Case Report: Inactivating PTH/PTHrP Signaling Disorder Type 1 Presenting With PTH Resistance

Abstract

PTH resistance is characterized by elevated parathyroid hormone (PTH) levels, hypocalcemia, hyperphosphatemia and it is classically associated with GNAS locus genetic or epigenetic defects. Inactivating PTH/PTHrP signaling disorders (iPPSD) define overlapping phenotypes based on their molecular etiology. iPPSD1 is associated with PTH1R variants and variable phenotypes including ossification anomalies and primary failure of tooth eruption but no endocrine disorder. Here we report on a 10-month-old child born from consanguineous parents, who presented with mild neurodevelopmental delay, seizures, enlarged fontanelles, round face, and bilateral clinodactyly. Hand x-rays showed diffuse delayed bone age, osteopenia, short metacarpal bones and cone-shaped distal phalanges. A diagnosis of PTH resistance was made on the basis of severe hypocalcemia, hyperphosphatemia, elevated PTH and normal vitamin D levels on blood sample. The patient was treated with calcium carbonate and alfacalcidol leading to rapid bio-clinical improvement. Follow-up revealed multiple agenesis of primary teeth and delayed teeth eruption, as well as Arnold-Chiari type 1 malformation requiring a ventriculoperitoneal shunt placement. GNAS gene analysis showed no pathogenic variation, but a likely pathogenic homozygous substitution c.723C>G p.(Asp241Glu) in PTH1R gene was found by trio-based whole exome sequencing. We studied the deleterious impact of the variant on the protein conformation with bioinformatics tools. In conclusion, our study reports for the first time PTH resistance in a child with a biallelic PTH1R mutation, extending thereby the clinical spectrum of iPPSD1 phenotypes.

Keywords: Albright hereditary osteodystrophy; GNAS; PTH1R; alfacalcidol; epilepsy; iPPSD1; parathyroid hormone; pseudohypoparathyreoidism.

Figure 1

The patient presented with severe hypocalcemia and hyperphosphatemia which improved after treatment initiation. The work-up highlighted an Arnold-Chiari type 1 malformation (CM1) and delayed ossification on brain magnetic resonance imaging (MRI) and hands X-ray, respectively. (A) Overview of the patient’s diagnostic and therapeutic journey. Plasma (B) ionized calcium, (C) total calcium, (D) phosphate and (E) parathyroid hormone (PTH) concentrations measured at diagnosis and during follow-up. Of note, the dramatic PTH level decrease was associated with hypercalcemia resulting from an iatrogenic event (excessive alfacalcidol administration). Grey zone highlights normal value range. (F) Sagittal T1-weighted, (G) transversal T2-weighted, and (H) coronal T2-weighted brain MRI highlighting the CM1 (arrowhead, tonsillar herniation measured 10 mm below the drawn McRae line) and a 6 mm lacune in the posterior part of the left lentiform nucleus (arrow, stable on control MRI 6 months later). Hand X-ray at (I) 12 months and (J) 32 months of age showing diffuse osteopenia, sparse trabeculae, absence of carpal bones ossification, short metacarpal bones (only described at 32 months), absence (or hypotrophy) of the medial phalange ossification centers, cone-shaped distal phalanges and clinodactyly of the fifth digit.

Figure 2

Genealogy, sequencing and bioinformatics supported the pathogenicity of the c.723C>G p.(Asp241Glu) variant in the PTH1R gene (NM_000316.3). (A) Pedigree of the family showing the index case (arrow), who is homozygous for the PTH1R variant (+/+), his unaffected (white symbol) parents (I.1 and I.2) and sister (II.1), who are heterozygous carriers of the variant (+/-), and his unaffected sister (II.2) who does not carry the PTH1R variant (-/-). (B) Sanger chromatograms confirming the homozygous and heterozygous status of our patient and his parents. (C) Next-generation sequencing uncovering the homozygous transversion, cytosine to guanine, with a sequencing depth of 132 x in the proband (upper panel) compared to heterozygous carrier status in his parents (lower panels). (D) Protein sequence alignment of 12 vertebrates species highlighting conservation of the amino-acid residue (red rectangle) up to the zebrafish. (E) WebLogo analysis of the region 234-248 among PTH1R sequences showing the high conservation of the position D241. The picture was produced via the web server weblogo with 979 protein sequences collected by a Blast search in the UniRef100 database and aligned with the clustal2.1 algorithm. (F) Ribbon representation of the crystal complex structure (PDB 6FJ3) between the PTH1R protein (in blue) and its peptide ligand (parathyroid hormone, in pink) highlighting the patient’s affected residue (D241, labeled and depicted in sphere representation) in close vicinity with the peptide hormone. Four other affected PTH1R residues previously reported in iPPSD1 patients are also labeled and showed in the 3D structure (see Supplementary Table 3 ). In residue sphere representation, atoms of carbon, oxygen and nitrogen are colored in green, red and blue, respectively. The C-terminal extremity of peptide hormone PTH is indicated. TMD and ECD mean transmembrane domain and extra-cellular domain.