Synonyms: CDKN2a, p16INK4a, Cyclin-dependent kinase inhibitor 2A, CDKN2A, Multiple tumor suppressor 1

by Jan Klos

Background

The CDKN2a (p16) gene is located on is located on the short arm of chromosome 9 (9p21) and encodes for the protein with the same name. P16 protein functions as an inhibitor of Cyclin-Dependent Kinases (CDK). This is important mechanism regulating of the cell cycle. When inactive complex of P16 and CDK4/6 dissociates, the free CDK4/6 molecule binds to Cyclin D and forms an active complex capable to phosphorylate the retinoblastoma protein (pRb). The phosphorylated pRb dissociates from the complex med transcription factor E2F1. Free E2F1 protein enters the cell nucleus and activates the transcription of target genes essential for transcription from G1 to S phase. So, p16 is acting as tumor suppressor by slowing down the progression of the cell cycle from the G1 phase to the S phase. Insufficient amount/lack of p16 protein as a result of gene deletion or mutation of P16 gene resulting in non-functional p16 protein, leads to acceleration of the cell cycle and this mechanism is observed in many types of cancer.
Human papilloma virus (HPV) is an important pathogenetic factor behind the development of dysplasia in uterine cervix and subsequent carcinoma. The viral oncogenes, E6 and E7, interact with various cell cycle-regulating proteins, leading to inactivation of retinoblastoma protein (pRB) which controls transcription of p16. Missing effect of pRB leads to increased production and accumulation of p16 in nucleus and cytoplasm.

Positive staining in normal cells

Control tissue: Scattered germinal center macrophages should a distinct nuclear and cytoplasmic staining

Positive staining in tumors

a number of different benign and malignant tumors show positive staining being result of different oncogenic mechanism. Only in certain types like: squamous and glandular dysplasia in uterine cervix  oropharyngeal squamous cell carcinoma, cervical cancer including neuroendocrine cervical carcinoma  (86% HPV+ and p16+ very good correlation) and esophageal cancer the staining is associated with HPV infection.

Staining pattern

Nuclear and cytoplasmic.

Application

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 favors low grade squamous epithelial lesion (LSIL).
Strong p16 expression is usually found in squamous cell carcinoma of uterine cervix, all subtypes of endocervical adenocarcinoma and adenocarcinoma in situ. Commonly the expression of P16 is associated with infection with oncogenic HPV types but p16 positivity does not correlate with HPV status in cases of non-usual type of endocervical carcinoma (clear cell carcinoma, gastric-type adenocarcinoma, minimal deviation adenocarcinoma, mesonephric adenocarcinoma, serous adenocarcinoma).
p16 can be used as a biomarker to improve the histological diagnostic accuracy of  cervical intraepithelial neoplasia grade 3 (CIN3)
Melanocytic. The immunohistochemical identification of p16 is particularly relevant … P16 is also implicated in the protection against development of melanoma,
ØNH cancers…………….
Mesothelioma: The detection of CDKN2a (p16) homozygous deletion by FISH analysis may be performed on FFPE and is observed in more than 40% of epithelioid mesotheliomas, compared with sarcomatoid mesotheliomas, while immunohistochemistry for P16 is not conclusive in cases of mesothelioma. Results of FISH for P16 are often used in combination with immunohistochemistry for BAP1.
No staining as a rule is reported in adenocarcinoma of appendix 0% (7 cases), adenocarcinoma of pancreatobiliary tract 0% (17 cases), adenolymphoma 0% (12 cases), adenomyoma, of the uterus  0%  (1 case), angiolipoma, NOS 0% (4 cases)
Branchial Cleft Cysts  0%  105 Carcinoid Tumor, Intestine 0%  35 Carcinoma, Adenosquamous, Pancreas 0% 8
Carcinoma, Adenosquamous, Penis  0%  1
Carcinoma, Squamous Cell, Bladder 0%  4
Carcinoma, undifferentiated, ENT  0%  2
Chronic Sinusitis 0%  32
Dysplasia, Gallbladder 0% 2
Malignant Lymphoma, Plasmablastic, AIDS-Related 0% 8
Myeloma, Plasmablastic Plasma Cell 0% 5
Neuroendocrine Tumor  0% 55
Normal Anal Transition Zone 0% 34
Normal Fallopian Tube Epithelium 0% 31
Normal Squamous Cells, Exocervix 0%  10
Pleomorphic Liposarcoma  0%  2
Reactive Endocervical Glands  0% 15
Squamous Papilloma 0% 8
Tunnel Clusters, Cervical

Selected references

1. Alejo M¹, Alemany L², Clavero O³, et al. Contribution of Human papillomavirus in neuroendocrine tumors from a series of 10,575 invasive cervical cancer cases. Papillomavirus Res. 2018 Jun;5:134-142. doi: 10.1016/j.pvr.2018.03.005. Epub 2018 Mar 17. Dabbs DJ. Diagnostic Immunohistochemistry: Theranostic and Genomic Applications. 5th edition, 2018
2. https://en.wikipedia.org/wiki/P16
3. McCluggage WG¹, Kennedy K, Busam KJ. An immunohistochemical study of cervical neuroendocrine carcinomas: Neoplasms that are commonly TTF1 positive and which may express CK20 and P63. Am J Surg Pathol. 2010 Apr;34(4):525-32. doi: 10.1097/PAS.0b013e3181d1d457.
4. Park KJ¹, Kiyokawa T, Soslow RA, et al. Unusual endocervical adenocarcinomas: an immunohistochemical analysis with molecular detection of human papillomavirus. Am J Surg Pathol. 2011 May;35(5):633-46. doi: 10.1097/PAS.0b013e31821534b9.