p16, also designated p16(INK4a) and cyclin-dependent kinase inhibitor 2A (CDKN2A), is a tumor suppressor protein, which in humans is encoded by the CDKN2A gene at chromosome 9p21. p16 plays an important role in regulating the cell cycle. Increased expression of the p16 gene, which is seen as organisms age, reduces the proliferation of stem cells. This reduction in the division and production of stem cells protects against cancer while increasing the risks associated with cellular senescence. When over expressed, the protein is detected both in the nucleus and cytoplasm.
Mutations in the p16 gene - associated with loss or over expression of the protein - are associated with increased risk of a wide range of cancers and cancer precursor lesions. The immunohistochemical identification of p16 is particularly relevant in uterine cervical lesions: Development of dysplasia is closely related to human papilloma virus (HPV) infection. Two viral oncogenes, E6 and E7, both interact with various cell cycle-regulating proteins. Among these is the retinoblastoma gene product pRB, which is inactivated by E7. pRB inhibits transcription of p16 causing an increased immunohistochemical expression. Strong and full thickness staining of p16 in the cervix epithelium is highly supportive of high grade squamous epithelial lesion (HSIL), while weak and intermediate staining favours low grade squamous epithelial lesion (LSIL). Also in cervical adenocarcinoma and adenocarcinoma in situ, a strong p16 expression is usually found.
p16 detection is particularly used in the classification of cervical dysplasia and in the differential diagnosis of cervical adenocarcinoma versus endometrial adenocarcinoma (which is usually p16 negative).
Tonsil appears to be a recommendable positive and negative tissue control. Scattered reticulated crypt epithelial cells must show a moderate to strong nuclear and cytoplasmic staining reaction. Dispersed germinal centre macrophages/dendritic cells must show an at least weak but distinct nuclear and cytoplasmic staining reaction.
No staining reaction should be seen in the vast majority of lymphocytes and normal superficial squamous epithelial cells.
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