In Situ Hybridization of Feline Leukemia Virus in a Case of Osteochondromatosis

Abstract

Osteochondromatosis, also known as multiple cartilaginous exostosis, polyostotic osteochondroma, and multiple osteochondromas, comprises one-fifth of all primary bone tumors in cats, with no breed or sex predisposition or hereditary pattern. Unlike in dogs, horses, and humans, it is predominantly seen in young cats (2-4 years old), after the maturation of the skeleton. Although the pathogenesis of osteochondromatosis is not fully understood, it is considered to be related to infection by feline leukemia virus (FeLV) or other retroviruses, such as the feline sarcoma virus. However, the presence of viral particles within tumor lesions has only been demonstrated by electron microscopy. The malignant transformation of osteochondromas, most typically to osteosarcoma or chondrosarcoma, has also been attributed to the viral infection. Here we report the case of osteochondromatosis in a 3.5-year-old male domestic European shorthair cat with concurrent FeLV infection confirmed by polymerase chain reaction. Viral RNA was visualized in representative tissues (spleen, mesenteric lymph node, liver, kidney, lung, brain) and in the osteochondromas with RNAscope in situ hybridization, which supports that FeLV infection may be involved in the pathogenesis of osteochondromatosis.

Keywords: FeLV; feline; in situ hybridization; osteochondromatosis.

Figure 1

Radiographic imaging showed calcified, moderately demarcated tumorous lesions (asterisks) protruding into the thoracic cavity, deforming normal skeletal structures.

Figure 2

Lesions in osteochondromatosis: (A) tumors originating from bones (vertebrae and ribs); (B) cut surface of the same lesion showing mostly osseus substance, but also gray-to-tan cartilaginous tissue could be observed.

Figure 3

Histopathologic picture of feline osteochondromatosis (hematoxylin and eosin staining, 100× magnification, bar = 200 µm). Trabeculae of bone (arrows) are surrounded by islands of cartilage (asterisks).

Figure 4

Tissue samples from a cat euthanized due to osteochondromatosis, FeLV ISH (magnification 100× (A,C), bar = 200 µm; 50× (B,D); bar = 500 µm). Red chromogen precipitates can be seen in (A) liver, (B) spleen, (C) brain, and (D) mesenteric lymph node tissues, in FeLV-infected cells.

Figure 5

Tumor sample from a cat euthanized due to osteochondromatosis, FeLV ISH, magnification 400×, bar = 50 µm. Arrows show two positively labeled cells (presumably osteoblasts) in the tumor. Inset: same cells with higher magnification: red chromogen precipitates can be seen, showing the presence of FeLV RNA. (Magnification 1000×, bar = 20 µm).