Uracil DNA glycosylase (UNG) loss enhances DNA double strand break formation in human cancer cells exposed to pemetrexed

Misincorporation of genomic uracil and formation of DNA double strand breaks (DSBs) are known consequences of exposure to TS inhibitors such as pemetrexed. Uracil DNA glycosylase (UNG) catalyzes the excision of uracil from DNA and initiates DNA base excision repair (BER). To better define the relationship between UNG activity and pemetrexed anticancer activity, we have investigated DNA damage, DSB formation, DSB repair capacity, and replication fork stability in UNG^+/+ and UNG^−/− cells. We report that despite identical growth rates and DSB repair capacities, UNG^−/− cells accumulated significantly greater uracil and DSBs compared with UNG^+/+ cells when exposed to pemetrexed. ChIP-seq analysis of γ-H2AX enrichment confirmed fewer DSBs in UNG^+/+ cells. Furthermore, DSBs in UNG^+/+ and UNG^−/− cells occur at distinct genomic loci, supporting differential mechanisms of DSB formation in UNG-competent and UNG-deficient cells. UNG^−/− cells also showed increased evidence of replication fork instability (PCNA dispersal) when exposed to pemetrexed. Thymidine co-treatment rescues S-phase arrest in both UNG^+/+ and UNG^−/− cells treated with IC₅₀-level pemetrexed. However, following pemetrexed exposure, UNG^−/− but not UNG^+/+ cells are refractory to thymidine rescue, suggesting that deficient uracil excision rather than dTTP depletion is the barrier to cell cycle progression in UNG^−/− cells. Based on these findings we propose that pemetrexed-induced uracil misincorporation is genotoxic, contributing to replication fork instability, DSB formation and ultimately cell death.

Production of human factor VIII-FL in 293T cells using the bicistronic MGMT(P140K)-retroviral vector

Hemophilia A is the most common X-linked bleeding disorder; it is caused by deficiency of coagulation factor VIII (FVIII). Replacement therapy with rFVIII produced from human cell line is a major goal for treating hemophilia patients. We prepared a full-length recombinant FVIII (FVIII-FL), using the pMFG-P140K retroviral vector. The IRES DNA fragment was cloned upstream to the P140K gene, providing a 9.34-kb bicistronic vector. FVIII-FL cDNA was then cloned upstream to IRES, resulting in a 16.6-kb construct. In parallel, an eGFP control vector was generated, resulting in a 10.1- kb construct. The 293T cells were transfected with these constructs, generating the 293T-FVIII-FL/P140K and 293T-eGFP/P140K cell lines. In 293T-FVIII-FL/P140K cells, FVIII and P140K mRNAs levels were 4,410 (±931.7)- and 295,400 (±75,769)-fold higher than in virgin cells. In 293T-eGFP/P140K cells, the eGFP and P140K mRNAs levels were 1,501,000 (±493,700)- and 308,000 (±139,300)-fold higher than in virgin cells. The amount of FVIII-FL was 0.2 IU/mL and 45 ng/mL FVIII cells or 4.4 IU/μg protein. These data demonstrate the efficacy of the bicistronic retroviral vector expressing FVIII-FL and MGMT(P140K), showing that it could be used for producing the FVIII-FL protein in a human cell line.

Removal of uracil by uracil DNA glycosylase limits pemetrexed cytotoxicity: overriding the limit with methoxyamine to inhibit base excision repair

Uracil DNA glycosylase (UDG) specifically removes uracil bases from DNA, and its repair activity determines the sensitivity of the cell to anticancer agents that are capable of introducing uracil into DNA. In the present study, the participation of UDG in the response to pemetrexed-induced incorporation of uracil into DNA was studied using isogenic human tumor cell lines with or without UDG (UDG(+/+)/UDG(-/-)). UDG(-/-) cells were very sensitive to pemetrexed. Cell killing by pemetrexed was associated with genomic uracil accumulation, stalled DNA replication, and catastrophic DNA strand breaks. By contrast, UDG(+/+) cells were >10 times more resistant to pemetrexed due to the rapid removal of uracil from DNA by UDG and subsequent repair of the resultant AP sites (abasic sites) via the base excision repair (BER). The resistance to pemetrexed in UDG(+/+) cells could be reversed by the addition of methoxyamine (MX), which binds to AP sites and interrupts BER pathway. Furthermore, MX-bound AP sites induced cell death was related to their cytotoxic effect of dual inactivation of UDG and topoisomerase IIα, two genes that are highly expressed in lung cancer cells in comparison with normal cells. Thus, targeting BER-based therapy exhibits more selective cytotoxicity on cancer cells through a synthetic lethal mechanism.

A phase I study of daily oral TRC102 (methoxyamine) to enhance the therapeutic effects of pemetrexed in patients with advanced refractory cancer

2552

Background: TRC102 (methoxyamine) is a small molecule inhibitor of base-excision repair (BER) that enhances the cytotoxicity of alkylator and antimetabolite chemotherapy and reverses chemotherapy resistance. TRC102 inhibits BER by stabilizing abasic sites produced by glycosylase removal of damaged DNA bases. Methods: A phase 1 trial evaluated the safety, PK, and PD of oral TRC102 combined with intravenous pemetrexed in patients with advanced refractory cancer. Patients were required to have ECOG PS ≤ 1 and adequate organ function. TRC102 was escalated in cohorts of 3–6 patients in combination with standard dose pemetrexed. All patients received TRC102 alone on Days 1–4 of a 2 week cycle followed by the combination of pemetrexed and TRC102 every 3 weeks thereafter. Results: Eight patients were treated with a total of 22 cycles of TRC102 at 15 mg/m2 (n=3) and 30 mg/m2 (n=5). No adverse events were attributed to TRC102 alone, and grade 3–4 neutropenia without fever, grade 3 thrombocytopenia, and grade 1–2 fatigue and asthenia were considered possibly related to the combination of TRC102 and pemetrexed. One patient at 30 mg/kg TRC102 experienced grade 3 anemia during Cycle 2 that was considered a dose-limiting toxicity. He recovered after 2 units PRBCs and a 50% dose-reduction of TRC102, and remained on study through Cycle 5. Clinical PK analyses indicate that TRC102 plasma concentrations required for in vivo activity are achievable with daily oral administration (Cmax = 49–62 ng/mL, half-life = 29–40 hr), and PD studies confirm that TRC102 stabilizes pemetrexed-induced abasic sites in these patients. Conclusions: Daily oral TRC102 is tolerated at doses that potentiate the clinical activity of pemetrexed in patients with advanced refractory cancer. Phase 2 studies are planned in multiple indications.

[Table: see text]

DNA repair modulates the vulnerability of the developing brain to alkylating agents

Neurons of the developing brain are especially vulnerable to environmental agents that damage DNA (i.e., genotoxicants), but the mechanism is poorly understood. The focus of the present study is to demonstrate that DNA damage plays a key role in disrupting neurodevelopment. To examine this hypothesis, we compared the cytotoxic and DNA damaging properties of the methylating agents methylazoxymethanol (MAM) and dimethyl sulfate (DMS) and the mono- and bifunctional alkylating agents chloroethylamine (CEA) and nitrogen mustard (HN2), in granule cell neurons derived from the cerebellum of neonatal wild type mice and three transgenic DNA repair strains. Wild type cerebellar neurons were significantly more sensitive to the alkylating agents DMS and HN2 than neuronal cultures treated with MAM or the half-mustard CEA. Parallel studies with neuronal cultures from mice deficient in alkylguanine DNA glycosylase (Aag(-/-)) or O(6)-methylguanine methyltransferase (Mgmt(-/-)), revealed significant differences in the sensitivity of neurons to all four genotoxicants. Mgmt(-/-) neurons were more sensitive to MAM and HN2 than the other genotoxicants and wild type neurons treated with either alkylating agent. In contrast, Aag(-/-) neurons were for the most part significantly less sensitive than wild type or Mgmt(-/-) neurons to MAM and HN2. Aag(-/-) neurons were also significantly less sensitive than wild type neurons treated with either DMS or CEA. Granule cell development and motor function were also more severely disturbed by MAM and HN2 in Mgmt(-/-) mice than in comparably treated wild type mice. In contrast, cerebellar development and motor function were well preserved in MAM-treated Aag(-/-) or MGMT-overexpressing (Mgmt(Tg+)) mice, even as compared with wild type mice suggesting that AAG protein increases MAM toxicity, whereas MGMT protein decreases toxicity. Surprisingly, neuronal development and motor function were severely disturbed in Mgmt(Tg+) mice treated with HN2. Collectively, these in vitro and in vivo studies demonstrate that the type of DNA lesion and the efficiency of DNA repair are two important factors that determine the vulnerability of the developing brain to long-term injury by a genotoxicant.

Time-dependent effect of non-Hodgkin's lymphoma grade on disease-free survival of relapsed/refractory patients treated with high-dose chemotherapy plus autotransplantation

Evaluation of time to event outcomes usually is examined by the Kaplan-Meier method and Cox proportional hazards models. We developed a modified statistical model based on histologic grade and other variables to describe the time-dependent outcome for autologous stem cell transplant (autotransplant) performed for non-Hodgkin's lymphoma (NHL) based on histologic grade and other variables. One hundred and fourteen relapsed or refractory NHL patients were treated using BCNU 600 mg/m2, etoposide 2400 mg/m2, and cisplatin 200 mg/m2 IV followed by autotransplant. Median age was 53.5 (range: 25-70) years, 78 patients had aggressive NHL and 36 indolent NHL. Seventy-five patients received involved-field radiotherapy just prior to transplant. At a median follow-up of 33 (range: 3 to 118) months, the estimated 5-year Kaplan-Meier probabilities of overall survival and disease-free survival were 61% and 51%, respectively. Cox proportional hazards model analysis showed that proportionality did not hold for lymphoma grade, indicating that the relationship between the grade and disease-free survival differed over time. By piece-wise Cox model, the relative risk for experiencing relapse or death after 1 year in patients with indolent compared with patients with aggressive NHL was 2.81 (p=0.019) with 95% confidence interval (1.19, 6.65). The time-dependent effect of lymphoma grade on disease-free survival suggests the need for early (within first year) incorporation of novel therapeutic approaches in management of patients with indolent NHL undergoing autotransplant.

Phase I trial of VNP40101M in children with recurrent brain tumors—A Pediatric Brain Tumor Consortium (PBTC) study

2059

Background: VNP40101M, a novel DNA alkylating agent and a potent inhibitor of alkylguanine alkyl transferase (AGT) in pre- clinical studies, was evaluated in a phase I study in children with recurrent brain tumors. Methods: VNP40101M was given intravenously (i.v) daily for 5 days q 6 weeks up to 8 cycles. Using the continual reassessment method, dose escalation was performed independently in pts stratified based on intensity of prior therapy (stratum I-less-heavily pre-treated; stratum II- heavily pre-treated). Dose limiting toxicities (DLTs) and responses were assessed at the end of the first and 2nd cycles of treatment, respectively. Correlative studies included pharmacokinetics and measurement of AGT activity in peripheral blood mononuclear cells (PBMC) before and after treatment. Results: 41 eligible pts (stratum I- 19, stratum II- 22; median age 9.3 yrs, range 0.9 to 21.5) were enrolled on this study. Dose levels (in mg/m2/day) evaluated in Stratum I were 45, 60, and 78 mg; In Stratum II 20, 30, 45, and 60 mg. DLT in evaluable pts was myelosuppression (grade IV neutropenia for > 7 days or any grade IV thrombocytopenia) and occurred in 4/16 pts in stratum I [45 mg (n=1/12), 60 mg (n=1/2), 78 mg (n= 2/2)] and 3/19 pts in stratum II [45 mg (n= 3/4), 60 mg (n=0/1), 30 mg (n= 0/12), and 20 mg (n= 0/2)] respectively. Other significant toxicities post first course included renal failure (n=2), pulmonary (n=2), and fatal infection (n=1). PK studies showed median (range) terminal half-life of 62 mins (7.3 to 522 min). The MTDs in stratum I and II were 45 mg/m2/day and 30 mg/m2/day daily for 5 days q 6 weeks, respectively. Objective responses were observed in one pt each with brain stem glioma and medulloblastoma respectively. PBMC AGT levels did not decrease following VNP40101M treatment. Conclusions: The recommended pediatric phase II dose of VNP40101M given i.v daily for 5 days q 6 weeks is 45 mg/m2/day in less-heavily treated and 30 mg/m2/day in heavily treated pts.

[Table: see text]

Prevention of base excision repair by TRC102 (methoxyamine) potentiates the anti-tumor activity of pemetrexed in vitro and in vivo

13005

Background: TRC102 (methoxyamine) reverses resistance to alkylating agents by inhibiting base excision repair (BER; a mechanism of DNA repair), thereby increasing DNA strand breaks and potentiating the anti-tumor activity of alkylating agents without additional toxicity, Based on these data, TRC102 is currently being studied in combination with temozolomide in a phase 1 trial. We hypothesized that inhibition of BER by TRC102 would also increase DNA strand breaks and improve the anti-tumor activity of anti-metabolite chemotherapeutics, including pemetrexed, because these agents also produce AP sites that are recognized and repaired by BER. Methods: Pemetrexed- induced AP sites and BER inhibition was quantified using an apurinic/apyrimidinic (AP) site assay in vitro. Single and double DNA strand breaks were quantified by the Comet assay in vitro and anti-tumor activity was assessed in an in vivo xenograft study of subcutaneously implanted H460 human lung cancer cells. Results: Pemetrexed induced and TRC102 reduced the number of available AP sites in pemetrexed- treated H460 cells (by 60–80%), indicating successful inhibition of BER. TRC102 treatment increased DNA strand breaks in pemetrexed-treated H460 cells (2 fold increase versus treatment with pemetrexed alone). Premetrexed treatment alone and in combination with TRC 102 delayed tumor growth in vivo (tumor growth delay of 4.7 days in the 150 mg/m2 pemetrexed alone group, 5.7 days in the 150 mg/m2 pemetrexed + 2 mg/m2 TRC102 group and 6.9 days in the 150 mg/m2 pemetrexed + 4 mg/m2 TRC102 group); in vivo systemic toxicity was not increased. TRC102 alone had no effect in vitro or in vivo. Conclusions: TRC102 effectively inhibits BER in lung cancer cells treated with pemetrexed. Inhibition of DNA repair by TRC102 results in an increase in DNA strand breaks and improved anti-tumor activity versus treatment with pemetrexed alone. Given its preclinical efficacy and safety profile, study of TRC102 combined with pemetrexed in a phase 1 trial is warranted.

No significant financial relationships to disclose.

Pre-mobilization therapy blood CD34+ cell count predicts the likelihood of successful hematopoietic stem cell mobilization

We examined pre-mobilization blood CD34+ cell count to predict ability to mobilize adequate peripheral blood progenitor cells (PBPC) in 106 cancer patients and 36 allogeneic donors. Mean pre-mobilization therapy blood CD34+ cell count was 3.1 cells x 10(6)/l (s.d. = 3.9, r = 0.3-37) and mean CD34+ cells collected were 5.3 x 10(6) cells/kg/leukapheresis procedure (s.d. = 7.0, r = 0.03-53). Yields correlated with pre-mobilization CD34+ cells x 10(6)/l (r = 0.37, P-value < 0.0001); correlation was stronger in allogeneic donors (r = 0.56, P-value = 0.0004) and males (r = 0.46, P-value < 0.0001). Based on classification and regression tree multivariate analysis, the predictive value of pre-mobilization blood CD34+ cell count was confounded by other variables, including age, gender, mobilization regimen and malignancy type. We generated an algorithm to predict a minimum PBPC yield of 1 x 10(6) CD34+ cells/kg/leukapheresis procedure after mobilization. A threshold pre-mobilization blood CD34+ cell count of 2.65 cells x 10(6)/l was the most important decision point in predicting successful mobilization. Only 2% of subjects with pre-mobilization blood CD34+ cell counts > 2.65 cells x 10(6)/l did not achieve the minimum per apheresis, whereas 24% with pre-mobilization values below threshold had inadequate mobilization. Prospectively identifying individuals at risk for mobilization failure would allow for improved treatment planning, resource utilization and time saving.

A phase I trial of PS-341 and fludarabine for relapsed and refractory indolent non-Hodgkin’s lymphoma and chronic lymphocytic leukemia

7580

Background: The safety and efficacy of PS-341 (bortezomib) as a single agent in relapsed and refractory hematologic malignancies has been well documented. Fludarabine is known to be active in indolent non-Hodgkin’s lymphoma (NHL) and chronic lymphocytic leukemia (CLL). PS-341 may potentiate fludarabine activity by inhibiting DNA repair and inducing apoptosis in Bcl-2 over-expressing cells. The safety of this combination was evaluated in an ongoing phase I study. An additional dose level including rituxumab has been added to potentially improve efficacy without significant additional toxicity. Methods: Dose levels were as outlined in the table below. Each cycle was 21 days. Results: To date, 13 patients have received at least one cycle of treatment and are evaluable for toxicity. Diagnoses of patients included CLL, Waldenstom’s macroglobulinemia and MALT, lymphoplasmacytoid, mantle cell and follicular lymphomas. Patients received a median of 3 prior treatments (range 1–6). DLT’s were not observed in the first 4 dose levels, however the first patient on dose level 5 experienced grade 4 neutropenia. Three patients had doses of PS-341 held due to toxicity (2 due to neuropathy and one leukopenia; doses were held during cycles 3, 4 and 2 respectively) and two required dose reductions for grade 2 neuropathy (on cycles 2 and 3). One CRu has been documented (follicular lymphoma), 1 PR (mantle cell) and 6 patients with SD for 1.5 to 3 months. One patient was taken off the study with disease progression after 1 cycle (CLL) and two discontinued treatment due to prolonged cytopenias (MCL) and patient decision. The most 2 most recent patients are not yet evaluable for response. Conclusions: Combination PS-341 and fludarabine appears to be well-tolerated and active in relapsed and refractory NHL and CLL. Enrollment and follow-up of patients on level 5 (addition of rituxan) continues. Initiation of a phase II study of this regimen is anticipated shortly.

[Table: see text]

No significant financial relationships to disclose.

Hematopoietic stem cell gene therapy of murine protoporphyria by methylguanine-DNA-methyltransferase-mediated in vivo drug selection

Erythropoietic protoporphyria (EPP) is an inherited defect of the ferrochelatase (FECH) gene characterized by the accumulation of toxic protoporphyrin in the liver and bone marrow resulting in severe skin photosensitivity. We previously described successful gene therapy of an animal model of the disease with erythroid-specific lentiviral vectors in the absence of preselection of corrected cells. However, the high-level of gene transfer obtained in mice is not translatable to large animal models and humans if there is no selective advantage for genetically modified hematopoietic stem cells (HSCs) in vivo. We used bicistronic SIN-lentiviral vectors coexpressing EGFP or FECH and the G156A-mutated O6-methylguanine-DNA-methyltransferase (MGMT) gene, which allowed efficient in vivo selection of transduced HSCs after O6-benzylguanine and BCNU treatment. We demonstrate for the first time that the correction and in vivo expansion of deficient transduced HSC population can be obtained by this dual gene therapy, resulting in a progressive increase of normal RBCs in EPP mice and a complete correction of skin photosensitivity. Finally, we developed a novel bipromoter SIN-lentiviral vector with a constitutive expression of MGMT gene to allow the selection of HSCs and with an erythroid-specific expression of the FECH therapeutic gene.

Hematopoietic stem cell gene therapy: progress toward therapeutic targets

The concept of hematopoietic stem cell gene therapy is as exciting as that of stem cell transplantation itself. The past 20 years of research have led to improved techniques for transferring and expressing genes in hematopoietic stem cells and preclinical models now routinely indicate the ease with which new genes can be expressed in repopulating stem cells of multiple species. Both modified murine oncoretroviruses and lentiviruses transmit genes into the genome of hematopoietic stem cells and allow expression in the host following transplantation. Using oncoretroviruses, therapeutic genes for severe combined immunodeficiency, common variable gamma chain immunodeficiency, chronic granulomatous disease, Hurler's and Gaucher's Disease have all been used clinically with only modest success except for the patients with immunodeficiency in whom a partial T-cell chimerism has been dramatic. Since stem cell selection in vivo appears important to the therapeutic success of gene transfer, drug resistance selection, most recently using the MGMT gene, has been developed and appears to be safe. Future trials combining a drug resistance and therapeutic gene are planned, as are trials using safety-modified lentiviruses. The therapeutic potential of hematopoietic stem cell gene therapy, particularly given recent advances in stem cell plasticity, remains an exceptionally exciting area of clinical research.

Marked inactivation of O6-alkylguanine-DNA alkyltransferase activity with protracted temozolomide schedules

Temozolomide, an oral DNA methylator that inactivates the DNA repair enzyme O(6)-alkylguanine-DNA alkyltransferase (AGAT), has demonstrated anticancer activity on protracted schedules. Protracted schedules may lead to an 'autoenhancement' of temozolomide's inherent cytotoxic potential by cumulative reduction of the cell's capacity for AGAT-mediated DNA repair and resistance. This study was undertaken to characterise AGAT inactivation and regeneration in the peripheral blood mononuclear cells (PBMCs) of patients treated on two protracted temozolomide schedules. O(6)-alkyl guanine-DNA alkyltransferase activity was measured in the PBMCs of patients treated on two phase I protracted temozolomide studies. Patients were treated daily for either 7 days every 2 weeks (Schedule A) or 21 days every 4 weeks (Schedule B). The effects of various temozolomide doses (75-175 mg m(-2)), treatment duration (7-21 days), and temozolomide plasma levels on AGAT inactivation and regeneration, as well as the relation between AGAT inactivation and toxicity, were studied. O(6)-alkyl guanine-DNA alkyltransferase activity in PBMCs was measured serially in 52 patients. Marked inactivation of AGAT occurred following 7 days of temozolomide treatment, with mean AGAT activity decreasing by 72% (P<0.0001). Similarly, mean AGAT activity decreased by 63 and 73% after 14 and 21 days of treatment, respectively (P<0.001 for both comparisons). O(6)-alkyl guanine-DNA alkyltransferase inactivation was greater after 7 days of treatment with higher doses of temozolomide than lower doses and remained markedly reduced 7 days post-treatment. However, AGAT inactivation following temozolomide treatment for 14 and 21 days was similar at all doses. On the continuous 21-day schedule, AGAT inactivation was significantly greater in patients who experienced severe thrombocytopenia than those who did not (90.3+/-5.5 vs 72.5+/-16.1%, P<0.045). In conclusion, protracted administration of temozolomide, even at relatively low daily doses, leads to significant and prolonged depletion of AGAT activity, which may enhance the antitumour activity of the agent.

Allogeneic mesenchymal stem cell infusion for treatment of metachromatic leukodystrophy (MLD) and Hurler syndrome (MPS-IH)

Patients with Hurler syndrome (mucopolysaccharidosis type-IH) and metachromatic leukodystrophy (MLD) develop significant skeletal and neurologic defects that limit their survival. Transplantation of allogeneic hematopoietic stem cells results in partial correction of the clinical manifestations. We postulated that some of these defects may be corrected by infusion of allogeneic, multipotential, bone marrow-derived mesenchymal stem cells (MSC). Patients with Hurler syndrome (n = 5) or MLD (n = 6) who previously underwent successful bone marrow transplantation from an HLA-identical sibling were infused with 2-10 x 10(6)/kg MSCs, isolated and expanded from a bone marrow aspirate of the original donor. There was no infusion-related toxicity. In most recipients culture-purified MSCs at 2 days, 30-60 days and 6-24 months after MSC infusion remained of host type. In two patients the bone marrow-derived MSCs contained 0.4 and 2% donor MSCs by FISH 60 days after MSC infusion. In four patients with MLD there were significant improvements in nerve conduction velocities after MSC infusion. The bone mineral density was either maintained or slightly improved in all patients. There was no clinically apparent change in patients' overall health, mental and physical development after MSC infusion. We conclude that donor allogeneic MSC infusion is safe and may be associated with reversal of disease pathophysiology in some tissues. The role of MSCs in the management of Hurler syndrome and MLD should be further evaluated.

G156A MGMT-transduced human mesenchymal stem cells can be selectively enriched by O6-benzylguanine and BCNU

Human bone marrow-derived mesenchymal stem cells (hMSCs) are being investigated for a potential therapeutic role as hematopoietic support cells following chemo-radiotherapy and as vehicles of gene delivery. Although hMSCs can be safely infused into humans and experimental animals, there is limited evidence regarding their engraftment and proliferation in vivo. We developed a drug resistance gene transfer strategy to mark and selectively enrich marked hMSCs using chemotherapy. We have determined that hMSCs are markedly sensitized to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) in vitro when pretreated with O(6)-benzylguanine (BG) resulting in a more than four-fold decrease in BCNU IC(90). The MFG retroviral vector encoding a bicistronic transcript for green fluorescent protein (GFP) and mutant (G156A)-methylguanine methyltransferase (G156A-MGMT), which encodes O(6)-alkylguanine-DNA alkyltransferase (AGT), conferring, BG plus BCNU resistance, transduced a high percentage of hMSCs. Transduced hMSCs had high expression of GFP and AGT and became significantly resistant to BG and BCNU. Furthermore, the proportion of GFP expressing transduced hMSCs increased from 32 +/- 14% to 70 +/- 14% following BG and BCNU treatment in vitro. Intravenously infused hMSCs were detected in NOD-SCID mice 8 weeks later by PCR analysis but could not be recultured from the bone marrow. GFP-expressing hMSCs inoculated into subcutaneous wounds in nonobese diabetic-severe combined immunodeficient (NOD-SCID) mouse could be recultured at a low frequency, but enriched by BG and BCNU treatment from 0.05 +/- 0.03% to 0.55 +/- 0.4 (p = 0.028, Welch t-test). Our results indicate that hMSCs are sensitive to BG and BCNU, predicting significant toxicity to the hematopoietic microenvironment with this therapy. G156A-MGMT is a powerful selectable gene for a second marker gene in hMSCs. Drug resistance gene transfer into hMSCs may allow in vivo enrichment of hMSCs when MSC homing and engraftment into target tissues is optimized.

Sequential tumor biopsies in early phase clinical trials of anticancer agents for pharmacodynamic evaluation

PURPOSE

In the setting of target-based anticancer drug development, it is critical to establish that the observed preclinical activity can be attributed to modulation of the intended target in early phase trials in human subjects. This paradigm of target modulation allows us to determine a Phase II or III dose (optimal biochemical/biological modulatory dose) that may not necessarily be the maximum tolerated dose. A major obstacle to target-based (often cytostatic) drug development has been obtaining relevant tumor tissue during clinical trials of these novel agents for laboratory analysis of the putative marker of drug effect.

EXPERIMENTAL DESIGN

From 1989 to present, we have completed seven clinical trials in which the end point was a biochemical or biological modulatory dose in human tumor tissues (not surrogate tissue). Eligibility enrollment required that patients have a biopsiable lesion either with computerized tomography (CT) guidance or direct visualization and consent to sequential (pre and posttreatment) biopsies.

RESULTS

A total of 192 biopsies were performed in 107 patients. All but 8 patients had sequential pre and posttreatment biopsies. Seventy-eight (73%) of the 107 patients had liver lesion biopsies. In eight patients, either one or both biopsies contained insufficient viable tumor tissue or no tumor tissue at all for analysis. Of a total of 99 patients in whom we attempted to obtain paired biopsies, a total of 87 (88%) were successful. Reasons for failure included patient refusal for a second biopsy (n = 2), vasovagal reaction with first biopsy precluding a second biopsy (n = 1), subcapsular hepatic bleeding (n = 1), and most commonly obtaining necrotic tumor, fibrous, or normal tissue in one of the two sequential biopsies (n = 8).

CONCLUSIONS

This is the first and largest reported series demonstrating that with adequate precautions and experience, sequential tumor biopsies are feasible and safe during early phase clinical trials.

MMTV promoter hypomethylation is linked to spontaneous and MNU associated c-neu expression and mammary carcinogenesis in MMTV c-neu transgenic mice

The erbB family of receptor tyrosine kinases is frequently implicated in neoplasia. Amplification and overexpression of erbB2/neu has been found in 20 to 40% of human breast cancers. Previous studies using MMTV/c-neu transgenic mice have linked rat neu overexpression to mammary tumor development. In this study, we provide evidence that rat neu overexpression in mammary tumors of MMTV/c-neu transgenic mice is always associated with demethylation of the MMTV promoter, whereas the normal mammary glands of these transgenic mice always contain specific methylated regions of the MMTV promoter. In addition, after exposure to N-methyl-N-nitrosourea (MNU), the latency of mammary tumor development is significantly reduced and again is also associated with MMTV promoter demethylation. Thus, the transition from methylation to hypomethylation of the MMTV promoter induces high-level expression of c-neu and appears to be a prerequisite for transformation from normal to malignant mammary epithelium, either spontaneously or after carcinogen exposure. Expression of transgenic c-neu from the demethylated MMTV promoter appears to be an early event that allows outgrowth of mammary epithelium predisposed to malignant transformation.

Overexpression of human O6-alkylguanine DNA alkyltransferase (AGT) prevents MNU induced lymphomas in heterozygous p53 deficient mice

O6-alkylguanine DNA alkyltransferase (AGT) is a key mechanism in the prevention against MNU induced malignant transformation by removal of O6 methyl guanine (O6mG) adducts. We asked whether heterozygous p53 deficient mice (p53+/-) would be more susceptible to MNU induced lymphomas than wild type mice, and whether O6mG adducts were responsible for this susceptibility. To determine whether MGMT overexpression would be protective, p53+/- mice were bred to human MGMT transgenic mice (MGMT+) and treated with 50 mg/kg MNU. MNU increased the incidence of thymic lymphomas in non-transgenic p53+/- mice from 23% (n=13) to 68% (n=22) and decreased the mean latency from 433 to 106 days (P=0.01 compared to untreated mice). Wild type mice had an incidence of 30% (n=38) and a mean latency of 135 days after MNU. Overexpression of MGMT in the thymus of p53+/- mice significantly reduced the lymphoma incidence from 68 to 28% (n=17) and increased the latency from 106 to 167 days (P=0.003). Similarly, the lymphoma incidence in MGMT+/wild type mice decreased from 30 to 8% (n=12) and the latency increased to 297 days (P=0.2). Loss of the wild type allele was found in only 2/17 lymphomas occurring in p53+/- mice and there were no significant point mutations in exons 5-8 of p53. Furthermore, there was no loss of p53 function in these mice. These data demonstrate that unrepaired O6mG lesions act cooperatively with the reduced p53 dose and lead to lymphomagenesis in p53+/- mice, but AGT overexpression and rapid removal of O6mG adducts is protective.

Differential degradation rates of inactivated alkyltransferase in blood mononuclear cells and tumors of patients after treatment with O(6)-benzylguanine

O(6)-Alkylguanine-DNA alkyltransferase (AGT) repairs O(6)-alkylating DNA adducts generated by alkylating therapeutic agents. Therefore, AGT activity may be an important marker of tumor and normal tissue sensitivity to chemotherapeutic agents and a predictor for the success of chemotherapeutic regimens. It is rapidly inactivated by O(6)-benzylguanine (BG) that mimics its substrates, O(6)-methylguanine and O(6)-chloroethylguanine DNA adducts. In a Phase I clinical trial, BG was given in increasing doses (from 10 to 120 mg/m(2)) by 1-h infusion. We previously reported depletion of AGT activity, and in this report, we demonstrate the relationship between degradation of BG-inactivated AGT protein and the depletion of AGT activity in peripheral blood mononuclear cells (PBMCs) and tumor samples obtained by computed tomography-guided cutting needle biopsy from patients prior to BG and either 2 or 18 h after BG. In PBMCs, BG inactivated AGT activity by over 95-100% at the end of a 1-h infusion, and depletion was maintained for 18 h. In contrast, AGT protein remained almost unchanged for up to 18 h after BG, suggesting that inactivated AGT proteins remain immunoreactive and are not rapidly degraded in PBMCs. In patient tumor biopsies, AGT activity was depleted approximately 90% 2 h after BG. Tumor AGT protein levels were reduced to approximately 40% of pretreatment values when detected by either Western blot or immunohistochemistry staining. In tumor samples obtained 18 h after BG, >95% inactivation of tumor AGT activity was observed at BG doses of 36-80 mg/m(2), and complete depletion of tumor AGT activity occurred at 120 mg/m(2) BG. However, residual AGT protein (5-10% of baseline) was detectable in all tumor samples. Therefore, the degradation of BG-inactivated AGT protein appeared to be much more rapid in tumors than that in PBMCs, which may impact on AGT regeneration rates as well. Because degradation of BG-inactivated AGT takes place slowly, antibody-based measurements of AGT protein correlate poorly with depletion of AGT activity immediately after BG. Thus, biochemical activity measurements remain the appropriate monitor of AGT during therapeutic modulation. These data provide the first and conclusive evidence of differential degradation rates of inactivated AGT in PBMCs and tumors of patients after treatment with BG and suggest that immunoreactive AGT measurements in PBMCs are a poor surrogate for AGT activity in tumor tissue.

Temozolomide: the effect of once- and twice-a-day dosing on tumor tissue levels of the DNA repair protein O(6)-alkylguanine-DNA-alkyltransferase

Temozolomide (TMZ) is a methylating agent of the imidotetrazine class, whose cytotoxic product is O(6)-methylguanine DNA adducts, which initiate a futile recycling of the mismatch repair pathway causing DNA strand breaks and apoptotic cell death in mismatch repair proficient cells. The DNA repair protein O(6)-alkylguanine DNA alkyltransferase (AGT) repairs these adducts in a suicide manner and reduces the cytotoxic action of TMZ. An antitumor threshold is reached when sufficient adducts are formed by TMZ to inactivate AGT. In this study, we evaluated the relation between TMZ dosing and AGT depletion in patients with deep visceral tumors and in peripheral blood mononuclear cells (PBMCs) to determine whether the dose of TMZ was sufficient to inactivate AGT and lead to therapeutic efficacy. To do so, we compared single dose therapy with a novel twice daily regimen in a laboratory correlate-driven Phase I dose escalation study. p.o. bolus dose TMZ 200 mg/m(2) daily times five was compared with the same bolus on day 1 followed by nine doses at 12-h intervals of 50, 75, 90, or 100 mg/m(2). Dose-limiting toxicity in the bid regimen (grade IV thrombocytopenia and neutropenia) was seen at 100 mg/m(2), cumulative dose 1100 mg/m(2), and the maximum tolerated dose was 1010 mg/m(2). The degree of tumor tissue AGT activity depletion measured in biopsies before and on day 5 of therapy varied widely, between 0 (in 3 patients) and 99% (in 1), with the majority of patients (10 of 15) having 52-84% tumor AGT depletion. In contrast, AGT activity in PBMCs fell rapidly during TMZ administration to undetectable levels in all dosage groups on day 5 but did not correlate with tumor AGT depletion. TMZ pharmacokinetics were dose proportional; no accumulation occurred >5-day period in the bid regimen. Two partial responses were seen, lasting 3 and 4 months. Five additional patients achieved prolonged stabilization of disease for 4-6 monthly cycles. This is the first study to document that at maximum tolerated doses, TMZ depletes PBMC AGT but only partially and variably depletes visceral tumor AGT in most patients, even during twice daily dosing. Drug combinations or schedules designed to maximally deplete tumor AGT might improve TMZ efficacy.

Hematopoietic engraftment and survival in adult recipients of umbilical-cord blood from unrelated donors

BACKGROUND

Umbilical-cord blood from unrelated donors who are not HLA-identical with the recipients can restore hematopoiesis after myeloablative therapy in children. We studied the use of transplantation of umbilical-cord blood to restore hematopoiesis in adults.

METHODS

Sixty-eight adults with life-threatening hematologic disorders received intensive chemotherapy or total-body irradiation and then transplants of HLA-mismatched umbilical-cord blood. We evaluated the outcomes in terms of hematologic reconstitution, the occurrence of acute and chronic graft-versus-host disease (GVHD), relapses, and event-free survival.

RESULTS

Of the 68 patients, 48 (71 percent) received grafts of umbilical-cord blood that were mismatched for two or more HLA antigens. Of the 60 patients who survived 28 days or more after transplantation, 55 had neutrophil engraftment at a median of 27 days (range, 13 to 59). The estimated probability of neutrophil recovery in the 68 patients was 0.90 (95 percent confidence interval, 0.85 to 1.0). The presence of a relatively high number of nucleated cells in the umbilical-cord blood before it was frozen was associated with faster recovery of neutrophils. Severe acute GVHD (of grade III or IV) occurred in 11 of 55 patients who could be evaluated within the first 100 days after transplantation. Chronic GVHD developed in 12 of 33 patients who survived for more than 100 days after transplantation. The median follow-up for survivors was 22 months (range, 11 to 51). Of the 68 patients, 19 were alive and 18 of these (26 percent) were disease-free 40 months after transplantation. The presence of a high number of CD34+ cells in the graft was associated with improved event-free survival (P=0.05).

CONCLUSIONS

Umbilical-cord blood from unrelated donors can restore hematopoiesis in adults who receive myeloablative therapy and is associated with acceptable rates of severe acute and chronic GVHD.

Methoxyamine potentiates DNA single strand breaks and double strand breaks induced by temozolomide in colon cancer cells

We have previously shown that human cancer cells deficient in DNA mismatch repair (MMR) are resistant to the chemotherapeutic methylating agent temozolomide (TMZ) and can be sensitized by the base excision repair (BER) blocking agent methoxyamine (MX) [21]. To further characterize BER-mediated repair responses to methylating agent-induced DNA damage, we have now evaluated the effect of MX on TMZ-induced DNA single strand breaks (SSB) by alkaline elution and DNA double strand breaks (DSB) by pulsed field gel electrophoresis in SW480 (O6-alkylguanine-DNA-alkyltransferase [AGT]+, MMR wild type) and HCT116 (AGT+, MMR deficient) colon cancer cells. SSB were evident in both cell lines after a 2-h exposure to equitoxic doses of temozolomide. MX significantly increased the number of TMZ-induced DNA-SSB in both cell lines. In contrast to SSB, TMZ-induced DNA-DSB were dependent on MMR status and were time-dependent. Levels of 50 kb double stranded DNA fragments in MMR proficient cells were increased after TMZ alone or in combination with O6-benzylguanine or MX, whereas, in MMR deficient HCT116 cells, only TMZ plus MX produced significant levels of DNA-DSB. Levels of AP endonuclease, XRCC1 and polymerase beta were present in both cell lines and were not significantly altered after MX and TMZ. However, cleavage of a 30-mer double strand substrate by SW480 and HCT116 crude cell extracts was inhibited by MX plus TMZ. Thus, MX potentiation of TMZ cytotoxicity may be explained by the persistence of apurinic/apyrimidinic (AP) sites not further processed due to the presence of MX. Furthermore, in MMR-deficient, TMZ-resistant HCT116 colon cancer cells, MX potentiates TMZ cytotoxicity through formation of large DS-DNA fragmentation and subsequent apoptotic signalling.

Applied molecular evolution of O6-benzylguanine-resistant DNA alkyltransferases in human hematopoietic cells

Applied molecular evolution is a rapidly developing technology that can be used to create and identify novel enzymes that nature has not selected. An important application of this technology is the creation of highly drug-resistant enzymes for cancer gene therapy. Seventeen O(6)-alkylguanine-DNA alkyltransferase (AGT) mutants highly resistant to O(6)-benzylguanine (BG) were identified previously by screening 8 million variants, using genetic complementation in Escherichia coli. To examine the potential of these mutants for use in humans, the sublibrary of AGT clones was introduced to human hematopoietic cells and stringently selected for resistance to killing by the combination of BG and 1,3-bis(2-chloroethyl)-1-nitrosourea. This competitive analysis between the mutants in human cells revealed three AGT mutants that conferred remarkable resistance to the combination of BG and 1,3-bis(2-chloroethyl)-1-nitrosourea. Of these, one was recovered significantly more frequently than the others. Upon further analysis, this mutant displayed a level of BG resistance in human hematopoietic cells greater than that of any previously reported mutant.

Phase I clinical and pharmacokinetic study of rebeccamycin analog NSC 655649 given daily for five consecutive days

PURPOSE

Rebeccamycin analog (NSC 655649) is active against a variety of both solid and nonsolid tumor cell lines. We performed a phase I trial to determine the maximum-tolerated dose (MTD) of rebeccamycin analog when given on a daily x 5 schedule repeated every 3 weeks, characterize the toxicity profile using this schedule, observe patients for antitumor response, and determine the pharmacokinetics of the agent and pharmacodynamic interactions.

PATIENTS AND METHODS

Thirty assessable patients received a total of 153 cycles according to the following dose escalation schema: 60, 80, 106, 141, and 188 mg/m(2)/d x 5 days.

RESULTS

Grade 2 phlebitis occurred in all patients before the use of central venous access, placed at dose level 4 and higher. Dose-limiting toxicity (DLT), grade 4 neutropenia, occurred at 188 mg/m(2)/d x 5 days in both previously treated and chemotherapy-naive patients. Pharmacokinetic analysis revealed a three-compartmental model of drug elimination and a long terminal half-life (154 +/- 55 hours). The percentage drop in absolute neutrophil count correlates with the area under the curve infinity. The presence of a second peak during the elimination phase as well as a high concentration of NSC 655649 in biliary fluid compared with the corresponding plasma measurement (one patient) is suggestive of enterohepatic circulation. Two partial responses, two minor responses, and six prolonged (> 6 months) cases of stable disease were observed. Of these, three patients with gallbladder cancer and one patient with cholangiocarcinoma experienced either a minor response or a significant period of freedom from progression.

CONCLUSION

The recommended phase II dose for NSC 665649 on a daily x 5 every 3 weeks schedule is 141 and 165 mg/m(2)/d for patients with prior and no prior therapy, respectively, with DLT being neutropenia. During this phase I trial, encouraging antitumor activity was been observed.

Acquired resistance to O6-benzylguanine plus chloroethylnitrosoureas in human breast cancer

PURPOSE

O(6)-benzylguanine (BG) is a pseudosubstrate inactivator of the DNA repair protein O(6)- alkylguanine-DNA alkyltransferase (AGT) that has entered clinical trials as a potentiator of the antitumor effect of 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). This study was designed to evaluate potential mechanisms of BG + BCNU resistance in breast cancer cells.

METHODS

We treated MCF-7 breast cancer cells three times with cytotoxic concentrations of BG + BCNU to isolate a population of MCF-7 cells possessing BG + BCNU resistance (BBR). We measured drug resistance, AGT activity, cross-resistance to other agents and sensitivity of AGT to BG inactivation.

RESULTS

When compared with the parent line, MCF-7BBR cells were no longer sensitized to BCNU by BG, resulting in three-fold resistance to BG + BCNU and fourfold resistance to BG + 1-(2-chloroethyl)-1-nitrosourea (CNU). In contrast, MCF-7 and MCF-7BBR cells had similar sensitivity to BCNU, CNU, temozolomide (each in the absence of BG), cisplatin, and UV light. Acquired resistance to BG + chloroethylnitrosoureas (CENU) in MCF-7BBR cells was not accompanied by an increase in AGT or altered kinetics of BG inactivation of AGT. While glutathione-S transferase activity was increased modestly, its contribution to resistance in this setting remains unclear.

CONCLUSIONS

This is the first report of acquired BG + CENU resistance in human tumor cells and the results indicate that selective pressure with BG + CENUs may result in nonresponsiveness to BG by one or more complex mechanisms. These results predict the development of acquired resistance to BG + BCNU in clinical trials.

Characterization of MLH1 and MSH2 DNA mismatch repair proteins in cell lines of the NCI anticancer drug screen

PURPOSE AND METHODS

The lack of a functional DNA mismatch repair (MMR) pathway has been recognized as a common characteristic of several different types of human cancers due to mutation affecting one of the MMR genes or due to promoter methylation gene silencing. These MMR-deficient cancers are frequently resistant to alkylating agent chemotherapy such as DNA-methylating or platinum-containing compounds. To correlate drug resistance with MMR status in a large panel of human tumor cell lines, we evaluated by Western blot the cellular levels of the two MMR proteins most commonly mutated in human cancers, MLH1 and MSH2, in the NCI human tumor cell line panel. This panel consists of 60 cell lines distributed among nine different neoplastic diseases.

RESULTS

We found that in most of these cell lines both MLH1 and MSH2 were expressed, although at variable levels. Five cell lines (leukemia CCRF-CEM, colon HCT 116 and KM12 and ovarian cancers SK-OV-3 and IGROV-1) showed complete deficiency in MLH1 protein. MSH2 protein was detected in all 57 cell lines studied. Absence of MLH1 protein was always linked to resistance to the methylating chemotherapeutic agent temozolomide. This resistance was independent of cellular levels of O6-alkylguanine DNA alkyltransferase. Based on data available for review in the NCI COMPARE database, cellular levels of MLH1 and MSH2 did not correlate significantly with sensitivity to any standard anticancer drug or with any characterized molecular target already tested against the same panel of cell lines.

CONCLUSION

Based on evaluation of 60 tumor cell lines in the NCI anticancer drug screen, MLH1 deficiency was more common than MSH2 deficiency and was always associated with a high degree of temozolomide resistance. These data will enable correlations with other drug sensitivities and molecular targets in the COMPARE database to evaluate linked processes in tumor drug resistance.

Systemic deficits in transporter for antigen presentation (TAP)-1 or proteasome subunit LMP2 have little or no effect on tumor incidence

Some tumor cells have deficits in class I MHC antigen processing, suggesting that T cells exert selective pressure on tumor cells. Previous studies have not revealed increased tumor incidence in mice with deficits in T-cell immunity, including mice lacking TAP1 (a subunit of the transporter for antigen presentation) or LMP2 (a regulated subunit of the 20S proteasome). The incidence of spontaneous tumors in these mice, however, is too low to assess differences in host resistance to tumors. To increase tumor incidence and better assess the role of systemic expression of TAP1 and LMP2 in responses to tumors, TAP1-/- and LMP2-/- mice were bred with p53-/- mice to create TAP1-/-p53-/- and LMP2-/-p53-/- double knockout mice. Lymphomas and sarcomas (malignant fibrous histiocytoma and angiosarcoma) occurred with high incidence in all p53-deficient populations. Tumor incidence and death rate were similar in TAP1-/-p53-/- mice and closely matched control TAP1+/+p53-/- mice. Tumor incidence and death rate were slightly accelerated in LMP2-/-p53-/- mice relative to control LMP2+/+p53-/- mice, but the biological significance of this difference was unclear. The relative incidence of lymphomas vs. sarcomas was not significantly altered by variation in TAP1 or LMP2. In conclusion, systemic absence of TAP1 did not alter tumor incidence, while absence of LMP2 was associated with only a slight acceleration of tumor incidence of uncertain significance. These observations are consistent with other evidence that normal T-cell responses do not effectively limit tumorigenesis. Even though T cells can attack some tumor cells, the ability of tumors to alter their immunogenicity and evade T-cell surveillance may render the native immune system ineffective at providing a rate-limiting barrier to tumorigenesis and preventing cancer.

MGMT expression in murine bone marrow is a major determinant of animal survival after alkylating agent exposure

Myelosuppression is commonly observed after alkylating agent chemotherapy due to low levels of O(6)-alkylguanine DNA alkyltransferase protein (AGT) in hematopoietic progenitors. Mice that lack AGT in all organs, O(6)-methylguanine-DNA methyltransferase gene knockout (MGMT(-/-)) mice are extremely hypersensitive to the methylating agent N-methyl-N-nitrosourea (MNU) and exhibit a 10-fold reduction in the LD(90). To determine whether bone marrow damage was the cause of the increased lethality, we transplanted 1 x 10(6) wild-type marrow into MGMT(-/-) mice and MGMT(-/-) marrow into wild-type mice and observed survival after MNU. Lethally irradiated MGMT(-/-) mice given > or = 25 mg/kg MNU 3 weeks after transplant of wild-type cells survived > 30 days (n = 11), whereas this dose was lethal to control MGMT(-/-) mice 9-12 days post treatment (n = 5). Conversely, lethally irradiated wild-type mice transplanted with MGMT(-/-) cells died after only 20-60 mg/kg MNU within 8-12 days (n = 6). No significant toxicities were found in other organs. Additionally, in an in vivo post transplant competition model, wild-type long-term repopulating cells had a > 200-fold competitive survival advantage over MGMT(-/-) cells, and after MNU treatment completely repopulated the mouse when transplanted at only one-tenth the cell number. We also observed a strong selection for transplanted marrow-derived wild-type stromal elements in the MGMT(-/-) background after drug treatment. These data indicate that alkylating agent hypersensitivity of MGMT(-/-) mice results from hematopoietic damage at the stem level. Thus, DNA repair involving AGT in hematopoietic cells is required for normal host survival following exposure to methylating and chloroethylating agents.

Human marrow-derived mesenchymal stem cells (MSCs) express hematopoietic cytokines and support long-term hematopoiesis when differentiated toward stromal and osteogenic lineages

Human mesenchymal stem cells (MSCs), bone marrow-derived pluripotent adherent cells of mesenchymal origin can differentiate along the osteogenic, chondrogenic, adipogenic, and tendonogenic lineages. In this report we characterize cytokine and growth factor gene expression by MSCs and investigate the modulation of cytokine expression that occurs during osteogenic and stromal differentiation. MSCs constitutively expressed mRNA for interleukin (IL)-6, IL-11, leukemia inhibitory factor (LIF), macrophage colony-stimulating factor (M-CSF), and stem cell factor (SCF). MSCs treated with IL-1alpha upregulated mRNA levels of IL-6, IL-11, and LIF, and began to express detectable levels of granulocyte colony-stimulating factor (G-CSF), granulocyte macrophage colony-stimulating factor (GM-CSF). mRNA levels of M-CSF and SCF did not change. MSCs cultured in osteogenic medium differentiated along the osteogenic lineage and downregulated mRNA levels of IL-6, IL-11 and LIF whereas, M-CSF and SCF expression were unchanged and G-CSF and GM-CSF remained undetectable. IL-3 was not detected in MSC culture under any conditions. MSCs precultured in control medium, IL-1alpha, or osteogenic medium maintained similar capacity to support long-term culture initiating cell (LT-CIC). Thus, primary and osteogenic differentiated MSCs produce important hematopoietic cytokines and support hematopoiesis in long-term cultures, suggesting that these cells may provide an excellent ex vivo environment for hematopoiesis during progenitor cell expansion and may be important for in vivo cell therapy.

Impaired growth and elevated fas receptor expression in PIGA(+) stem cells in primary paroxysmal nocturnal hemoglobinuria

The genetic defect underlying paroxysmal nocturnal hemoglobinuria (PNH) has been shown to reside in PIGA, a gene that encodes an element required for the first step in glycophosphatidylinositol anchor assembly. Why PIGA-mutated cells are able to expand in PNH marrow, however, is as yet unclear. To address this question, we compared the growth of affected CD59(-)CD34(+) and unaffected CD59(+)CD34(+) cells from patients with that of normal CD59(+)CD34(+) cells in liquid culture. One hundred FACS-sorted cells were added per well into microtiter plates, and after 11 days at 37 degrees C the progeny were counted and were analyzed for their differentiation pattern. We found that CD59(-)CD34(+) cells from PNH patients proliferated to levels approaching those of normal cells, but that CD59(+)CD34(+) cells from the patients gave rise to 20- to 140-fold fewer cells. Prior to sorting, the patients' CD59(-) and CD59(+)CD34(+) cells were equivalent with respect to early differentiation markers, and following culture, the CD45 differentiation patterns were identical to those of control CD34(+) cells. Further analyses of the unsorted CD59(+)CD34(+) population, however, showed elevated levels of Fas receptor. Addition of agonist anti-Fas mAb to cultures reduced the CD59(+)CD34(+) cell yield by up to 78% but had a minimal effect on the CD59(-)CD34(+) cells, whereas antagonist anti-Fas mAb enhanced the yield by up to 250%. These results suggest that expansion of PIGA-mutated cells in PNH marrow is due to a growth defect in nonmutated cells, and that greater susceptibility to apoptosis is one factor involved in the growth impairment.

Limiting numbers of G156A O(6)-methylguanine-DNA methyltransferase-transduced marrow progenitors repopulate nonmyeloablated mice after drug selection

The limited efficacy of hematopoietic gene therapy can be improved by in vivo selection for transduced long-term repopulating cells (LTRC). We selected for G156A MGMT (triangle upMGMT) transduced LTRC present in 5 x 10(4) to 100 x 10(4) marrow cells infused into nonmyeloablated mice by the administration of O(6)-benzylguanine (BG) and BCNU every 3 to 4 weeks. To facilitate engraftment, mice were given a nonablative dose of BG and BCNU before infusion. Without selection, triangle upMGMT was not detected in any hematopoietic colony-forming units (CFU) 24 to 30 weeks after infusion. After BG and BCNU, triangle upMGMT(+) CFU were frequently detected, and their proportions increased with each treatment cycle. After 2 to 3 cycles of BG and BCNU, many mice were stably reconstituted with 75% to 100% triangle upMGMT(+) CFU for at least 6 months, representing up to 940-fold enrichment. Thus, BG and BCNU stem cell toxicity allows triangle upMGMT-transduced LTRC to repopulate the bone marrow. This degree of selection pressure in nonmyeloablated mice is far greater than that observed in previous drug-resistance gene transfer studies. These data support our approved clinical trial to select for drug-resistant, transduced hematopoietic cells, potentially decreasing cumulative drug-induced myelosuppression in patients with cancer. These data also suggest that triangle upMGMT may be a potent, dominant, selectable marker for use in dual gene therapy.

Randomized cross-over trial of progenitor-cell mobilization: high-dose cyclophosphamide plus granulocyte colony-stimulating factor (G-CSF) versus granulocyte-macrophage colony-stimulating factor plus G-CSF

PURPOSE

Patient response to hematopoietic progenitor-cell mobilizing regimens seems to vary considerably, making comparison between regimens difficult. To eliminate this inter-patient variability, we designed a cross-over trial and prospectively compared the number of progenitors mobilized into blood after granulocyte-macrophage colony-stimulating factor (GM-CSF) days 1 to 12 plus granulocyte colony-stimulating factor (G-CSF) days 7 to 12 (regimen G) with the number of progenitors after cyclophosphamide plus G-CSF days 3 to 14 (regimen C) in the same patient.

PATIENTS AND METHODS

Twenty-nine patients were randomized to receive either regimen G or C first (G1 and C1, respectively) and underwent two leukaphereses. After a washout period, patients were then crossed over to the alternate regimen (C2 and G2, respectively) and underwent two additional leukaphereses. The hematopoietic progenitor-cell content of each collection was determined. In addition, toxicity and charges were tracked.

RESULTS

Regimen C (n = 50) resulted in mobilization of more CD34(+) cells (2.7-fold/kg/apheresis), erythroid burst-forming units (1.8-fold/kg/apheresis), and colony-forming units-granulocyte-macrophage (2.2-fold/kg/apheresis) compared with regimen G given to the same patients (n = 46; paired t test, P<.01 for all comparisons). Compared with regimen G, regimen C resulted in better mobilization, whether it was given first (P =.025) or second (P =.02). The ability to achieve a target collection of > or =2x10(6) CD34(+) cells/kg using two leukaphereses was 50% after G1 and 90% after C1. Three of the seven patients in whom mobilization was poor after G1 had > or =2x10(6) CD34(+) cells/kg with two leukaphereses after C2. In contrast, when regimen G was given second (G2), seven out of 10 patients failed to achieve the target CD34(+) cell dose despite adequate collections after C1. Thirty percent of the patients (nine of 29) given regimen C were admitted to the hospital because of neutropenic fever for a median duration of 4 days (range, 2 to 10 days). The higher cost of regimen C was balanced by higher CD34(+) cell yield, resulting in equivalent charges based on cost per CD34(+) cell collected.

CONCLUSION

We report the first clinical trial that used a cross-over design showing that high-dose cyclophosphamide plus G-CSF results in mobilization of more progenitors then GM-CSF plus G-CSF when tested in the same patient regardless of sequence of administration, although the regimen is associated with greater morbidity. Patients who fail to achieve adequate mobilization after regimen G can be treated with regimen C as an effective salvage regimen, whereas patients who fail regimen C are unlikely to benefit from subsequent treatment with regimen G. The cross-over design allowed detection of significant differences between regimens in a small cohort of patients and should be considered in design of future comparisons of mobilization regimens.

Heterozygous DNA mismatch repair gene PMS2-knockout mice are susceptible to intestinal tumor induction with N-methyl-N-nitrosourea

PMS2-deficient (PMS2(-/-)) mice are hypersensitive to N-methyl-N-nitrosourea (MNU)-induced thymic lymphomas based on the failure to initiate mismatch repair (MMR) at O(6)-methylguanine:T mismatches formed after MNU exposure. However, heterozygous PMS2 knockout (PMS2(+/-)) mice do not develop spontaneous tumors, suggesting that they have sufficient MMR function to prevent genomic instability. We hypothesized that in PMS2(+/-) mice, exogenous carcinogens may either mutationally knockout the remaining normal allele leading cells to develop tumors or introduce sufficient DNA adducts and mismatches to overload the lower capacity for MMR, leading in either case to an increased rate of tumor production. In the present study, PMS2(+/-) mice and their littermate PMS2(+/+) mice were monitored for tumor incidence following MNU treatment. Mice were given 50 mg MNU/kg i.p. when 5 weeks old. They demonstrated a similar incidence of thymic lymphomas, suggesting that expression of the single normal PMS2 allele is sufficient to protect the thymus and implying that a single dose of MNU may not efficiently knock out the remaining PMS2 allele in the thymus. Surprisingly, PMS2(+/-) mice were significantly more likely to develop intestinal tumors-both adenomas and adenocarcinomas-after MNU than were PMS2(+/+) mice (2.34 +/- 0.34 tumors per mouse versus 1.34 +/- 0.25 tumors per mouse; P < 0.05). The intestinal tumors were located mainly in the small intestine. However, these tumors in both the PMS2(+/-) mice and PMS2(+/+) mice did not show microsatellite instability characteristic of loss of MMR. These results suggest that a single normal PMS2 allele can protect thymus but not intestine from MNU carcinogenesis. Organ-specific factors might influence MMR- mediated resistance to methylating agents. Heterozygous PMS2 knockout mice may be used as a promising animal model for intestinal tumorigenesis studies involving environmental carcinogens.

CD34+ selection of hematopoietic blood cell collections and autotransplantation in lymphoma: overnight storage of cells at 4 degrees C does not affect outcome

The purpose of this study was to investigate whether storing mobilized peripheral blood progenitor cell (PBPC) collections overnight before CD34+ selection may delay platelet count recovery after high-dose chemotherapy and CD34+-enriched PBPC re-infusion. Lymphoma patients underwent PBPC mobilization with cyclophosphamide 4 g/m2 i.v. and G-CSF 10 microg/kg/day subcutaneously. Patients were prospectively randomized to have each PBPC collection enriched for CD34+ cells with the CellPro CEPRATE SC System either immediately or after overnight storage at 4 degrees C. Thirty-four patients were randomized to overnight storage and 34 to immediate processing of PBPC; 15 were excluded from analysis due to tumor progression or inadequate CD34+ cell mobilization. PBPC from 23 patients were stored overnight, while 30 subjects underwent immediate CD34+ selection and cryopreservation. Median yield of CD34+ enrichment was 43.6% in the immediate processing group compared to 39.1% in the overnight storage group (P = 0.339). Neutrophil recovery >500 x 10(9)/l occurred a median of 11 days (range 9-16 days) in the overnight storage group compared to 10.5 days (range 9-21 days) in the immediate processing group (P = 0.421). Median day to platelet transfusion independence was 13 (range 7-43) days in the overnight storage group vs 13.5 (range 8-35) days in those assigned to immediate processing (P = 0.933). We conclude that storage of PBPC overnight at 4 degrees C allows pooling of consecutive-day collections resulting in decreased costs and processing time without compromising neutrophil and platelet engraftment after infusion of CD34+-selected progenitor cells. Bone Marrow Transplantation(2000) 25, 559-566.

Rapid hematopoietic recovery after coinfusion of autologous-blood stem cells and culture-expanded marrow mesenchymal stem cells in advanced breast cancer patients receiving high-dose chemotherapy

PURPOSE

Multipotential mesenchymal stem cells (MSCs) are found in human bone marrow and are shown to secrete hematopoietic cytokines and support hematopoietic progenitors in vitro. We hypothesized that infusion of autologous MSCs after myeloablative therapy would facilitate engraftment by hematopoietic stem cells, and we investigated the feasibility, safety, and hematopoietic effects of culture-expanded MSCs in breast cancer patients receiving autologous peripheral-blood progenitor-cell (PBPC) infusion.

PATIENTS AND METHODS

We developed an efficient method of isolating and culture-expanding a homogenous population of MSCs from a small marrow-aspirate sample obtained from 32 breast cancer patients. Twenty-eight patients were given high-dose chemotherapy and autologous PBPCs plus culture-expanded MSC infusion and daily granulocyte colony-stimulating factor.

RESULTS

Human MSCs were successfully isolated from a mean +/- SD of 23.4 +/- 5.9 mL of bone marrow aspirate from all patients. Expansion cultures generated greater than 1 x 10(6) MSCs/kg for all patients over 20 to 50 days with a mean potential of 5.6 to 36.3 x 10(6) MSCs/kg after two to six passages, respectively. Twenty-eight patients were infused with 1 to 2.2 x 10(6) expanded autologous MSCs/kg intravenously over 15 minutes. There were no toxicities related to the infusion of MSCs. Clonogenic MSCs were detected in venous blood up to 1 hour after infusion in 13 of 21 patients (62%). Median time to achieve a neutrophil count greater than 500/microL and platelet count >/= 20,000/microL untransfused was 8 days (range, 6 to 11 days) and 8.5 days (range, 4 to 19 days), respectively.

CONCLUSION

This report is the first describing infusion of autologous MSCs with therapeutic intent. We found that autologous MSC infusion at the time of PBPC transplantation is feasible and safe. The observed rapid hematopoietic recovery suggests that MSC infusion after myeloablative therapy may have a positive impact on hematopoiesis and should be tested in randomized trials.

Characterization of the P140K, PVP(138-140)MLK, and G156A O6-methylguanine-DNA methyltransferase mutants: implications for drug resistance gene therapy

The G156A O6-alkylguanine-DNA alkyltransferase (AGT) mutant protein, encoded by the G156A O6-methylguanine-DNA methyltransferase gene (MGMT), is resistant to O6-benzylguanine (BG) inactivation and, after transduction into hematopoietic progenitors, transmits remarkable resistance to BG and BCNU. As a result, a clinical trial, in which the MGMT gene is transduced into CD34+ cells of patients with cancer, has been approved. A newly identified AGT mutation, P140K, generates dramatically increased BG resistance relative to G156A, and suggests that gene transfer of P140K may confer improved hematopoietic cell protection. To address this hypothesis, we measured BG + BCNU and BG + TMZ resistance in G156A, P140K, or P138M/V139L/P140K (MLK) MGMT-transduced K562 cells. In addition, we performed a detailed characterization of individual properties including BG resistance, activity, and protein stability of these mutants in human hematopoetic K562 cells and E86 retroviral producer cells. In K562 cell extracts, the MLK and P140K mutants retained full activity at doses up to 1 mM BG, while G156A had a BG ED50 of 15 microM, compared with 0.1 microM for wtAGT. In the absence of BG, the G156A protein possessed a 56% reduction in specific O6-methyltransferase activity compared with wtAGT. MLK, P140K, and wtAGT all possessed similar specific activities, although the O6-methyl repair rate of all mutants was reduced 4- to 13-fold relative to wtAGT. The wtAGT, MLK, and P140K proteins were stable, with half-lives of greater than 18 hr. In contrast, only 20% of the G156A protein was stable after 12 hr in cycloheximide and, interestingly, the remaining protein appeared to retain most of the activity present in non-cycloheximide-treated cells. Differences in BG resistance, activity, and stability between P140K, MLK, and G156A suggest that P140K may be the optimal mutant for drug resistance gene transfer. However, hematopoietic K562 cells transduced with MFG-G156A, P140K, or MLK had similar degrees of BG and BCNU as well as BG and TMZ resistance when treated with concentrations of BG (< or =25 microM) achieved in clinical trials, suggesting similar efficacy in many in vivo applications.

Bone marrow-derived mesenchymal stem cells remain host-derived despite successful hematopoietic engraftment after allogeneic transplantation in patients with lysosomal and peroxisomal storage diseases

Human bone marrow contains mesenchymal stem cells (MSCs) that can differentiate into various cells of mesenchymal origin. We developed an efficient method of isolating and culture expanding a homogenous population of MSCs from bone marrow and determined that MSCs express alpha-L-iduronidase, arylsulfatase-A and B, glucocerebrosidase, and adrenoleukodystrophy protein. These findings raised the possibility that MSCs may be useful in the treatment of storage disorders. To determine if donor derived MSCs are transferred to the recipients with lysosomal or peroxisomal storage diseases by allogeneic hematopoietic stem cell (HSC) transplantation, we investigated bone marrow derived MSCs of 13 patients 1-14 years after allogeneic transplantation. Highly purified MSCs were genotyped either by fluorescence in situ hybridization using probes for X and Y-chromosomes in gender mis-matched recipients or by radiolabeled PCR amplification of polymorphic simple sequence repeats. Phenotype was determined by the measurement of disease specific protein/enzyme activity in purified MSCs. We found that MSCs isolated from recipients of allogeneic HSC transplantation are not of donor genotype and have persistent phenotypic defects despite successful donor type hematopoietic engraftment. Whether culture expanded normal MSCs can be successfully transplanted into patients with storage diseases and provide therapeutic benefit needs to be determined.

Human mesenchymal stem cells provide stromal support for efficient CD34+ transduction

Human mesenchymal stem cells (hMSC)-nonhematopoietic cells within the bone marrow microenvironment that can be culture expanded to a uniform population of fibroblastic cells-have been shown to support long-term hematopoiesis of CD34+ cells. Because direct contact between stromal elements and CD34+ cells enhances long-term engraftment, we postulated that hMSC would be a good alternative to the more heterogeneous stroma currently used in gene transfer studies. We used hMSC to support retroviral gene transfer of the G156A MGMT (deltaMGMT) gene encoding an alkyltransferase (AGT), which confers drug resistance to a combination of O6-benzylguanine (BG) plus the alkylating agents BCNU and temozolomide (TMZ) in human hematopoietic progenitors. In the presence of IL-3, IL-6, SCF, or leukemia inhibitory factor (LIF) and Flt-3 ligand, hMSC facilitated expansion and retroviral transduction of human peripheral blood-mobilized CD34+ cells. Furthermore, the transduced cells expressed AGT in 29% of hematopoietic cells and were 5-fold more resistant to BCNU and TMZ than were untransduced cells. Unirradiated hMSC present as support cells were simultaneously transduced and expressed AGT in 26% of the cells. Thus, the homogeneous nature of hMSC, and their ability to support gene transfer and be transduced themselves suggest they may be useful in clinical gene transfer protocols and have broad therapeutic applications.

Pharmacologic disruption of base excision repair sensitizes mismatch repair-deficient and -proficient colon cancer cells to methylating agents

Previously we showed that a mismatch repair (MMR)-deficient cell line, HCT116 (hMLH1 mut), unlike a MMR wild-type cell line, SW480, was more resistant to the therapeutic methylating agent, temozolomide (TMZ), because the MMR complex fails to recognize TMZ-induced O6-methylguanine DNA adduct mispairings with thymine that arise after replication. TMZ also produces N7-methylguanine and N3-methyladenine adducts that are processed efficiently by the base excision repair (BER) system. After removal of the methylated base by methylpurine glycosylase, which creates the abasic or apurinic-apyrimidinic (AP) site, the phosphodiester bond is hydrolyzed immediately by AP endonuclease, initiating the repair of the AP site. Methoxyamine (MX) reacts with the abasic site and prevents AP endonuclease cleavage, disrupting DNA repair. MX potentiated the cytotoxic effect of TMZ with a dose modification factor (DMF) of 2.3+/-0.12 in SW480 and 3.1+/-0.16 in HCT116. When combined with O6-benzylguanine (BG), MX and TMZ dramatically increased TMZ cytotoxicity (65.8-fold) in SW480, whereas no additive effect was seen in HCT116. This suggests that N7-methylguanine and N3-methyladenine adducts are cytotoxic lesions in MMR-deficient and wild-type cells when BER is interrupted. Because poly(ADP-ribose) polymerase (PARP) aids in processing of DNA strand breaks induced during MMR and BER, we asked whether PARP inhibitors would also affect BER-mediated cell killing. We found that PARP inhibitors PD128763, 3-aminobenzimide, and 6-aminonicotinamide increased the sensitivity to TMZ in both HCT116 MMR-deficient cells and SW480 MMR wild-type cells. In HCT116 cells, PD128763 remarkably decreased resistance to TMZ, with a DMF of 4.7+/-0.2. However, the combination of PD128763, BG, and TMZ had no greater effect, indicating that persistent O6-methylguanine had no effect on cytotoxicity. In SW480, the DMF for TMZ cytotoxicity was 3.1+/-0.12 with addition of PD128763 and 36 with addition of PD128763 and BG. Synergy analysis by median effect plots indicated a high degree of synergy between TMZ and MX or PD128763. In contrast, 1,3-bis(2-chloroethyl)-1-nitrosourea combined with either MX or PD128763 showed little if any potentiation observed in the absence of BG in either cell line, suggesting that BER pathway has little impact on cytotoxic processing of 1,3-bis(2-chloroethyl)-1-nitrosourea-induced adducts. These studies indicate that targeting BER with MX or PARP inhibitors enhances the cytotoxicity of methylating agents, even in MMR-deficient cells.

Transgenic expression of human MGMT blocks the hypersensitivity of PMS2-deficient mice to low dose MNU thymic lymphomagenesis

Mice deficient in the DNA mismatch repair (MMR) gene, PMS2, develop spontaneous thymic lymphomas and sarcomas. We have previously shown that PMS2(-/-) mice were hypersensitive to a single i.p. injection of 50 mg/kg of N-methyl-N-nitrosourea (MNU) for thymic lymphoma induction. We postulated that MNU sensitivity was due to formation of O(6)-methylguanine (O(6)-mG), which, if unrepaired by O(6)-alkylguanine DNA alkyltransferase (AGT), leads to apoptosis in MMR competent cells and O(6)-mG:T mismatches in MMR deficient cells. Tumor induction is less in MMR(+/+) mice because cells with residual DNA adducts die, whereas mutagenized cells survive in MMR(-/-) mice. Overexpression of AGT (encoded by the methylguanine DNA methyltransferase-MGMT-gene) is known to block MNU induced tumorigenesis in mice with functional MMR. To further determine the sensitivity of PMS2(-/-) mice to MNU and the protective effect of hAGT overexpression, a low dose of MNU (25 mg/kg) was studied in PMS2(-/-) mice and PMS2(-/-)/hMGMT(+) mice. No thymic lymphomas were found in MNU-treated PMS2(+/+) and PMS2(+/-) mice. At 1 year, 46% of the MNU-treated PMS2(-/-) mice developed thymic lymphoma, compared with an incidence of 25% in both untreated PMS2(-/-) mice and MNU treated PMS2(-/-)/hMGMT(+) mice. In addition, a significantly shorter latency in the onset of thymic lymphomas was seen in MNU-treated PMS2(-/-) mice. K-ras mutations were detected almost equally in the thymic lymphomas induced by MNU in both PMS2(-/-) and PMS2(-/-)/hMGMT(+) mice, but not in the spontaneous lymphomas. These data suggest that PMS(-/-) mice are hypersensitive to MNU, that there are different pathways responsible for spontaneous and MNU induced thymic lymphomas in PMS2(-/-) mice, and that overexpression of hMGMT protects the mice by blocking non-K-ras pathways.

Mice defective in the DNA mismatch gene PMS2 are hypersensitive to MNU induced thymic lymphoma and are partially protected by transgenic expression of human MGMT

DNA mismatch repair (MMR) stabilizes the cellular genome. Mice defective in the MMR gene PMS2 are susceptible to spontaneous thymic lymphoma and sarcomas. To determine the sensitivity of PMS2 knockout mice to environmental carcinogens and the protective effect of O6-methylguanine DNA methyltransferase (MGMT), heterozygous PMS2 knockout mice and human MGMT (hMGMT) transgenic mice were mated and the PMS2-/- and PMS2+/+ with or without hMGMT offspring were treated at 5 weeks of age with 50 mg/kg N-methyl-N-nitrosourea (MNU). MNU produces carcinogenic O6-methylguanine (O6-meG) adducts, resulting in thymic lymphoma in mice, which can be prevented in normal mice by overexpression of hMGMT. A significantly higher incidence of thymic lymphomas was observed in MNU-treated PMS2-/- mice, compared to wildtype PMS2+/+ mice (100 vs 52%; P < 0.001). The mean latency of lymphomas was also significantly shortened in PMS2-/- mice (81 vs 102 days, P < 0.01). Transgenic expression of hMGMT significantly but incompletely blocked MNU lymphomagenesis in PMS2-/- mice. The incidence of lymphomas in PMS2-/-/hMGMT+ mice was reduced to 80% (P < 0.01) and mean latency increased to 91 days (P < 0.05). Thymic lymphomagenesis was efficiently blocked in PMS2+/+/hMGMT+ mice with rapid repair of O6-meG. Since O6-meG:T mismatches in MMR+ cells may trigger mismatch repair resulting in abortive repair and cell death whereas in the absence of MMR, these mismatches are converted to A:T, we predicted that G to A point mutations in codon 12 of the K-ras gene would occur. In this study, we found G to A point mutations in codon 12 of the K-ras gene in many tumors. Thus, in MMR deficient tissues, methylating agents induce point mutations in cells with a higher rate of cell survival which together are potently carcinogenic in the thymus. These data suggest that PMS2 defective lymphomas may arise by the concerted action of environmental and perhaps endogenous methylation of DNA coupled to genomic instability.

Human long-term culture initiating cells are sensitive to benzylguanine and 1,3-bis(2-chloroethyl)-1-nitrosourea and protected after mutant (G156A) methylguanine methyltransferase gene transfer

Human hematopoietic progenitors express low levels of O6-alkylguanine-DNA alkyltransferase and are sensitive to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), particularly following O6-benzylguanine (BG)-mediated O6-alkylguanine-DNA alkyltransferase inhibition. Expression of the BG-resistant mutant (G156A) methylguanine methyltransferase (deltaMGMT) gene in hematopoietic cells confers resistance to BG and BCNU. Because BCNU targets both early and late human hematopoietic cells and results in prolonged and cumulative myelosuppression, we attempted to protect early hematopoietic progenitors (long-term culture initiating cells (LTC-ICs)) by retroviral-mediated transfer of the deltaMGMTgene. A total of 33-56% of LTC-ICs were transduced with MFG-deltaMGMT retrovirus as determined by evidence of provirus in secondary colony-forming units at 5 weeks of culture under conditions optimal for the survival and proliferation of early hematopoietic progenitors. The addition of flt-3 ligand to cultures increased the transduction rate of LTC-ICs. Furthermore, 17.8 +/- 8.1% of deltaMGMT-transduced LTC-ICs survived doses of BG and BCNU; these doses allowed the survival of only 0-1% of untransduced LTC-ICs. This finding compares favorably with the 8-12% of CD34+ cell-derived colony-forming units that we previously showed became resistant to BG and BCNU after deltaMGMTgene transfer. Thus, deltaMGMT transduction of human early hematopoietic progenitor LTC-ICs confers resistance to BG and BCNU and may allow transduced LTC-ICs selective survival and enrichment over untransduced cells in patients undergoing BG and BCNU chemotherapy.

Mice over-expressing human O6 alkylguanine-DNA alkyltransferase selectively reduce O6 methylguanine mediated carcinogenic mutations to threshold levels after N-methyl-N-nitrosourea

While it is well known that MNU induces thymic lymphomas in the mouse, it remains unclear which pre-mutagenic lesions are responsible for lymphomagenic transformation. One lesion thought to play a critical role is O6methylguanine[O6mG]which initiates G: C to A:T transition mutations in K-ras and other oncogenes. O6alkylguanine-DNA alkyltransferase (AGT), encoded by the methylguanine methyltransferase gene [MGMT], removes the methyl group thereby preventing the mutation from occurring. When overexpressed in the thymus, MGMT protects mice from MNU-induced thymic lymphomas. To determine whether MGMT overexpression reduced G: C to A: T mutation frequency after MNU, Big Blue lacI and MGMT+/Big Blue mice were treated with MNU and analysed for mutations in the lacI and K-ras genes. The incidence of MNU-induced lymphomas was 84% in Big Blue lacI mice compared to 14% in MGMT+Big Blue lacI mice. Sixty-two per cent of the lymphomas had a GGT to GAT activating mutation in codon 12 of K-ras consistent with O6mG adduct-mediated point mutagenesis. LacI mutation frequency in thymus of MNU treated Big Blue mice was 45-fold above background whereas it was 11-fold above background in MNU treated MGMT+/Big Blue mice. Most lacI mutations were G:C to A:T transitions, implicating O6mG even in the MGMT+mice. No mutations were attributable to chromosomal aberrations or rearrangements. Thus, O6mG adducts account for the carcinogenic effect of MNU and MGMT overexpression is selectively able to reduce O6methylguanine adducts below a carcinogenic threshold. Other adducts are mutagenic but appear to contribute much less to malignant transformation or oncogene activation.