Loss of the Mismatch Repair Protein MSH6 in Human Glioblastomas Is Associated with Tumor Progression during Temozolomide Treatment

Abstract

Purpose: Glioblastomas are treated by surgical resection followed by radiotherapy [X-ray therapy (XRT)] and the alkylating chemotherapeutic agent temozolomide. Recently, inactivating mutations in the mismatch repair gene MSH6 were identified in two glioblastomas recurrent post-temozolomide. Because mismatch repair pathway inactivation is a known mediator of alkylator resistance in vitro, these findings suggested that MSH6 inactivation was causally linked to these two recurrences. However, the extent of involvement of MSH6 in glioblastoma is unknown. We sought to determine the overall frequency and clinical relevance of MSH6 alterations in glioblastomas. Experimental Design: The MSH6 gene was sequenced in 54 glioblastomas. MSH6 and O 6 -methylguanine methyltransferase (MGMT) immunohistochemistry was systematically scored in a panel of 46 clinically well-characterized glioblastomas, and the corresponding patient response to treatment evaluated. Results: MSH6 mutation was not observed in any pretreatment glioblastoma (0 of 40), whereas 3 of 14 recurrent cases had somatic mutations (P = 0.015). MSH6 protein expression was detected in all pretreatment (17 of 17) cases examined but, notably, expression was lost in 7 of 17 (41%) recurrences from matched post^XRT + temozolomide cases (P = 0.016). Loss of MSH6 was not associated with O 6 -methylguanine methyltransferase status. Measurements of in vivo tumor growth using three-dimensional reconstructed magnetic resonance imaging showed that MSH6-negative glioblastomas had a markedly increased rate of growth while under temozolomide treatment (3.17 versus 0.04 cc/mo for MSH6-positive tumors; P = 0.020). Conclusions: Loss of MSH6 occurs in a subset of post^XRT + temozolomide glioblastoma recurrences and is associated with tumor progression during temozolomide treatment, mirroring the alkylator resistance conferred by MSH6 inactivation in vitro. MSH6 deficiency may therefore contribute to the emergence of recurrent glioblastomas during temozolomide treatment.

Acquired Resistance to Temozolomide in Glioma Cell Lines: Molecular Mechanisms and Potential Translational Applications

“…Recent research confirmed loss of MMR protein hMSH6 in human glioblastomas associated with tumor progression during TMZ treatment, as well as demonstrating hMSH6 loss during TMZ treatment regardless of MGMT status [38,47]. Our data are consistent with clinical observations reporting hMSH6 loss as one phenotype acquired in glioma patients treated with TMZ.…”

“…In contrast, impaired MMR through loss of hMSH6 expression and enhanced expression of a key BER gene developed in SNB19VR cells. Our observations parallel in vitro and clinical studies reporting that alkylator resistance can be mediated either by MGMT over-expression or MMR deficiency [34,38,47]. Because tumors readily acquire the ability to repair or tolerate TMZ-induced lesions, treatment is rendered ineffective.…”

“…However, enhanced sensitivity to TMZ may be a consequence of increased MMR expression following 5-AZA treatment. Indeed, Cahill et al [38] argue that induction of MGMT expression may not be frequent during TMZ treatment in clinical practice. Their data indicated that while pre-treatment tumor MGMT levels are linked to primary resistance, acquisition of further increased expression does not seem to be a common cause of emergent TMZ resistance.…”

Differential expression of miR200a-3p and miR21 in grade II–III and grade IV gliomas

“…10,11 This suggests that additional mechanisms are involved in tumor resistance to 12 Thus, resistance of GBM cells to TMZ includes various events, such as loss of the phosphatase and tensin homolog (PTEN), leading to the activation of the phosphoinositide-3-kinase (PI3K)/Akt pathway, deficiency in the DNA mismatch repair system, mutations in the pro-apoptotic p53 protein, overexpression of the anti-apoptotic protein Bcl-2, or the selection of less-differentiated pre-existing resistant cells in the parental tumor. [13][14][15][16][17][18][19] Dysregulation of the expression of microRNAs (miRNAs) is emerging as an important step in early processes during tumorigenesis as well as disease progression/metastasis in various malignancies including GBMs. [20][21][22] The miRNAs function mainly through their ability to bind to the 3′ untranslated regions of mRNAs, leading to their degradation or suppression of their translation.…”

Certain Imidazotetrazines Escape O6-Methylguanine-DNA Methyltransferase and Mismatch Repair

“…Given the negative clinical impact of acquired resistance, GBM cell lines were generated possessing acquired resistance to TMZ [20]. Molecular mechanisms of resistance to TMZ in vitro were identified, MMR loss and MGMT upregulation, consistent with acquired resistance, emerging clinically during TMZ treatment and tumor progression [21,22,23]. In this paper, we advance the possibility that imidazotetrazines structurally related to TMZ but with different N-3 monofunctional alkylation potential might generate cytotoxic lesions in DNA which would not be repaired by MGMT and would be unaffected by MMR status.…”