1p/19q Codeletion: Definition

1p/19q codeletion is a specific type of genetic change found in a kind of brain tumor called oligodendroglioma. The term “1p/19q codeletion” means that a tumor cell has lost part of two chromosomes—the short arm of chromosome 1 (called 1p) and the long arm of chromosome 19 (called 19q). This genetic change helps pathologists confirm the diagnosis of oligodendroglioma, especially when combined with a mutation in the IDH1 or IDH2 gene.

Why is 1p/19q codeletion important?

1p/19q codeletion plays an important role in the diagnosis, prognosis, and treatment of brain tumors:

• It is required to make the diagnosis of oligodendroglioma, IDH-mutant and 1p/19q-codeleted, which is a specific type of tumor recognized by the World Health Organization.

• Tumors with this genetic change usually grow more slowly and respond better to treatment than those without the codeletion.

• It helps doctors select the most appropriate therapies, especially chemotherapy.

How do pathologists test for 1p/19q codeletion?

Pathologists use molecular tests to look for the 1p/19q codeletion in tumor tissue.

The most common tests include:

• Fluorescence in situ hybridization (FISH): This test uses fluorescent markers to detect the presence or absence of DNA in specific areas of chromosomes 1 and 19. It is often used because it works on routine pathology slides. A positive result means the 1p and 19q parts are missing (codeleted).
• Polymerase chain reaction-based loss of heterozygosity (PCR-LOH): This method compares the DNA from the tumor to normal tissue to look for missing regions on chromosomes 1p and 19q. It is very accurate and widely used.
• Next-generation sequencing (NGS) and SNP array testing: These newer tests analyze many genes and chromosomes at once. They are increasingly used because they can detect codeletions and many other changes in a single test.
• Methylation arrays: Primarily used to classify brain tumors based on their DNA methylation patterns, these arrays can also detect chromosome changes such as 1p/19q codeletion.

What does it mean if my tumor has a 1p/19q codeletion?

If your pathology report says your tumor has a 1p/19q codeletion, it supports a diagnosis of oligodendroglioma. This type of tumor generally has a better outcome compared to similar tumors without the codeletion. It also suggests that your tumor may respond well to specific types of chemotherapy, such as temozolomide.

What is the difference between absolute and relative codeletion?

Most tumors with 1p/19q codeletion lose both chromosome arms completely. This is called an absolute codeletion. In other cases, extra copies of chromosomes may be present, making it look like a partial loss—this is called a relative codeletion. Absolute codeletions are more strongly associated with better outcomes.

What types of tumors are tested for 1p/19q codeletion?

1p/19q codeletion testing is done almost exclusively on gliomas, especially those suspected to be oligodendrogliomas. It is not typically used for other types of brain tumors or cancers in other parts of the body.

Sample pathology report

• Brain, right frontal lobe, biopsy: Oligodendroglioma, IDH-mutant and 1p/19q-codeleted, WHO grade 2
• Comment: Molecular testing confirms the presence of an IDH1 mutation and a 1p/19q codeletion. These findings support the diagnosis of oligodendroglioma. This type of tumor generally has a favorable prognosis and may respond well to chemotherapy and radiation.

Questions to ask your doctor

• What type of brain tumor do I have?

• Does my tumor have a 1p/19q codeletion?

• How does this result affect my treatment options?

• Should I also be tested for an IDH mutation?

• What are the benefits and limitations of the testing method used?