Do papillomaviruses cause feline cutaneous squamous cell carcinoma?

[...]

Appraisal, application and reflection

Feline papillomaviruses (especially Felis catus papillomavirus type 2 – FcaPV-2) have been suspected as a potential oncogenic factor in the development of feline cutaneous squamous cell carcinomas (SCC), similar to human papillomaviruses (HPV) (O'Neill et al., 2011; Murphy, 2013; Oh et al., 2018; Munday & Aberdein, 2012; Altamura et al., 2016b; Geisseler et al., 2016; Munday et al., 2019; and Munday et al., 2008). The aim of this Knowledge Summary was to review the primary evidence available in order to assess the strength of evidence supporting FcaPV-2 involvement in SCC oncogenesis. This may open up a new pathway to reduce the incidence of the SCC cases by preventing PV infections, such as through vaccination (Thomson et al., 2019).

Early studies into this topic focused mainly on demonstrating an association between the presence of PV and SCC lesions (Munday et al., 2008; Munday et al., 2007; O'Neill et al., 2011; and Nespeca et al., 2006). This was primarily achieved through PCR amplification of PV DNA (including FcaPV-2) from SCC lesions (Munday et al., 2008; O'Neill et al., 2011; Munday et al., 2007; and Nespeca et al., 2006). Whilst a significant association was demonstrated between the presence of FcaPV-2 within SCC lesions, causation is not established by this evidence. FcaPV-2 is still isolated from a high proportion of normal and clinically healthy cats (Munday & Kiupel, 2010; Thomson et al., 2016; and Munday et al., 2019). PCR alone cannot differentiate whether the presence of FcaPV-2 in SCC lesions is a contaminant, transient infection or an active carcinogen (Hoggard et al., 2018). Thus, whilst these studies were important to identify that initial significant association, they provided weak evidence to support whether FcaPV-2 is actively oncogenic in the malignant transformation of SCC.

Stronger evidence to support the oncogenic role of FcaPV-2 came from studies that aimed to demonstrate mechanisms that FcaPV-2 utilises in neoplastic transformation. These studies analysed either the effect of FcaPV-2 on host cell tumour suppressor genes and proteins – p16, p53 and retinoblastoma (pRb); and/or the oncogenic and transformative activity of FcaPV-2 viral proteins and E6/E7 oncogenes.

The roles of host cell tumour suppressor genes and proteins – p16, p53 and pRb – were investigated using immunohistochemistry in three studies included in this review (Munday & Aberdein, 2012; Munday et al., 2011a; and Munday et al., 2011b). Increased p16 levels were significantly associated with the presence of PV DNA in SCC lesions (Munday & Aberdein, 2012; Munday et al., 2011b; and Munday et al., 2011a). Additionally, elevated p16 levels were significantly associated with reductions in pRb in PV-infected cutaneous neoplastic lesions (including SCC); and decreases in pRb were significantly associated with presence of PV DNA in cutaneous lesions (Munday & Aberdein, 2012). Interestingly, p53 levels were unaffected in PV-infected cutaneous neoplastic lesions (Munday & Aberdein, 2012).

Accordingly, the transformative mechanisms of FcaPV-2 draw some similarities with HPV in that both PVs influence cell regulation by degrading and inactivating pRb (Munday & Aberdein, 2012; and Munday et al., 2011a). p16 regulates cell division utilising a mechanism that is dependent on pRb, and thus decreased levels of pRb in turn results in increased p16 levels (Munday & Aberdein, 2012; Coleman, 2010; and Parry et al., 1995). Even though p16 is a tumour suppressor protein, elevated levels in cells due to FcaPV-2 degradation of pRb will not cause tumour suppression because the function of p16 is dependent on pRb (Ohtani et al., 2004). However, unlike HPV, p53 is not degraded by FcaPV-2 (Munday & Aberdein, 2012). These studies provided evidence that FcaPV-2 shares a similar transformative pathway to HPV, by disrupting the pRb/p16 cell regulation pathway, with the exception of p53 degradation.

If FcaPV-2 was actively involved in the oncogenesis of SCC, then it would be expected that the virus would be present in the lesion at high viral loads and with active transcription, especially of E6 and E7 genes, because their products can influence neoplastic transformation and cell proliferation (Wilczynski et al., 1998; and Thomson et al., 2016). In the HPV model of oncogenesis, a key event in malignant transformation involves the deregulation of PV E6 and E7 oncogenes, which are normally expressed within the suprabasal epidermal layers (Flores et al., 2000). Overexpression of PV E6/E7 oncogenes in the basal cells results in abnormal cell proliferation and the acquisition of new mutations which can ultimately progress to cancer (Isaacson Wechsler et al., 2012).

Four studies included in this summary aimed to demonstrate how FcaPV-2 proteins and E6/E7 oncogenes are involved in the neoplastic transformation of cells into SCC lesions (Altamura et al., 2016b; Thomson et al., 2016; Altamura et al., 2016a; and Hoggard et al., 2018). These studies provided evidence to support the carcinogenic properties of FcaPV-2. Altamura et al. (2016a) and Altamura et al. (2016b) utilised in vitro models in their studies (specifically cultured feline kidney epithelial cells), which may mean that the results are somewhat challenging to extrapolate to living cats. Furthermore, whilst the studies utilised feline epithelial cells, these cells are independent of the tissue environment that skin epithelial cells would be. As such, the molecular pathways examined in these studies may not accurately represent the molecular pathways in skin epithelial cells and thus provide a moderate level of evidence to support the PICO question.

Thomson et al. (2016) demonstrated that transcriptionally active FcaPV-2 expressing E6/E7 oncogenes was present in one-third of the study’s FFPE SCC sample size which was also significantly associated with increased p16 levels. Continuing on from those findings, Altamura et al. (2016b) showed in vivo that biologically active FcaPV-2 can express the oncogenic viral genes E2, E6 and E7 in FFPE SCC lesions; and the transformative effects of E6/E7 oncogenes in disrupting p53 and pRb pathways in vitro. This seemingly conflicts with findings by Munday & Aberdein (2012) which found that FcaPV-2 does not degrade p53. A completely in vitro study by Altamura et al. (2016a) revealed a difference between the pathogenesis of FcaPV-2 and the HPV model of pathogenesis. FcaPV-2 E6 oncogene enhances the activation of MAPK and Akt pathways independently of EGFR expression, which contrasts with the HPV model of pathogenesis which is EGFR-dependent (Altamura et al., 2016a). MAPK is a key regulator of the Ras-MAPK signalling route which is involved in cellular proliferation, and Akt is involved in the inhibition of apoptosis (Ranieri et al., 2013). Hoggard et al. (2018) demonstrated the presence of E6/E7 mRNA, and unregulated E6/E7 transcription in SCC lesions. This is a similar finding to the HPV model of pathogenesis (Hoggard et al., 2018; and Boscolo-Rizzo et al., 2016).

In conclusion, there is evidence in the literature to say with moderate confidence that FcaPV-2 is actively involved in carcinogenesis and the development of SCC. The pathogenesis of FcaPV-2 and neoplastic transformation processes share many similar characteristics with the HPV model of pathogenesis; however, they are not identical.

[…]