Glioblastoma Multiforme (GBM)

OVERVIEW

Glioblastoma multiforme (GBM) is the most common and most aggressive form of glial tumour in adults1. Classified as a WHO grade IV astrocytoma, GBM is very infiltrative and can spread to other parts of the brain rapidly, resulting in a median survival of less than 15 months from initial diagnosis2. The current standard treatment for GBM is surgical resection, followed by concurrent radiation and chemotherapy with the alkylating drug temozolomide2.

Temozolomide triggers the death of tumour cells by damaging DNA through base mispairings and double-stranded breaks; however, some tumour cells are able to escape apoptosis by expressing the DNA-repair protein O6-methylguanine-DNA methyltransferase encoded by the MGMT gene3. Accordingly, loss of MGMT expression in tumour cells by epigenetic silencing and hypermethylation of the MGMT promoter region has been associated with improved response to temozolomide4.

Hence, MGMT promoter methylation status is an important prognostic factor for patients with glioblastoma multiforme (GBM). Patients with methylated MGMT have longer overall survival when they receive alkylating chemotherapy with TMZ in addition to radiotherapy.

INDICATION

MGMT promoter methylation status testing is offered for patients over 60 years of age with a diagnosis of glioblastoma multiforme (GBM)

TEST REQUIREMENTS

  • Tumour block
  • H&E that matches the current block surface
  • The H&E should be circled with the area to sample for molecular testing – multiple circles can be made but each should be larger than 5mm in diameter
  • For the circled area, in the space provided on the requisition, provide the tumour content (i.e. the proportion of cells in the circle that are tumour cells) and the tumour cellularity (i.e. the proportion of the circled area occupied by nucleated cells).  NOTE: 10% tumour content is the minimum requirement for this assay.
  • Completed CGL requisition – current requisitions can be found here: https://cancergeneticslab.ca/requisitions/

TURN-AROUND TIME

Approximately 14 days from receipt of specimen and completed, signed requisition form.

METHOD

Genomic DNA is extracted from the submitted specimen and bisulfite treated. Bisulfite-treated DNA is assayed for MGMT promoter hypermethylation using methylation-specific PCR amplification. The resulting PCR products are resolved using capillary electrophoresis.

This assay detects methylation of CpGs 76 to 80 and 84 to 87 (see Malley et al. (2011)), with a limit of detection estimated to be 1% methylated DNA relative to total genomic DNA.

REFERENCES

  1. Holland, E. Glioblastoma multiforme: The terminator. Proc Natl Acad Sci USA. 2000 Jun 6; 97(12): 6242–6244.
  2. Fernandes C., et al. Current Standards of Care in Glioblastoma Therapy. In: De Vleeschouwer S, editor. Glioblastoma [Internet]. Brisbane (AU): Codon Publications; 2017 Sep 27. Chapter 11.
  3. Malstrom, A. et al. Temozolomide versus standard 6-week radiotherapy versus hypofractionated radiotherapy in patients older than 60 years with glioblastoma: the Nordic andomized, phase 3 trial. Lancet Oncol. 2012; 13: 916-926.
  4. Christians, A. et al. Prognostic Value of Three Different Methods of MGMT Promoter Methylation Analysis in a Prospective Trial on Newly Diagnosed Glioblastoma. PLoS ONE. 2012; 7(3): e33449.
  5. Malley, D. et al. A distinct region of the MGMT CpG island critical for transcriptional regulation is preferentially methylated in glioblastoma cells and xenografts. Acta Neuropathol. 2011 May; 121(5): 651-61.
  6. Cankovic, M. et al. The Role of MGMT Testing in Clinical Practice: A Report of the Association for Molecular Pathology. The Journal of Molecular Diagnostics. 2013; 15(5).
  7. Hegi, M. et al. MGMT Gene Silencing and Benefit from Temozolomide in Glioblastoma. The New England Journal of Medicine. 2005; 352: 997-1003.