HPV16 E7 Nucleotide Variants Found in Cancer-Free Subjects Affect E7 Protein Expression and Transformation

SIMPLE SUMMARY: Human papillomaviruses (HPV) are one of the deadliest viruses causing cancer resulting in over 300,000 deaths annually in the world. HPV16 is the most oncogenic form of HPV and is responsible for over half of all HPV-driven tumors. The viral E7 gene encodes a powerful oncoprotein that can cause infected cells to have uncontrolled growth. A previous study showed that HPV16 isolates with amino acid alterations in the E7 gene were nearly always found in women with HPV infection but no cancer. To study the role of these variants, we expressed them in cultured cells. Most variants produced lower levels of protein, and selected variants had reduced activity in allowing cells to grow in assays that measure tumor cell invasiveness. The data indicate that inhibition of E7 could be a practical approach to treating HPV-driven cancers. ABSTRACT: The human papillomavirus (HPV) type 16 E7 oncogene is critical to carcinogenesis and highly conserved. Previous studies identified a preponderance of non-synonymous E7 variants amongst HPV16-positive cancer-free controls compared to those with cervical cancer. To investigate the function of E7 variants, we constructed full-length HPV16 E7 genes and tested variants at positions H9R, D21N, N29S, E33K, T56I, D62N, S63F, S63P, T64M, E80K, D81N, P92L, and P92S (found only in controls); D14E, N29H cervical intraepithelial neoplasia (CIN2), and P6L, H51N, R77S (CIN3). We determined the steady-state level of cytoplasmic and nuclear HPV16 E7 protein. All variants from controls showed a reduced level of E7 protein, with 7/13 variants having lower protein levels. In contrast, 2/3 variants from the CIN3 precancer group had near-wild type E7 levels. We assayed the activity of representative variants in stably transfected NIH3T3 cells. The H9R, E33K, P92L, and P92S variants found in control subjects had lower transforming activity than D14E and N29H variants (CIN2), and the R77S (CIN3) had activity only slightly reduced from wild-type E7. In addition, R77S and WT E7 caused increased migration of NIH3T3 cells in a wound-healing assay compared with H9R, E33K, P92L, and P92S (controls) and D14E (CIN2). These data provide evidence that the E7 variants found in HPV16-positive cancer-free women are partially defective for transformation and cell migration, further demonstrating the importance of fully active E7 in cancer development.