O6-methylguanine-DNA methyltransferase activity and promoter methylation status in pediatric rhabdomyosarcoma

Construction of a chemiluminescent biosensor based on enzymatic extension and click chemistry for sensitive measurement of MGMT activity in human breast tissues

O6-methylguanine methyltransferase (MGMT) is responsible for dealkylation of naturally occurring O6-methylguanines, and it is closely related with DNA replication, transcription, and cancers. Herein, we develop a chemiluminescent biosensor based on enzymatic extension and click chemistry for sensitive measurement of MGMT activity. When MGMT is present, the MGMT-catalyzed demethylation reaction initiates the cleavage of biotinylated dumbbell probes by PvuII restrictive enzyme, releasing two DNA fragments with 3'-OH end. The resultant DNA fragments can trigger terminal transferase (TdT)- and click chemistry-assisted isothermal amplification to obtain abundant G-rich sequences. The G-rich sequences can be captured by magnetic beads to produce a high chemiluminescence signal. This biosensor can greatly amplify the chemiluminescence signal, facilitating label-free and template-free measurement of MGMT. Especially, the introduction of dumbbell probe and PvuII enzyme can efficiently eliminate the false positive and improve the assay specificity. This biosensor possesses high sensitivity with a detection limit of 1.4 × 10-9 ng/μL, and it may accurately quantify the intracellular MGMT. Importantly, this biosensor can be used to screen the MGMT inhibitors and distinguish the MGMT level in breast tumor tissues and normal tissues, with great potential in drug discovery and cancer diagnosis.

A "Clickable" Probe for Active MGMT in Glioblastoma Demonstrates Two Discrete Populations of MGMT

Various pathways can repair DNA alkylation by chemotherapeutic agents such as temozolomide (TMZ). The enzyme O⁶-methylguanine methyltransferase (MGMT) removes O⁶-methylated DNA adducts, leading to the failure of chemotherapy in resistant glioblastomas. Because of the anti-chemotherapeutic activities of MGMT previously described, estimating the levels of active MGMT in cancer cells can be a significant predictor of response to alkylating agents. Current methods to detect MGMT in cells are indirect, complicated, time-intensive, or utilize molecules that require complex and multistep chemistry synthesis. Our design simulates DNA repair by the transfer of a clickable propargyl group from O⁶-propargyl guanine to active MGMT and subsequent attachment of fluorescein-linked PEG linker via "click chemistry." Visualization of active MGMT levels reveals discrete active and inactive MGMT populations with biphasic kinetics for MGMT inactivation in response to TMZ-induced DNA damage.

Successful treatment of metastatic alveolar rhabdomyosarcoma with MGMT gene promoter methylation by temozolomide-based combination chemotherapy

A 3-year-old male presented with a large retroperitoneal mass and multiple metastases. Biopsy results suggested alveolar rhabdomyosarcoma bearing a methylated O6-methylguanine-DNA methyltransferase (MGMT) gene promoter. Serum microRNA-206 levels were elevated and remained high after three cycles of vincristine, dactinomycin, and cyclophosphamide (VAC). Replacement of vincristine, irinotecan, and temozolomide (VIT) for VAC induced a marked tumor reduction and normalization of the miR-206 levels. The patient completed 14 cycles of VIT with local radiotherapy and has been in remission for 31 months. Temozolomide could be effective for tumors with a methylated MGMT gene promoter. Individualized therapy is warranted for such patients.

O(6)-Methylguanine-DNA methyltransferase (MGMT) in normal tissues and tumors: enzyme activity, promoter methylation and immunohistochemistry

O(6)-Methylguanine-DNA methyltransferase (MGMT) is a suicide enzyme that repairs the pre-mutagenic, pre-carcinogenic and pre-toxic DNA damage O(6)-methylguanine. It also repairs larger adducts on the O(6)-position of guanine, such as O(6)-[4-oxo-4-(3-pyridyl)butyl]guanine and O(6)-chloroethylguanine. These adducts are formed in response to alkylating environmental pollutants, tobacco-specific carcinogens and methylating (procarbazine, dacarbazine, streptozotocine, and temozolomide) as well as chloroethylating (lomustine, nimustine, carmustine, and fotemustine) anticancer drugs. MGMT is therefore a key node in the defense against commonly found carcinogens, and a marker of resistance of normal and cancer cells exposed to alkylating therapeutics. MGMT also likely protects against therapy-related tumor formation caused by these highly mutagenic drugs. Since the amount of MGMT determines the level of repair of toxic DNA alkylation adducts, the MGMT expression level provides important information as to cancer susceptibility and the success of therapy. In this article, we describe the methods employed for detecting MGMT and review the literature with special focus on MGMT activity in normal and neoplastic tissues. The available data show that the expression of MGMT varies greatly in normal tissues and in some cases this has been related to cancer predisposition. MGMT silencing in tumors is mainly regulated epigenetically and in brain tumors this correlates with a better therapeutic response. Conversely, up-regulation of MGMT during cancer treatment limits the therapeutic response. In malignant melanoma, MGMT is not related to the therapeutic response, which is due to other mechanisms of inherent drug resistance. For most cancers, studies that relate MGMT activity to therapeutic outcome following O(6)-alkylating drugs are still lacking.

Inactivation of O6-methylguanine-DNA methyltransferase in soft tissue sarcomas: association with K-ras mutations

The DNA-repair protein O(6)-methylguanine-DNA methyltransferase removes alkyl adducts from the O(6)-position of guanine. The adducts can mispair with T during DNA replication, resulting in a G-to-A mutation. Epigenetic inactivation of O(6)-methylguanine-DNA methyltransferase has been found in human neoplasia and is considered one of the implicated factors in chemoresistance. Sixty-two patients with soft tissue sarcomas were analyzed with regard to the status of O(6)-methylguanine-DNA methyltransferase protein expression status using immunohistochemistry and promoter hypermethylation of the MGMT gene using methylation-specific PCR. G-to-A transitions in codons 12 and 13 of the K-ras oncogene were investigated using PCR and direct automated sequencing analysis. A loss of O(6)-methylguanine-DNA methyltransferase expression was noted in 20 (32.3%) cases of 62 total soft tissue sarcomas. The MGMT promoter hypermethylation rate was 33.9% (21/62 cases). Of the 54 sarcomas evaluated, K-ras mutations were found in only 2 (3.7%) cases. Loss of O(6)-methylguanine-DNA methyltransferase expression and MGMT promoter hypermethylation showed a significant association with high American Joint Committee on Cancer stage, high French Federation of Cancer Centers grade, and aggressive behavior. On multivariate analysis, these were not an independently significant prognostic factors. However, when the group receiving chemotherapy was analyzed (n = 27), loss of O(6)-methylguanine-DNA methyltransferase expression was correlated with worse survival on multivariate analysis (P = .024). MGMT promoter hypermethylation status had a strong correlation with loss of O(6)-methylguanine-DNA methyltransferase expression (P = .000). Our results suggest that MGMT promoter hypermethylation and loss of O(6)-methylguanine-DNA methyltransferase expression tend to be associated with poor prognosis and that the loss of O(6)-methylguanine-DNA methyltransferase protein expression frequently occurs via MGMT promoter hypermethylation. However, MGMT promoter hypermethylation was not significantly associated with point mutations of K-ras at codons 12 and 13 in sarcomas.

Phase I pharmacokinetic and pharmacodynamic study of temozolomide in pediatric patients with refractory or recurrent leukemia: a Children's Oncology Group Study

PURPOSE

To determine the tolerability, pharmacokinetics, and mechanisms of temozolomide resistance in children with relapsed or refractory leukemia.

PATIENTS AND METHODS

Cohorts of three to six patients received 200 or 260 mg/m2/d of temozolomide by mouth daily for 5 days every 28 days. Toxicities, clinical response, and pharmacokinetics were evaluated. Pretreatment leukemia cell O6-methylguanine-DNA methyltransferase (MGMT) activity, tumor and plasma MGMT promoter methylation, and microsatellite instability (MSI) were examined in 14 of 16 study patients and in tissue bank samples from children with acute leukemia not treated with temozolomide (MGMT, n = 67; MSI, n = 65).

RESULTS

Sixteen patients (nine female, seven male; acute lymphoblastic leukemia [ALL], n = 8; acute myeloid leukemia [AML], n = 8), median age 11 years (range, 1 to 19 years), received either 200 mg/m2/d (nine enrolled, three assessable for toxicity) or 260 mg/m2/d (seven enrolled, three assessable for toxicity) of temozolomide. Temozolomide was well tolerated and no dose-limiting toxicities occurred. The mean clearance of temozolomide was 107 mL/min/m2, with a volume of distribution of 20 L/m2 and half-life of 109 minutes. MGMT activity in leukemia cells was quite variable and was highest in patients with relapsed ALL. Only one patient had MSI. Two patients had a partial response. Both of these patients had no detectable MGMT activity; both also had methylated MGMT promoters and were MSI stable.

CONCLUSION

Temozolomide was well tolerated at doses as high as 260 mg/m2/d for 5 days in children with relapsed or refractory leukemia. Increased MGMT activity may account for the temozolomide resistance in children with relapsed leukemia. Leukemia cell MGMT activity was higher in pediatric ALL than AML (P < .0001).