Thursday, July 2, 2009

MGMT status of glioblastomas: Is PCR or IHC better?

Back in February '08 I wrote a post arguing that MGMT testing on glioblastomas is still not ready for clinical use outside of clinical trials. Since not all of our oncologist colleagues are Neuropathology Blog disciples, I continue to occasionally get a request from a clinician for MGMT testing on glioblastoma specimens. MGMT stands for O(6)-methylguanine DNA methyltransferase. Why are they asking for this test? Alkylating agents, like temazolamide, are more effective when MGMT is not active in a tumor because MGMT normally acts to remove the toxic methylguanine adducts provided by temozolamide. So MGMT silencing, usually due to gene promoter hypermethylation, predicts better response to temazolamide.

What's the best method for assessing MGMT activity in a tumor? According to a review article in the July 2009 issue of Archives of Pathology and Laboratory Medicine by Drs. Peter Pytel (pictured) and Rimas Lukas (Vol133:1062-1077) "immunohistochemical staining for MGMT does not offer a reliable way to stratify glioblastomas, and polymerase chain reaction-based assays are therefore necessary." Pytel and Lukas, of the University of Chicago Medical Center, provide the following two references for this position:

-- Yip S, Iafrate AJ, Louis DN. Molecular diagnostic testing in malignant gliomas: a practical update on predictive markers. J Neuropathol Exp Neurol. 2008;67:1-15.
-- Preusser M, Janzer RC, Felsberg J, et al. Anti-O6-methylguanine methyltransferase (MGMT) immunohistochemistry in glioblastoma multiforme: observer variability and lack of association with patient survival impede its use as a clinical biomarker. Brain Pathol. 2008;18(4):520-532.


On the other hand, some would argue that immunohistochemistry picks up more cases of MGMT than does PCR. Since its still really an experimental test, the issue ultimately may come down to what insurance will pay for. Immunohistochemistry is cheaper, so the patient may have an easier time getting his or her insurance to pay for it. My position is this: I will continue to discourage clinicians from getting the test until it becomes standard practice. If they insist on getting it, I would recommend immunohistochemistry for cost reasons and send it out to the Duke University laboratory of Dr. Roger McLendon to have it done. If, however, PCR is specifically demanded, I will send it to LabCorp, a commercial reference laboratory, which offers the MGMT PCR assay on paraffin-embedded tissue.

8 comments:

shipcolldoc said...

This is an interesting problem. There is no doubt that the absence of MGMT expression in a high grade glioma connotes a better response to alkylating agent chemotherapy, especially Temozolomide, and probably a better long-term survival, but the published data also show that patients survive longer after radiation and chemotherapy even if their tumors express MGMT. So denial of treatment based on this test is not currently justifiable. That being the case, why do the test? Just for prognostic purposes? If we had better alternative treatments for high grade gliomas, I could see doing the test, but the current state of treatment doesn't support a clinical use of this test, whether IHC or PCR. As for the choice of tests, a negative IHC test could imply no MGMT expression OR failure of the test...one needs a reliable internal positive control, and that isn't always so easy either. So this remains a thorny ethical as well as practical clinical-pathological problem.--DCM

Brian E. Moore, MD said...

All good points, Shipcolldoc. I should note that infiltrating microglia do stain positively for MGMT on immunohistochemistry. Of course, rather than a reliable internal positve control, they are more likely to serve as a source of confusion. I know that the lab at Duke is working on this issue to see if it can be resolved.

harold said...

Hi, Dr. Moore,

I'm a GBM patient, not a doctor.

But, I have a question, if you don't mind answering it.

Are you saying that when MGMT is not active in a tumor cell, it is good because, when the MGMT is active, it acts to removes the toxic methylguanine (whatever that is)?

And this is one the benefits of Temodar - introducting the toxic methylguanine to the tumor cell?

Brian E. Moore, MD said...

To Harold:
You have it exactly right. MGMT inactivity is a good thing since MGMT would otherwise act to mitigate the effects of alkylating chemotherapeutic agents.
Best to you,
BEM

harold said...

Dr. Moore,

I'm sorry - I have a couple more - I think related - questions.

Is there a correlation between having the MGMT expression and being EGFRvIII positive?

As is either of the two related to the "unmethalyated status" (if I'm using the term correctly?)

Brian E. Moore, MD said...

To Harold,
EGFR is another genetic test that can be done on brain tumor specimens, but it is not related to MGMT status per se. Although not mechanistically related, I am not sure if there is a statistical correlation between a patient's MGMT status and the same patient's EGFR status. I'll look into this issue and publish it as a separate blog post. Stay tuned... Regarding your second question, methylation is the most common way (but not the only way) that MGMT is inactivated.

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