Characterization of HPV16 oncogenes : genetic variation, localization and transactivation function

• Main Author: Kulthida Vaeteewoottacharn
• Other Authors: Mathurose Ponglikitmongkol
• Language: English
• Published: Mahidol University. Mahidol University Library and Knowledge Center 2023
• Subjects: Cervix Neoplasms; Oncogenes; Papillomavirus Human; Trans-Activation (Genetics); Variation (Genetics)
• Online Access: https://repository.li.mahidol.ac.th/handle/123456789/88694
• Institution: Mahidol University

Cervical cancer is the most common cancer in Thai women. Of the causes of cervical cancer, an infection of HPV16 ranks highest in clinical samples. Overexpression of E6 and E7 oncogenes following viral integration results in the interference of host homeostasis. The functions of viral proteins in modulating apoptosis leading to drug resistance are of special interest as it affects the treatment outcome. HPV infection in 82 Thai invasive cervical cancers was investigated. HPV16 was the most common type, found in 57.3% of the samples. Characterization of HPV16 E6 sequences grouped the variations into 5 subtypes, of which the most prevalent, found in 58.1% of samples, was an alteration of T to G at nucleotide position 178. Identification of E7 mutations showed that the mutation at nucleotide position 647, a change of A to G, was the most frequent. Interestingly, co-mutation of these two most common mutants of E6 and E7 was predominantly found in cervical cancers that were studied suggesting its functional significance. Notably, this co-mutation was first reported in this study. The effect of HPV16 oncoproteins on the transcription activation of an antiapoptotic gene, Bcl-xL, was also studied. No clear correlations were observed between the expression of HPV16 oncogenes and Bcl-xL gene in transiently transfected cells. However, when the assay was performed in C33a cells stably transfected with HPV16 oncogenes; E6*I (splice form of E6), E6 or E7 gene, the increases of both Bcl-xL transcript and protein were evident in all stable cells expressing HPV16 oncogenes. Yet, the increased level of Bcl-xL protein could not rescue the transfected cells from apoptosis induced by etoposide. Study on protein localization revealed that E6 was dispersed throughout cells, but was predominantly found in the nucleus. A similar result was seen with E6*I whereas E6MT (unsplice variant of E6) localized mainly in the nucleus. None of these HPV oncoproteins were found in the nucleoli. Co-localizations of E6 and E6AP (E6 associated protein; ubiquitin ligase enzyme) as well as E6*I and E6AP were mainly in the cytoplasm, rarely in the nucleus. On the other hand, E6MT rarely co-localized with E6AP in the nucleus and perinuclear region. These differential localizations implied a role of E6*I protein in interfering with E6-E6AP binding. Taken together, genetic variation, transcription function and protein localization of HPV oncogenes have been described in this study. The results indicate the importance of co-mutation variant of E6 and E7 genes and the functional relevance of E6*I protein in modulating host homeostasis.