Radiologic vignette. X-linked hypophosphatemia (familial vitamin D-resistant rickets)

Mineralizing enthesopathy is a common feature of renal phosphate-wasting disorders attributed to FGF23 and is exacerbated by standard therapy in hyp mice

We have previously confirmed a paradoxical mineralizing enthesopathy as a hallmark of X-linked hypophosphatemia. X-linked hypophosphatemia is the most common of the phosphate-wasting disorders mediated by elevated fibroblast growth factor 23 (FGF23) and occurs as a consequence of inactivating mutations of the PHEX gene product. Despite childhood management of the disease, these complications of tendon and ligament insertion sites account for a great deal of the disease's morbidity into adulthood. It is unclear whether the enthesopathy occurs in other forms of renal phosphate-wasting disorders attributable to high FGF23 levels. Here we describe two patients with autosomal recessive hypophosphatemic rickets due to the Met1Val mutation in dentin matrix acidic phosphoprotein 1 (DMP1). In addition to the biochemical and skeletal features of long-standing rickets with elevated FGF23 levels, these individuals exhibited severe, debilitating, generalized mineralized enthesopathy. These data suggest that enthesophytes are a feature common to FGF23-mediated phosphate-wasting disorders. To address this possibility, we examined a murine model of FGF23 overexpression using a transgene encoding the secreted form of human FGF23 (R176Q) cDNA (FGF23-TG mice). We report that FGF23-TG mice display a similar mineralizing enthesopathy of the Achilles and plantar facial insertions. In addition, we examined the impact of standard therapy for phosphate-wasting disorders on enthesophyte progression. We report that fibrochondrocyte hyperplasia persisted in Hyp mice treated with oral phosphate and calcitriol. In addition, treatment had the untoward effect of further exacerbating the mineralization of fibrochondrocytes that define the bone spur of the Achilles insertion. These studies support the need for newer interventions targeted at limiting the actions of FGF23 and minimizing both the toxicities and potential morbidities associated with standard therapy.

An atypical degenerative osteoarthropathy in Hyp mice is characterized by a loss in the mineralized zone of articular cartilage

Patients with X-linked hypophosphatemia (XLH) develop enthesophytes and osteophytes secondary to articular cartilage degeneration and together are the primary cause of morbidity in adult patients so afflicted. We have previously characterized the enthesopathy in Hyp mice, a murine model of XLH. We now extend these studies to the synovial joint in order to characterize potential cellular changes in articular cartilage that may predispose patients to the osteoarthropathy of XLH. We report that, despite highly elevated levels of alkaline phosphatase activity throughout articular cartilage, there is a complete loss in the mineralized zone of articular cartilage as assessed by von Kossa staining of mineral and as quantified by EPIC-microCT analysis and evidence of vascular invasion. We also identify the downregulation of extracellular matrix (ECM) factors identified as regulators of terminally differentiated mineralizing articular chondrocytes. There is also a striking increase in the histochemical staining of sulfated proteoglycans, a change that may reflect the loss of a transitional tissue that reduces mechanical stress at the interface between cartilage and subchondral bone. The failure of mineralizing articular chondrocytes to develop in the hypophosphatemic state suggests that phosphate may be a key regulator of chondrocyte mineralization. Accordingly, we find that the appropriate zonal arrangement and phenotypic markers of articular cartilage are significantly reestablished by phosphate-replacement therapy. Given the turnover and maintenance of articular cartilage ECM, the identification of early and abnormal cellular changes unique to XLH will undoubtedly aid in a more effective management of this disease to minimize the onset of degenerative osteoarthropathy.

Survey of the enthesopathy of X-linked hypophosphatemia and its characterization in Hyp mice

X-linked hypophosphatemia (XLH) is characterized by rickets and osteomalacia as a result of an inactivating mutation of the PHEX (phosphate-regulating gene with homology to endopeptidases on the X chromosome) gene. PHEX encodes an endopeptidase that, when inactivated, results in elevated circulating levels of FGF-23, a novel phosphate-regulating hormone (a phosphatonin), thereby resulting in increased phosphate excretion and impaired bone mineralization. A generalized and severe mineralizing enthesopathy in patients with XLH was first reported in 1985; we likewise report a survey in which we found evidence of enthesopathy in fibrocartilaginous insertion sites, as well as osteophyte formation, in the majority of patients. Nonetheless, there has been very little focus on the progression and pathogenesis underlying the paradoxical heterotopic calcification of tendon and ligament insertion sites. Such studies have been hampered by lack of a model of mineralizing enthesopathy. We therefore characterized the involvement of the most frequently targeted fibrocartilaginous tendon insertion sites in Hyp mice, a murine model of the XLH mutation that phenocopies the human syndrome in every detail including hypophosphatemia and elevated FGF-23. Histological examination of the affected entheses revealed that mineralizing insertion sites, while thought to involve bone spur formation, were not due to bone-forming osteoblasts but instead to a significant expansion of mineralizing fibrocartilage. Our finding that enthesis fibrocartilage cells specifically express fibroblast growth factor receptor 3 (FGFR3)/Klotho suggests that the high circulating levels of FGF-23, characteristic of XLH and Hyp mice, may be part of the biochemical milieu that underlies the expansion of mineralizing enthesis fibrocartilage.