Common Histological Features Suggesting Enchondral Ossification Pathways in Calciphylaxis of Various Origins: A Study of Human Subcutaneous Tissue Biopsies

Calciphylaxis is a rare, yet underdiagnosed condition causing high mortality in patients with severe renal and cardiovascular disease. Since knowledge of the pathophysiology of calciphylaxis is limited, a differential analysis of histological alterations in patient subgroups with various comorbidities might expose different disease phenotypes and allow deeper insights into the pathophysiology of the condition. Histological markers of osteogenesis and calcification were investigated in a group of 18 patients with clinically and histologically verified calciphylaxis, using immunohistochemical staining. Analysis of staining intensity and distribution of marker proteins in histological structures was performed to evaluate distinct patterns between subgroups with different clinical comorbidities in comparison with a control group. In all cases, immunohistochemical staining for bone matrix proteins, bone-morphogenic proteins and matrix-Gla proteins co-localized with subcutaneous vascular and interstitial calcifications. Significant expression of bone-morphogenic protein-7 and active matrix-Gla protein was observed. Mortality was associated with renal comorbidities and increased expression of bone-morphogenic protein-7. However, no distinct histological patterns were found between subgroups with renal disease, warfarin intake or coexisting micro- and macro-angiopathies. The upregulation of osteogenic markers (including bone-morphogenic protein-7) plays a major role in the development of calciphylaxis. Clinical outcome correlates with kidney function and phosphate handling, suggesting different pathophysiological mechanisms. However, biopsy at late-stage disease shows a common histological phenotype, involving enchondral ossification. SIGNIFICANCE Calciphylaxis is a rare, calcifying, occlusive vessel disease with different aetiopathological origins and high mortality. Previous histological studies have shown upregulation of bone-morphogenic proteins, bone matrix proteins, and dysbalance of calcification inhibitors. This study confirmed the upregulation of osteogenic markers, including the first evidence of upregulation of bone-morphogenic-protein 7, in calciphylaxis. Renal function, sequential comorbidities, phosphate handling and high expression of bone-morphogenic protein-7 were identified as predictors of clinical outcome. However, the histological appearance did not differ between patients with different comorbidities. Therefore, disease evolution may have different pathophysiological drivers with a common pathophysiological endpoint. This underlines the need for individualization of multimodal treatment in calciphylaxis.