Unravelling the Molecular Pathways of Feline Diffuse Iris Melanoma Progression

Feline diffuse iris melanoma (FDIM) is the most common primary ocular tumour in cats, with high metastatic potential. Greater intraocular invasion correlates with increased mortality. No effective therapeutics exist for metastatic disease, partly due to a lack of known molecular targets associated with aggressive tumour behaviour. Here, we define the transcriptomic landscape of FDIM in treatment-naïve cats using bulk RNA sequencing on laser capture microdissection and core biopsy specimens from formalin-fixed paraffin-embedded tissue. Samples included ‘iris melanosis’ (dysplastic melanocytes confined to the anterior iris; n = 7), ‘early FDIM’ (neoplastic melanocytes confined to the iris stroma; n = 13), and ‘late FDIM’ (neoplastic infiltration into the ciliary body and sclera; n = 13). Iris melanosis exhibited genetic overlap with early FDIM, supporting its reclassification as ‘melanoma in situ.’ Early FDIM showed upregulation of genes linked to tumour initiation, immune recruitment, and motility (e.g., STOX1, PEG3, XIAP, MCAM, VIM ). Late FDIM exhibited immune microenvironment remodelling, immune evasion, and apoptosis inhibition (e.g., BIRC2, BIRC5, CCL2, HAVCR2 ), with downregulation of FOX1, FOXC2 , and SOX11 . These results provide critical biomarkers of disease progression, which may aid in the development of more accurate prognostic tests and more effective targeted therapies for FDIM.