Equine genital and ocular squamous cell carcinomas: clinical, histopathological, molecular and viral characterization with proposed histopathological classification system.

Overview

• This study characterizes different types of equine squamous cell carcinomas (eSCCs) and explores their relationships with viral infections, molecular markers, and clinical outcomes to improve diagnosis and prognosis.
• It establishes a histopathological classification system and identifies potential prognostic indicators for equine genital and ocular SCCs.

Introduction and Background

• Equine squamous cell carcinomas (eSCCs) are common tumors that affect the genital and ocular regions of horses.
• A subset of these cancers may be induced by equine papillomavirus type 2 (EcPV-2), which is known to play a role in viral oncogenesis.
• Currently, there is no recognized system for histopathological classification of eSCCs, and predicting clinical outcomes remains challenging.
• Understanding the tumor subtypes, molecular characteristics, and viral status could help identify prognostic markers and improve treatment strategies.

Study Aims

• Histopathologically subtype a large series of equine genital and ocular SCCs.
• Correlate tumor subtypes with expression of p16 and HER-2 proteins, presence of EcPV-2 infection, and clinical and histopathological features.
• Develop prognostic criteria based on these correlations to better predict tumor behavior and survival outcomes.
• Compare findings with human SCC classifications to explore parallels and differences.

Methods

• 185 equine SCC tissue samples were histologically examined and classified.
• Immunohistochemistry (IHC) was performed to detect p16 and HER-2 protein expression on select cases.
• In situ hybridization (ISH) targeted EcPV-2 E6/E7 oncogenes to assess viral presence within tumors.
• Follow-up clinical data including survival were collected and analyzed to assess prognosis.
• Histological subtypes were compared to human squamous cell carcinoma categorizations established by the WHO.

Results: Histopathological Subtypes

• Six distinct histopathological subtypes of eSCC were identified:
• Usual/invasive (most common subtype)
• Verrucous
• Pseudoglandular
• Papillary
• Warty
• Basaloid
• This classification, based on WHO human SCC guidelines, was reported for the first time in horses.
• Different subtypes showed distinct histological features and varied prognostic significance, indicating that subtype classification could inform clinical decision-making.

Results: Molecular and Viral Findings

• EcPV-2 DNA was detected by ISH in the majority of cases, confirming a frequent association with eSCCs.
• Notably, the papillary subtype lacked detectable EcPV-2 genetic material, suggesting these may include mature rather than pre-cancerous lesions.
• HER-2 protein was widely expressed across different tumor subtypes, although this expression was not linked to prognosis.
• p16 expression did not correlate with EcPV-2 infection status but showed potential as a prognostic marker worth further investigation.

Clinical Implications

• The identification of histopathological subtypes provides a foundation for improved prognostication of eSCCs.
• Widespread HER-2 expression suggests this receptor could be a therapeutic target in equine SCC, similar to some human cancers.
• p16 may serve as a prognostic biomarker independent of EcPV-2 status, which could help stratify equine tumors by aggressiveness.
• Papillary eSCCs lacking viral DNA highlight the need to consider subtype-specific molecular pathways in diagnosis and treatment.

Conclusion

• This study successfully classified equine genital and ocular SCCs into six histopathological subtypes using criteria adapted from human oncology.
• It demonstrated a strong association between EcPV-2 and most eSCC types, except papillary tumors.
• Although HER-2 and EcPV-2 status were not linked to prognosis, p16 expression showed promise as a prognostic factor.
• The findings pave the way for tailored diagnostic and treatment approaches for horses with SCC, including exploration of HER-2 targeted therapies.
• Overall, the proposed histopathological classification system improves understanding of eSCC and guides future research into molecular drivers and clinical management.