Higher expression of MGMT and a trend toward less frequent methylation of the promoter region of MGMT gene may partly account for relative resistance to temozolomide in pediatric gliomas

Temozolomide constitutes current standard of care for adult patients with high-grade gliomas. However, results for pediatric gliomas are rather disappointing. In order to investigate the molecular differences between pediatric and adult gliomas that could affect sensitivity to temozolomide, we studied 23 pediatric non-ependymal, non-pilocytic gliomas in comparison to 59 consecutive adult gliomas for the expression of O6-methylguanine methyltransferase (MGMT) and the DNA mismatch repair protein, mutS homolog 6 (MSH6) by immunohistochemistry, as well as for the presence or absence of promoter methylation of the MGMT gene by methylationspecific PCR. The expression of MGMT in pediatric gliomas was significantly higher than in adult gliomas, as shown by immunohistochemistry (p=0.00004). This association was conserved if statistical analysis was carried out only in astrocytic tumors (diffuse astrocytoma, anaplastic astrocytoma and glioblastoma, p=0.00007), or in oligodendroglial tumors (oligodendroglioma and anaplastic oligodendroglioma, p=0.020). Although methylation-specific PCR was successfully performed only in 15 pediatric gliomas, it also showed a trend toward less frequent methylation in pediatric as opposed to adult gliomas (p=0.242). MSH6 was almost equally expressed in pediatric and adult gliomas. Pediatric gliomas appear to have a distinct molecular profile associated with resistance to temozolomide. Higher expression of MGMT and a trend toward less frequent methylation of the promoter region of MGMT gene may partly account for relative resistance to temozolomide in pediatric gliomas as compared to adult gliomas. Introduction Temozolomide, an oral methylating agent, has better activity and less toxicity than conventional agents, including nitrosoureas in the treatment of gliomas (1,2). Indeed, concomitant and adjuvant temozolomide has improved progression-free survival and overall survival compared to radiotherapy alone in adult patients with newly diagnosed supratentorial glioblastoma, and constitutes current standard of care for those patients (3,4). Nowadays, the majority of clinical trials for gliomas comprises protocols, including temozolomide. However, the efficacy of temozolomide is virtually based on reports on adult gliomas, and results for pediatric gliomas are rather disappointing. Verschuur et al treated 20 children with recurrent high-grade gliomas, including 11 grade III and 8 grade IV gliomas. Progression-free survival at 6 months was 18 and 12.5% for each grade (5). Those figures are less favorably compared to those reported in adult studies (1,2). Ruggiero et al and Lashford et al also reported lower response rate to temozolomide in recurrent pediatric, high-grade gliomas as compared to adult gliomas (6,7). Moreover, in a previous phase II trial (ACNS0126) for newly diagnosed pediatric high-grade gliomas, chemoradiotherapy with temozolomide only provided comparable results to the historical control with BCNU (8). Temozolomide rapidly hydrolyzed into 5-amino1Himidazole-4-carboxamide and methyl diazonium ion at physiological pH via 5-[(1z)-3-Methyl-1-en-1-yl]-1H-imidazole-4-carboxamide. The cytotoxity of temozolomide is caused by methyl diazonium ion (9,10). The methylating activity of the methyl diazonium ion is exerted on the N7-position (70%), N3-position (10%) and O6-position of guanine (9%) (11). However, methylation of the N7-guanine is, in most cases, repaired by the base excision repair system in cancer cells. However, methylation of the O6-guanine leads to a mispaired thymine (GT-mismatch) in DNA reproduction (12), and finally leads to DNA strand break through the futile mismatch repair cycle (13). Therefore, a defect in the mismatch repair system as well as the increased expression of O6-methylguanine methyltransferase (MGMT), the enzyme that removes alkyl group from O6-guanine, appear to be associated with resistance to temozolomide in gliomas. Indeed, the expression of the mismatch repair protein, mutS homolog 6 (MSH6), is lost in 41% of recurrent glioblastomas in comparison to 0% of pre-treatment glioblastomas, and loss Molecular characteristics of pediatric non-ependymal, non-pilocytic gliomas associated with resistance to temozolomide TAKETO EZAKI1, HIKARU SASAKI1, YUICHI HIROSE2, TOMORU MIWA1, KAZUNARI YOSHIDA1 and TAKESHI KAWASE1 1Department of Neurosurgery, Keio University School of Medicine, Tokyo 160-8582; 2Department of Neurosurgery, Fujita Health University School of Medicine, Aichi 470-1192, Japan Received April 7, 2011; Accepted August 2, 2011 DOI: 10.3892/mmr.2011.573 Correspondence to: Dr Hikaru Sasaki, Department of Neurosurgery, Keio University School of Medicine, 35 Shinanomachi, Shinjyuku-ku, Tokyo 160-8582, Japan E-mail: [email protected]