Human papillomavirus (HPV) infection is a common sexually transmitted disease, with approximately 50–80% of sexually active adolescents being infected within 2–3 years of initiating intercourse [1]. Most HPV infections are latent by immune regression, while about 10% of the infections are proliferative, which is associated with cervical cancer development [2]. The International Agency for Research on Cancer divided the HPV genotypes into the following groups according to their carcinogenesis: the highly carcinogenic Group 1 (HPVs 16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, and 59); the probably carcinogenic Group 2A (HPV 68); and the possibly carcinogenic Group 2B (HPVs 26, 30, 34, 53, 66, 67, 69, 70, 73, 82, 85, and 97) [3]. Continuous expression of HPV E6 and E7 oncogenes, mainly caused by integration of the HPV genome into the human genome, is critical in cervical cancer progression [4]. HR-HPV E6 and E7 are likely to transform CIN lesion to cancer. Especially, HPV 16 and 18 are the most carcinogenic. The prevalence of HPV 16 and 18 in cervical cancer and cervical intraepithelial neoplasia (CIN) are quite different from other HR-HPV. About 50% and 15% of cervical cancer are positive for HPV 16 and 18, whereas about 40% and 3–7% of high-grade CIN (CIN2/3) are positive, respectively [5, 6]. Other data show that the rate of progression of HPV 16- or 18-infected cervical epithelium to CIN3 or more is around 15% at 10 years post-infection, which is much higher compared to other HR-HPV [7]. Further, HPV 18 is likely to integrate the viral genome into the host genome compared to HPV 16 [8]. Furthermore, there are HPV type-dependent features among cancer histological types. Most HPV 16-positive cancers are squamous cell carcinomas, whereas around 50% of HPV 18-positive cancers are adenocarcinomas [5].
HPV generates numerous viral transcripts via differential RNA splicing. For example, at least 13 transcripts are derived from eight HPV genes in HPV 16-infected W12E cells [9]. There are six genes (E6, E7, E1, E2, E4, and E5) located in the early region of the HPV genome, and two genes (L1 and L2) in the late region. Expression of these genes is altered during epithelial differentiation and/or CIN progression. E6 and E7 are oncogenes encoding proteins that suppress p53 and pRb activation, respectively [10]. E6* is a splicing isoform of E6, which is the main E6 isoform in cervical cancer, and might facilitate E7 expression [11]. Although the roles of E1^E4 are not explicitly defined, E1^E4 is considered to be associated with viral replication [12]. The L1 and L2 proteins are components of the viral capsid and are associated with HPV infection [13].
The expression patterns of HPV-derived transcripts vary depending on CIN grade. For example, the expression of E6 and E7 is higher in high-grade squamous intraepithelial lesions (high-grade SILs) than in low-grade SILs [14]. In contrast, expression of the L1 protein is lower in high-grade SILs [15, 16]. The expression patterns of HPV-derived transcripts also differ among HPV genotypes. For example, among E6 isoforms, HPV 18 cancers exhibit significantly higher ratios of the non-spliced isoform of E6 oncoprotein than HPV 16 cancers [17]. Furthermore, Griffin et al. demonstrated that CIN3 with HPV 18 exhibited no E4 protein expression [18].
In this study, we analyzed each HPV-derived transcript to gain a better understanding of HPV genotype-dependent carcinogenesis. To represent the viral life cycle, we focused on the expression levels of HPV-derived transcripts E6/E6*, E1^E4, and L1. E6/E6* are oncogenes regulated by the early promoter, the E1^E4 splicing site relates to both early and late gene expression and can contribute to viral replication, and L1 expression is observed in the late phase of viral differentiation, which is regulated by the late promoter [19]. In addition to HPV 16 and 18, we focused on HPV 52 and 58, which are highly prevalent in East Asia [20].