Overexpression of p16^INK4a as an indicator for human papillomavirus oncogenic activity in cervical squamous neoplasia

Overexpression of p16^INK4a has been observed when retinoblastoma protein is inactivated by high-risk human papillomavirus (HPV) oncoprotein E7. We investigated overexpression of p16^INK4a and HPV infection in cervical squamous neoplasia to evaluate the oncogenic potential among various HPV subtypes. The high-risk HPV was detected by PCR in 69.8% (37/53), 97.5% (39/40), 91.7% (44/48), and 100% (16/16) of cervical intraepithelial neoplasia (CIN)1, CIN2, CIN3, and squamous cell carcinoma (SCC), respectively. The p16^INK4a overexpression was investigated immunohistochemically using a p16^INK4a-specific monoclonal antibody (clone E6H4). In high-risk HPV positive cases, 32.4% (12/37) of CIN1, 82.1% (32/39) of CIN2, 93.2% (41/44) of CIN3, and all (16/16) SCC showed p16^INK4a overexpression. The incidence of p16^INK4a overexpression was significantly different between CIN1 and CIN2, suggesting that the disorder of cell cycle regulation by HPV frequently occurred from CIN2. As for CIN1 cases, p16^INK4a overexpression was observed more frequently in HPV16 and HPV52 than in HPV51 and HPV35. Using p16^INK4a as a bio marker of HPV oncogenic activity, we demonstrate that the level of pRb dysfunction by high-risk HPV varied from subtypes and was getting more frequent from CIN2.