MGMT: its role in cancer aetiology and cancer therapeutics

O6-benzylguanine and BCNU in multiple myeloma: a phase II trial

PURPOSE

Carmustine (BCNU) is known to have modest activity in multiple myeloma; however, resistance to BCNU manifests by the activity of O6-methylguanine methyltransferase (MGMT). The objective of this study was to determine the safety and efficacy of depletion of MGMT activity in plasma cells using O6-benzylguanine (O6-BG) with BCNU in patients with multiple myeloma.

METHODS

Patients with previously treated or untreated multiple myeloma were eligible. Cycles of O6-BG at a dose of 120 mg/m2 and BCNU at a dose of 40 mg/m2 were repeated every 6 weeks.

RESULTS

Seventeen patients were enrolled on the study, with a median follow-up of 24.5 (range 5-69) months. One complete response (7%) and 3 partial responses (20%) were observed. Nine patients (60%) had stable disease. Bone marrow studies demonstrated 94% depletion of MGMT activity in CD38+ marrow cells. The most frequent grade 3 and 4 adverse events were neutropenia (71%), lymphocytopenia (53%), and thrombocytopenia (53%).

CONCLUSIONS

Chemotherapy utilizing the MGMT inhibitor O6-benzylguanine and BCNU results in inhibition of MGMT activity in malignant plasma cells and produces meaningful responses in a modest proportion of patients with multiple myeloma. Hematologic toxicity with this regimen is significant and dose-limiting.

Sporadic colorectal cancer and individual susceptibility: a review of the association studies investigating the role of DNA repair genetic polymorphisms

Mutations in one of the DNA repair genes are one of the most common reasons for cancer, and it may be assumed that the individual genetic background modulating the DNA repair capacity may affect the susceptibility to cancer. Numerous polymorphisms (mainly SNPs) have been identified for DNA repair genes, although their functional outcome and phenotypic effect is often unknown. The aim of the present review is to evaluate the studies investigating a possible influence of DNA repair polymorphisms in the risk of sporadic colorectal cancer and/or adenoma. Overall, no relevant common findings emerge among the studies, except for some statistically significant associations between polymorphisms in the XRCC1 and XPD genes, mainly for colorectal adenoma risk. Other individual associations remain to be confirmed. This inconclusive data may suggest that the modulation of cancer risk depends not only on a single gene/SNP, but also on a joint effect of multiple polymorphisms (or haplotypes) within different genes or pathways, in close interaction with environmental factors. The relevance of many low-penetrance genes in cancer susceptibility is supposed to be very subtle. Several reviewed association studies revealed weaknesses in their design. However, there has been a progressive improvement over the years in aspects such as simultaneous genotyping and combined analyses of different polymorphisms in larger numbers of patients and controls, as well as stratification of results by ethnicity, gender, and tumor localization. This gained experience shows that only carefully designed studies of a sufficient statistical power may resolve the relationships between polymorphisms and colorectal cancer risk.

Breast cancer prognostication and prediction in the postgenomic era

Expanding knowledge, together with implementation of new techniques, has fuelled the area of translational medical research aiming at improving prognostication as well as prediction in cancer therapy. At the same time, new discoveries have revealed a biological complexity we were unaware of only a decade ago. Thus, we are faced with novel challenges with respect to how we may explore issues such as prognostication and predict drug resistance in vivo. While microarray analysis exploring expression of thousands of genes in concert represents a major methodological advancement, discoveries such as the finding of different mechanisms of epigenetic silencing, intronic mutations, that most gene transcripts in the human genome are subject to alternative splicing and that hypersplicing seems to be a tumour-related phenomenon, exemplifies a complex pathology that may not be explored with use of single analytical methods only. This paper discusses clinical settings for studying drug resistance in vivo together with a discussion of contemporary biology in this field. Notably, each individual parameter which has been found correlated to drug resistance in vivo so far represents either a direct drug target or a factor involved in DNA repair or apoptosis. On the basis of these findings, we suggest drug resistance may be explored on the basis of upfront biological hypotheses.

Oncolytic viruses and DNA-repair machinery: overcoming chemoresistance of gliomas

The current standard of care for malignant gliomas is surgical resection and radiotherapy followed by extended adjuvant treatment with the alkylating agent temozolomide. Temozolomide causes DNA damage, which induces cell death. Through changes in the DNA-repair machinery, glioma cells develop resistance to temozolomide, compromising the therapeutic effect of the drug. Oncolytic viruses, such as herpes simplex viruses and adenoviruses, are being introduced into clinical trials as a new treatment for this malignancy. Biological studies have revealed that these viruses use mechanisms to either inactivate (adenovirus) or take advantage of (herpes simplex virus) the cellular DNA-repair machinery to achieve productive replication. Adenoviruses express proteins from the early genes to either downregulate the damage-repair enzyme, O(6)-methylguanine-DNA methyltransferase, or degrade poly (ADP-ribose) polymerase or the Mre11-Rad50-NBS1 complex, which detects DNA strand breaks. Temozolomide enhances herpes simplex virus oncolysis by upregulating the DNA repair-related genes growth arrest DNA damage 34 and ribonucleotide reductase. The interactions between viruses and the DNA-repair machinery suggest that a combined temozolomide and viral therapy will overcome the limitations of a single therapy by diminishing chemoresistance or enhancing oncolysis. This hypothesis has been supported by promising findings from preclinical and clinical studies.

A distinct epigenetic signature at targets of a leukemia protein

Background

Human myelogenous leukemia characterized by either the non random t(8; 21)(q22; q22) or t(16; 21)(q24; q22) chromosome translocations differ for both their biological and clinical features. Some of these features could be consequent to differential epigenetic transcriptional deregulation at AML1 targets imposed by AML1-MTG8 and AML1-MTG16, the fusion proteins deriving from the two translocations. Preliminary findings showing that these fusion proteins lead to transcriptional downregulation of AML1 targets, marked by repressive chromatin changes, would support this hypothesis. Here we show that combining conventional global gene expression arrays with the power of bioinformatic genomic survey of AML1-consensus sequences is an effective strategy to identify AML1 targets whose transcription is epigenetically downregulated by the leukemia-associated AML1-MTG16 protein.

Results

We interrogated mouse gene expression microarrays with probes generated either from 32D cells infected with a retroviral vector carrying AML1-MTG16 and unable of granulocyte differentiation and proliferation in response to the granulocyte colony stimulating factor (G-CSF), or from 32D cells infected with the cognate empty vector. From the analysis of differential gene expression alone (using as criteria a p value < 0.01 and an absolute fold change > 3), we were unable to conclude which of the 37 genes downregulated by AML1-MTG16 were, or not, direct AML1 targets. However, when we applied a bioinformatic approach to search for AML1-consensus sequences in the 10 Kb around the gene transcription start sites, we closed on 17 potential direct AML1 targets. By focusing on the most significantly downregulated genes, we found that both the AML1-consensus and the transcription start site chromatin regions were significantly marked by aberrant repressive histone tail changes. Further, the promoter of one of these genes, containing a CpG island, was aberrantly methylated.

Conclusion

This study shows that a leukemia-associated fusion protein can impose a distinct epigenetic repressive signature at specific sites in the genome. These findings strengthen the conclusion that leukemia-specific oncoproteins can induce non-random epigenetic changes.

A changing paradigm of glioma biology

The past 30 years have witnessed a major paradigm shift in brain tumor research with the development of a wide variety of molecular biology techniques. These methods have permitted a better understanding of the pathogenesis of gliomas including the finding of neural stem cells that contribute to the establishment and continuous population of brain tumors. Molecular biology has contributed to our understanding of prognosis in these tumors with findings of genetic correlations to patient age, response to treatment, and outcome. Gene therapy has been made possible by molecular techniques that contribute to new treatment options. Nevertheless, if these advances are to make substantial clinical improvements, attention must be paid to issues of tumor evolution, local versus general pathogenesis, tumor heterogeneity, both general and regional, and the development of resistance to treatment. Appropriate clinical trials will be needed to test these new findings.

The Fanconi anemia (FA) pathway confers glioma resistance to DNA alkylating agents

DNA alkylating agents including temozolomide (TMZ) and 1,3-bis[2-chloroethyl]-1-nitroso-urea (BCNU) are the most common form of chemotherapy in the treatment of gliomas. Despite their frequent use, the therapeutic efficacy of these agents is limited by the development of resistance. Previous studies suggest that the mechanism of this resistance is complex and involves multiple DNA repair pathways. To better define the pathways contributing to the mechanisms underlying glioma resistance, we tested the contribution of the Fanconi anemia (FA) DNA repair pathway. TMZ and BCNU treatment of FA-proficient cell lines led to a dose- and time-dependent increase in FANCD2 mono-ubiquitination and FANCD2 nuclear foci formation, both hallmarks of FA pathway activation. The FA-deficient cells were more sensitive to TMZ/BCNU relative to their corrected, isogenic counterparts. To test whether these observations were pertinent to glioma biology, we screened a panel of glioma cell lines and identified one (HT16) that was deficient in the FA repair pathway. This cell line exhibited increased sensitivity to TMZ and BCNU relative to the FA-proficient glioma cell lines. Moreover, inhibition of FA pathway activation by a small molecule inhibitor (curcumin) or by small interference RNA suppression caused increased sensitivity to TMZ/BCNU in the U87 glioma cell line. The BCNU sensitizing effect of FA inhibition appeared additive to that of methyl-guanine methyl transferase inhibition. The results presented in this paper underscore the complexity of cellular resistance to DNA alkylating agents and implicate the FA repair pathway as a determinant of this resistance.

Human Monocytes, but not Dendritic Cells Derived from Them, Are Defective in Base Excision Repair and Hypersensitive to Methylating Agents

Monocytes and dendritic cells are key players in the immune response. Because dendritic cells drive the tumor host defense, it is important that monocytes and dendritic cells survive cytotoxic tumor therapy. Although most of the anticancer drugs target DNA, the DNA repair capacity of monocytes and dendritic cells has not yet been investigated. We studied the sensitivity of monocytes and monocyte-derived dendritic cells against various genotoxic agents and found monocytes to be more sensitive to overall cell kill and apoptosis upon exposure to methylating agents (e.g., N-methyl-N'-nitro-N-nitrosoguanidine, methyl methanesulfonate, and the anticancer drug temozolomide). On the other hand, upon treatment with the cross-linking chemotherapeutics fotemustine, mafosfamide, and cisplatin, monocytes and dendritic cells responded in the same way. Monocytes were also more sensitive than lymphocytes. The data indicate a defect in the repair of DNA methylation damage in monocytes. Because the expression of the repair protein O(6)-methylguanine-DNA methyltransferase was higher in monocytes than in dendritic cells, and because its inhibition by O(6)-benzylguanine had no effect on the sensitivity of monocytes, we investigated the base excision repair (BER) pathway. In contrast to dendritic cells, monocytes are unable to perform BER following exposure to methylating agents. Expression studies revealed that monocytes lack XRCC1 and ligase IIIalpha, whereas dendritic cells, similar to human lymphocytes, express these repair proteins at a high level. The data revealed a DNA repair defect in a specific human cell population. The BER defect in monocytes may cause them to be selectively killed during tumor therapy with alkylating agents, provoking hematotoxicity and sustained immunosuppression.

DNA repair in neurons: so if they don't divide what's to repair?

Neuronal DNA repair remains one of the most exciting areas for investigation, particularly as a means to compare the DNA repair response in mitotic (cancer) vs. post-mitotic (neuronal) cells. In addition, the role of DNA repair in neuronal cell survival and response to aging and environmental insults is of particular interest. DNA damage caused by reactive oxygen species (ROS) such as generated by mitochondrial respiration includes altered bases, abasic sites, and single- and double-strand breaks which can be prevented by the DNA base excision repair (BER) pathway. Oxidative stress accumulates in the DNA of the human brain over time especially in the mitochondrial DNA (mtDNA) and is proposed to play a critical role in aging and in the pathogenesis of several neurological disorders including Parkinson's disease, ALS, and Alzheimer's diseases. Because DNA damage accumulates in the mtDNA more than nuclear DNA, there is increased interest in DNA repair pathways and the consequence of DNA damage in the mitochondria of neurons. The type of damage that is most likely to occur in neuronal cells is oxidative DNA damage which is primarily removed by the BER pathway. Following the notion that the bulk of neuronal DNA damage is acquired by oxidative DNA damage and ROS, the BER pathway is a likely area of focus for neuronal studies of DNA repair. BER variations in brain aging and pathology in various brain regions and tissues are presented. Therefore, the BER pathway is discussed in greater detail in this review than other repair pathways. Other repair pathways including direct reversal, nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination and non-homologous end joining are also discussed. Finally, there is a growing interest in the role that DNA repair pathways play in the clinical arena as they relate to the neurotoxicity and neuropathy associated with cancer treatments. Among the numerous side effects of cancer treatments, major clinical effects include neurocognitive dysfunction and peripheral neuropathy. These symptoms occur frequently and have not been effectively studied at the cellular or molecular level. Studies of DNA repair may help our understanding of how those cells that are not dividing could succumb to neurotoxicity with the clinical manifestations discussed in the following article.

Loss of expression of DNA repair enzyme MGMT in oral leukoplakia and early oral squamous cell carcinoma. A prognostic tool?

MGMT is a specific DNA repair enzyme that removes alkylating lesions and therefore plays an important role in maintaining normal cell physiology and genomic stability. Loss of expression of MGMT is associated with increased carcinogenic risk and sensitivity to methylating agents in different types of tumours. The expression of MGMT was immunohistochemically assessed in 12 normal oral mucosa, 38 oral leukoplakias and 33 early oral squamous cell carcinomas. The results were correlated with clinicopathological data. We found a significant loss of MGMT protein expression from leukoplakia when compared with early squamous cell carcinoma. We also observed a statistically significant relationship between smoking and the loss of MGMT protein expression. Loss of MGMT expression could be considered an early event in oral carcinogenesis with possible prognostic implications.

Cloretazine (VNP40101M), a Novel Sulfonylhydrazine Alkylating Agent, in Patients Age 60 Years or Older With Previously Untreated Acute Myeloid Leukemia

Purpose

Cloretazine (VNP40101M) is a sulfonylhydrazine alkylating agent with significant antileukemia activity. A multicenter phase II study of cloretazine was conducted in patients 60 years of age or older with previously untreated acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome (MDS).

Patients and Methods

Cloretazine 600 mg/m2was administered as a single intravenous infusion. Patients were stratified by age, performance score, cytogenetic risk category, type of AML, and comorbidity.

Results

One hundred four patients, median age 72 years (range, 60 to 84 years), were treated on study. Performance status was 2 in 31 patients (30%) and no patient had a favorable karyotype. Forty-seven patients (45%) had cardiac disease, 25 patients (24%) had hepatic disease, and 19 patients (18%) had pulmonary disease, defined as per the Hematopoietic Cell Transplantation–Specific Comorbidity Index, at study entry. The overall response rate was 32%, with 29 patients (28%) achieving complete response (CR) and four patients (4%) achieving CR with incomplete platelet recovery. Response rates in 44 de novo AML patients, 45 secondary AML patients, and 15 high-risk MDS patients were 50%, 11%, and 40%, respectively. Response by cytogenetic risk category was 39% in 56 patients with intermediate cytogenetic risk and 24% in 46 patients with unfavorable cytogenetic risk. Nineteen (18%) patients died within 30 days of receiving cloretazine therapy. Median overall survival was 94 days, with a 1-year survival of 14%; the median duration of survival was 147 days, with a 1-year survival of 28% for those who achieved CR.

Conclusion

Cloretazine has significant activity and modest extramedullary toxicity in elderly patients with AML or high-risk MDS. Response rates remain consistent despite increasing age and comorbidity.

Temozolomide 3 weeks on and 1 week off as first-line therapy for recurrent glioblastoma: phase II study from gruppo italiano cooperativo di neuro-oncologia (GICNO)

The efficacy of temozolomide strongly depends on O⁶-alkylguanine DNA-alkyl transferase (AGAT), which repairs DNA damage caused by the drug itself. Low-dose protracted temozolomide administration can decrease AGAT activity. The main end point of the present study was therefore to test progression-free survival at 6 months (PFS-6) in glioblastoma patients following a prolonged temozolomide schedule. Chemonaïve glioblastoma patients with disease recurrence or progression after surgery and standard radiotherapy were considered eligible. Chemotherapy cycles consisted of temozolomide 75 mg/m²/daily for 21 days every 28 days until disease progression. O⁶-methyl-guanine-DNA-methyl-tranferase (MGMT) was determined in 22 patients (66.7%). A total of 33 patients (median age 57 years, range 31–71) with a median KPS of 90 (range 60–100) were accrued. The overall response rate was 9%, and PFS-6 30.3% (95% CI:18–51%). No correlation was found between the MGMT promoter methylation status of the tumours and the overall response rate, time to progression and survival. In 153 treatment cycles delivered, the most common grade 3/4 event was lymphopoenia. The prolonged temozolomide schedule considered in the present study is followed by a high PFS-6 rate; toxicity is acceptable. Further randomised trials should therefore be conducted to confirm the efficacy of this regimen.

Temozolomide: a milestone in neuro-oncology and beyond?

Temozolomide (Temodal, Temodar), an imidazol derivative, is a second-generation alkylating agent. The orally available prodrug with the capacity of crossing the blood-brain barrier received accelerated US FDA approval in 1999. Three pivotal Phase II trials showed modest activity in the treatment of recurrent anaplastic astrocytoma glioblastoma. In 2005, the FDA and the European Agency for the Evaluation of Medicinal Products approved temozolomide for use in newly diagnosed glioblastoma, in conjunction with radiotherapy, based on an European Organisation for Research and Treatment of Cancer/National Cancer Institute of Canada Phase III trial. The adverse events associated with temozolomide are mild-to-moderate and generally predictable; the most serious are noncumulative and reversible myelosuppression and, in particular, thrombocytopenia, which occurs in less than 5% of patients. Continuous temozolomide administration is associated with profound CD4-selective lymphocytopenia. Molecular studies have suggested that the benefit of temozolomide chemotherapy is restricted to patients whose tumors have a methylated methylguanine methyltransferase gene promotor and are thus unable to repair some of the chemotherapy-induced DNA damage. Temozolomide is under investigation for other disease entities, in particular lower-grade glioma, brain metastases and melanoma.

Local intracerebral administration of O(6)-benzylguanine combined with systemic chemotherapy with temozolomide of a patient suffering from a recurrent glioblastoma

The DNA repair protein O(6)-methylguanine-DNA methyltransferase (MGMT) is a major determinant of methylating anticancer drug resistance. Inactivation of MGMT by pseudosubstrate inhibitors, such as O(6)-benzylguanine (O(6)BG), sensitizes tumor cells to O(6)-alkylating agents. However, systemic administration of O(6)BG causes depletion of MGMT in all tissues of the body. Therefore, dose reduction of O(6)-alkylating drugs administered together with O(6)BG is required in order to avoid unwished toxic side effects. To attenuate the increased systemic toxicity caused by MGMT inhibitors, local MGMT inactivation would be desirable. Here, we report on intracerebral treatment with O(6)BG of a patient suffering from glioblastoma. O(6)BG was administered weekly in the tumor cavity by means of an Ommaya reservoir. This application was well tolerated. Concomitant treatment with temozolomide (Temodal) was associated with transient tumor stabilization without detectable side effects. Although evidence is still lacking that local O(6)BG administration caused MGMT to be depleted in the residual tumor, the trial shows that intracerebral treatment with O(6)BG is feasible. It might be a safe strategy for improving glioma therapy by treatment with temozolomide (and presumably also other O(6)-alkylating drugs) concomitant with O(6)BG without augmenting drug-induced systemic side effects.

Epigenetics as a mechanism driving polygenic clinical drug resistance

Aberrant methylation of CpG islands located at or near gene promoters is associated with inactivation of gene expression during tumour development. It is increasingly recognised that such epimutations may occur at a much higher frequency than gene mutation and therefore have a greater impact on selection of subpopulations of cells during tumour progression or acquisition of resistance to anticancer drugs. Although laboratory-based models of acquired resistance to anticancer agents tend to focus on specific genes or biochemical pathways, such ‘one gene : one outcome’ models may be an oversimplification of acquired resistance to treatment of cancer patients. Instead, clinical drug resistance may be due to changes in expression of a large number of genes that have a cumulative impact on chemosensitivity. Aberrant CpG island methylation of multiple genes occurring in a nonrandom manner during tumour development and during the acquisition of drug resistance provides a mechanism whereby expression of multiple genes could be affected simultaneously resulting in polygenic clinical drug resistance. If simultaneous epigenetic regulation of multiple genes is indeed a major driving force behind acquired resistance of patients' tumour to anticancer agents, this has important implications for biomarker studies of clinical outcome following chemotherapy and for clinical approaches designed to circumvent or modulate drug resistance.

O6-methylguanine-DNA methyltransferase expression strongly correlates with outcome in childhood malignant gliomas: results from the CCG-945 Cohort

PURPOSE

O6-methylguanine-DNA methyltransferase (MGMT) functions to counteract the cytotoxic effects of alkylating agents, such as nitrosoureas, which play a central role in the treatment of childhood malignant gliomas. Epigenetic silencing of MGMT has been associated with prolonged survival in adults with malignant gliomas, although the association between MGMT expression status and outcome in pediatric malignant gliomas has not been defined.

METHODS

We examined the association between MGMT expression and survival duration using tumor samples from the Children's Cancer Group 945 study, the largest randomized trial for childhood malignant gliomas completed to date. All patients received alkylator-based chemotherapy as a component of adjuvant therapy. Archival histopathologic material yielded tissue of sufficient quality for immunohistochemical assessment of MGMT expression status in 109 specimens.

RESULTS

Twelve of the 109 samples demonstrated overexpression of MGMT compared with normal brain. Five-year progression-free survival was 42.1% +/- 5% in the 97 patients whose tumors had low levels of MGMT expression versus 8.3% +/- 8% in the 12 patients whose tumors overexpressed MGMT (P = .017, exact log-rank test). The association between MGMT overexpression and adverse outcome remained significant after stratifying for institutional histologic diagnosis (eg, anaplastic astrocytoma or glioblastoma multiforme), as well as age, amount of residual tumor, and tumor location.

CONCLUSION

Overexpression of MGMT in childhood malignant gliomas is strongly associated with an adverse outcome in children treated with alkylator-based chemotherapy, independently of a variety of clinical prognostic factors.

New tricks for old drugs: the anticarcinogenic potential of DNA repair inhibitors

Defective or abortive repair of DNA lesions has been associated with carcinogenesis. Therefore it is imperative for a cell to accurately repair its DNA after damage if it is to return to a normal cellular phenotype. In certain circumstances, if DNA damage cannot be repaired completely and with high fidelity, it is more advantageous for an organism to have some of its more severely damaged cells die rather than survive as neoplastic transformants. A number of DNA repair inhibitors have the potential to act as anticarcinogenic compounds. These drugs are capable of modulating DNA repair, thus promoting cell death rather than repair of potentially carcinogenic DNA damage mediated by error-prone DNA repair processes. In theory, exposure to a DNA repair inhibitor during, or immediately after, carcinogenic exposure should decrease or prevent tumorigenesis. However, the ability of DNA repair inhibitors to prevent cancer development is difficult to interpret depending upon the system used and the type of genotoxic stress. Inhibitors may act on multiple aspects of DNA repair as well as the cellular signaling pathways activated in response to the initial damage. In this review, we summarize basic DNA repair mechanisms and explore the effects of a number of DNA repair inhibitors that not only potentiate DNA-damaging agents but also decrease carcinogenicity. In particular, we focus on a novel anti-tumor agent, beta-lapachone, and its potential to block transformation by modulating poly(ADP-ribose) polymerase-1.

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 O(6)-methylguanine-DNA methyltransferase (MGMT) and depletion of MGMT by O(6)-benzylguanine showed that, in gliomas, the apoptotic signal originates from O(6)-methylguanine (O(6)MeG) and that repair of O(6)MeG by MGMT prevents apoptosis. We further demonstrate that O(6)MeG-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. O(6)MeG-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. O(6)MeG-induced apoptosis in glioma cells was preceded by the formation of DNA double-strand breaks (DSBs), as measured by gammaH2AX formation. Glioma cells mutated in DNA-PK(cs), 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.

Polymorphisms in O 6-methylguanine DNA methyltransferase and breast cancer risk

OBJECTIVE

Endogenous and exogenous estrogens influence breast cancer risk by interacting with estrogen receptor (ER). The O-methylguanine DNA methyltransferase (MGMT) gene has a dual role in repairing alkylation damage and in inhibiting ER-mediated cell proliferation. We assessed the two MGMT polymorphisms, Leu84Phe and Ile143Val, with breast cancer risk. We also evaluated the potential interactions between the two polymorphisms and estrogen-related risk factors and cigarette smoking on breast cancer risk.

METHODS

We conducted a nested case-control study within the Nurses' Health Study (1311 cases, 1760 controls).

RESULTS

Compared with the 84Leu/Leu genotype, the Phe/Phe genotype had a multivariate odds ratio (OR) of 1.68 (95% confidence interval (CI), 0.98-2.88). This positive association was magnified among postmenopausal women with body mass index>25 (OR, 3.01; 95% CI, 1.30-6.94), those in the highest tertile of pre-diagnostic plasma endogenous estradiol levels (Phe carriers versus non-carriers, OR, 2.42; 95% CI, 1.49-3.94), non-current postmenopausal hormone users (OR, 2.60; 95% CI, 1.19-5.64), and possibly estrogen receptor-positive cases (OR, 1.82; 95% CI, 0.99-3.35). We did not observe a main effect of the Ile143Val polymorphism or its interactions with these factors. No interaction was observed between either of the polymorphisms and cigarette smoking on breast cancer risk.

CONCLUSIONS

These data suggest that the Leu84Phe polymorphism affect the capacity of MGMT to inhibit estrogen receptor-mediated cell proliferation and is associated with breast cancer risk.

hMLH1 and MGMT inactivation as a mechanism of tumorigenesis in monoclonal gammopathies

Monoclonal gammopathies are a group of disorders characterized by clonal proliferation and accumulation of immunoglobulin-producing plasma cells. Multiple myeloma and monoclonal gammopathy of undetermined significance are the most common monoclonal gammopathies; the two comprise a spectrum of disorders, ranging from a relatively benign disease, monoclonal gammopathy of undetermined significance, to a malignant disease, multiple myeloma. Aberrant promoter methylation represents a primary mechanism of gene silencing during tumorigenesis. DNA repair systems act to maintain genome integrity in the presence of replication errors, environmental insults, and the cumulative effects of aging. The methylation patterns of two genes implicated in DNA repair, O6 methylguanine DNA methyl-transferase (MGMT) and human mutL homologue1 (hMLH1) have been detected in various solid tumours. With the purpose of studying the gene silencing of MGMT and hMLH1 in plasma cell disorders, we investigated the methylation status and expression of both genes in: 29 cases of multiple myeloma; one case of plasma cell leukaemia; 13 cases of monoclonal gammopathy of undetermined significance; and two cases of polyclonal plasmacytosis, using methylation-specific polymerase-chain reaction and immunohistochemical techniques. Methylation frequencies for MGMT were 23% in multiple myeloma and 8% in monoclonal gammopathy of undetermined significance. It was 10% for hMLH1 in multiple myeloma. None of the patients diagnosed with monoclonal gammopathy of undetermined significance had hMLH1 hypermethylated. In addition, 50% of myeloma cases had a loss of hMLH1 expression, whereas silencing of MGMT was observed in 43% of myeloma and 36% of samples with monoclonal gammopathy of undetermined significance. This study indicates that repair pathway defects play a role in the pathogenesis and evolution of monoclonal gammopathies, and suggests that inactivation of hMLH1 could be implicated in multiple myeloma tumorigenesis.

Emerging cancer therapeutic opportunities target DNA-repair systems

DNA-damaging agents have a central role in non-surgical cancer treatment. The balance between DNA damage and repair determines the final therapeutic consequences. An elevated DNA-repair capacity in tumor cells leads to drug or radiation resistance and severely limits the efficacy of these agents. Interference with DNA repair has emerged as an important approach in combination therapy against cancer. Anticancer targets in DNA-repair systems have emerged, against which several small-molecule compounds are currently undergoing clinical trials.

A pilot study of dose-intensified procarbazine, CCNU, vincristine for poor prognosis brain tumors utilizing fibronectin-assisted, retroviral-mediated modification of CD34+ peripheral blood cells with O6-methylguanine DNA methyltransferase

Administration of chemotherapy is often limited by myelosuppression. Expression of drug-resistance genes in hematopoietic cells has been proposed as a means to decrease the toxicity of cytotoxic agents. In this pilot study, we utilized a retroviral vector expressing methylguanine DNA methyltransferase (MGMT) to transduce hematopoietic progenitors, which were subsequently used in the setting of alkylator therapy (procarbazine, CCNU, vincristine (PCV)) for poor prognosis brain tumors. Granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood progenitor cells were collected by apheresis and enriched for CD34+ expression. Nine subjects were infused with CD34+-enriched cells treated in a transduction procedure involving a 4-day exposure to cytokines with vector exposure on days 3 and 4. No major adverse event was related to the gene therapy procedure. Importantly, the engraftment kinetics of the treated product was similar to unmanipulated peripheral blood stem cells, suggesting that the ex vivo manipulation did not significantly reduce engrafting progenitor cell function. Gene-transduced cells were detected in all subjects. Although the level and duration was limited, patients receiving cells transduced using fibronectin 'preloaded' with virus supernatant appeared to show improved in vivo marking frequency. These findings demonstrate the feasibility and safety of utilizing MGMT-transduced CD34+ peripheral blood progenitor cells in the setting of chemotherapy.

Novel synthesis of O6-alkylguanine containing oligodeoxyribonucleotides as substrates for the human DNA repair protein, O6-methylguanine DNA methyltransferase (MGMT)

The human DNA repair protein O6-methylguanine DNA methyltransferase (MGMT) dealkylates mutagenic O6-alkylguanine lesions within DNA in an irreversible reaction which results in inactivation of the protein. MGMT also provides resistance of tumours to alkylating agents used in cancer chemotherapy and its inactivation is therefore of particular clinical importance. We describe a post-DNA synthesis strategy which exploits the novel, modified base 2-amino-6-methylsulfonylpurine and allows access for the first time to a wide variety of oligodeoxyribonucleotides (ODNs) containing O6-alkylguanines. One such ODN containing O6-(4-bromothenyl)guanine is the most potent inactivator described to date with an IC50 of 0.1 nM.

Phase I study of cloretazine (VNP40101M), a novel sulfonylhydrazine alkylating agent, combined with cytarabine in patients with refractory leukemia

PURPOSE

Cloretazine (VNP40101M) is a novel sulfonylhydrazine alkylating agent with significant antileukemia activity. A phase I study of cloretazine combined with cytarabine (1-beta-d-arabinofuranosylcytosine, ara-C) was conducted in patients with refractory disease.

DESIGN

Ara-C was given i.v. at a fixed dose of 1.5 gm/m(2)/d by continuous infusion for 4 days (patients ages <65 years at time of diagnosis) or 3 days (patients ages > or =65 years). Cloretazine was given i.v. over 15 to 60 minutes on day 2 at a starting dose of 200 mg/m(2), with escalation in 100 mg/m(2) increments in cohorts of three to six patients until a maximum tolerated dose was established. The DNA repair enzyme O(6)-alkylguanine DNA alkyltransferase (AGT) was measured at baseline.

RESULTS

Forty patients, including 32 with acute myeloid leukemia, received 47 courses of treatment. Complete responses were seen at cloretazine dose levels of > or =400 mg/m(2) in 10 of 37 (27%) evaluable patients, and in this patient subset, AGT activity was significantly lower in patients that responded to treatment than in patients who did not (P < or = 0.027). Dose-limiting toxicities (gastrointestinal and myelosuppression) were seen with 500 and 600 mg/m(2) of cloretazine combined with the 4-day ara-C schedule but not seen with the 3-day schedule.

CONCLUSION

The recommended cloretazine dose schedule for future studies is 600 mg/m(2) combined with 1.5 gm/m(2)/d continuous infusion of ara-C for 3 days. The cloretazine and ara-C regimen has significant antileukemic activity. AGT activity may be a predictor of response to cloretazine.

Temozolomide in metastatic breast cancer (MBC): a phase II trial of the National Cancer Institute of Canada - Clinical Trials Group (NCIC-CTG)

BACKGROUND

Temozolomide is an oral alkylating agent that crosses the blood-brain barrier, and has preclinical activity in breast cancer. This phase II trial sought to determine the activity and toxicity of temozolomide in metastatic breast cancer (MBC). Temozolomide was administered in a dose dense schedule of 150 mg/m(2) on days 1-7 and 15-21 in a 28-day cycle.

MATERIALS AND METHODS

Patients had unidimensional disease for response assessment by RECIST criteria, up to two prior chemotherapy regimens for MBC, and may have had brain metastases if radiation was not expected to be required within 4 weeks.

RESULTS

Nineteen women were entered on the study. All were evaluable for toxicity and 18 were evaluable for response. The median age was 54 years; 14 had prior chemotherapy for MBC and 12 had prior hormones. Sites of disease included bone, brain, liver and lung. Treatment was well tolerated with 14/19 receiving >90% planned dose intensity. Common grade 1-3 drug-related effects included nausea, fatigue, vomiting, anorexia and skin rash. Grade 3-4 hematologic toxicities included granulocytopenia and thrombocytopenia. Of the 18 evaluable patients, there were no objective responses; three had stable disease and 15 progressive disease.

CONCLUSIONS

No responses to temozolomide were documented in these heavily pretreated women with extensive MBC including brain metastases.

Towards hematopoietic stem cell-mediated protection against infection with human immunodeficiency virus

The failure of pharmacological approaches to cure infection with the human immunodeficiency virus (HIV) has renewed the interest in gene-based therapies. Among the various strategies that are currently explored, the blockade of HIV entry into susceptible T cells and macrophages promises to be the most powerful intervention. For long-term protection of both of these lineages, genetic modification of hematopoietic stem cells (HSCs) would be required. Here, we tested whether HSCs and their progeny can be modified to express therapeutic levels of M87o, a gammaretroviral vector encoding an artificial transmembrane molecule that blocks fusion-mediated uptake of HIV. In serial murine bone marrow transplantations, efficient and multilineage expression of M87o was observed for more than 1 year (range 37-75% of mononuclear cells), without signs of toxicity related to the transmembrane molecule. To allow enrichment of M87o-modified HSCs after transplant, we constructed vectors coexpressing the P140K mutant of O(6)-methylguanine-DNA-methyltransferase (MGMT-P140K). This clinically relevant selection marker mediates a survival advantage in HSCs if exposed to combinations of methylguanine-methyltransferase (MGMT) inhibitors and alkylating agents. A bicistronic vector mediated sufficient expression of both M87o and MGMT to confer a selective survival advantage in the presence of HIV and alkylating agents, respectively. These data encourage further investigations in large animal models and clinical trials.

CpG island hypermethylation in progression of esophageal and gastric cancer

The upper gastrointestinal (GI) cancers have various carcinogenic pathways and precursor lesions, such as dysplasia for esophageal squamous cell carcinoma, Barrett esophagus for esophageal adenocarcinoma, and intestinal metaplasia for the intestinal-type of gastric cancer. Recently, many epigenetic events in carcinogenic pathways have been revealed, along with genomic and genetic alterations. This information has provided deeper insight into an understanding of the mechanisms of upper GI carcinogenesis. Moreover, detection methods of aberrant methylation have been applied to clinical fields to stratify high-risk groups, detect early cancer, and to predict clinical outcomes. In this review, a variety of information is summarized regarding gene hypermethylation in esophageal and gastric cancer.

Recent advances in the treatment of malignant astrocytoma

Malignant gliomas, including the most common subtype, glioblastoma multiforme (GBM), are among the most devastating of neoplasms. Their aggressive infiltration in the CNS typically produces progressive and profound disability--ultimately leading to death in nearly all cases. Improvement in outcome has been elusive despite decades of intensive clinical and laboratory research. Surgery and radiotherapy, the traditional cornerstones of therapy, provide palliative benefit, while the value of chemotherapy has been marginal and controversial. Limited delivery and tumor heterogeneity are two fundamental factors that have critically hindered therapeutic progress. A novel chemoradiotherapy approach, consisting of temozolomide administered concurrently during radiotherapy followed by adjuvant systemic temozolomide, has recently demonstrated a meaningful, albeit modest, improvement in overall survival for newly diagnosed GBM patients. As cell-signaling alterations linked to the development and progression of gliomas are being increasingly elucidated, targeted therapies have rapidly entered preclinical and clinical evaluation. Responses to therapies that function via DNA damage have been associated with specific mediators of resistance that may also be subject to targeted therapies. Other approaches include novel locoregional delivery techniques to overcome barriers of delivery. The simultaneous development of multiple advanced therapies based on specific tumor biology may finally offer glioma patients improved survival.

Recent developments in the use of chemotherapy in brain tumours

Several recent studies have further clarified the role of chemotherapy in newly diagnosed anaplastic glioma. For newly diagnosed glioblastoma, combined daily radiotherapy with daily temozolomide followed by six cycles of adjuvant temozolomide improves overall survival. This benefit is especially observed in patients with a methylated promotor of the MGMT gene which encodes an alkyltransferase; this observation however, needs confirmation. Although oligodendroglial tumours are sensitive to chemotherapy, classical adjuvant nitrosourea-based chemotherapy does not improve overall survival in newly diagnosed anaplastic oligodendroglioma, even in the subset of 1p/19q loss tumours. It may increase progression-free survival however, and further studies must show if combined modality treatment with daily chemotherapy during radiotherapy increases survival. Trials exploring the role of chemotherapy in low-grade glioma are ongoing. No standard chemotherapy is currently available for highly anaplastic glioma failing first-line temozolomide-based therapy.

Stringent Regulation of DNA Repair During Human Hematopoietic Differentiation: A Gene Expression and Functional Analysis

For the lymphohematopoietic system, maturation-dependent alterations in DNA repair function have been demonstrated. Because little information is available on the regulatory mechanisms underlying these changes, we have correlated the expression of DNA damage response genes and the functional repair capacity of cells at distinct stages of human hematopoietic differentiation. Comparing fractions of mature (CD34-), progenitor (CD34+ 38+), and stem cells (CD34+ 38-) isolated from umbilical cord blood, we observed: 1) stringently regulated differentiation-dependent shifts in both the cellular processing of DNA lesions and the expression profiles of related genes and 2) considerable interindividual variability of DNA repair at transcriptional and functional levels. The respective repair phenotype was found to be constitutively regulated and not dominated by adaptive response to acute DNA damage. During blood cell development, the removal of DNA adducts, the resealing of repair gaps, the resistance to DNA-reactive drugs clearly increased in stem or mature compared with progenitor cells of the same individual. On the other hand, the vast majority of differentially expressed repair genes was consistently upregulated in the progenitor fraction. A positive correlation of repair function and transcript levels was found for a small number of genes such as RAD23 or ATM, which may serve as key regulators for DNA damage processing via specific pathways. These data indicate that the organism might aim to protect the small number of valuable slow dividing stem cells by extensive DNA repair, whereas fast-proliferating progenitor cells, once damaged, are rather eliminated by apoptosis.

Genotoxic effects of myosmine in a human esophageal adenocarcinoma cell line

The incidence of esophageal adenocarcinoma is rapidly rising in Western populations. Gastroesophageal reflux disease (GERD) is thought to be one of the most important risk factors. However, the mechanisms by which GERD enhances tumor formation at the gastroesophageal junction are not well understood. Myosmine is a tobacco alkaloid which has also a wide spread occurrence in human diet. It is readily activated by nitrosation and peroxidation giving rise to the same hydroxypyridylbutanone-releasing DNA adducts as the esophageal carcinogen N'-nitrosonornicotine. Therefore, the genotoxicity of myosmine was tested in a human esophageal adenocarcinoma cell line (OE33). DNA damage was assessed by single-cell gel electrophoresis (Comet assay). DNA strand breaks, alkali labile sites and incomplete excision repair were expressed using the Olive tail moment (OTM). The Fapy glycosylase (Fpg) enzyme was incorporated into the assay to reveal additional oxidative DNA damage. DNA migration was determined after incubation of the cells for 1-24h. Under neutral conditions high myosmine concentrations of 25-50mM were necessary to elicit a weak genotoxic effect. At pH 6 genotoxicity was clearly enhanced giving a significant increase of OTM values at 5mM myosmine. Lower pH values could not be tested because of massive cytotoxicity even in the absence of myosmine. Co-incubation of 25 mM myosmine with 1mM H(2)O(2) for 1h significantly enhanced the genotoxicity of H(2)O(2) but not the oxidative lesions additionally detected with the Fpg enzyme. In the presence of the peroxynitrite donor 3-morpholinosydnonimine (SIN-1) a dose-dependent significant genotoxic effect was obtained with 1-10mM myosmine after 4h incubation. NS-398, a selective inhibitor of cyclooxygenase 2, did not affect the SIN-1 stimulated genotoxicity of myosmine. Finally, the 23 h repair of N-methyl-N'-nitro-N-nitrosoguanidine-induced DNA lesions was significantly inhibited in the presence of 10mM myosmine. In conclusion, myosmine exerts significant genotoxic effects in esophageal cells under conditions which may prevail in GERD such as increased oxidative and nitrosative stress resulting from chronic inflammation.

Changing Paradigms—An Update on the Multidisciplinary Management of Malignant Glioma

Treatment of malignant glioma requires a multidisciplinary team. Treatment includes surgery, radiotherapy, and chemotherapy. Recently developed agents have demonstrated activity against recurrent malignant glioma and efficacy if given concurrently with radiotherapy in the upfront setting. Oligodendroglioma with 1p/19q deletions has been recognized as a distinct pathologic entity with particular sensitivity to radiotherapy and chemotherapy. Randomized trials have shown that early neoadjuvant or adjuvant administration of procarbazine, lomustine, and vincristine chemotherapy prolongs disease-free survival; however, it has no impact on overall survival. Temozolomide, a novel alkylating agent, has shown modest activity against recurrent glioma. In combination with radiotherapy in newly diagnosed patients with glioblastoma, temozolomide significantly prolongs survival. Molecular studies have demonstrated that the benefit is mainly observed in patients whose tumors have a methylated methylguanine methyltransferase gene promoter and are thus unable to repair some of the chemotherapy-induced DNA damage. For lower-grade glioma, the use of chemotherapy remains limited to recurrent disease, and first-line administration is the subject of ongoing clinical trials. Irinotecan and agents like gefitinib, erlotinib, and imatinib targeting the epidermal growth factor receptor and platelet-derived growth factor receptor have shown some promise in recurrent malignant glioma. This review summarizes recent developments, focusing on the clinical management of patients in daily neuro-oncology practice.

Targeted Modulation of MGMT: Clinical Implications: Fig. 1

O(6)-Methylguanine DNA methyltransferase (MGMT) has been studied for >20 years as a gene that is associated with the mutagenicity and cytotoxicity induced by either methylating carcinogens or alkylating (methylating and chloroethylating) therapeutic agents. Pioneering studies of alkylating agents identified alkylated guanine at the O(6) position, the substrate of MGMT, as a potentially promutagenic and lethal toxic DNA lesion. MGMT plays a prominent role in DNA adduct repair that limits the mutagenic and cytotoxic effect of alkylating agents. Because of its role in cancer etiology and chemotherapy resistance, MGMT is of particular interest. In this article, the clinical effect of MGMT expression and targeted modulation of MGMT will be summarized.

Sensitization of human carcinoma cells to alkylating agents by small interfering RNA suppression of 3-alkyladenine-DNA glycosylase

One of the major cytotoxic lesions generated by alkylating agents is DNA 3-alkyladenine, which can be excised by 3-alkyladenine DNA glycosylase (AAG). Inhibition of AAG may therefore result in increased cellular sensitivity to chemotherapeutic alkylating agents. To investigate this possibility, we have examined the role of AAG in protecting human tumor cells against such agents. Plasmids that express small interfering RNAs targeted to two different regions of AAG mRNA were transfected into HeLa cervical carcinoma cells and A2780-SCA ovarian carcinoma cells. Stable derivatives of both cell types with low AAG protein levels were sensitized to alkylating agents. Two HeLa cell lines with AAG protein levels reduced by at least 80% to 90% displayed a 5- to 10-fold increase in sensitivity to methyl methanesulfonate, N-methyl-N-nitrosourea, and the chemotherapeutic drugs temozolomide and 1,3-bis(2-chloroethyl)-1-nitrosourea. These cells showed no increase in sensitivity to UV light or ionizing radiation. After treatment with methyl methanesulfonate, AAG knockdown HeLa cells were delayed in S phase but accumulated in G2-M. Our data support the hypothesis that ablation of AAG activity in human tumor cells may provide a useful strategy to enhance the efficacy of current chemotherapeutic regimens that include alkylating agents.

Effect of chemotherapy-induced DNA repair on oncolytic herpes simplex viral replication

BACKGROUND

Gliomas treated with the alkylating agent temozolomide have incomplete responses in part because of tumoral repair of chemotherapy-induced DNA damage. Data from phase I trials suggest that G207, an oncolytic herpes simplex virus (HSV) with mutated ribonucleotide reductase (RR) and gamma34.5 genes, is safe but needs greater viral oncolysis to be effective. We hypothesized that temozolomide and G207 treatment limitations could be jointly addressed using temozolomide-induced tumor-protective DNA repair pathways to enhance viral replication.

METHODS

Human glioblastoma cells (U87, T98, and U373) and U87 cells transfected with the gene for the DNA repair enzyme O6-methylguanine DNA methyltransferase (MGMT) were treated with G207 and/or temozolomide. Drug interactions, expression of the growth arrest DNA damage 34 (GADD34) and RR transcripts before and after their knockdown with short interfering RNAs, DNA strand breaks, and apoptosis were measured using Chou-Talalay analysis, real-time reverse transcription-polymerase chain reaction, the comet assay, and flow cytometry, respectively. Survival of mice (groups of ten) with intracranial U87 xenograft tumors treated with temozolomide and/or G207 was analyzed using Kaplan-Meier analysis.

RESULTS

Temozolomide exhibited strong synergy with G207 in both MGMT-negative and the MGMT inhibitor O6-benzylguanine-treated MGMT-expressing gliomas (Chou-Talalay combination indices = 0.005 to 0.39) and induced GADD34 expression primarily in nonapoptotic MGMT-negative U87 glioma cells (fold difference = 16, 95% confidence interval [CI] = 12.6 to 20.4, compared with untreated cells). MGMT-expressing T98 and U87/MGMT cells treated with temozolomide plus O6-benzylguanine had higher RR expression than untreated cells (fold difference =14.9, 95% CI = 10.1 to 22.0 [T98]; 9.9, 95% CI = 7.0 to 13.8 [U87/MGMT]). GADD34 and RR knockdown increased temozolomide-induced DNA damage and inhibited the synergy of G207 and temozolomide in U87 and O6-benzylguanine-treated U87/MGMT cells. Mice bearing intracranial U87 tumors survived longer after combination therapy (100% survival at 90 days) than after single-agent therapy (median survival = 46 and 48 days with G207 and temozolomide treatment, respectively).

CONCLUSIONS

Temozolomide-induced DNA repair pathways vary with MGMT expression and enhance HSV-mediated oncolysis in glioma cells. These findings unveil the potential of HSV to target cells surviving temozolomide treatment.

Post-transplant lymphoproliferative disorders: molecular basis of disease histogenesis and pathogenesis

Post-transplant lymphoproliferative disorders (PTLD) represent a serious complication of solid organ and allogeneic bone marrow transplantation. PTLD generally display B-cell lineage derivation, involvement of extranodal sites, aggressive histology and clinical behaviour, and frequent association with EBV infection. The occurrence of IgV mutations in the overwhelming majority of PTLD documents that malignant transformation targets germinal centre (GC) B-cells and their descendants both in EBV-positive and EBV-negative cases. Analysis of phenotypic markers of B-cell histogenesis, namely BCL6, MUM-1 and CD138, allows further distinction of PTLD histogenetic categories. PTLD expressing the BCL6(+)/MUM1(+/-)/CD138(-) profile reflect B-cells actively experiencing the GC reaction and comprise diffuse large B-cell lymphoma (DLBCL) centroblastic and Burkitt lymphoma. PTLD expressing the BCL6(-)/MUM1(+)/CD138(-) phenotype putatively derive from B-cells that have concluded the GC reaction and comprise the majority of polymorphic PTLD and a fraction of DLBCL. A third group of PTLD is reminiscent of post-GC and pre- terminally differentiated B-cells that show the BCL6(-)/MUM1(+)/CD138(+) phenotype and are morphologically represented by either polymorphic PTLD or DLBCL immunoblastic. The molecular pathogenesis of PTLD involves infection by oncogenic viruses, namely Epstein-Barr virus, as well as genetic or epigenetic alterations of several cellular genes. At variance with lymphoma arising in immunocompetent hosts, whose genome is relatively stable, a fraction of PTLD are characterized by microsatellite instability as a consequence of defects in the DNA mismatch repair mechanism. Apart from microsatellite instability, molecular alterations of cellular genes recognized in PTLD include alterations of c-MYC, BCL-6, p53, DNA hypermethylation, and aberrant somatic hypermutation of proto-oncogenes.

Smoking is associated with a decrease ofO6-alkylguanine-DNA alkyltransferase activity in bronchial epithelial cells

O6-alkylguanine-DNA alkyltransferase (MGMT) represents the first line of defense against the toxic, mutagenic and carcinogenic effects of O6-alkylguanine adducts in DNA. These adducts mediate the biological activity from a series of alkylating agents, such as the tobacco-specific nitrosamines, believed to contribute to the carcinogenicity of tobacco smoke. There have been conflicting reports on the effects of smoking on MGMT activity in lung and other tissues. Here, we investigate MGMT activity in peripheral blood mononuclear cells (PBMC) and lung bronchial epithelial cells (BEC), extracted by lung brushings, from smokers and nonsmokers attending a bronchoscopy clinic. MGMT activity was significantly lower in BECs (geometric mean; 95% confidence interval 1.02; 0.86-1.20 fmol/microg DNA) than in PBMCs (7.86; 6.70-9.59 fmol/microg DNA; p < 0.001), suggesting that bronchial epithelia may be particularly sensitive to alkylation damage. More importantly our results indicate that activity in BECs is significantly decreased in samples from current smokers (0.71; 0.54-0.93 fmol/microg DNA) compared to nonsmokers (1.25; 1.03-1.51 fmol/microg DNA; p = 0.002). This could represent an important contribution to the carcinogenicity of tobacco smoke.

The syntheses and properties of tricyclic pyrrolo[2,3-d]pyrimidine analogues of S6-methylthioguanine and O6-methylguanine

The syntheses of novel tricyclic pyrrolo[2,3-d]pyrimidine analogues of S6-methylthioguanine are described. The crystal structures and pKa values of these and related O6-methylguanine analogues are reported. All compounds display higher pKa values than O6-methylguanine with the sulfur-containing analogues being the more basic and exhibiting higher stability in aqueous solution. In a standard substrate assay with the human repair protein O6-methylguanine-DNA methyltransferase (MGMT) only the oxygen-containing analogue displayed activity.

Correlative studies in neuro-oncology trials: Should they influence treatment?

Recent molecular correlative studies accompanying clinical trials in glioma have provided strong evidence for prognostic markers and predictive factors for treatment response. However, to what extent can these markers influence the limited choice of therapeutic options? Do we further validate the markers in the next trials or move on, incorporate the markers for patient selection or stratification, aim at improving the modestly effective treatments by adding new drugs, and develop alternative therapy strategies for patients selected for their bad predictor?

O6-(4-bromothenyl)guanine reverses temozolomide resistance in human breast tumour MCF-7 cells and xenografts

Tumour resistance to chemotherapy involving methylating agents such as DTIC (dacarbazine) and temozolomide is linked to expression of the DNA repair protein O(6)-alkylguanine-DNA alkyltransferase (MGMT). There is considerable interest in improving the efficacy of such O(6)-alkylating chemotherapy by the prior inactivation of MGMT. We have examined the effect of the modified guanine base, O(6)-(4-bromothenyl)guanine (PaTrin-2, Patrin, Lomeguatrib) on MGMT activity and cell or xenograft tumour growth inhibition by temozolomide in the human breast carcinosarcoma cell line, MCF-7. PaTrin-2 effectively inactivated MGMT in MCF-7 cells (IC(50) approximately 6 nM) and in xenografts there was complete inactivation of MGMT within 2 h of dosing (20 mg kg(-1) i.p.) and only slight recovery by 24 h. MGMT inactivation in a range of murine host tissues varied between complete and approximately 60%, with extensive recovery by 24 h. PaTrin-2 (10 microM) substantially increased the growth inhibitory effects of temozolomide in MCF-7 cells (D(60)=10 microM with PaTrin-2 vs 400 microM without). In MCF-7 xenografts, neither temozolomide (100 mg kg(-1) day(-1) for 5 days) nor PaTrin-2 (20 mg kg(-1) day(-1) for 5 days) had any significant effect on tumour growth. In contrast, the PaTrin-2-temozolomide combination produced a substantial tumour growth delay: median tumour quintupling time was increase by 22 days (P<0.005) without any significant increase in toxicity as assessed from animal weight. A PaTrin-2-temozolomide combination may therefore be beneficial in the treatment of human breast cancers.

Signalling cell cycle arrest and cell death through the MMR System

Loss of DNA mismatch repair (MMR) in mammalian cells, as well as having a causative role in cancer, has been linked to resistance to certain DNA damaging agents including clinically important cytotoxic chemotherapeutics. MMR-deficient cells exhibit defects in G2/M cell cycle arrest and cell killing when treated with these agents. MMR-dependent cell cycle arrest occurs, at least for low doses of alkylating agents, only after the second S-phase following DNA alkylation, suggesting that two rounds of DNA replication are required to generate a checkpoint signal. These results point to an indirect role for MMR proteins in damage signalling where aberrant processing of mismatches leads to the generation of DNA structures (single-strand gaps and/or double-strand breaks) that provoke checkpoint activation and cell killing. Significantly, recent studies have revealed that the role of MMR proteins in mismatch repair can be uncoupled from the MMR-dependent damage responses. Thus, there is a threshold of expression of MSH2 or MLH1 required for proper checkpoint and cell-death signalling, even though sub-threshold levels are sufficient for fully functional MMR repair activity. Segregation is also revealed through the identification of mutations in MLH1 or MSH2 that provide alleles functional in MMR but not in DNA damage responses and mutations in MSH6 that compromise MMR but not in apoptotic responses to DNA damaging agents. These studies suggest a direct role for MMR proteins in recognizing and signalling DNA damage responses that is independent of the MMR catalytic repair process. How MMR-dependent G2 arrest may link to cell death remains elusive and we speculate that it is perhaps the resolution of the MMR-dependent G2 cell cycle arrest following DNA damage that is important in terms of cell survival.

The current status of gene therapy in autologous transplantation

Autologous hematopoietic cells have been used as targets of gene transfer, with applications in inherited disorders, cell therapy, and acquired immunodeficiency. The types of cells include hematopoietic progenitor cells, lymphocytes, and mesenchymal stem cells. The inherited disorders thus far approached in clinical trials include severe combined immunodeficiency, common variable gamma-chain immunodeficiency, chronic granulomatous disease, and Gaucher disease. Preclinical studies are vigorously under way in thalassemia, sickle cell anemia, Wiskott-Aldrich syndrome and Fanconi anemia. Clinical trials of immunological therapy with gene-modified lymphocytes are under study in the treatment of malignancies. Clinical trials using anti-viral strategies for HIV infection in combination with autologous transplantation have begun, with additional approaches being developed. Gene therapy vectors are being developed to eliminate tumor cells contaminating autologous stem cell products. However, the risk of insertional mutagenesis and the potential for development of leukemia was highlighted by the first gene therapy trials in inherited immunodeficiency syndromes that achieved a therapeutic effect. Despite the slow progress of the field to date, there is extraordinary promise for gene therapy in the future.

Temozolomide: a milestone in the pharmacotherapy of brain tumors

Temozolomide (TMZ) is an alkylating agent earlier approved for recurrent anaplastic astrocytoma and approved for the treatment of newly diagnosed glioblastoma in the USA and Europe in 2005. TMZ shows good blood–brain barrier penetration and exhibits a favorable side effect profile. Its key mode of action appears to be methylation at N7 and O6-positions of guanine. The level of expression and activity of the DNA repair enzyme O6-methylguanine DNA methyltransferase is thought to be a major predictor of response to TMZ. The demonstration of prolonged survival when TMZ was added to radiotherapy in the European Organisation for Research and Treatment of Cancer 26981/22981/NCIC CE.3 trial has been a breakthrough in the treatment of newly diagnosed glioblastoma. The early preliminary evidence for activity in recurrent malignant gliomas further resulted in a broad evaluation of TMZ for other tumors in neuro-oncology, mainly low-grade gliomas, brain metastases and primary cerebral lymphomas.

The emerging role of DNA repair proteins as predictive, prognostic and therapeutic targets in cancer

Advanced cancer is the second leading cause of death in the western world. Chemotherapy and radiation are the two main treatment modalities currently available to improve patient outcomes, but treatment related toxicity and the emergence of resistance limit their effectiveness. Hence there is an urgent need to develop novel treatment strategies. Rapid advances in cancer biology have identified key pathways involved in the repair of DNA damage induced by chemotherapeutic agents and irradiation. Efficient DNA repair in the cancer cell is an important mechanism for therapeutic resistance. Up to 130 genes have been identified that are associated with human DNA repair. Several of these proteins are emerging as important predictive and prognostic factors in solid tumours. Inhibition of DNA repair has the potential to enhance the efficacy of currently available DNA damaging agents. In recent years, several promising drug targets have been identified and novel drugs synthesised that target specific DNA repair proteins. These agents have shown impressive anti-cancer effects in preclinical studies in combination with chemotherapy or irradiation. Their role in human cancer is now being investigated in early phase clinical trials in combination with chemotherapy. MGMT inhibitors, PARP inhibitors and methoxyamine are currently in early stages of clinical development. Innovative clinical trial designs are essential to evaluate the potential of DNA repair inhibitor in cancer therapy.

Epigenetic silencing of the MGMT gene in cancer

Silencing of the O6-methylguanine-DNA methyltransferase (MGMT) gene, a key to DNA repair, plays a critical role in the development of cancer. The gene product, functioning normally, removes a methyl group from mutagenic O6-methylguanine, which is produced by alkylating agents and can make a mismatched pair with thymine, leading to transition mutation through DNA replication. MGMT is epigenetically silenced in various human tumors. It is well known that DNA hypermethylation at the promoter CpG island plays a pivotal role in the epigenetic silencing of tumor suppressor genes. MGMT silencing, however, occurs without DNA hypermethylation in some cancer cells. Dimethylation of histone H3 lysine 9 and binding of methyl-CpG binding proteins are common and essential in MGMT-silenced cells. Silencing of MGMT has been shown to be a poor prognostic factor but a good predictive marker for chemotherapy when alkylating agents are used. In this review, we describe recent advances in understanding the silencing of MGMT and its role in carcinogenesis; epigenetic mechanisms; and clinical implications.

High expression of a new marker PCA-1 in human prostate carcinoma

PURPOSE

Identifying the genetic factors involved in prostate carcinogenesis is critical. Novel cancer-specific markers aid in early detection, in differentiating between cancer and nonmalignant disorders, and in monitoring clinical of prostate disease. We therefore examined differential gene displays in an attempt to identify genes that may be involved in prostate carcinogenesis.

EXPERIMENTAL DESIGN

Applying fluorescent differential display analysis to human prostate carcinomas, we have identified and cloned several cDNA transcripts. Antisera were raised against synthetic peptides and used in Western blot and immunohistochemical analyses. The mRNAs were also analyzed by real-time reverse transcription-PCR. For functional analysis, we assessed methylmethane sulfonate (MMS)-induced toxicity in COS-7 cells after cDNA transfection.

RESULTS

We identified a gene, designated prostate cancer antigen-1 (pca-1), which shows high mRNA expression in prostate carcinoma. Database analysis of the deduced amino acid sequence of PCA-1 indicated high similarity to Escherichia coli AlkB, a DNA alkylation damage repair enzyme. By immunohistochemical analysis, PCA-1 was expressed in a high number of both prostate carcinoma samples and in the atypical cells within high-grade prostatic intraepithelial neoplasias but not in benign prostatic hyperplasia or normal adjacent tissues. PCA-1-transfected COS-7 cells further showed resistance against MMS-induced cell death.

CONCLUSIONS

These findings suggest that PCA-1 could be a useful diagnostic marker. Furthermore, because this human counterpart of AlkB exhibits a protective function against alkylation damage in mammalian cells, PCA-1 may also serve as a therapeutic target molecule for prostate cancer.

Molecular features of adult glioma associated with patient race/ethnicity, age, and a polymorphism in O6-methylguanine-DNA-methyltransferase

BACKGROUND

Risk factors for adult glioma in the San Francisco Bay Area include well-known demographic features such as age and race/ethnicity, and our previous studies indicated that these characteristics are associated with the TP53 mutation status of patients' tumors. We enlarged our study to assess the relationships of risk factors with TP53 as well as epidermal growth factor receptor (EGFR) and murine double minute-2 (MDM2) gene amplification and expression and the germ line Leu84Phe polymorphism in the DNA repair protein O6-methylguanine-DNA-methyltransferase (MGMT). MGMT expression may depend on the TP53 status of cells.

METHODS

Molecular analyses were carried out on 556 incident astrocytic tumors. MGMT genotype data were collected on germ line DNA from 260 of these cases.

RESULTS

The tumor data confirm the inverse relationships between TP53 mutation and MDM2 (P = 0.04) or EGFR (P = 0.004) amplification and that patients whose tumors contain TP53 mutations are younger than those without (P < 0.001). Although there was little difference in age of patient by EGFR amplification or expression among glioblastoma multiforme cases, EGFR gene amplification was associated with much older age of onset of anaplastic astrocytoma; for example, EGFR-amplified anaplastic astrocytoma cases were on average 63 years old compared with 48 years for nonamplified cases (P = 0.005). An increased prevalence of TP53 mutation positive glioblastoma multiforme was noted among nonwhites (African American and Asian) compared with whites (Latino and non-Latino; P = 0.004). Carriers of the MGMT variant 84Phe allele were significantly less likely to have tumors with TP53 overexpression (odds ratio, 0.30; 95% confidence interval, 0.13-0.71) and somewhat less likely to have tumors with any TP53 mutation (odds ratio, 0.47; 95% confidence interval, 0.13-1.69) after adjusting for age, gender, and ethnicity. Interestingly, EGFR gene amplification and EGFR protein overexpression were also inversely associated with the MGMT 84Phe allele.

CONCLUSIONS

Our results are consistent with ethnic variation in glioma pathogenesis. The data on MGMT show that an inherited factor involving the repair of methylation and other alkylation damage, specifically to the O6 position of guanine, may be associated with the development of tumors that proceed in their development without TP53 mutations or accumulation of TP53 protein and possibly also those that do not involve amplification of the EGFR locus.

Equal potency of gammaretroviral and lentiviral SIN vectors for expression of O6-methylguanine-DNA methyltransferase in hematopoietic cells

Severe adverse events related to insertional mutagenesis have reinforced interest in self-inactivating (SIN) retroviral vectors lacking enhancer-promoter sequences in the long terminal repeats (LTRs). Here, we have compared the potency of gammaretroviral and lentiviral vectors expressing the P140K mutant of O(6)-methylguanine-DNA methyltransferase (MGMT). MGMT-P140K is a clinically relevant selection marker that mediates a strong survival advantage in hematopoietic cells exposed to alkylating agents. We designed gammaretroviral and lentiviral vectors that contained identical enhancer-promoter sequences located either in the LTR or downstream of the packaging region, for internal initiation of transcription from SIN backbones. Gammaretroviral vectors with intact LTRs containing enhancer-promoter sequences showed both higher titers and higher expression levels than the lentiviral counterparts, likely a result of suboptimal RNA processing of the lentiviral leader region. In the SIN context, gammaretroviral and lentiviral vectors with comparable internal cassettes had similar expression properties. Interestingly, gammaretroviral SIN vectors pseudotyped with RD114/TR had a higher transduction efficiency on proliferating human CD34(+) cells than lentiviral counterparts. These results encourage further investigations into the formation of retroviral hybrid vectors that combine the desired properties of high efficiency and increased biosafety.

DNA repair inhibition: a selective tumour targeting strategy

Advanced cancer is a leading cause of death in the developed world. Chemotherapy and radiation are the two main treatment modalities currently available. The cytotoxicity of many of these agents is directly related to their propensity to induce DNA damage. However, the ability of cancer cells to recognize this damage and initiate DNA repair is an important mechanism for therapeutic resistance and has a negative impact upon therapeutic efficacy. Pharmacological inhibition of DNA repair, therefore, has the potential to enhance the cytotoxicity of a diverse range of anticancer agents. Moreover, the use of inhibitors of DNA repair or DNA damage signalling pathways appears to provide an exciting opportunity to target the genetic differences that exist between normal and tumour tissue.