Regulation of Human Papillomavirus Type 16 Late L1 mRNA Splicing

Abstract

Human papillomaviruses (HPVs) cause almost half of the human cancers that are attributable to viruses. HPV type 16 is the most carcinogenic type among the HPVs and is detected in 50% of all cervical cancers. HPV-16 infects epithelial cells and HPV-16 gene expression is tightly linked to the differentiation stage of the infected cells. Early HPV-16 genes are expressed in basal layers of the epithelium whereas the late genes which encode highly immunogenic viral structural proteins are only expressed in the suprabasal layers. HPV-16 infections are normally cleared within 18-24 months, but HPV-16 can establish persistent infections that progress to cancer. Such HPV-infected cancer cells express early HPV-16 genes but never expressed the late genes. We speculate that inhibition of HPV-16 late gene expression is a prerequisite for viral persistence and progression to cervical cancer.

HPV-16 uses alternative splicing to regulate expression of early and late genes. HPV RNA elements and cellular factors control the expression level of viral proteins by regulating alternative splicing. This project was carried out to enhance our understanding of the regulation of HPV-16 gene expression, in particular at the RNA splicing level. The goal of this thesis was to identify viral RNA elements and cellular factors that regulate the processing of HPV-16 early and late mRNA splicing. These studies may also contribute to the identification of diagnostic biomarkers for premalignant infections at risk of progressing to cervical cancer.

We identified a splicing silencer that interacts with hnRNP D proteins and hnRNP A2/B1 to suppress HPV-16 late gene expression in mitotic cells, including cervical cancer cells. We also characterized a splicing enhancer that promotes HPV-16 early gene expression, thereby indirectly inhibiting late gene expression. Mutation in this enhancer reduced its binding to the ASF/SF2 splicing factor. This resulted in decreased expression of the viral oncogenes E6 and E7 and a reduced ability of HPV-16 to immortalize human epithelial cells, thereby, linking HPV-16 mRNA splicing regulation to its pathogenic prospects. We also identified the hnRNP G protein binding to this enhancer and has opposite effects to ASF/SF2 on splicing matched by antagonism in RNA binding.

Abstract (Swedish)

Popular Abstract in English

Each year an estimated 530 000 new cases of cervical cancer are diagnosed and more than 275 000 women die from cervical cancer worldwide (2). Cervical cancer is caused by sexually-acquired infections with a subset of human papillomavirus (HPV) (359, 360). HPV is the most common viral infection of the anogenital tract (2). Almost all sexually active individuals will be infected by HPV at least once but probably multiple times during their lives. Most HPV infections do not cause any symptoms or disease as they spontaneously resolve, often within one to two years after infection. However, infections with specific types of HPVs (most frequently type 16 and 18) may persist and lead to precancerous lesions or cancer if they are not appropriately treated.

More than 40 types of HPVs are sexually transmitted and infect the anogenital region. HPV types that infect the genital mucosa are divided into two groups: High-risk (HR) types like HPV-16, 18, 31 and 33 that can cause genital, mouth, or oro-pharynx cancer, and low-risk (LR) types like HPV-6 and 11 that may cause warts, but are not found in malignancies. HR HPV-16 and 18 are the most frequently detected HPV types in cervical cancer (244).

Although vaccines against HR HPV-16 and 18, and LR HPV-6 and 11, are available nowadays, there is a large number of new cases of cervical cancer and deaths each year, especially in low and middle income countries (2). Vaccination is only freely available for girls aged 9-13 years in a few developed countries. Papanicolaou test and liquid-based cytology are two available methods in cervical cancer screening (44, 247). However, there are no efficient biomarkers or medicines available for diagnostic or treatment of HPV infections at risk of progressing to cancer. Therefore, it is important to fully understand the mechanism of HPV persistence in order to uncover novel biomarkers for disease, or targets for antiviral treatment. In this study, we have investigated how HPV-16 late gene expression is regulated at the level of RNA processing. These results will enhance our understanding of the ability of HPV-16 to hide from the immune system by using a highly regulated gene expression program. This is highly significant since establishment of HPV-16 persistence is one of the most important risk-factors for development of cervical cancer.

• author: Li, Xiaoze
• supervisor: Stefan Schwartz; Magnus Abrahamson
• opponent: Prof. Öhman, Marie, Department of Molecular Biology and Functional Genomics. Stockholm University. Sweden
• organization: Division of Medical Microbiology
• publishing date: 2013
• type: Thesis
• publication status: published
• subject: Microbiology in the medical area
• keywords: HPV-16, cervical cancer, RNA processing, alternative splicing, splicing enhancer, splicing silencer, viral persistance, SR proteins, hnRNP proteins
• categories: Higher Education
• in: Lund University Faculty of Medicine Doctoral Dissertation Series
• volume: 2013:133
• pages: 158 pages
• defense location: BMC, I1346, Lund University
• defense date: 2013-12-11 09:00:00
• ISSN: 1652-8220
• ISBN: 978-91-87651-08-3
• language: English
• LU publication?: yes
• id: bbd425df-9de5-4605-a1d0-48e5f1c24f46 (old id 4153601)
• date added to LUP: 2016-04-01 14:07:01
• date last changed: 2023-04-18 20:22:09