The Role of Ataxia Telangiectasia Mutant and Rad3-Related DNA Damage Response in Pathogenesis of Human Papillomavirus

Abstract

Human papillomavirus (HPV) infection leads to a variety of benign lesions and malignant tumors such as cervical cancer and head and neck squamous cell carcinoma. Several HPV vaccines have been developed that can help to prevent cervical carcinoma, but these vaccines are only effective in individuals with no prior HPV infection. Thus, it is still important to understand the HPV life cycle and in particular the association of HPV with human pathogenesis. HPV production requires activation of the DNA damage response (DDR), which is a complex signaling network composed of multiple sensors, mediators, transducers, and effectors that safeguard cellular DNAs to maintain the host genome integrity. In this review, we focus on the roles of the ataxia telangiectasia mutant and Rad3-related (ATR) DNA damage response in HPV DNA replication. HPV can induce ATR expression and activate the ATR pathway. Inhibition of the ATR pathway results in suppression of HPV genome maintenance and amplification. The mechanisms underlying this could be through various molecular pathways such as checkpoint signaling and transcriptional regulation. In light of these findings, other downstream mechanisms of the ATR pathway need to be further investigated for better understanding HPV pathogenesis and developing novel ATR DDR-related inhibitors against HPV infection.

Keywords: DNA damage response (DDR); HPV DNA replication; Human papillomavirus (HPV); cervical cancer; genome amplification; life cycle; the ATR pathway.

Figure 1

The schematic HPV 31 genome structure. The linearized HPV 31 genome is 8000 bp in size and encodes gene products in three regions: the early, late, and upstream regulatory regions (URR). The genes on the early region encode E1, E2, E4, E5, E6, and E7. The late genes represent capsid proteins L1 and L2. The URR contains the replication starting point and transcription factor binding sites. HPV31 has two well-characterized promoter elements known as early promoter (p97) and late promoter (p742). poly AE and poly AL indicate the positions of polyadenylation early and late sites shown as arrows.

Figure 2

The schematic illustration of the HPV life cycle. Epithelial differentiation processes are compared between normal and HPV-infected keratinocytes. HPVs infect keratinocytes in the basal layer when wounded. Early genes E1, E2, E6, E7 are expressed at low levels for genome maintenance and cell transformation. The viral genome is maintained at the basal layer. Upon differentiation, HPV late genes such as E1, E4^E5, L1, and L2 are expressed. The viral genomes are greatly amplified, assembled, and newly synthesized virions are released from the uppermost layers of epithelium.

Figure 3

The schematic overview of the classical ATM and ATR DDR. The ATM/CHK2 pathway is activated by double-strand breaks, while ATR/CHK1 pathway is induced by single-strand breaks or replication stress. Activated ATM or ATR phosphorylates critical targets such as p53, CHK2, and CHK1, which are essential for regulating cell cycle checkpoints. Activated p53 leads to G1-phase cell cycle arrest and induces apoptosis. Phosphorylation of Cdc25 mediated by CHK2 and CHK1 results in cell cycle arrest in either S phase or G2/M phase. FANCD2 and FANCI, as the key components of the Fanconi anemia (FA) pathway, can be activated by ATM or ATR. In addition, activation of the ATM and ATR pathways play critical roles in regulation of DNA repair, apoptosis, and replication fork stability.

Figure 4

The ATR signaling pathway is involved in the HPV life cycle. HPV phosphorylates STAT-5 in the absence of exogenous DNA damage agents to increase TopBP1 levels, which activate the ATR signaling pathway. HPV also phosphorylates TopBP1 to regulate E2F1 transcriptional activities, which is important for HPV replication. ATR activation is necessary for HPV genomes replication through various mechanisms. The ATR/CHK1 DDR can regulate cell cycle arrest through E2F1 in HPV-positive cells. ATR possibly modulates immune response by GATA4-driven inflammation. ATR is also suggested to be responsible for induction of autophagy, which correlates with the GATA4 signaling via autophagy cargo protein p62. The ATR/CHK1 pathway can act with RRM2 to promote HPV DNA synthesis. The solid arrow means positive regulation, whereas the blocking arrow signifies inhibition. The dotted line represents that the role of the pathway remains uncertain.