MGMT Promoter Methylation: A Glioma Treatment Biomarker

A Repair Protein That Sacrifices Itself

MGMT transfers the offending alkyl group from O6-methylguanine onto one of its own cysteine residues, restoring the guanine. This inactivates the MGMT molecule permanently, so repair capacity depends on how fast the cell can make new MGMT protein.

If O6-methylguanine persists into replication, it mispairs with thymine, and futile mismatch-repair cycling triggers strand breaks and cell death. Tumours with abundant MGMT resist this; tumours with little MGMT do not.

Why Promoter Methylation Matters

Methylation of CpG sites in the MGMT promoter reduces transcription and lowers MGMT protein. In newly diagnosed glioblastoma treated with temozolomide, a methylated promoter is associated with better outcomes, and it is both prognostic and predictive of alkylating-agent benefit.

Trials have used methylation status to stratify treatment; for example, adding lomustine to temozolomide has been studied specifically in methylated tumours. Status does not guarantee an individual response and is one input among several.

Assay Caveats

MGMT methylation is typically measured by methylation-specific PCR, pyrosequencing or methylation array, and results near the threshold can be discordant between methods. There is no single universally agreed cut-off.

Sample quality, tumour purity and the specific CpG sites interrogated all affect the call, so the method and any quantitative value should be kept with the result.

How Status Feeds Treatment Decisions

In newly diagnosed glioblastoma, methylation status informs rather than dictates treatment. Most patients still receive temozolomide with radiotherapy regardless of status, but a methylated promoter strengthens the case for alkylating-agent-based approaches and has been used to select patients for intensified regimens such as adding lomustine. In older or frailer patients an unmethylated promoter may shift the balance toward radiotherapy alone or a shorter course.

Status is prognostic as well as predictive, so it also shapes conversations about likely course and trial eligibility. It is one input alongside IDH status, age, performance status and the extent of surgical resection.

Key Takeaways

• ·MGMT reverses O6-guanine alkylation in a self-inactivating, stoichiometric reaction.
• ·Promoter methylation silences MGMT and predicts greater benefit from temozolomide in glioblastoma.
• ·Methylation assays vary near the threshold, so method and cut-off belong with the report.