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Published on: September 25, 2011
High MGMT expression identifies aggressive colorectal cancer with distinct genomic features and immune evasion
Janie Yue Zhang1,2, Barani Kumar P Rajendran3, Shruti S Desai3
1Division of Malignant Hematology and Medical Oncology, Department of Medicine, University of Pittsburgh School of Medicine and UPMC Hillman Cancer Center, Pittsburgh, Pennsylvania, USA.
Introduction:
The epigenetic silencing of O6-methylguanine DNA methyltransferase (MGMT) is associated with reduced DNA repair capacity, carcinogenesis and increased sensitivity to alkylating chemotherapy. However, the biological role and clinical significance of MGMT overexpression in cancer remains poorly understood.
Methods:
Using multiplexed quantitative immunofluorescence we measured the localized levels of MGMT protein, γH2AX and CD8+ T cells in multiple retrospective colorectal cancer (CRC) cohorts. Genomic and transcriptomic features of selected cases were also studied with whole exome DNA sequencing and genome-wide methylation analysis. MGMT-methylated human CRC cells SW620 were transfected with an MGMT-containing plasmid and co-cultured with allogeneic peripheral blood mononuclear cells.
Results:
A subset of CRCs showed MGMT protein upregulation associated with lower γH2AX, reduced CD8+ tumor infiltrating lymphocytes (TILs), mismatch repair proficient (pMMR) status and shorter survival. CD8+ TILs were more distant from MGMT-expressing cells than MGMT-negative cells and the MGMT promoter methylation status did not highly correlate with MGMT protein levels in CRC. In genomic/transcriptomic analysis, high MGMT expression was associated with a lower nonsynonymous somatic mutational burden, higher transition-to-transversion mutation ratio, increased deleterious TP53 variants and distinct transcriptomic profiles. The exogenous expression of MGMT in SW620 CRC cells reduced the number of spontaneous nonsynonymous mutations, reproduced mutational features of MGMT-high CRC and limited the in vitro T-cell-mediated killing of malignant cells induced by proinflammatory cytokines in tumor/immune cell co-cultures.
Conclusions:
MGMT overexpression identifies a previously undescribed subset of CRCs with distinct biological and clinical properties including reduced mutagenesis, adaptive immune evasion, predominantly pMMR phenotype and aggressive clinical course. Direct, quantitative assessment of MGMT protein expression using spatially resolved analysis is more reliable than inference of MGMT expression by promoter methylation status in CRC.
Insights
O-6-methylguanine DNA methyltransferase (MGMT) overexpression in colorectal cancer (CRC) is linked to immune evasion and aggressive disease. Direct protein measurement is more reliable than promoter methylation for assessing MGMT in CRC.
Area of Science:
- Oncology
- Molecular Biology
- Cancer Genomics
Background:
- O-6-methylguanine DNA methyltransferase (MGMT) epigenetic silencing is linked to DNA repair, cancer, and chemotherapy sensitivity.
- The role of MGMT overexpression in cancer is not well understood.
Purpose of the Study:
- To investigate the biological role and clinical significance of MGMT protein overexpression in colorectal cancer (CRC).
- To compare the reliability of direct MGMT protein assessment versus promoter methylation status in CRC.
Main Methods:
- Multiplexed quantitative immunofluorescence to measure MGMT, γH2AX, and CD8+ T cells in CRC cohorts.
- Whole exome sequencing and genome-wide methylation analysis.
- Transfection of MGMT-expressing plasmid in CRC cells and co-culture with immune cells.
Main Results:
- MGMT overexpression in a subset of CRCs correlated with lower γH2AX, reduced CD8+ tumor-infiltrating lymphocytes (TILs), proficient mismatch repair (pMMR) status, and shorter survival.
- MGMT protein levels showed poor correlation with MGMT promoter methylation status in CRC.
- Exogenous MGMT expression in CRC cells reduced mutations, mimicked MGMT-high CRC features, and impaired T-cell-mediated killing.
Conclusions:
- MGMT overexpression defines a distinct CRC subset characterized by reduced mutagenesis, immune evasion, pMMR phenotype, and aggressive clinical course.
- Spatially resolved MGMT protein assessment is more reliable than promoter methylation for predicting MGMT status in CRC.
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