HER2

What is HER2?

HER2 is a type of protein called a receptor. It helps the cell respond to signals that tell it to grow and divide to create new cells. The protein sits on the outside of a cell and it can be switched on or off by proteins made by other nearby cells. HER2 is made by normal, healthy cells throughout the body. Another name for this protein is Her2/neu.

Cancers that make HER2

Some cancers make extra amounts of HER2 receptor which allows them to grow faster and divide more often than normal cells. Patients with tumours that produce extra HER2 will likely benefit from medications that block the action of HER2 such as trastuzumab.

Cancers that make extra HER2 include:

• Invasive ductal carcinoma of the breast
• Adenocarcinoma of the esophagus
• Adenocarcinoma of the stomach
• Salivary duct carcinoma of the salivary glands

How do pathologists test for HER2?

There are two tests that are commonly performed to measure the amount of HER2 in tumour cells. The first test is called immunohistochemistry and it allows your pathologist to see the HER2 protein on the surface of the cell. This test is given a score of 0 through 3.

HER2 immunohistochemistry score:

• Negative (0 and 1) – A score of 0 or 1 means the tumour cells are not making extra HER2 protein.
• Equivocal (2) – A score of 2 means the cells may be making extra HER2 protein and another test called fluorescence in situ hybridization (see below) will need to be performed to confirm the results.
• Positive (3) – A score of 3 means the cells are making extra HER2 protein.

The second test that is used to measure HER2 is called fluorescence in situ hybridization (FISH). This test is usually only performed after a score of 2 on the immunohistochemistry test. Instead of looking for HER2 on the outside of the cell, FISH uses a probe that sticks to the HER2 gene inside the nucleus of the cell. Normal cells have 2 copies of the HER2 gene in the nucleus of the cell. The purpose of the HER FISH test is to identify tumour cells that have more copies of the HER2 gene which allows them to make more copies of the HER2 protein.

Tumour cells that make extra HER2 will also have more DNA instructions for HER2. Pathologists call this change a translocation.

HER2 FISH score:

• Positive (amplified) – The tumour cells have extra copies of the HER2 gene. These cells are most likely making extra HER2 protein.
• Negative (not amplified) – The tumour cells do not have extra copies of the HER2 gene. These cells are most likely not making extra HER2 protein.