Proliferative Cartilaginous Lesions in the Calcaneal Tendons of huNOG Mice

NOD/Shi-scid/IL-2Rγnull (NOG) mice are humanized with CD34+ hematopoietic cells (huNOG mice) and are commonly utilized for biological or medical research on human therapeutics. In the present study, nine 26-week-old, female huNOG mice were utilized for testing proprietary immune–modulating drugs over a 3-week period at the University of Washington. Two of the 9 mice developed unilateral swelling of a tibiotarsal joint with associated paresis of the affected limb. Full necropsies were performed after euthanasia at experimental end point, and routine tissues and affected tibiotarsal joints were evaluated. An expansile, multilobular mass composed primarily of chondroid cells associated with the calcaneal tendon was present within 1 tibiotarsal joint of both mice. A small focus of well-differentiated bone was present within the mass of 1 mouse. In addition, the calcaneal periosteum was expanded by a chondroid mass in 1 mouse. The cartilaginous masses associated with the calcaneal tendon were interpreted as a hyperplastic or low-grade neoplastic process accompanied by endochondral ossification, and the mass associated with the calcaneal periosteum was interpreted as chondroid metaplasia. Although the etiology of these lesions is unclear, their prevalence in 2/9 (22%) huNOG mice is interesting and may have biological significance for future studies involving the huNOG mouse model.