is involved in non-homologous end-joining, were more sensitive to TMZ-induced apoptosis, supporting the involvement of DSBs as a downstream apoptosis triggering lesion. Overall, the data demonstrate that cell death induced by TMZ in gliomas is due to apoptosis and that determinants of sensitivity of gliomas to TMZ are MGMT, p53, proliferation rate and DSB repair.

In Situ Genetic Correction of the Sickle Cell Anemia Mutation in Human Induced Pluripotent Stem Cells Using Engineered Zinc Finger Nucleases

The combination of induced pluripotent stem cell (iPSC) technology and targeted gene modification by homologous recombination (HR) represents a promising new approach to generate genetically corrected, patient‐derived cells that could be used for autologous transplantation therapies. This strategy has several potential advantages over conventional gene therapy including eliminating the need for immunosuppression, avoiding the risk of insertional mutagenesis by therapeutic vectors, and maintaining expression of the corrected gene by endogenous control elements rather than a constitutive promoter. However, gene targeting in human pluripotent cells has remained challenging and inefficient. Recently, engineered zinc finger nucleases (ZFNs) have been shown to substantially increase HR frequencies in human iPSCs, raising the prospect of using this technology to correct disease causing mutations. Here, we describe the generation of iPSC lines from sickle cell anemia patients and in situ correction of the disease causing mutation using three ZFN pairs made by the publicly available oligomerized pool engineering method (OPEN). Gene‐corrected cells retained full pluripotency and a normal karyotype following removal of reprogramming factor and drug‐resistance genes. By testing various conditions, we also demonstrated that HR events in human iPSCs can occur as far as 82 bps from a ZFN‐induced break. Our approach delineates a roadmap for using ZFNs made by an open‐source method to achieve efficient, transgene‐free correction of monogenic disease mutations in patient‐derived iPSCs. Our results provide an important proof of principle that ZFNs can be used to produce gene‐corrected human iPSCs that could be used for therapeutic applications. STEM CELLS 2011;29:1717–1726

Telomere shortening and loss of self-renewal in dyskeratosis congenita induced pluripotent stem cells

The differentiation of patient-derived induced pluripotent stem cells (iPSCs) to committed fates such as neurons, muscle and liver is a powerful approach for understanding key parameters of human development and disease. Whether undifferentiated iPSCs themselves can be used to probe disease mechanisms is uncertain. Dyskeratosis congenita is characterized by defective maintenance of blood, pulmonary tissue and epidermal tissues and is caused by mutations in genes controlling telomere homeostasis. Short telomeres, a hallmark of dyskeratosis congenita, impair tissue stem cell function in mouse models, indicating that a tissue stem cell defect may underlie the pathophysiology of dyskeratosis congenita. Here we show that even in the undifferentiated state, iPSCs from dyskeratosis congenita patients harbour the precise biochemical defects characteristic of each form of the disease and that the magnitude of the telomere maintenance defect in iPSCs correlates with clinical severity. In iPSCs from patients with heterozygous mutations in TERT, the telomerase reverse transcriptase, a 50% reduction in telomerase levels blunts the natural telomere elongation that accompanies reprogramming. In contrast, mutation of dyskerin (DKC1) in X-linked dyskeratosis congenita severely impairs telomerase activity by blocking telomerase assembly and disrupts telomere elongation during reprogramming. In iPSCs from a form of dyskeratosis congenita caused by mutations in TCAB1 (also known as WRAP53), telomerase catalytic activity is unperturbed, yet the ability of telomerase to lengthen telomeres is abrogated, because telomerase mislocalizes from Cajal bodies to nucleoli within the iPSCs. Extended culture of DKC1-mutant iPSCs leads to progressive telomere shortening and eventual loss of self-renewal, indicating that a similar process occurs in tissue stem cells in dyskeratosis congenita patients. These findings in iPSCs from dyskeratosis congenita patients reveal that undifferentiated iPSCs accurately recapitulate features of a human stem cell disease and may serve as a cell-culture-based system for the development of targeted therapeutics.

TPP1 OB-Fold Domain Controls Telomere Maintenance by Recruiting Telomerase to Chromosome Ends

Telomere synthesis in cancer cells and stem cells involves trafficking of telomerase to Cajal bodies, and telomerase is thought to be recruited to telomeres through interactions with telomere-binding proteins. Here, we show that the OB-fold domain of the telomere-binding protein TPP1 recruits telomerase to telomeres through an association with the telomerase reverse transcriptase TERT. When tethered away from telomeres and other telomere-binding proteins, the TPP1 OB-fold domain is sufficient to recruit telomerase to a heterologous chromatin locus. Expression of a minimal TPP1 OB-fold inhibits telomere maintenance by blocking access of telomerase to its cognate binding site at telomeres. We identify amino acids required for the TPP1-telomerase interaction, including specific loop residues within the TPP1 OB-fold domain and individual residues within TERT, some of which are mutated in a subset of pulmonary fibrosis patients. These data define a potential interface for telomerase-TPP1 interaction required for telomere maintenance and implicate defective telomerase recruitment in telomerase-related disease.

Differential Sensitivity of Malignant Glioma Cells to Methylating and Chloroethylating Anticancer Drugs: p53 Determines the Switch by Regulating xpc, ddb2, and DNA Double-Strand Breaks

Glioblastoma multiforme is the most severe form of brain cancer. First line therapy includes the methylating agent temozolomide and/or the chloroethylating nitrosoureas [1-(2-chloroethyl)-1-nitrosourea; CNU] nimustine [1-(4-amino-2-methyl-5-pyrimidinyl)methyl-3-(2-chloroethyl)-3-nitrosourea; ACNU], carmustine [1,3-bis(2-chloroethyl)-1-nitrosourea; BCNU], or lomustine [1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea; CCNU]. The mechanism of cell death after CNU treatment is largely unknown. Here we show that ACNU and BCNU induce apoptosis in U87MG [p53 wild-type (p53wt)] and U138MG [p53 mutant (p53mt)] glioma cells. However, contrary to what we observed previously for temozolomide, chloroethylating drugs are more toxic for p53-mutated glioma cells and induce both apoptosis and necrosis. Inactivation of p53 by pifithrin-alpha or siRNA down-regulation sensitized p53wt but not p53mt glioma cells to ACNU and BCNU. ACNU and BCNU provoke the formation of DNA double-strand breaks (DSB) in glioma cells that precede the onset of apoptosis and necrosis. Although these DSBs are repaired in p53wt cells, they accumulate in p53mt cells. Therefore, functional p53 seems to stimulate the repair of CNU-induced cross-links and/or DSBs generated from CNU-induced lesions. Expression analysis revealed an up-regulation of xpc and ddb2 mRNA in response to ACNU in U87MG but not U138MG cells, indicating p53 regulates a pathway that involves these DNA repair proteins. ACNU-induced apoptosis in p53wt glioma cells is executed via both the extrinsic and intrinsic apoptotic pathway, whereas in p53mt glioma cells, the mitochondrial pathway becomes activated. The data suggest that p53 has opposing effects in gliomas treated with methylating or chloroethylating agents and, therefore, the p53 status should be taken into account when deciding which therapeutic drug to use.

Inhibition of Pluripotency Networks by the Rb Tumor Suppressor Restricts Reprogramming and Tumorigenesis

Mutations in the retinoblastoma tumor suppressor gene Rb are involved in many forms of human cancer. In this study, we investigated the early consequences of inactivating Rb in the context of cellular reprogramming. We found that Rb inactivation promotes the reprogramming of differentiated cells to a pluripotent state. Unexpectedly, this effect is cell cycle independent, and instead reflects direct binding of Rb to pluripotency genes, including Sox2 and Oct4, which leads to a repressed chromatin state. More broadly, this regulation of pluripotency networks and Sox2 in particular is critical for the initiation of tumors upon loss of Rb in mice. These studies therefore identify Rb as a global transcriptional repressor of pluripotency networks, providing a molecular basis for previous reports about its involvement in cell fate pliability, and implicate misregulation of pluripotency factors such as Sox2 in tumorigenesis related to loss of Rb function.

p53 mutant human glioma cells are sensitive to UV-C-induced apoptosis due to impaired cyclobutane pyrimidine dimer removal.

The p53 protein is a key regulator of cell responses to DNA damage, and it has been shown that it sensitizes glioma cells to the alkylating agent temozolomide by up-regulating the extrinsic apoptotic pathway, whereas it increases the resistance to chloroethylating agents, such as ACNU and BCNU, probably by enhancing the efficiency of DNA repair. However, because these agents induce a wide variety of distinct DNA lesions, the direct importance of DNA repair is hard to access. Here, it is shown that the induction of photoproducts by UV light (UV-C) significantly induces apoptosis in a p53-mutated glioma background. This is caused by a reduced level of photoproduct repair, resulting in the persistence of DNA lesions in p53-mutated glioma cells. UV-C-induced apoptosis in p53 mutant glioma cells is preceded by strong transcription and replication inhibition due to blockage by unrepaired photolesions. Moreover, the results indicate that UV-C-induced apoptosis of p53 mutant glioma cells is executed through the intrinsic apoptotic pathway, with Bcl-2 degradation and sustained Bax and Bak up-regulation. Collectively, the data indicate that unrepaired DNA lesions induce apoptosis in p53 mutant gliomas despite the resistance of these gliomas to temozolomide, suggesting that efficiency of treatment of p53 mutant gliomas might be higher with agents that induce the formation of DNA lesions whose global genomic repair is dependent on p53. (Mol Cancer Res 2009;7(2):237–46)

Understanding telomere diseases through analysis of patient-derived iPS cells

A unique characteristic of tissue stem cells is the ability to self-renew, a process that enables the life-long maintenance of many organs. Stem cell self-renewal is dependent in part on the synthesis of telomere repeats by the enzyme telomerase. Defects in telomerase and in genes in the telomere maintenance pathway result in diverse disease states, including dyskeratosis congenita, pulmonary fibrosis, aplastic anemia, liver cirrhosis and cancer. Many of these disease states share a tissue failure phenotype, such as loss of bone marrow cells or failure of pulmonary epithelium, suggesting that stem cell dysfunction is a common pathophysiological mechanism underlying these telomere diseases. Studies of telomere diseases in undifferentiated iPS cells have provided a quantitative relationship between the magnitude of biochemical defects in the telomerase pathway and disease severity in patients, thereby establishing a clear correlation between genotype and phenotype in telomere disease states. Modeling telomere diseases in iPS cells has also revealed diverse underlying disease mechanisms, including reduced telomerase catalytic activity, diminished assembly of the telomerase holoenzyme and impaired trafficking of the enzyme within the nucleus. These studies highlight the need for therapies tailored to the underlying biochemical defect in each class of patients.

Effect of the anti-neoplastic drug doxorubicin on XPD-mutated DNA repair-deficient human cells

Doxorubicin (DOX), a member of the anthracycline group, is a widely used drug in cancer therapy. The mechanisms of DOX action include topoisomerase II-poisoning, free radical release, DNA adducts and interstrand cross-link (ICL) formation. Nucleotide excision repair (NER) is involved in the removal of helix-distorting lesions and chemical adducts, however, little is known about the response of NER-deficient cell lines to anti-tumoral drugs like DOX. Wild type and XPD-mutated cells, harbouring mutations in different regions of this gene and leading to XP-D, XP/CS or TTD diseases, were treated with this drug and analyzed for cell cycle arrest and DNA damage by comet assay. The formation of DSBs was also investigated by determination of gammaH2AX foci. Our results indicate that all three NER-deficient cell lines tested are more sensitive to DOX treatment, when compared to wild type cells or XP cells complemented by the wild type XPD cDNA, suggesting that NER is involved in the removal of DOX-induced lesions. The cell cycle analysis showed the characteristic G2 arrest in repair-proficient MRC5 cell line after DOX treatment, whereas the repair-deficient cell lines presented significant increase in sub-G1 fraction. The NER-deficient cell lines do not show different patterns of DNA damage formation as assayed by comet assay and phosphorylated H2AX foci formation. Knock-down of topoisomerase IIalpha with siRNA leads to increased survival in both MRC5 and XP cells, however, XP cell line still remained significantly more sensitive to the treatment by DOX. Our study suggests that the enhanced sensitivity is due to DOX-induced DNA damage that is subject to NER, as we observed decreased unscheduled DNA synthesis in XP-deficient cells upon DOX treatment. Furthermore, the complementation of the XPD-function abolished the observed sensitivity at lower DOX concentrations, suggesting that the XPD helicase activity is involved in the repair of DOX-induced lesions.

Photorepair of RNA polymerase arrest and apoptosis after ultraviolet irradiation in normal and XPB deficient rodent cells.

Cyclobutane pyrimidine dimers (CPDs) are directly involved in signaling for UV-induced apoptosis in mammalian cells. Failure to remove these lesions, specially those located at actively expressing genes, is critical, as cells defective in transcription coupled repair have increased apoptotic levels. Thus, the blockage of RNA synthesis by lesions is an important candidate event triggering off active cell death. In this work, wild-type and XPB mutated Chinese hamster ovary (CHO) cells expressing a marsupial photolyase, that removes specifically CPDs from the damaged DNA, were generated, in order to investigate the importance of this lesion in both RNA transcription blockage and apoptotic induction. Photorepair strongly recovers RNA synthesis in wild-type CHO cell line, although the resumption of transcription is decreased in XPB deficient cells. This recovery is accompanied by the prevention of cells entering into apoptosis. These results demonstrate that marsupial photolyase has access to CPDs blocking RNA synthesis in vivo, and this may be affected by the presence of a mutated XPB protein.