Growth pattern in children with X-linked hypophosphatemia treated with burosumab and growth hormone.

BACKGROUND: X-linked hypophosphatemia (XLH) is characterized by increased serum concentrations of fibroblast growth factor 23 (FGF23), hypophosphatemia and insufficient endogenous synthesis of calcitriol. Beside rickets, odonto- and osteomalacia, disproportionate short stature is seen in most affected individuals. Vitamin D analogs and phosphate supplements, i.e., conventional therapy, can improve growth especially when started early in life. Recombinant human growth hormone (rhGH) therapy in XLH children with short stature has positive effects, although few reports are available. Newly available treatment (burosumab) targeting increased FGF23 signaling leads to minimal improvement of growth in XLH children. So far, we lack data on the growth of XLH children treated with concomitant rhGH and burosumab therapies. RESULTS: Thirty-six patients received burosumab for at least 1 year after switching from conventional therapy. Of these, 23 received burosumab alone, while the others continued rhGH therapy after switching to burosumab. Children treated with burosumab alone showed a minimal change in height SDS after 1 year (mean ± SD 0.0 ± 0.3 prepubertal vs. 0.1 ± 0.3 pubertal participants). In contrast, rhGH clearly improved height during the first year of treatment before initiating burosumab (mean ± SD of height gain 1.0 ± 0.4); patients continued to gain height during the year of combined burosumab and rhGH therapies (mean ± SD height gain 0.2 ± 0.1). As expected, phosphate serum levels normalized upon burosumab therapy. No change in serum calcium levels, urinary calcium excretion, or 25-OHD levels was seen, though 1,25-(OH)2D increased dramatically under burosumab therapy. CONCLUSION: To our knowledge, this is the first study on growth under concomitant rhGH and burosumab treatments. We did not observe any safety issue in this cohort of patients which is one of the largest in Europe. Our data suggest that continuing treatment with rhGH after switching from conventional therapy to burosumab, if the height prognosis is compromised, might be beneficial for the final height.

Elevation of phosphate levels impairs skeletal myoblast differentiation.

Hyperphosphatemic conditions such as chronic kidney disease are associated with severe muscle wasting and impaired life quality. While regeneration of muscle tissue is known to be reliant on recruitment of myogenic progenitor cells, the effects of elevated phosphate loads on this process have not been investigated in detail so far. This study aims to clarify the direct effects of hyperphosphatemic conditions on skeletal myoblast differentiation in a murine in vitro model. C2C12 murine muscle progenitor cells were supplemented with phosphate concentrations resembling moderate to severe hyperphosphatemia (1.4-2.9 mmol/l). Phosphate-induced effects were quantified by RT-PCR and immunoblotting. Immunohistochemistry was performed to count nuclear positive cells under treatment. Cell viability and metabolic activity were assessed by XTT and BrdU incorporation assays. Inorganic phosphate directly induced ERK-phosphorylation in pre-differentiated C2C12 myoblast cells. While phosphate concentrations resembling the upper normal range significantly reduced Myogenin expression (- 22.5%, p = 0.015), severe hyperphosphatemic conditions further impaired differentiation (Myogenin - 61.0%, p < 0.0001; MyoD - 51.0%; p < 0.0001). Analogue effects were found on the protein level (Myogenin - 42.0%, p = 0.004; MyoD - 25.7%, p = 0.002). ERK inhibition strongly attenuated phosphate-induced effects on Myogenin expression (p = 0.002). Metabolic activity was unaffected by the treatments. Our data point to a phosphate-induced inhibition of myoblast differentiation without effects on cell viability. Serum phosphate levels as low as the upper normal serum range significantly impaired marker gene expression in vitro. Investigation of cellular effects of hyperphosphatemia may help to better define serum cutoffs and modify existing treatment approaches of phosphate binders, especially in patients at risk of sarcopenia.

Alterations of bone material properties in adult patients with X-linked hypophosphatemia (XLH).

X-linked hypophosphatemia (XLH) caused by PHEX mutations results in elevated serum FGF23 levels, renal phosphate wasting and low 1,25-dihydroxyvitamin D. The glycophosphoprotein osteopontin, a potent inhibitor of mineralization normally degraded by PHEX, accumulates within the bone matrix. Conventional therapy consisting of supplementation with phosphate and vitamin D analogs is burdensome and the effects on bone material poorly characterized. We analyzed transiliac bone biopsies from four adult patients, two of them severely affected due to no diagnosis and no treatment until adulthood. We used light microscopy, qBEI and FTIRI to study histology, histomorphometry, bone mineralization density distribution, properties of the organic matrix and size of hypomineralized periosteocytic lesions. Non-treatment resulted in severe osteomalacia, twice the amount of mineralized trabecular volume, multiple osteon-like perforations, continuity of lamellae from mineralized to unmineralized areas and distinctive patches of woven bone. Periosteocytic lesions were larger than in treated patients. The latter had nearly normal osteoid thicknesses, although surface was still elevated. The median calcium content of the matrix was always within normal range, although the percentage of lowly mineralized bone areas was highly increased in non-treated patients, resulting in a marked heterogeneity in mineralization. Divalent collagen cross-links were evident independently of the mineral content of the matrix. Broad osteoid seams lacked measurable pyridinoline, a mature trivalent cross-link and exhibited considerable acidic lipid content, typically found in matrix vesicles. Based on our results, we propose a model that possibly integrates the relationship between the observed mineralization disturbances, FGF23 secretion and the known osteopontin accumulation in XLH.

A Pediatric Patient with a CYP24A1 Mutation: Four Years of Clinical, Biochemical, and Imaging Follow-Up.

BACKGROUND: A female infant was admitted to hospital due to failure to thrive. She presented hypercalcemia (4.09 mmol/L, normal range: 2.2-2.65 mmol/L), high 25-hydroxyvitamin D (283 nmol/L, normal range: 75-250 nmol/L), 1,25-dihydroxyvitamin D in the upper normal range, and low parathyroid hormone. Vitamin D intoxication was suspected. The patient had received routine rickets prophylaxis. METHODS: Williams-Beuren syndrome was genetically excluded. Sequencing of CYP24A1 showed 2 mutations: c.443T>C and c.1186C>T. RESULTS: The patient's clinical status improved after intravenous rehydration, cessation of supplementation, and on a low-calcium diet. 25-Hydroxyvitamin D concentrations normalized within days, while 1,25-dihydroxyvitamin D remained in the upper normal range. We also investigated our patient's bone health. CONCLUSION: The patient was hospitalized initially on suspicion of vitamin D intoxication but proved to be a case of compound heterozygosity. Data on the long-term clinical and biochemical evolution of patients with idiopathic infantile hypercalcemia are sparse. Our follow-up showed seasonal variations of vitamin D and calcium parameters, with no influence on kidney function or bone health for the investigated period.