Cyclin-dependent kinase-4, CDK4-I, p16INK4a (inhibitor of cyclin-dependent kinase 4A)

CDK4-I is the product of the INK-4 gene. It binds to cyclin 4/6 complexes to control the cell cycle at G1S. In the absence of p16, CDK4 phosphorylates the retinoblastoma gene which in turn releases E2F, inducing DNA synthesis. p16 therefore acts as a tumour suppressor gene.

Immunohistochemical expression

Staining is both nuclear and cytoplasmic.

In the cervix, high-risk human papilloma virus types become integrated into the host genome. This results in increased expression of the E6 and E7 oncogenes. E7 binds to and inactivates the retinoblastoma protein, with resulting increased expression of p16INK4a. There is increased expression of p16 in both high-grade cervical intra-epithelial neoplasia and in low-grade neoplasia associated with high-risk HPV. There is also immunoreactivity in both cervical glandular intra-epithelial neoplasia and cervical adenocarcinoma4. The technique is applicable to ThinPrep cytological specimens5.

Diffuse strong staining for p16INK4a is more sensitive (and more easily performed) than HPV ISH for the detection of high or intermediate-risk HPV in diagnostically challenging atypical squamous epithelium in the cervix.

p16 overexpression identifies HPV-positive vulval squamous carcinoma6.

Ovarian high grade serous carcinoma shows stronger and more extensive staining than do low grade serous carcinoma or serous borderline tumours7:

Positivity for p16 identifies endocervical adenocarcinoma metastatic to the ovary and differentiates from primary ovarian adenocarcinoma and metastases from other sites9:

In most non-small cell carcinomas of lung, inactivation of p16 and/or upregulation of cyclin D1 leads to disruption of the retinoblastoma-mediated cell cycle arrest. Aberrant p16 methylation is an early event in pulmonary non-small cell carcinogenesis, leading to loss of both p16 activity and expression2.

p16 may be helpful in discriminating between benign and malignant uterine smooth muscle tumours:

Diagnostic value

• Identification of both high grade cervical intraepithelial squamous neoplasia and glandular neoplasia.

• Endometrial versus endocervical adenocarcinoma: diffuse positivity favours endocervical carcinoma. Note that benign squamous morules in endometrial adenocarcinoma are commonly positive for p16.

• Identification of HPV-positive endocervical carcinoma metastatic to the ovary.

• Small cell versus poorly differentiated squamous cell carcinoma of lung2.

• In combination with Ki67 in the identification of anal intraepithelial neoplasia.

• Possibly to identify malignancy in uterine smooth muscle tumours.

1 McCluggage, W. G. (2004). "A critical appraisal of the value of immunohistochemistry in diagnosis of uterine neoplasms." Adv Anat Pathol 11(3): 162-71.

2 Zhang H, Liu J, Cagle PT, Allen TC, Laga AC,Zander DS Distinction of pulmonary small cell carcinoma from poorly differentiated squamous cell carcinoma: an immunohistochemical approach. Mod Pathol 2005; 18:111-8

3 Kalof AN, Evans MF, Simmons-Arnold L, et al. p16INK4A immunoexpression and HPV in situ hybridization signal patterns: potential markers of high-grade cervical intraepithelial neoplasia. Am J Surg Pathol 2005; 29:674-9

4 Negri G, Egarter-Vigl E, Kasal A, et al. p16INK4a is a useful marker for the diagnosis of adenocarcinoma of the cervix uteri and its precursors: an immunohistochemical study with immunocytochemical correlations. Am J Surg Pathol 2003; 27:187-93

5 Bibbo M, Klump WJ, DeCecco J, et al. Procedure for immunocytochemical detection of P16INK4A antigen in thin-layer, liquid-based specimens. Acta Cytol 2002; 46:25-9

6 Santos M, Landolfi S, Olivella A, et al. p16 overexpression identifies HPV-positive vulvar squamous cell carcinomas. Am J Surg Pathol 2006; 30:1347-56

7 O'Neill C J, McBride HA, Connolly LE, et al. High-grade ovarian serous carcinoma exhibits significantly higher p16 expression than low-grade serous carcinoma and serous borderline tumour. Histopathology 2007; 50:773-9

8 Kong CS, Balzer BL, Troxell ML, et al. p16INK4A immunohistochemistry is superior to HPV in situ hybridization for the detection of high-risk HPV in atypical squamous metaplasia. Am J Surg Pathol 2007; 31:33-43

9 Vang R, Gown AM, Farinola M, et al. p16 expression in primary ovarian mucinous and endometrioid tumors and metastatic adenocarcinomas in the ovary: utility for identification of metastatic HPV-related endocervical adenocarcinomas. Am J Surg Pathol 2007; 31:653-63

10 Atkins KA, Arronte N, Darus CJ, et al. The Use of p16 in enhancing the histologic classification of uterine smooth muscle tumors. Am J Surg Pathol 2008; 32:98-102

11 Bodner-Adler B, Bodner K, Czerwenka K, et al. Expression of p16 protein in patients with uterine smooth muscle tumors: an immunohistochemical analysis. Gynecol Oncol 2005; 96:62-6

This page last revised 18.5.2008.

©SMUHT/PW Bishop