Synovial Pathology in Camptodactyly-Arthropathy-Coxa Vara-Pericarditis Syndrome

At least 25 families with camptodactyly-arthropathy-coxa vara-pericarditis (CACP syndrome) have been reported, with descriptions of a distinctive synovial pathology based largely on light microscopy. Although described as “proliferative,” with numerous multinucleated giant cells, the natures of proliferating cells and giant cells have not been determined. To clarify the pathogenesis of this disorder, we studied 3 patients who had CACP syndrome and underwent synovial biopsy. Cells in the biopsies were studied by immunohistochemistry and electron microscopy. Giant cells were identified as macrophage in origin based on CD68 expression and electron microscopic features of macrophages. Most cells in the synovium were CD68 positive, in keeping with macrophages. The degree of proliferation in synovial biopsies was estimated by MIB1 immunostaining, which showed that up to 30% of cells were cycling compared with fewer than 10% in control synovial biopsies. None of the giant cells was cycling. By double immunostaining, proliferating cells were determined to be fibroblastic synoviocytes rather than macrophages. Thus the proliferative synovitis in this CACP syndrome can be more accurately thought of as hypercellularity by infiltrating macrophages with a contribution by proliferating fibroblastic synoviocytes. The synoviocyte proliferation is likely a response to the underlying genetic mutations involving the proteoglycan-4 (or CACP) gene. The encoded protein normally acts as a lubricant and possibly controls cell proliferation. Loss of one or another of these functions may be a possible mechanism that leads to synoviocyte proliferation in this disease, but the exact pathophysiology leading to this change requires further study.