Prognostic Value of Primary Tumor Volume Changes on kV-CBCT during Definitive Chemoradiotherapy for Stage III Non-Small Cell Lung Cancer

INTRODUCTION

Kilovoltge cone beam computed tomography (kV-CBCT) allows for tumor localization and response assessment during definitive chemoradiotherapy for locally advanced NSCLC. We hypothesize that significant tumor volume loss occurs early during radiotherapy and that the extent of volume loss correlates with clinical outcomes.

METHODS

A total of 52 patients with locally advanced NSCLC treated with definitive chemoradiotherapy were reviewed. kV-CBCT images were used to contour primary gross tumor volumes at four time points during treatment. Patients were dichotomized according to absolute and relative volume changes at each time point. Statistical analyses were performed to evaluate correlations between volume changes and clinical outcomes.

RESULTS

The median gross tumor volumes were 77.1, 48.3, 42.5, and 29.9 cm³ for fractions 1, 11, 21, and final, respectively. Greater relative volume loss between fractions 1 and 21 correlated with improved distant control (hazard ratio [HR] = 0.35, 95% confidence interval [CI]: 0.13-0.94, p = 0.038) and overall survival (HR = 0.40, 95% CI: 0.16-0.98, p = 0.046). Greater relative volume loss between fractions 11 and 21 correlated with improved progression-free survival (HR = 0.39, 95% CI: 0.17-0.88, p = 0.02) and trended toward improved overall survival (HR = 0.43, 95% CI: 0.17-1.06, p = 0.07). On multivariate analysis, greater relative volume loss between fractions 11 and 21 correlated with improved progression-free survival (HR = 0.39, 95% CI: 0.16-0.97, p = 0.041) and overall survival (HR = 0.31, 95% CI: 0.11-0.88, p = 0.027).

CONCLUSIONS

Significant primary tumor volume loss occurs early during radiotherapy for locally advanced NSCLC. Greater relative tumor volume loss during treatment correlates with improved disease control and overall survival. Thus, kV-CBCT has potential to be used as a practical prognostic imaging marker.

Patterns of major wound complications following multidisciplinary therapy for lower extremity soft tissue sarcoma

BACKGROUND AND OBJECTIVES

The purpose of this study was to determine the pattern and timing of major wound complications (MWCs) in patients at our institution who received multimodality treatment for lower extremity soft tissue sarcoma (LE-STS) and to evaluate the impact of MWCs on tumor control and patient outcomes.

METHODS

The medical records of 102 LE-STS patients treated with limb-sparing surgery and radiation therapy were reviewed. MWCs were defined as secondary operations with anesthesia, seroma/hematoma aspiration, admission for IV antibiotics, or persistent deep packing.

RESULTS

MWCs occurred in 22% of patients, with 45% of events occurring >120 days after resection. On multivariate analysis, preoperative external beam radiation therapy (EBRT) (OR 4.29, 95% CI 1.06-17.40, P = 0.042) and skin graft placement (OR 6.39, 95% CI 1.37-29.84, P = 0.018) were found to be independent predictors of MWCs. MWC occurrence did not predict for chronic toxicity and did not impact tumor control or survival.

CONCLUSIONS

A considerable proportion of MWCs occur >120 days from surgical resection with preoperative EBRT and skin graft placement independent predictors for MWCs. While an additional source of morbidity, MWC occurrence did not impact tumor control, nor did it predict for chronic toxicity. J. Surg. Oncol. 2016;114:385-391. © 2016 Wiley Periodicals, Inc.

Inter-Fraction Tumor Volume Response during Lung Stereotactic Body Radiation Therapy Correlated to Patient Variables

Purpose

Analyze inter-fraction volumetric changes of lung tumors treated with stereotactic body radiation therapy (SBRT) and determine if the volume changes during treatment can be predicted and thus considered in treatment planning.

Methods and Materials

Kilo-voltage cone-beam CT (kV-CBCT) images obtained immediately prior to each fraction were used to monitor inter-fraction volumetric changes of 15 consecutive patients (18 lung nodules) treated with lung SBRT at our institution (45–54 Gy in 3–5 fractions) in the year of 2011–2012. Spearman's (ρ) correlation and Spearman's partial correlation analysis was performed with respect to patient/tumor and treatment characteristics. Multiple hypothesis correction was performed using False Discovery Rate (FDR) and q-values were reported.

Results

All tumors studied experienced volume change during treatment. Tumor increased in volume by an average of 15% and regressed by an average of 11%. The overall volume increase during treatment is contained within the planning target volume (PTV) for all tumors. Larger tumors increased in volume more than smaller tumors during treatment (q = 0.0029). The volume increase on CBCT was correlated to the treatment planning gross target volume (GTV) as well as internal target volumes (ITV) (q = 0.0085 and q = 0.0039 respectively) and could be predicted for tumors with a GTV less than 22 mL. The volume increase was correlated to the integral dose (ID) in the ITV at every fraction (q = 0.0049). The peak inter-fraction volume occurred at an earlier fraction in younger patients (q = 0.0122).

Conclusions

We introduced a new analysis method to follow inter-fraction tumor volume changes and determined that the observed changes during lung SBRT treatment are correlated to the initial tumor volume, integral dose (ID), and patient age. Furthermore, the volume increase during treatment of tumors less than 22mL can be predicted during treatment planning. The volume increase remained significantly less than the overall PTV expansion, and radiation re-planning was therefore not required for the purpose of tumor control. The presence of the studied correlations suggests that the observed volumetric changes may reflect some underlying biologic process rather than random fluctuations.

Imaging across the life span: innovations in imaging and therapy for gynecologic cancer

The focus of this article is radiation therapy for gynecologic cancers, with emphasis on imaging-based treatment planning and delivery. For the various gynecologic cancers, radiation oncologists rely on essential clinical information to triage treatment options, and various imaging studies are performed for treatment planning and radiation therapy delivery. A practical approach is provided to help radiologists tailor their reports for the needs of their radiation oncology and gynecologic oncology colleagues, to optimize multidisciplinary care for patients with gynecologic cancer. Template radiology reports are proposed to address the specific information needs of oncologists at each phase-before, during, and after treatment. Fueled by the rapid progress in engineering and computer sciences during the past 2 decades, remarkable advances have been made in anatomic, functional, and molecular imaging and in radiation treatment planning and delivery in patients with gynecologic cancer. Radiation therapy has evolved from a nontargeted approach to a precisely targeted, highly conformal treatment modality, to further improve treatment outcomes and reduce morbidity. High-quality imaging has become essential for staging of the disease, delineation of tumor extent for treatment planning and delivery, and monitoring therapy response. Anatomic and functional imaging has also been shown to provide prognostic information that allows clinicians to tailor therapy on the basis of personalized patient information. This field is an area of active research, and future clinical trials are warranted to validate preliminary results in the field.

Primary Cutaneous B-Cell Lymphoma: Management and Patterns of Recurrence at the Multimodality Cutaneous Lymphoma Clinic of The Ohio State University

BACKGROUND

The increasing incidence of primary cutaneous B-cell lymphomas (PCBCLs) presents new challenges for clinicians. Despite advances in the clinical and pathologic characterization of PCBCL, the significance of the current staging approach as a risk profiling tool and the effect of various treatments on outcome remain unclear.

MATERIALS AND METHODS

We retrospectively reviewed patients who presented with a diagnosis of PCBCL seen at The Ohio State University between 1998 and 2012. We reviewed the initial presentation and treatment modality. We then assessed whether the treatment modality (conservative skin-directed vs. definitive radiation with or without systemic therapy), stage (T1 or ≥T2), or histologic subtype (primary cutaneous follicle center lymphoma [PCFCL] vs. primary cutaneous marginal zone B-cell lymphoma [PCMZL]) affected the risk of recurrence.

RESULTS

We identified 67 patients referred with an initial diagnosis of PCBCL. After imaging, 12 did not meet the criteria for PCBCL and were classified as having systemic B-cell lymphoma with cutaneous involvement. The remaining 55 patients included 25 with PCMZL, 24 with PCFCL, 2 with primary cutaneous large B-cell lymphoma leg type, and 4 with unclassifiable disease. According to the International Society of Cutaneous Lymphoma-European Organization for Research and Treatment of Cancer staging, 30 cases were T1 (55%), 14 T2 (25%), and 11 T3 (20%). Comparing the time to first recurrence (TFR) by indolent PCBCL subtypes, we found no difference in the recurrence risk for either stage (T1, p = .51 vs. T2/T3, p = .30). Comparing TFR by treatment modality, we found no difference in TFR within T1 patients (p = .34) or T2/T3 patients (p = .44).

CONCLUSION

Our limited analysis highlights the importance of complete staging at diagnosis and suggests that the treatment modality does not affect the risk of recurrence in T1 indolent PCBCL.

Minimal Inter-Fractional Fiducial Migration during Image-Guided Lung Stereotactic Body Radiotherapy Using SuperLock Nitinol Coil Fiducial Markers

Objectives

Stereotactic body radiotherapy (SBRT) is being increasingly used for the treatment of patients with lung cancer or lung metastasis who are medically unfit to undergo resection. In order to improve accuracy and confidence in targeting tumors, many centers rely on fiducial implantation. We evaluated the migration of a novel fiducial marker specifically designed for lung tissue implanted via electromagnetic navigation bronchoscopy (ENB).

Methods

We retrospectively quantified the individual and group migrations of SuperLock nitinol coil fiducials for 15 patients receiving lung stereotactic body radiotherapy (SBRT), in order to evaluate the reliability of using these fiducials as a target surrogate for cases where tumors cannot be clearly delineated on cone beam CTs (CBCTs). For each fraction, we compared the individual and group migrations of the fiducials between the planning CT and the acquired CBCT. The group migration was defined as the distance between the centroids of the fiducial group and GTV.

Results

A total of 16 lung targets were included in our study for these 15 patients (one patient with two targets). Of 55 fiducials placed, we observed a 100% retention rate. The mean individual migration was 1.87 mm (range, 0.63–5.25 mm) with a standard deviation of 1.26 mm. The mean group migration was 1.94 mm (range, 0.03–6.19 mm) with a standard deviation of 1.45 mm. Overall, there was minimal change in the relative locations of the markers with respect to each other, as well as to the target.

Conclusions

We found that the SuperLock nitinol coil fiducial marker positions are stable throughout the radiation treatment, and can be used as a reliable surrogate to target, and to avoid geometric misses during gated treatments.

Treating cutaneous T-cell lymphoma with highly irregular surfaces with photon irradiation using rice as tissue compensator

PURPOSE

Cutaneous T-cell lymphoma (CTCL) is known to have an excellent response to radiotherapy, an important treatment modality for this disease. In patients with extremity and digit involvement, the irregular surface and depth variations create difficulty in delivering a homogenous dose using electrons. We sought to evaluate photon irradiation with rice packing as tissue equivalence and determine clinical tolerance and response.

MATERIALS AND METHODS

Three consecutive CTCL patients with extensive lower extremity involvement including the digits were treated using external beam photon therapy with rice packing for tissue compensation. The entire foot was treated to 30-40 Gy in 2-3 Gy per fraction using 6 MV photons prescribed to the mid-plane of an indexed box filled with rice in which the foot was placed. Treatment tolerance and response were monitored with clinical evaluation.

RESULTS

All patients tolerated the treatment without treatment breaks. Toxicities included grade 3 erythema and desquamation with resolution within 4 weeks. No late toxicities were observed. All patients had a partial response by 4 weeks after therapy with two patients achieving a complete response. Patients reported improved functionality after treatment. No local recurrence has been observed.

CONCLUSION

Tissue compensation with rice packing offers a convenient, inexpensive, and reproducible method for the treatment of CTCL with highly irregular surfaces.

Downregulation of peroxiredoxin-1 by β-elemene enhances the radiosensitivity of lung adenocarcinoma xenografts

β-elemene, the active component of elemene (1-methyl-1-vinyl-2,4-diisopropenyl-cyclohexane), is a naturally occurring compound isolated from the traditional Chinese medicinal herb Curcuma wenyujin. Studies have confirmed that β-elemene enhances the radiosensitivity of lung cancer cell lines such as A549, by multiple pathways; however, their underlying mechanisms and pathways are yet to be elucidated. In the present study, two-dimensional differential in-gel electrophoresis and matrix-assisted laser desorption/ionization time-of-flight tandem mass spectrometry were used to profile the different proteins in A549 cell xenograft models of both treatment groups. The protein/mRNA expression was assessed by reverse transcription-polymerase chain reaction and western blotting techniques in tumor samples from all treatment groups. As a critical player in redox regulation of cancer cells, inhibition of peroxiredoxin-1 (Prx-1) may be an effective option for enhancing the tumor response to radiation. We further verified Prx-1 expression at the transcription and translation levels. β-elemene at a dose of 45 mg/kg had little effect on the Prx-1 protein expression, which was correlated with a moderate antitumor effect. However, a 45 mg/kg dose of β-elemene significantly inhibited the Prx-1 mRNA expression, thereby suggesting a possible influence on the transcriptional process, and radiation significantly increased the Prx-1 mRNA/protein expression compared to the control group (p<0.01). Notably, Prx-1 mRNA/protein expression was significantly lower in the β-elemene/radiation co-treatment group compared to the baseline levels in the control group (p<0.01). These results suggest that radiation-induced Prx-1 expression is directly or indirectly suppressed by β-elemene, thus suggesting a new pathway by which to reverse radioresistance.

The role of modern radiation therapy in the management of extremity sarcomas

For nearly half a decade, surgery and radiation therapy have been used in combination to achieve the goal of limb preservation in extremity soft tissue sarcoma, with success rates in excess of 90%. Common decision points in therapeutic radiation delivery for sarcoma are discussed, including preoperative versus postoperative irradiation, the postoperative boost, and when irradiation might be unnecessary. We describe specialized techniques, such as brachytherapy and intraoperative irradiation. The data driving current practice is summarized.

Combining targeted agents with modern radiotherapy in soft tissue sarcomas

Improved understanding of soft-tissue sarcoma (STS) biology has led to better distinction and subtyping of these diseases with the hope of exploiting the molecular characteristics of each subtype to develop appropriately targeted treatment regimens. In the care of patients with extremity STS, adjunctive radiation therapy (RT) is used to facilitate limb and function, preserving surgeries while maintaining five-year local control above 85%. In contrast, for STS originating from nonextremity anatomical sites, the rate of local recurrence is much higher (five-year local control is approximately 50%) and a major cause of death and morbidity in these patients. Incorporating novel technological advancements to administer accurate RT in combination with novel radiosensitizing agents could potentially improve local control and overall survival. RT efficacy in STS can be increased by modulating biological pathways such as angiogenesis, cell cycle regulation, cell survival signaling, and cancer-host immune interactions. Previous experiences, advancements, ongoing research, and current clinical trials combining RT with agents modulating one or more of the above pathways are reviewed. The standard clinical management of patients with STS with pretreatment biopsy, neoadjuvant treatment, and primary surgery provides an opportune disease model for interrogating translational hypotheses. The purpose of this review is to outline a strategic vision for clinical translation of preclinical findings and to identify appropriate targeted agents to combine with radiotherapy in the treatment of STS from different sites and/or different histology subtypes.

Abstract 854: Inhibition of PRMT5 results in radiosensitization in lung cancer cell lines

Background: Protein arginine methylation is a post translational modification that influences signal transduction, mRNA splicing, gene transcription and DNA repair. Among the PRMT family members, PRMT5 is a type II enzyme that symmetrically methylates histone H4 at Arginine 3 and histone H3 at Arginine 8. Studies have recently linked this modification to carcinogenesis and metastasis. The function of PRMT5 in carcinogenesis is related to cell proliferation through modulation of E2F1, p53, EGFR, and CRAF. It is known to accelerate progression through the G1 phase of cell cycle by influencing proteins like CDK4 and CDK6. Previous work on human lung cancer specimens has demonstrated an overexpression of PRMT5 in cancerous tissue when compared to normal lung parenchyma. Suppression of PRMT5 significantly inhibits cell proliferation in lung cancer cell lines A549 and H1299. We hypothesized inhibition of PRMT5 can lead to increased radiosensitivity in lung cancer cells.

Method: Several lung cancer cell lines were used in the experiments, including A549, H1299 and H23. SiRNA (Dharmacon) and lentiviral shRNA (Sigma) were used to knock down (KD) PRMT5 levels transiently or stably in A549 cell line in which p53 is present in its wild type form. Forty eight hours after transient transfection, cells were plated for clonogenic survival assay and subsequently exposed to ionizing radiation at 0, 2, and 8 Gy. Cellular PRMT5 protein levels were estimated by western blotting analysis for PRMT5 KD and scramble control cell lines. The scramble control and siRNA knockdown cells were subjected to cell cycle analysis by flow cytometry. We also tested specific PRMT5 inhibitors with and without radiation therapy in the lung cancer cell lines to see if PRMT5 inhibitors could lead to increased radiosensitivity.

Results: We observed a &gt;90% PRMT5 KD in transiently transfected cells at 48 h and 72 h post transfection as verified by western blot analysis. This transient KD lead to a small but significant decrease in colony survival after radiation. This radiosensitization was not observed in cells selected for stable KD of PRMT5 protein by lentiviral RNA transfection. There is an increase of cell population in G1 arrest in PRMT5 transient KD cells but not in stable KD cells. Additionally, cells treated with PRMT5 specific inhibitors (“cpd5” or “cpd65”) demonstrated increased radiosensitivity in A549 cells but not in H1299 suggesting that this effect may be p53-dependent.

Conclusion: PRMT5 inhibition by siRNA or its specific inhibitors lead to radiosensitivity in A549 lung cancer cell line. This effect may be partially dependent on p53-dependent cell cycle arrest. Further work to inhibit PRMT5 in other lung cancer cell lines with different p53 activities will be investigated.

Citation Format: Smitha Sharma, X Wu, P Smith, N Denko, C Li, H Lai, F Yan, K Shilo, A Chakravarti, S Sif, R Baiocchi, G Otterson, Meng Xu-Welliver. Inhibition of PRMT5 results in radiosensitization in lung cancer cell lines. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 854. doi:10.1158/1538-7445.AM2014-854

Dosimetric consequences of pancreatic tumor motion when predetermined treatment margins are employed during intensity-modulated radiation therapy

PURPOSE

To quantify the dosimetric consequences of pancreatic tumor motion on the pancreatic intensity-modulated radiation therapy (IMRT) plans.

METHODS

Dose map of IMRT plans for 5 patients with pancreatic cancer were measured using a 2D diode array placed on a computer-controlled platform to simulate 2D pancreatic tumor motion. Dosimetric analysis was then performed to obtain IMRT quality assurance (QA) passing rates. The convolution method, which used a motion kernel to simulate 2D pancreatic motion, was also applied to the treatment and phantom verification plans for a wide range of magnitudes of motion (0.8-2.0 cm). The resulting motion-convolved verification dose maps (VDMs) were compared with the dynamic measurements to evaluate IMRT QA passing rates as well as the dose-volume histogram, the V95% of the planning target volume (PTV) and V98% of the clinical target volume (CTV).

RESULTS

While CTV coverage was maintained when the simulated pancreatic tumor drifted inside the PTV with magnitudes of 1.0 cm and 1.5 cm, the V95% of the PTV was reduced by 10% and 17%, respectively. We also found that the differences between the measurements and the static VDMs increased proportional to the amplitude of motion, while the agreement between the measurements and the motion-convolved VDMs was excellent for any magnitude of motion.

CONCLUSIONS

When the 4D technique is not available, predetermined margins must be used carefully to avoid possible under-dose to the target. Additionally, the phantom results show that the kernel convolution method provides an accurate evaluation of the dosimetric impact due to tumor motion and it should be employed in the planning process.

Down-regulation of survivin and hypoxia-inducible factor-1 α by β-elemene enhances the radiosensitivity of lung adenocarcinoma xenograft

Elemene (1-methyl-1-vinyl-2,4-diisopropenyl-cyclohexane) is a naturally occurring compound that can be isolated from the traditional Chinese medicinal herb Curcuma wenyujin. β-elemene, its active component, has recently been demonstrated to enhance the radiosensitivity of human cancer cell lines in vitro and of one animal tumor in vivo. The underlying mechanism, however, is still unclear. In this study, we demonstrated for the first time that β-elemene significantly improves the radiosensitivity of A549 lung adenocarcinoma xenograft in vivo as measured by tumor regrowth delay experiments. Our results showed that β-elemene, at 45 mg/kg, significantly inhibited radiation-induced expression of survivin and hypoxia-inducible factor (HIF)-1 α proteins. Because HIF-1 α is known to regulate survivin transcription and acts as upstream regulator of survivin, it is possible that β-elemene regulates the transcription of survivin through HIF-1 α. Our study suggests that β-elemene is a promising drug to enhance tumor radioresponse, and survivin and HIF-1 α are novel targets of β-elemene.

The adaptive imbalance in base excision-repair enzymes generates microsatellite instability in chronic inflammation

Chronic infection and associated inflammation are key contributors to human carcinogenesis. Ulcerative colitis (UC) is an oxyradical overload disease and is characterized by free radical stress and colon cancer proneness. Here we examined tissues from noncancerous colons of ulcerative colitis patients to determine (a) the activity of two base excision-repair enzymes, AAG, the major 3-methyladenine DNA glycosylase, and APE1, the major apurinic site endonuclease; and (b) the prevalence of microsatellite instability (MSI). AAG and APE1 were significantly increased in UC colon epithelium undergoing elevated inflammation and MSI was positively correlated with their imbalanced enzymatic activities. These latter results were supported by mechanistic studies using yeast and human cell models in which overexpression of AAG and/or APE1 was associated with frameshift mutations and MSI. Our results are consistent with the hypothesis that the adaptive and imbalanced increase in AAG and APE1 is a novel mechanism contributing to MSI in patients with UC and may extend to chronic inflammatory or other diseases with MSI of unknown etiology.

The adaptive imbalance in base excision-repair enzymes generates microsatellite instability in chronic inflammation

Chronic infection and associated inflammation are key contributors to human carcinogenesis. Ulcerative colitis (UC) is an oxyradical overload disease and is characterized by free radical stress and colon cancer proneness. Here we examined tissues from noncancerous colons of ulcerative colitis patients to determine (a) the activity of two base excision-repair enzymes, AAG, the major 3-methyladenine DNA glycosylase, and APE1, the major apurinic site endonuclease; and (b) the prevalence of microsatellite instability (MSI). AAG and APE1 were significantly increased in UC colon epithelium undergoing elevated inflammation and MSI was positively correlated with their imbalanced enzymatic activities. These latter results were supported by mechanistic studies using yeast and human cell models in which overexpression of AAG and/or APE1 was associated with frameshift mutations and MSI. Our results are consistent with the hypothesis that the adaptive and imbalanced increase in AAG and APE1 is a novel mechanism contributing to MSI in patients with UC and may extend to chronic inflammatory or other diseases with MSI of unknown etiology.

The role of p53 in base excision repair following genotoxic stress

The p53 tumor suppressor protein is involved in apoptosis and cell cycle checkpoints. We have shown recently that p53 also facilitates base excision repair (BER). To further examine p53 involvement in the regulation of BER we chose to focus on 3-methyladenine DNA glycosylase (3-MeAde DNA glycosylase), the first enzyme acting in the BER pathway. 3-MeAde DNA glycosylase activity was found to be modulated by the p53 protein. This modulation was dependent on the type of genotoxic stress used. Gamma-irradiation damage resulted in activation of glycosylase, which was enhanced by p53. Doxorubicin and hydrogen peroxide (H2O2) treatment, although inducing p53 stabilization, did not cause the activation of glycosylase. Nitric oxide (NO) resulted in activation of 3-MeAde DNA glycosylase. Surprisingly this activation was down regulated by wild-type p53. The down regulation of 3-MeAde DNA glycosylase activity was due to trans repression of glycosylase mRNA by p53. Furthermore, we found that AP endonuclease (APE) activity was not altered by NO. Our study provides evidence for a possible antimutagenic role for p53 following exposure of cells to NO species. In the absence of p53, NO exposure results in elevation of 3-MeAde DNA glycosylase activity that results in elevation in the number of AP sites in DNA. At the same time, APE activity does not rise and removal of the AP sites is not further processed resulting in a mutator phenotype. When p53 is present, it down regulates the transcription of 3-MeAde DNA glycosylase. This provides a new model by which p53 prevents the creation of a mutator phenotype.

Inactivation and degradation of O(6)-alkylguanine-DNA alkyltransferase after reaction with nitric oxide

O(6)-Alkylguanine-DNA alkyltransferase (AGT) plays a critical role in protection from the carcinogenic effects of simple alkylating agents by repairing O(6)-alkylguanine adducts via a direct transfer reaction. Nitric oxide (NO) or species derived from it are known to be able to initiate neoplastic growth and cannot only damage DNA, either directly or via the formation of intermediates such as nitrosamines, but can also inhibit some DNA repair processes. We have studied the inactivation of AGT by NO in detail in vitro and in vivo using wild-type human AGT (hAGT) and mutants at key residues. Our results show that hAGT is readily but reversibly inactivated by the formation of S-nitrosylcysteine at Cys-145, which is the alkyl acceptor site. The facile reaction of this cysteine residue with NO is attributable to its interaction with other residues in hAGT including His-146 and Glu-172 that activate the sulfhydryl group of Cys-145 to allow its nucleophilic attack on DNA adducts. Although the S-nitrosylcysteine adduct in hAGT is readily reversible by reaction with other cellular thiols, the formation of S-nitrosocysteine at Cys-145 was found to lead to the rapid degradation of the hAGT protein in vivo. This degradation is brought about by the ubiquitin/proteasomal system. The formation of an S-nitrosylcysteine at Cys-145 in hAGT in response to NO led to a large increase in the ubiquitination of the protein. This NO-mediated increase did not occur with the C145S or C145A mutants. A conformational change in hAGT, which involves opening of an asparagine hinge, normally occurs after alkylation of the protein in its role in DNA repair and causes degradation of the alkylated hAGT. Our results indicate that a similar effect occurs after reaction of the protein with NO. Thus, exposure to NO causes an irreversible loss of DNA repair capacity for alkylation adducts. This may contribute toward the potential development of tumors in cells upon chronic exposure to NO because of inflammation or infection. This may be of particular importance because such exposure may also lead to the formation of N-nitroso compounds that can act as alkylating agents.

Degradation of the alkylated form of the DNA repair protein, O(6)-alkylguanine-DNA alkyltransferase

O(6)-Alkylguanine-DNA alkyltransferase (AGT) is a DNA repair protein that removes alkyl groups from DNA by transferring them to an internal Cys-145 residue. As the S-alkylcysteine is not converted back to cysteine, the protein can only act once and the resulting alkylated AGT molecule is rapidly degraded. The mechanism underlying the disappearance of the alkylated AGT has been studied in vivo in CHO cells and in vitro in reticulocyte lysates by using the pseudosubstrate O(6)-benzylguanine (BG) and mutant forms of AGT. The wild-type AGT was stable but was ubiquitinated and degraded rapidly by the proteasome after treatment with BG or with an oligodeoxyribonucleotide, which contained O(6)-methylguanine. Mutants C145F (and other mutants with bulky substituents at position 145), which have alterations that cause a steric alteration at the active site and also prevent hydrogen bonding involving Cys-145 resembled the alkylated AGT and were ubiquitinated and degraded rapidly irrespective of treatment with BG. Mutant M134F, which causes a steric alteration without interfering directly with the hydrogen-bonding network involving Cys-145, partially destabilized AGT and its degradation was increased further by reaction with BG. Mutant C145S, which maintains the hydrogen-binding network and causes no distortion, was not rapidly degraded. The results indicate that the conformational change resulting in the opening of the asparagine hinge region in the structure, which is brought about by formation of an S-alkyl adduct, leads to an increased recognition by a ubiquitin ligase targeting the protein for degradation. This is a novel type of post-translational modification causing ubiquitination.

Effect of O6-benzylguanine on alkylating agent-induced toxicity and mutagenicity. In Chinese hamster ovary cells expressing wild-type and mutant O6-alkylguanine-DNA alkyltransferases

The DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) has been shown to protect cells from the toxic and mutagenic effect of alkylating agents by removing lesions from the O6 position of guanine. O6-Benzylguanine (BG) is a potent inactivator of AGT, resulting in an increase in the sensitivity of cells to the toxic effects of chemotherapeutic alkylating agents. Chinese hamster ovary (CHO) cells and CHO cells transfected with wild-type AGT (CHOWTAGT) and a mutant AGT [P138 M/V139I/P140K (CHOMIK)] known to be resistant to BG were treated with BG and various alkylating agents. BG treatment alone dramatically decreased AGT activity in CHOWTAGT cells but resulted in no depletion in AGT activity in CHOMIK cells. In the absence of AGT, these cells are highly sensitive to the toxic and mutagenic effects of temozolomide and 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), and no further sensitization occurs in the presence of BG. In contrast, CHOWTAGT cells are resistant to temozolomide and BCNU, and treatment with BG resulted in a significantly higher cell killing and mutation frequency. CHOMIK cells were completely resistant to temozolomide or BCNU in the presence and absence of BG. Both cell killing and mutation frequency of 4-hydroperoxycyclophosphamide (4-HC) in CHO, CHOWTAGT, and CHOMIK cells were increased in the presence of BG. 4-HC generates two active metabolites, phosphoramide mustard (PM) and acrolein. BG had no effect on 4hydroperoxydidechlorocyclophosphamide (which generates acrolein and a nonalkylating form of PM) in CHO cells and CHOMIK cells, but enhancement of toxicity was observed with PM in both these cell lines. Therefore, we attribute the enhancement to the PM metabolite of 4-HC. Our results demonstrate that wild-type AGT plays an important role in protecting against the toxic and mutagenic effect of O6 alkylating agents and that a mutant AGT resistant to inactivation by BG effectively prevents BG-enhanced toxicity and mutagenicity induced by these agents. Expression of the AGT protein contributes to resistance of 4-HC. BG also enhances the toxicity of 4-HC and PM by a mechanism that may not involve the AGT repair protein.

Direct reversal of DNA damage by mutant methyltransferase protein protects mice against dose-intensified chemotherapy and leads to in vivo selection of hematopoietic stem cells

Direct reversal of O6 adducts caused by chemotherapy agents is accomplished in mammalian cells by the protein O6-methylguanine DNA methyltransferase (MGMT). Some tumors overexpress MGMT and are resistant to alkylator therapy. One future approach to treatment of these tumors may rely on concurrent pharmacological depletion of tumor MGMT with O6-benzylguanine (6-BG) and protection of sensitive tissues, such as hematopoietic stem and progenitor cells, using genetic modification with 6-BG-resistant MGMT mutants. We have used retroviral-mediated gene transfer to transduce murine hematopoietic bone marrow cells with MGMT point mutants showing resistance to 6-BG depletion in vitro. These mutants include proline to alanine and proline to lysine substitutions at the 140 position (P140A and P140K, respectively), which show 40- and 1000-fold resistance to 6-BG compared with wild-type (WT) MGMT. Lethally irradiated mice were reconstituted with murine stem cells transduced with murine stem cell virus retrovirus expressing each mutant, WT MGMT, or mock-infected cells and then treated with a combination of 30 mg/kg 6-BG and 10 mg/kg 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) or with 40 mg/kg BCNU alone. Compared with mice treated with BCNU alone, significant myeloid toxicity and death occurred in mice reconstituted with mock-infected or WT MGMT (<0.1 probability of survival) or the P140A mutant (0.13 probability of survival) MGMT cDNAs. In contrast, after an initial period of mild cytopenia, mice reconstituted with the P140K mutant (0.83 probability of survival) recovered nearly normal blood counts, even during continued treatment. Comparison of peripheral blood neutrophils after completion of 5 weekly treatments in these animals showed a direct correlation between the treatment and in vivo selection for progeny of transduced cells (pretreatment, approximately 8-12% transduced cells; no treatment, approximately 6% transduced cells; BCNU only, 51% transduced cells; 6-BG/BCNU, 93% transduced cells). To determine whether this selection occurred at the stem cell level, bone marrow from each treatment group was infused into secondary recipients. Whereas animals that received bone marrow from untreated animals reconstituted with 2% transduced cells, animals receiving marrow from 6-BG/BCNU-treated animals reconstituted with 94% transduced cells, demonstrating nearly complete selection for stem cells in the primary animals. Mice reconstituted with marrow from animals treated with BCNU only demonstrated 23% transduced cells, consistent with partial selection of stem cells in the primary mice. The levels of transduced cells also correlated with survival during a second round of intensive combination chemotherapy (probability of survival: 6-BG/BCNU, 1.0; BCNU alone, >0.70; no treatment, <0.1). These data demonstrate that mutant MGMT expressed in the bone marrow can protect mice from time- and dose-intensive chemotherapy and that the combination of 6-BG and BCNU leads to uniform selection of transduced stem cells in vivo in mice.

The role of human O(6)-alkylguanine-DNA alkyltransferase in promoting 1,2-dibromoethane-induced genotoxicity in Escherichia coli

The expression of the DNA repair protein human O(6)-alkylguanine-DNA alkyltransferase (AGT) in Escherichia coli strains GWR109 or TRG8 that lack endogenous AGT greatly increased the toxicity and mutagenicity of 1,2-dibromoethane (DBE). Pretreatment of strain TRG8 expressing human AGT, which is permeable to exogenous drugs, with the AGT inhibitor O(6)-benzylguanine (BG) abolished the lethal and mutagenic effects of DBE, indicating that an active AGT is required for promoting DBE genotoxicity. This was confirmed by the observation that E. coli expressing either the C145A AGT mutant, which is inactive due to loss of the alkyl acceptor site, or mutants Y114E and R128A, which are inactive due to alteration of the DNA binding domain, did not enhance the action of DBE. However, the AGT mutant protein P138M/V139L/P140K, which is active in repairing methylated DNA but is totally resistant to inactivation by BG due to alterations in the active site pocket, was unable to enhance the genotoxicity of DBE. Similarly, other mutants, G156P, Y158H and K165R that are strongly resistant to BG, were much less effective than wild type AGT in mediating the genotoxicity of DBE. Mutant P140A, which is moderately resistant to BG, did increase mutations in response to DBE but was less active than wild type. These results suggest that human AGT is able to interact with a DNA lesion produced by DBE but, instead of repairing it, converts it to a more genotoxic adduct. This interaction is prevented by mutations that modify the active site of AGT to exclude BG.