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Feline oral squamous cell carcinoma (FOSCC) is the most common oral cancer in cats. This cancer is very aggressive and has the ability to invade into the adjacent bone resulting in poor quality of life and a grave prognosis in affected cats. FOSCC has been shown to be comparable to human OSCC in many aspects. However, the pathogenesis of FOSCC remains unclear. Similar to human OSCC, FOSCCs respond poorly to the available standard therapy. Therefore, novel molecular targets as well as effective therapies are required for this cancer. The usefulness of p16, p53, and pRb immunohistochemistry (IHC) for human OSCC classification has been reported. Low p16 frequently associates with non-viral-associated OSCCs, while high p16 with low p53 and pRb usually occurs in HPV-associated OSCC. In addition, a favorable prognosis was found in patients with HPV-associated OSCC and a grave prognosis was observed in patients with non-viral-associated OSCC. In this study, the IHC pattern of p16, p53, and pRb in FOSCC was investigated. We found that low p16 IHC is common in FOSCCs and high p16 IHC is present in a subset of patients with this cancer. However feline papillomavirus was not detected in these samples. These results indicate that FOSCCs have more than one type of classification and a variation in pathogenesis may occur in FOSCC.Increased osteoprotegerin (OPG) has been found to reduce tumor size and bone invasion in many human bone-invasive cancers. Low levels of feline OPG (fOPG) mRNA expression were previously observed in a bone-invasive FOSCC cell line (SCCF2). The role of fOPG in bone-invasive FOSCC in vitro and in vivo was investigated using a novel bone-invasive FOSCC cell line with high fOPG mRNA expression (SCCF2-OPGv.1).We found that overexpression of fOPG reduced tumor growth and inhibited bone invasion in bone-invasive FOSCC in vivo. Thus, fOPG could be effective adjuvant therapy for FOSCC-associated bone lysis. Telomerase, a protein complex with cell immortalization activity, has been speculated as a novel therapeutic target for human cancers. Telomerase transcriptase catalytic subunit (TERT) and the non-coding RNA template are the main components of this protein. Pre-mRNA alternative splicing occurs in human TERT gene and more than 20 human TERT alternative splice variants were identified. In this study, high levels of telomerase activity were observed in FOSCC cells (SCCF1, SCCF2, and SCCF3) and in 2 of 3 FOSCC tumors. Full-length and 10 novel feline TERT (fTERT) alternative splice variants were identified. Full-length and the deletion of exon 10 (Del-e10) splice variant of fTERT were the two common variants in FOSCCs with high telomerase activity found in this study. These findings suggest that alternative splicing is important for the regulation of telomerase activity and provide critical information to further the understanding of telomerase in feline cancers.

Feline oral squamous cell carcinoma (FOSCC) is an aggressive head and neck cancer in cats. The survival rate for FOSCC is less than one year and little is known about the mechanisms that induce this cancer. In humans, head and neck squamous cell carcinoma (HNSCC) can be categorized as human papillomavirus (HPV)-induced and non-HPV- induced HNSCC. Non-HPV HNSCCs are associated with mutated tumor suppressor genes, especially TP53 and p16 (CDKN2A/INK4A). Immunohistochemistry (IHC) for p16 was reported to be an effective tool to classify HNSCC. Increased p16 with decreased p53 and pRb immunoreactivity were suspected to be markers of HPV-induced SCC, while decreased p16 was reported to correlate with mutations of the CDKN2A gene in non-HPV- induced HNSCC. The roles of p16 and PVs have not been investigated in FOSCC. In this study, we searched for feline domestic papillomavirus type II (FdPV2) and other PV DNA and measured p16, p53, and pRb immunoreactivity in FOSCCs. Expression of p16 mRNA was measured in 3 FOSCC cell lines (F1, F2, and F3) by quantitative reverse transcriptase PCR (Q-RT-PCR). Four published primers were used to detect PV DNA in three FOSCC cell lines and one sample from a feline Bowenoid in situ carcinoma (FBISC) was used as a positive control. Immunoreactivity for p16, p53, and pRb were also examined in 43 FOSCC paraffin-embedded biopsy specimens, three FOSCC cell line xenografts, and one FBISC. We found that 13% (6/43) of FOSCC samples had increased p16 protein for IHC, while 40% (17/43) and 46% (20/43)iiiexpressed low and moderate levels, respectively. Almost all FOSCC samples classified as high intensity immunostaining for p16 protein had low p53 IHC staining with moderate pRb immunoreactivity. A high level of p16 mRNA level was demonstrated in the F1 cell line, which also expressed increased p16 protein by IHC. In contrast, decreased expression of p16 immunoreactivity was found in the F2 and F3 cell lines, which had low levels of p16 mRNA. PV DNA was not amplified from the F1, F2, or F3 cells. These data indicate that p16 mRNA levels in FOSCC correlated well with p16 immunoreactivity, but the pattern of FOSCC immunoreactivity for p16, p53, and pRb in FOSCC was different compared to HNSCC. The RT-PCR sequences amplified from SCCF1 and normal cat gingiva had 100% homology to the predicted transcript of feline p16 cDNA. The sequence of the L1 gene was not identified by PCR in FOSCC using consensus or FdPV2-specific sequences.