Degradation of the retinoblastoma tumor suppressor protein by the human papillomavirus-16 E7 variants

Human papillomavirus type 16 (HPV-16) plays an etiological role in benign and malignant epithelial tumors. A critical event in HPV transformation of human cells is the inactivation of retinoblastoma protein (pRB) by the E7 protein. The metabolic half-life of pRB is decreased in cells that express high-risk HPV E7 proteins. The present study investigated the frequency of HPV-16 E7 variants in Korean women and compared the pRB degradation activity of E7 variant proteins. Of the 40 HPV-positive specimens from a total of 91 tissue specimens, 21 HPV-16 positive specimens were studied by sequencing analysis to determine the variation of E7 gene. The most frequent E7 variant was N29S (57%). The HPV-16 E7 variant was more prevalent in invasive cervical cancer tissue specimens than in those from low grade clinical stage. The degradation of pRB in HaCaT cells by HPV-16 E7 variant proteins was investigated by western blot analysis. There was no significant difference in pRB degradation activity between the HPV-16 E7 prototype protein and E7 variant proteins. The pRB degradation activity did not differ among HPV-16 E7 variants. These results suggest that the E7-rnduced degradation of pRB is important in cervical tumorigenesis; however, there was no relation between the pRB degradation activity and the variations in HPV-16 E7 protein among Korean women.