Progression of Mineral Ion Abnormalities in Patients With Jansen Metaphyseal Chondrodysplasia

Abstract

Context: Five different activating PTH/PTH-related peptide (PTHrP) receptor (PTHR1) mutations have been reported as causes of Jansen metaphyseal chondrodysplasia (JMC), a rare disorder characterized by severe growth plate abnormalities and PTH-independent hypercalcemia.

Objectives: Assess the natural history of clinical and laboratory findings in 24 patients with JMC and characterize the disease-causing mutant receptors in vitro.

Patients and methods: The H223R mutation occurred in 18 patients. T410P, I458R and I458K each occurred in single cases; T410R was present in a father and his two sons. Laboratory records were analyzed individually and in aggregate.

Results: Postnatal calcium levels were normal in most patients, but elevated between 0.15 and 10 years (11.8 ± 1.37 mg/dL) and tended to normalize in adults (10.0 ± 1.03 mg/dL). Mean phosphate levels were at the lower end of the age-specific normal ranges. Urinary calcium/creatinine (mg/mg) were consistently elevated (children, 0.80 ± 0.40; adults, 0.28 ± 0.19). Adult heights were well below the 3rd percentile for all patients, except for those with the T410R mutation. Most patients with JMC had undergone orthopedic surgical procedures, most had nephrocalcinosis, and two had advanced chronic kidney disease. The five PTHR1 mutants showed varying degrees of constitutive and PTH-stimulated cAMP signaling activity when expressed in HEK293 reporter cells. The inverse agonist [L11,dW12,W23,Y36]PTHrP(7-36) reduced basal cAMP signaling for each PTHR1 mutant.

Conclusions: Except for T410R, the other PTHR1 mutations were associated with indistinguishable mineral ion abnormalities and cause similarly severe growth impairment. Hypercalciuria persisted into adulthood. An inverse agonist ligand effectively reduced in vitro PTH-independent cAMP formation at all five PTHR1 mutants, suggesting a potential path toward therapy.

Figure 1.

Serum calcium concentrations of multiple patients with different PTHR1 mutations from the newborn period until the sixth decade of life; eight patients with measurements within the first 1.5 mo of life are indicated at the left of the axis break. Patients are depicted by open or closed symbols of different colors. The patients are identified in Supplemental Table 1. Patients with the H223R mutation are represented by open or closed circles; black-filled circles represent patients for whom only one measurement was available; colored open or closed circles represent patients for whom multiple measurements were available. Consecutive measurements for patient H223R-17 are depicted with red circles/line. Data for three patients with the T410R-PTHR1 mutation at different ages (father, black triangle; his two sons, blue and red triangles, respectively), and measurements for the patients with the T410P-PTHR1 mutation (diamonds), I458K-PTHR1 mutation (trapezoids), and I458R-PTHR1 mutation (pentagons) are shown. Dashed lines represent the upper/lower end of the adult normal range for total calcium levels (8.6 to 10.2 mg/dL). The reference range for infants is 8.4 to 10.6 mg/dL.

Figure 2.

(A) Estimated glomerular filtration rates (eGFRs) as calculated by the Schwartz formula are presented for eight adult patients with three different PTHR1 mutations. For the patient with the T410P mutation (diamonds), three measurements are shown that were obtained during adulthood prior to hemodialysis that was initiated at age 37 y. For patient H223R-16 (filled circles) numerous measurements were performed after the age of 38 y showing the progressive decline in renal function. (B) Latest abdominal CT of patient H223R-16 at age 55 y showing extensive renal calcifications (arrows).

Figure 3.

Serum and urinary calcium measurements for multiple children [0.15 to 10 y; n = 22 for serum calcium, n = 15 for urinary calcium/creatinine (Ca/Cr) ratio] and multiple adults (17 to 38 y; n = 11 for serum calcium, n = 8 for urinary Ca/Cr ratio) with Jansen disease due to different PTHR1 mutations. Each data point represents the mean when a patient had multiple measurements during the two observation periods. (A) Total calcium levels; dashed lines represent the upper/lower end of the adult normal range (8.6 to 10.2 mg/dL). (B) Urinary Ca/Cr ratio; all individual data points are shown. Means ± SD are for patients with the H223R mutation. Dashed line represents the upper end of normal for adult patients (<0.2). Children and adults showed no significant difference in the urinary Ca/Cr ratio.

Figure 4.

Serum phosphate levels and PTH levels at different ages for multiple patients affected by Jansen disease due to different PTHR1 mutations. The means are shown when patients had multiple measurements during the two observation periods. (A) Phosphate levels for infants (<1 y), children between 1 and 12 y of age, and patients >15 y of age). The lower limits of the age-dependent reference ranges for phosphate are: 0 to 6 mo, 1.8 mmol/L (5.6 mg/dL); 6 to 12 mo, 1.6 mmol/L (4.9 mg/dL); 1 to 10 y, 1.2 mmol/L (3.8 mg/dL); and >15 y, 0.8 mmol/L (2.5 mg/dL). Individual data points are shown. Means ± SD are for patients with the H223R mutation. (B) PTH levels for children (0.15 to 10 y) and adults (17 to 38 y). Lower end of the adult reference range is 10 pg/mL (dashed line). Individual data points and means ± SD for patients with the H223R mutation are shown. Serum PTH levels were not significantly different for affected children and adults.

Figure 5.

Height data for different patients affected by Jansen disease due to different PTHR1 mutations. (A) Individual final heights for 13 adult patients with JMC. Means ± SD are shown for the final heights of patients with the H223R mutation; the red broken lines indicates the 3rd percentile for normal adult heights. (B) Individual height z scores for eight children.

Figure 6.

Functional evaluation of the wild-type and different PTHR1 mutants in HEK-293/Glosensor (GS22A) cells. For some data points, the error bars are small and thus within the height of the symbol. (A) Basal cAMP production in GS22A cells that were transiently transfected with increasing amounts of plasmid DNA (10, 20, 40, 80, and 160 ng per well) encoding either a mutant or the wild-type PTHR1. (B) PTH-stimulated cAMP accumulation in cells transfected with plasmid DNA (100 ng per well) encoding either wild-type or mutant PTHR1s. Data are shown as the AUC of cAMP accumulation (mean ± SEM. (C) Functional evaluation of the inverse agonist [L¹¹,dW¹²,W²³,Y36]PTHrP(7–36) in GS22A cells expressing the wild-type PTHR1 or different JMC mutants. The cAMP-dependent luminescence responses in cells transfected with plasmid DNA (100 ng per well) encoding either wild-type or mutant receptor are shown. Data are shown as the AUC of cAMP-dependent luminescence measured over time after addition (t = 0) of either buffer (open symbols) or inverse agonist (filled symbols); all data were corrected for time 0 (mean ± SEM).