Circulating levels of soluble klotho and FGF23 in X-linked hypophosphatemia: circadian variance, effects of treatment, and relationship to parathyroid status

Abstract

Context: Circulating fibroblast growth factor (FGF)-23 is variably elevated in individuals with X-linked hypophosphatemia (XLH), and klotho has recently been shown to effect renal phosphate handling, yet limited data are available on circulating FGF23 and klotho in XLH.

Objective: The objective of the study was to characterize circulating FGF23 and klotho in XLH.

Design: Children and adults with XLH withheld medication for 14 d. Fasting serum FGF23, PTH, klotho, phosphate, and 1,25 dihydroxyvitamin D were obtained. Treated adults were also sampled, and circadian sampling was performed in selected individuals.

Setting: The study was conducted at a hospital research unit at an academic medical center.

Patients and other participants: Participants included 23 individuals with XLH and eight controls.

Interventions: There were no interventions.

Main outcome measures: Serum klotho and FGF23 were measured.

Results: FGF23 was greater in XLH than in controls and greater in treated XLH subjects compared with XLH subjects not receiving medical therapy. Children had higher klotho levels than adults, but values in XLH were similar to controls. A strong positive correlation between FGF23 and PTH was found in XLH subjects, whereas there was no relationship between these variables in controls. Circulating klotho, but not FGF23, has a diurnal pattern.

Conclusions: Serum klotho declines with age and demonstrates circadian variation but is normal in XLH. Serum FGF23 is similar in children and adults, is elevated in XLH, further increases with therapy, and demonstrates no diurnal variation. The direct relationship between FGF23 and PTH in subjects with XLH suggests that FGF23 regulation of PTH secretion is aberrant in this disorder.

Figure 1

A–F, Fasting morning values (mean and sem) for serum P (A), TMP/GFR (B), serum PTH (C), serum 1,25(OH)₂D (D), serum klotho (E), and serum FGF23 (F) in children with XLH (first column of each panel), unaffected control children (second column), adults (third column), and unaffected adult controls (fourth column). Affected subjects were untreated at the time of sample collection. Mean and sem are shown. A single asterisk above a column representing values for XLH individuals indicates a significant difference from controls of the same age group (significance levels reported in text). A double asterisk above a column representing children indicates a significant difference from adults of the same disease/control group. G–L, Serum klotho (G), FGF23 (H), PTH (I), P (J), 1,25(OH)₂D (K), and TMP/GFR (L) in adult subjects off therapy (left column) compared with on therapy (right column). Mean and sem are shown. A single asterisk above a column representing values for treated XLH individuals indicates a significant difference from untreated XLH individuals. M, The relationship between serum PTH and FGF23 values in XLH subjects are shown. There is a positive correlation observed in subjects with XLH (PTH = 5.3 + 0.40 × FGF23). To convert serum phosphate concentration and TMP/GFR from mass units (milligrams per deciliter) to standard international units (millimoles per liter), divide by 3.1. To convert 1,25(OH)₂D units from mass units (picograms per milliliter) to standard international units (picomoles per liter), multiply by 2.4. Normal ranges include: serum P (A and J) in 9–20 yr olds: 3.1–6.2 mg/dl, and in adults, 2.7–4.4 mg/dl; TMP/GFR (B and L) in 9–20 yr olds: 2.2–5.3 mg/dl, and in adults, 2.1–4.8 mg/dl; PTH (C, I, and M), 10–25 nanoliter-equivalents/ml (nlEq/ml); 1,25(OH)₂D (D and K), 25–66 pg/ml; FGF23 less than 50 pg/ml (F, H, and M); klotho (E and G) does not have an established reference range apart from the data presented in the control subject data above.

Figure 1

A–F, Fasting morning values (mean and sem) for serum P (A), TMP/GFR (B), serum PTH (C), serum 1,25(OH)₂D (D), serum klotho (E), and serum FGF23 (F) in children with XLH (first column of each panel), unaffected control children (second column), adults (third column), and unaffected adult controls (fourth column). Affected subjects were untreated at the time of sample collection. Mean and sem are shown. A single asterisk above a column representing values for XLH individuals indicates a significant difference from controls of the same age group (significance levels reported in text). A double asterisk above a column representing children indicates a significant difference from adults of the same disease/control group. G–L, Serum klotho (G), FGF23 (H), PTH (I), P (J), 1,25(OH)₂D (K), and TMP/GFR (L) in adult subjects off therapy (left column) compared with on therapy (right column). Mean and sem are shown. A single asterisk above a column representing values for treated XLH individuals indicates a significant difference from untreated XLH individuals. M, The relationship between serum PTH and FGF23 values in XLH subjects are shown. There is a positive correlation observed in subjects with XLH (PTH = 5.3 + 0.40 × FGF23). To convert serum phosphate concentration and TMP/GFR from mass units (milligrams per deciliter) to standard international units (millimoles per liter), divide by 3.1. To convert 1,25(OH)₂D units from mass units (picograms per milliliter) to standard international units (picomoles per liter), multiply by 2.4. Normal ranges include: serum P (A and J) in 9–20 yr olds: 3.1–6.2 mg/dl, and in adults, 2.7–4.4 mg/dl; TMP/GFR (B and L) in 9–20 yr olds: 2.2–5.3 mg/dl, and in adults, 2.1–4.8 mg/dl; PTH (C, I, and M), 10–25 nanoliter-equivalents/ml (nlEq/ml); 1,25(OH)₂D (D and K), 25–66 pg/ml; FGF23 less than 50 pg/ml (F, H, and M); klotho (E and G) does not have an established reference range apart from the data presented in the control subject data above.

Figure 2

Circadian pattern of circulating klotho (A), FGF23 (B), and P (C). The observed mean and sem are plotted at each time point. For klotho, values from all subjects are presented as the percent of the 0800 h value on d 1. For FGF23 and P, data from XLH subjects off therapy are shown in the dotted lines with open squares, data from XLH subjects receiving standard therapy are shown in dashed lines with open triangles, and data from unaffected control subjects are shown in solid lines with open circles. At midnight, there is an approximately 40% reduction in circulating klotho with a gradual return to near baseline values in the early morning hours. *, Value significantly different from baseline (P < 0.05). This correlates with the midnight increase in serum P observed in treated and untreated patients with XLH. (Midnight values were less than baseline for the treated and untreated XLH subjects, but the difference did not achieve statistical significance for unaffected controls). There is no significant diurnal change in serum FGF23 levels, but values are higher in patients with XLH receiving treatment than in untreated patients (P = 0.0004). Irrespective of treatment, FGF23 levels are much higher in patients with XLH than in normal individuals. To convert serum phosphate concentration from mass units (milligrams per deciliter) to standard international units (millimoles per liter), divide by 3.1. Normal ranges are: serum FGF23 less than 50 pg/ml (B); serum P (C): 2.7–6.2 mg/dl; serum klotho (A) does not have an established reference range, apart from data presented in the control subjects here and in Fig. 1.