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

Objectives: To determine the activity of the DNA repair protein O6-methylguanine-DNA methyltransferase (MGMT) and MGMT promoter methylation status of pediatric rhabdomyosarcoma (RMS) and examine MGMT in RMS tumors from different prognostic groups.

Methods: Fifteen samples each of the alveolar (ARMS) and embryonal (ERMS) subtypes were obtained for analysis of MGMT activity and promoter methylation status. MGMT activity was assayed by measuring the removal of O6-[3H] methylguanine from [3H]-methylated substrate by a tumor extract containing the enzyme. Promoter methylation status was examined using methylation-specific polymerase chain reaction (PCR).

Results: MGMT activity was successfully assayed from 25 samples, 10 ERMS and 15 ARMS. All ERMS and 11 of the 15 ARMS samples displayed high activity levels. There was significant intertumor variability among both subtypes but no significant difference in mean activity between the two histologic groups. There were trends toward increased activity in ERMS tumors and tumors from anatomically unfavorable locations. Only one tumor was hypermethylated at the MGMT promoter region.

Conclusions: This analysis suggests that a low percentage of RMS samples are hypermethylated at the MGMT promoter and that most have significant MGMT activity, implying that clinical trials with MGMT-modulating agents may have a role in the treatment of these tumors. This analysis does not support MGMT activity as an explanation of the differential response to chemotherapy demonstrated by ARMS and ERMS, but does suggest that MGMT may be involved in RMS treatment failure regardless of subtype and in the poorer response shown by tumors from unfavorable locations.