Research & Publications
My lab is involved in the study of a heredity form of osteomalacia (rickets in children) called X-linked hypophosphatemia (XLH). While XLH is characterized by defective bone mineralization, a paradoxical and severe mineralizing enthesopathy (entheses are tendon and ligament insertion sites) develops in many adult patients. We have made significant progress in understanding the pathogenesis of the enthesopathy of XLH, as reported in our most recent publication (G Liang, LD Katz, KL Insogna, TO Carpenter, CM Macica. A Study of the Enthesopathy of X-linkedHypophosphatemia. Calcified Tissue International, Volume 85 (3) 235-242, 2009). Here, we describe the first comprehensive patient survey of the enthesopathy of XLH, which provided valuable insight into the pathogenesis of the enthesopathy. In this study, we utilized the Hyp mice, the murine model of XLH. Indeed, we have identified the Hyp mouse as the first valid model of enthesopathy by examining those fibrocartilagenous insertion sites most frequently targeted in the XLH patient population studied. The primary histologic feature of the enthesopathy of the Achilles and patellar insertion sites in the Hyp mouse was a dramatic cellular expansion and thickening of mineralized enthesis fibrocartilage. We have thus identified an enthesopathy in Hyp mice that closely phenocopies the human disease. We also provide the first evidence that an expansion of mineralized fibrocartilage may define the enthesopathy and is not due to ectopic osteoblast-driven bone formation, as previously thought. We additionally found that fibrocartilage cells express the fibroblast growth factor receptor FGFR3, as well as the co-receptor, Klotho. This finding suggests that the high circulating levels of FGF-23, characteristic of XLH and Hyp mice, (and an FGFR3 ligand), may be part of the biochemical milieu that underlies the expansion of mineralizing enthesis fibrocartilage. Because the pathology of XLH enthesopathy and of age and use-dependent injuries to tendon and ligament insertion sites all involve excessive mineralization or “hardening” of these sites, our newest data focuses on the molecules that regulate this process. Analysis of enthesis and articular cartilage alkaline phosphatase (ALP) activity, the enzyme that catalyzes the deposition of mineral, revealed significantly higher levels of activity in Hyp mice during periods of both growth and adulthood. This is consistent with the elevated serum ALP activity reported in patients with XLH and in Hyp mice, a finding we have confirmed. Immunohistochemical analysis of ALP protein suggests the presence of two biochemically distinct isoforms of ALP that we refer to as osteoblast ALP and chondrocyte ALP. We propose that because articular cartilage and fibrocartilage are avascular, limited phosphoester substrate availability would favor an enzyme with a low Michaelis constant (Km) for phosphoester substrates. This, along with an increase in ALP activity, may account for the ability of chondrocytes to hypermineralize in a rachitic environment. We are working in collaboration with the Keck facility at Yale to characterize the alkaline phosphatase enzyme we have isolated from the long bones of Hyp mice. Finally, mineralizing fibrocartilage cells and articular cartilage specifically express osteopontin (OPN), a highly phosphorylated osteoblast glycoprotein, the levels of which are regulated by extracellular pyrophosphate (ePPi). It has recently been shown that phosphorylated OPN and ePPi act together to synergistically inhibit mineralization. We found that while OPN levels are highly elevated in osteoblasts of Hyp mice, OPN is absent in the mineralized cartilage of Hyp mice. We speculate that, together with the increase in ALP activity, the loss of OPN from cartilage might underlie the hypermineralization of cartilages by reducing inhibitory constraint of OPN, explaining the paradox of inappropriate mineralization of cartilage tissue in a rachitic environment.
Education & Training
- Postdoctoral fellowshipYale University (2000)