p16



p16 is a protein made by normal cells throughout the body. It plays an important role in protecting cells from becoming cancerous, so it is known as a “tumour suppressor” protein. Its job is to help control how cells grow and divide, preventing them from growing uncontrollably.

Where is p16 found in the cell?

After it is made, most of the p16 protein is found in the cell’s nucleus, which is the part of the cell that holds most of our genetic information. Some p16 may also be found in the surrounding part of the cell, called the cytoplasm.

How do pathologists test for p16?

Pathologists use immunohistochemistry (IHC) to look for p16 inside cells. This test involves using special dyes that bind to the p16 protein, making it visible under the microscope. Pathologists can gather important information about the tissue being examined by determining the amount of p16 present and where it is located in the cell.

p16 expression in HPV associated squamous cell carcinoma.

Why do pathologists test for p16?

Pathologists perform IHC for p16 because the results can help confirm the diagnosis of certain types of tumours. In some areas of the body, p16 acts as a “surrogate” marker for human papillomavirus (HPV) infection. HPV infection can cause cells to produce large amounts of p16, so detecting high levels of this protein can indicate the presence of an HPV-related tumour.

Testing for p16 is particularly helpful in diagnosing:

  • Precancerous and cancerous conditions of the cervix: Pathologists look for p16 in high grade squamous intraepithelial lesions (HSIL) and squamous cell carcinoma to help confirm that these conditions are related to HPV.
  • HPV-associated squamous cell carcinoma of the oropharynxThis is a type of throat cancer caused by long-standing infection with HPV, and p16 testing is commonly used to confirm the diagnosis.
  • Other tumour types: p16 testing can also provide helpful information on cancers unrelated to HPV, such as certain tumours in the urinary tract or gastrointestinal system.

What does p16 testing tell us about the tumour?

When a tumour shows strong p16 staining on IHC, it often means the tumour cells are overproducing the p16 protein. In some cancers, this is a sign that the tumour is related to HPV infection. For example, HPV-related cancers of the cervix or oropharynx often have strong, diffuse p16 staining, meaning nearly all the tumour cells show p16. This helps pathologists confirm the diagnosis and provide information that may guide treatment.

For tumours unrelated to HPV, p16 staining can still be helpful in better understanding the tumour’s behaviour and distinguishing it from other types of cancer.

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