For instance, methylation of the DNA repair gene MGMT promoter was observed in 51% to 66% of glioblastoma specimens. In addition, in some glioblastomas, the MGMT protein is deficient due to another type of epigenetic alteration. MGMT protein expression may also be reduced due to increased levels of a microRNA that inhibits the ability of the MGMT messenger RNA to produce the MGMT protein. Zhang et al. found, in the glioblastomas without methylated MGMT promoters, that the level of microRNA miR-181d is inversely correlated with protein expression of MGMT and that the direct target of miR-181d is the MGMT mRNA 3UTR (the three prime untranslated region of MGMT messenger RNA).Epigenetic reductions in expression of another DNA repair protein, ERCC1, were found in an assortment of 32 gliomas. For 17 of the 32 (53%) of the gliomas tested, ERCC1 protein expression was reduced or absent. In the case of 12 gliomas (37.5%) this reduction was due to methylation of the ERCC1 promoter. For the other 5 gliomas with reduced ERCC1 protein expression, the reduction could have been due to epigenetic alterations in microRNAs that affect ERCC1 expression.When expression of DNA repair genes is reduced, DNA damages accumulate in cells at a higher than normal level, and such excess damages cause increased frequencies of mutation. Mutations in gliomas frequently occur in either isocitrate dehydrogenase (IDH) 1 or 2 genes. One of these mutations (mostly in IDH1) occurs in about 80% of low grade gliomas and secondary high-grade gliomas. Wang et al.
Results show the evidence is insufficient to make a reliable decision. The relative effectiveness of surgical resection compared to biopsy for people with malignant glioma (high-grade) is unknown.For high-grade gliomas, a 2003 meta-analysis compared radiotherapy with radiotherapy and chemotherapy. It showed a small but clear improvement from using chemotherapy with radiotherapy. A 2019 meta-analysis suggested that for people with less aggressive gliomas, radiotherapy may increase the risk of long term neurocognitive side effects. Whilst, evidence is uncertain on whether there are long term neurocognitive side effects associated with chemoradiotherapy.Temozolomide is effective for treating Glioblastoma Multiforme (GBM) compared to radiotherapy alone. A 2013 meta-analysis showed that Temozolomide prolongs survival and delays progression, but is associated with an increase in side effects such as blood complications, fatigue, and  infection. For people with recurrent GBM, when comparing temozolomide with chemotherapy, there may be an improvement in the time-to-progression and the persons quality of life, but no improvement in overall survival, with temozolomide treatment. Evidence suggests that for people with recurrent high-grade gliomas who have not had chemotherapy before, there are similar survival and time-to-progression outcomes between treatment with temozolomide or the chemotherapy multidrug known as PCV (procarvazine, lomustine and vincristine).A mutational analysis of 23 initial low-grade gliomas and recurrent tumors from the same patients has challenged the benefits and usage of Temozolomide. The study showed that when lower-grade brain tumors of patients are removed and patients are further treated with Temozolomide, 6 out of 10 times the recurrent tumors were more aggressive and acquired alternative and more mutations.