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FANCD2 Binds Human Papillomavirus Genomes and Associates with a Distinct Set of DNA Repair Proteins to Regulate Viral Replication

2017-02-14 13:08:49

mBio;February 14 2017: /DOI:10.1128/mBio.02340-16



Chelsey C. Spriggs, Laimonis A. Laimins



Abstract


The life cycle of human papillomavirus (HPV) is dependent on the differentiation state of its host cell. HPV genomes are maintained as low-copy episomes in basal epithelial cells and amplified to thousands of copies per cell in differentiated layers. Replication of high-risk HPVs requires the activation of the ataxia telangiectasia-mutated (ATM) and ATM and Rad3-related (ATR) DNA repair pathways. The Fanconi anemia (FA) pathway is a part of the DNA damage response and mediates cross talk between the ATM and ATR pathways. Our studies show that HPV activates the FA pathway, leading to the accumulation of a key regulatory protein, FANCD2, in large nuclear foci. These HPV-dependent foci colocalize with a distinct population of DNA repair proteins, including ATM components γH2AX and BRCA1, but infrequently with p-SMC1, which is required for viral genome amplification in differentiated cells. Furthermore, FANCD2 is found at viral replication foci, where it is preferentially recruited to viral genomes compared to cellular chromosomes and is required for maintenance of HPV episomes in undifferentiated cells. These findings identify FANCD2 as an important regulator of HPV replication and provide insight into the role of the DNA damage response in the differentiation-dependent life cycle of HPV.



Introduction


Human papillomaviruses (HPVs) are the causative agents of cervical cancer along with most anogenital and many oropharyngeal cancers. Over 200 types of HPV have been identified, and approximately 10 of these, including types 16, 18, and 31, are referred to as high risk due to their association with the development of cancers. HPVs infect the basal layer of stratified epithelia and establish their double-stranded DNA genomes as nuclear episomes at approximately 100 copies per cell. Upon epithelial differentiation, HPV-infected cells override cell cycle checkpoint controls to reenter S/G2 phase and amplify their genomes to thousands of copies per cell. HPV genomes are approximately 8 kb in size and encode eight open reading frames. In infected basal cells, early gene expression is controlled by the p97 promoter, which is regulated by viral and cellular factors through binding at sequences in the viral upstream regulatory region (URR). The early promoter directs transcription of polycistronic messages that encode proteins that contribute to the stable maintenance of HPV genomes, including the E1 and E2 replication proteins and the E6 and E7 viral oncoproteins. The late promoter, p742, is activated upon differentiation and controls expression of the L1 and L2 capsid proteins along with E1, E1^E4, E2, and E5, which are involved in regulating genome amplification and late gene expression.



The productive life cycle of HPV is dependent upon activation of both the ataxia-telangiectasia mutated (ATM) and the ATM and Rad3-related (ATR) DNA repair pathways. The ATM pathway is activated in response to DNA double-stranded breaks, while ATR responds to replication stress and the presence of single-stranded DNA at stalled replication forks. High-risk HPVs have been shown to selectively activate and repress components of these signaling pathways to promote viral replication; however, which members of these pathways are involved in regulating episomal maintenance as well as differentiation-dependent genome amplification is still not fully understood.




Empire Genomic's Hybridization Buffer was used in this publication.



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