Expression of FGF23 is correlated with serum phosphate level in isolated fibrous dysplasia

Abstract

Fibrous dysplasia (FD) patients sometimes suffer from concomitant hypophosphatemic rickets/osteomalacia, resulting from renal phosphate wasting. It was recently reported that FD tissue in the patients with McCune–Albright syndrome (MAS) expressed fibroblast growth factor-23 (FGF-23), which is now known to be as a pathogenic phosphaturic factor in patients with oncogenic osteomalacia and X-linked hypophosphatemic rickets. Since it remains controversial whether serum phosphate levels are influenced by FGF23 expressions in FD tissue, isolated FD patients without MAS syndrome were examined for the relationship between FGF23 expressions, circulating levels of FGF-23 and phosphate to negate the effects of MAS-associated endocrine abnormalities on serum phosphate. Eighteen paraffin embedded FD tissues and 2 frozen tissues were obtained for the study. Sixteen of 18 isolated FD tissues were successfully analyzed GNAS gene, which exhibited activated mutations observed in MAS. Eight of 16 FD tissues, which exhibited GNAS mutations, revealed positive staining for FGF-23. These evidence indicate that postzygotic activated mutations of GNAS is necessary for the FD tissue formation by mosaic distribution of mutated osteogenic cell lineage, but is not sufficient to elevate FGF23 expression causing generalized osteomalacia with severe renal phosphate wasting. The expression level of FGF23 in isolated FD tissue with hypophosphatemic osteomalacia determined by real-time PCR was abundant close to the levels in OOM tumors. Osteoblasts/osteocytes in woven bone were predominant source of circulating FGF-23 in FD tissues by immunohistochemistry. A negative correlation of the intensity of FGF-23 staining with serum inorganic phosphate levels indicated that the expression of FGF23 in focal FD tissues could be a prominent determinant of serum phosphate levels in isolated FD patient. These data provide novel insights into the regulatory mechanism of serum inorganic phosphate levels in isolated FD patients and extend the notion that FGF-23 originating from FD tissue may cause hypophosphatemia not only in isolated FD patients but also in the patients with MAS syndrome.

Introduction

Fibroblast growth factor-23 (FGF-23), a member of the FGF family, is now known to be a pathogenic factor in certain types of hypophosphatemic rickets, such as X-linked hypophosphatemia (XLH) and oncogenic osteomalacia (OOM) (Jonsson et al., 2003, White et al., 2001b, Yamazaki et al., 2002). Autosomal dominant hypophosphatemic rickets, a similar disorder characterized by renal phosphate wasting, has been reported to be associated with missense mutations of FGF-23 (The ADHR Consortium, 2000) which prevent its cleavage (White et al., 2001a). Administration of recombinant FGF-23 decreased serum phosphate levels in mice via its phosphaturic effect (Shimada et al., 2001). Mice bearing a Chinese hamster ovary cell line stably expressing FGF-23 also exhibited characteristics of hypophosphatemic rickets in vivo (Shimada et al., 2001), indicating the importance of FGF-23 in the pathogenesis of hypophosphatemic rickets/osteomalacia. Elevated circulating FGF-23 concentration was observed in primary (Kobayashi et al., in press) and secondary (Imanishi et al., 2004) hyperparathyroidism, suggesting that hypercalcemia and/or PTH oversecretion could stimulate FGF-23 secretion via intracellular signal transduction such as cyclic adenosine monophosphate.

McCune–Albright syndrome (MAS), a rare sporadic genetic disorder caused by postzygotic somatic activated mutations in the GNAS gene, which encodes an á subunit of the stimulatory G protein, is characterized by the presence of polyostotic fibrous dysplasia of bone (FD), café au lait skin pigmentation, sexual precocity, and other hyperfunctioning endocrinopathies (Weinstein et al., 1991). Hypophosphatemic rickets/osteomalacia is sometimes observed as a complication of MAS (Lee et al., 1986). The occurrence of rickets/osteomalacia in FD/MAS patients has been suggested to be analogous to the hypophosphatemic bone disease in OOM (Collins et al., 2001, Dent and Gertner, 1976). Recently, Riminucci et al. demonstrated local expression of FGF23 in FD tissues which played an important role in the renal phosphate wasting in FD/MAS patients (Riminucci et al., 2003). Interestingly, human trabecular bone cells cultured primarily from FD tissue expressed FGF23 to the same degree as cells from normal trabecular bone (Riminucci et al., 2003), suggesting the involvement of circulating factors in stimulating secretion of FGF23 from fibrous tissue into the circulation. Activating missense mutations in GNAS observed in FD/MAS are known to play roles in the pathogenesis of hyperfunctional endocrinopathies (Landis et al., 1989). Indeed, MAS patients often have endocrine disorders, such as hyperthyroidism, Cushing syndrome, acromegaly, and hyperprolactinemia (Weinstein et al., 1991). These findings together suggest that certain circulating factors, targeted to FGF-23-producing cells to upregulate increase FGF23 expression, may contribute to the pathogenesis of hypophosphatemic rickets/osteomalacia in MAS. However, the involvement of abnormalities in other endocrine systems in MAS syndrome in increasing serum FGF-23 levels should be ruled out. Although it has been reported that patients with isolated FD develop hypophosphatemic rickets/osteomalacia, expression of FGF23 has not been demonstrated in association with their symptoms (Dent and Gertner, 1976, Ryan et al., 1968).

In the present study, to determine the role of local expression of FGF23 in FD tissue on serum phosphate levels, we examined the relationships between FGF23 expression in FD tissue determined by immunohistochemistry and quantitative real-time PCR, and circulating levels of FGF-23 and phosphate in isolated FD patients without other endocrine disorders observed in MAS syndrome.