Oligodendroglioma

What is the 1p/19q test?

The test is useful in diagnosing oligodendrogliomas and, therefore, in making decisions about the most appropriate treatment for you.

How does the test work and what does it show?

Our bodies are made up of cells. Each cell has 23 pairs of chromosomes, which carry your genes (DNA). One chromosome of each pair is inherited from your mother, and the other from your father. Each pair of chromosomes are numbered 1 to 22, then XX or XY.

The 1p/19q test looks at genetic changes to chromosomes 1 and 19 to see whether they have a section missing. If the sections called the p section (p for “petite”, i.e. the shorter) of chromosome 1 and the q section (q has been chosen because it is the next letter in the alphabet) of chromosome 19 are missing, this means that the genes carried in those sections are also missing.

These genes seem to be involved in resistance to chemotherapy drugs. So if you’re found to be missing both those sections, you’re more likely to have a better response to chemotherapy and longer overall survival.

What is the IDH1/IDH2 test?

This test is useful in diagnosing oligodendrogliomas and in making decisions about the most appropriate treatment for you.

It might also be useful in predicting how effective a particular treatment is likely to be.

How does the test work and what does it show?

IDH1 and IDH2 are genes. A change (mutation) in the IDH1 or IDH2 genes has been found in almost all oligodendrogliomas and astrocytomas.

Tumours that have the change are known as IDH–mutant, and those without the change are known as IDH-wildtype.

Previously, a type of brain tumour called glioblastoma was divided into primary and secondary types. Secondary glioblastomas were those that had developed from a lower grade tumour and had an IDH mutation, whilst primary glioblastomas were those that developed without a lower grade tumour found beforehand, and were IDH-wildtype. It is now known that any adult-type diffuse gliomas (oligodendrogliomas, astrocytomas and glioblastomas) that have an IDH mutation tend to have better long-term survival.

So, in the most up to date tumour classification system, only oligodendrogliomas and astrocytomas will have IDH mutations and all glioblastomas are IDH-wildtype.

It’s important to be aware that people with gliomas that have the IDH1/IDH2 mutation tend to be younger adults and older children, which may partially account for their longer survival.

What is the MGMT methylation test?

For people with oligodendroglioma, the MGMT methylation test helps to predict how effective chemotherapy treatment is likely to be, although there are many other factors that also affect response to treatment. This can be used to help plan a suitable, individualised treatment plan.

What does the test show?

In summary, the test looks at the amount (percentage) of something called methylation. If your tumour is found to be:

• MGMT methylated, you’re more likely to respond well to TMZ chemotherapy.
• MGMT unmethylated, you’re less likely to respond to TMZ chemotherapy.

This helps your healthcare team decide on the best course of treatment for you. For example, if your tumour is unmethlyated, giving you high doses of chemotherapy may give you lots of unpleasant side-effects, but with little benefit in terms of reducing your tumour.

If you do decide to have the test, you need to be aware that it may be found to be unmethylated. Speak to your healthcare team about what would happen next if this turned out to be the case.

There are other factors that influence the effectiveness of chemotherapy, so there’ll always be ‘good’ and ‘poor’ responders in both the methylated and unmethylated groups.

TERT promoter mutation test

When found with 1p/19q co-deletion and IDH-mutant biomarkers, the TERT promoter mutation suggests a diagnosis of oligodendroglioma, and predicts greater benefit from chemotherapy and radiotherapy and longer survival, particularly in grade 2 and 3 gliomas.

However, when the TERT mutation is found on its own in grade 2 and 3 gliomas without mutations in the IDH1 or IDH2 gene, it usually predicts poorer survival. This suggests the need for early, more aggressive treatment.