Novel PTH Gene Mutations Causing Isolated Hypoparathyroidism

Abstract

Context: Parathyroid hormone (PTH) gene mutations represent a rare cause of familial isolated hypoparathyroidism (FIH). These defects can cause hypoparathyroidism with increased or decreased serum levels of PTH through 1) impaired PTH synthesis; 2) induction of parathyroid cell apoptosis; or 3) secretion of bioinactive PTH molecules. Eight pathogenic mutations of this gene have been described previously.

Objective: Through describing 2 novel mutations of the PTH gene, we aim to extend the molecular basis for FIH and further refine the proposed mechanisms by which PTH mutations cause hypoparathyroidism.

Methods: Proband case reports were compiled with extended family analysis. The probands in both kindreds presented before age 10 days with hypocalcemia and elevated phosphate levels. Proband A had low PTH levels, whereas these levels were elevated in Proband B. Proband B was initially diagnosed with pseudohypoparathyroidism. Methylation analysis was performed of CpG dinucleotides within 3 GNAS differentially methylated regions; whole-genome sequencing; and PTH infusion with analysis of nephrogenous 3',5'-cyclic adenosine 5'-monophosphate.

Results: Proband A had a novel heterozygous sequence change in exon 2 of the PTH gene, c.46_47delinsAA (p.Ala16Lys), and proband B had a novel homozygous nucleotide transition in PTH exon 3 (c.128G > A; p.G43E) that led to replacement of glycine by glutamic acid at position 12 of PTH 1-84. PTH 1-34 infusion demonstrated that renal responsiveness to PTH was intact and not antagonized by circulating bioinactive PTH.

Conclusion: PTH gene mutations are uncommon causes of hypoparathyroidism, but can be misdiagnosed as disorders of gland development or receptor function if PTH levels are decreased or elevated, respectively. Genetic testing should be considered early in the diagnostic approach to these presentations.

Keywords: PTH; bioinactive; genetic; hypocalcemia; hypoparathyroidism; parathyroid.

Figure 1.

Pedigrees of the 2 investigated families. Individuals with biochemical hypoparathyroidism are depicted by solid-colored symbols; unaffected individuals are depicted by open symbols. Those participants who underwent genetic testing have genotypes below the symbol; WT is a wild-type parathyroid hormone (PTH) gene allele and M is a mutant allele. Squares denote males and circles denote females. A, The proband, denoted by the arrow, is heterozygous for the PTH gene mutation and her parents are homozygous for WT PTH alleles. B, The proband, denoted by the arrow, and 2 siblings are homozygous for the PTH gene mutation, whereas his parents and 1 brother are heterozygous.

Figure 2.

Schematic representation of the parathyroid hormone (PTH) protein. The complete preproparathyroid hormone (preproPTH) molecule is shown in the center with positions of point mutations that affect the protein shown beneath the figure; positions of the 3 mutations that cause bioinactive PTH are also depicted above the figure in the mature, PTH 1-84 molecule. The lower figure shows preproPTH, which consists of 115 amino acids; the specific regions comprise a 25-amino acid pre sequence (–31 to –7), a 6-amino acid pro sequence (–6 to –1), and 84-amino acid mature PTH molecule (+1 to 84). The upper figure shows epitopes used for detection of whole PTH (1-4) or intact PTH (13-34) as well as the epitope used to capture PTH fragments (39-84) in conventional sandwich assays.

Figure 3.

Evaluation of the preproparathyroid hormone (preproPTH) as a signal peptide using the SignalP hidden Markov model program. A, Wild-type (WT) parathyroid hormone 1-84; B, p.A16K; C, p.M14K; D, p.C18R proteins; black arrows denote replaced amino acids. A highly relevant probability score was obtained for likelihood of N-terminal signal sequence (dotted red line) and likelihood is predicted for cleavage site (dotted green line). The WT protein (A) has a strongly predicted signal peptide cleavage site between amino acids 25 and 26. Residues are numbered conventionally, with the first residue (M) of the preproPTH polypeptide designated as +1. See text for “Materials and Methods”.

Figure 4.

Secondary structure predicted by the Chou-Fasman algorithm. We used the CFSPP online server to analyze the predicted locations of alpha-helix and beta-strand from the amino acid sequence of A, wild-type (WT), and B, p.G43E parathyroid hormone proteins. In each panel, a black arrow denotes either the WT amino acid or the substituted amino acid at position 43. Alpha helices are shown in light grey and beta strands are shown in dark grey.

Figure 5.

Teriparatide infusion in 2 affected individuals from family B. Effects of teriparatide on affected individuals VI-2 (left) and VI-3 (right) are shown. Teriparatide (40 µg) was administered subcutaneously at time 0. A, Upper panels show the changes in urinary excretion of 3′,5′-cyclic adenosine 5′-monophosphate (cAMP) and tubular maximum phosphate reabsorption per glomerular filtration rate (TMP/GFR) after infusion; B, lower panels show changes in serum levels of calcium, phosphate, and intact parathyroid hormone (PTH) as well as urinary calcium to creatinine ratio.