Differences Between Pediatric and Adult Malignant Astrocytomas
Most recent treatment protocols that have attempted to use alkylator-based chemotherapy regimens for the treatment of pediatric malignant astrocytoma employ temozolomide. Temozolomide works by alkylating three specific residues on replicating DNA – the O position of guanine, the N position of adenine and the N position of guanine. When the O position of guanine is methylated, the methylguanine residue incorrectly pairs with thymidine, and the mismatch repair system is triggered. Activity of this system results in repeated and incorrect reinsertion of thymidine. DNA replication cannot take place, strand breaks occur and eventually apoptosis is signaled. This process can be overcome by activity of the enzyme methylguanine methyltransferase (MGMT), which stoichiometrically removes the methyl group from the O position of guanine and allows proper replication. Work by many groups has shown the presence of MGMT (determined either by gene promoter methylation status or immunohistochemistry) can distinguish a group of patients and tumors with a poorer prognosis. Depletion of this enzyme, most notably through the use of the substrate analog O-benzylguanine has been shown to resensitize the tumor cells to temozolomide both in vitro and in vivo. Resistance to temozolomide has also been shown to exist in tumor cells deficient in the DNA mismatch repair system, where tolerance develops to O-methylguanine and the presence of persistent DNA damage.
As with many of the discoveries regarding molecular genetics in malignant astrocytomas, the bulk of the early work on MGMT has been undertaken in adult populations. Of note, work within the past 5 years has been performed on malignant pediatric astrocytomas. In 109 pediatric HGG samples using immunohistochemistry, 11% (12 of 109 samples) demonstrated overexpression of MGMT, with higher levels of expression correlating with decreased survival. Another study of ten snap-frozen pediatric glioblastoma samples identified MGMT promoter methylation in four samples. Comparing adult and pediatric malignant astrocytomas, Ezaki et al. showed that MGMT expression via immunohistochemistry was significantly higher in pediatric gliomas, while other markers of mismatch repair (MSH6) were approximately equal. Further methylation-specific PCR showed a trend toward less frequent methylation (and thus less frequent epigenetic silencing of MGMT) in pediatric versus adult malignant gliomas. These differences highlight the therapeutic history of temozolomide in the pediatric malignant glioma population. In spite of these differences, temozolomide is routinely used for newly diagnosed adult malignant glioma based on improvement in outcome when compared with radiation therapy alone. A recently completed single-arm clinical trial in pediatric patients with HGG showed acceptable results with radiation and temozolomide that appear approximately equivalent to other alkylator-based approaches. Unfortunately, both adult and pediatric patients treated with radiation and temozolomide will eventually succumb to disease. Pontine gliomas that are typically anaplastic astrocytoma or GBM in histology (often taken at the time of autopsy) achieved no benefit with the addition of temozolomide to radiation therapy, a further example of the heterogeneity of GBMs in the pons and supratentorial compartment.
Methylguanine Methyltransferase
Most recent treatment protocols that have attempted to use alkylator-based chemotherapy regimens for the treatment of pediatric malignant astrocytoma employ temozolomide. Temozolomide works by alkylating three specific residues on replicating DNA – the O position of guanine, the N position of adenine and the N position of guanine. When the O position of guanine is methylated, the methylguanine residue incorrectly pairs with thymidine, and the mismatch repair system is triggered. Activity of this system results in repeated and incorrect reinsertion of thymidine. DNA replication cannot take place, strand breaks occur and eventually apoptosis is signaled. This process can be overcome by activity of the enzyme methylguanine methyltransferase (MGMT), which stoichiometrically removes the methyl group from the O position of guanine and allows proper replication. Work by many groups has shown the presence of MGMT (determined either by gene promoter methylation status or immunohistochemistry) can distinguish a group of patients and tumors with a poorer prognosis. Depletion of this enzyme, most notably through the use of the substrate analog O-benzylguanine has been shown to resensitize the tumor cells to temozolomide both in vitro and in vivo. Resistance to temozolomide has also been shown to exist in tumor cells deficient in the DNA mismatch repair system, where tolerance develops to O-methylguanine and the presence of persistent DNA damage.
As with many of the discoveries regarding molecular genetics in malignant astrocytomas, the bulk of the early work on MGMT has been undertaken in adult populations. Of note, work within the past 5 years has been performed on malignant pediatric astrocytomas. In 109 pediatric HGG samples using immunohistochemistry, 11% (12 of 109 samples) demonstrated overexpression of MGMT, with higher levels of expression correlating with decreased survival. Another study of ten snap-frozen pediatric glioblastoma samples identified MGMT promoter methylation in four samples. Comparing adult and pediatric malignant astrocytomas, Ezaki et al. showed that MGMT expression via immunohistochemistry was significantly higher in pediatric gliomas, while other markers of mismatch repair (MSH6) were approximately equal. Further methylation-specific PCR showed a trend toward less frequent methylation (and thus less frequent epigenetic silencing of MGMT) in pediatric versus adult malignant gliomas. These differences highlight the therapeutic history of temozolomide in the pediatric malignant glioma population. In spite of these differences, temozolomide is routinely used for newly diagnosed adult malignant glioma based on improvement in outcome when compared with radiation therapy alone. A recently completed single-arm clinical trial in pediatric patients with HGG showed acceptable results with radiation and temozolomide that appear approximately equivalent to other alkylator-based approaches. Unfortunately, both adult and pediatric patients treated with radiation and temozolomide will eventually succumb to disease. Pontine gliomas that are typically anaplastic astrocytoma or GBM in histology (often taken at the time of autopsy) achieved no benefit with the addition of temozolomide to radiation therapy, a further example of the heterogeneity of GBMs in the pons and supratentorial compartment.