Autosomal dominant and recessive forms of isolated hypoparathyroidism are caused by mutations of the PTH gene, which consists of three exons and is located on chromosome 11p15 (Figure 1). Exon 1 of the PTH gene is untranslated, whereas exons 2 and 3 encode the 115- amino acid pre- pro- PTH peptide. Exon 2 encodes the initiation (ATG) codon, the prehormone sequence and part of the prohormone sequence, while exon 3 encodes the remainder of the prohormone sequence, the mature 84- amino acid PTH peptide, and the 3’ untranslated region (Figure 1).
Fig1. Schematic representation of the PTH gene. The PTH gene consists of three exons and two introns; the peptide is encoded by exons 2 and 3. The PTH peptide is synthesized as a precursor which contains a pre- and a pro- sequence. The mature PTH peptide, which contains 84 amino acids, and larger carboxy- terminal PTH fragments are secreted from the parathyroid cell. The polymorphic sites associated with the PTH gene are indicated. Two restriction fragment length polymorphisms (RFLPs) are associated with the PTH gene, and the TaqI polymorphic site is within intron 2 and the PstI polymorphic site is 1.7 kbp downstream in the 3′ direction of the gene. Two other polymorphisms of the PTH gene designated Mir1 and Mir2 are located in intron 1 and exon 3, respectively, and the tetranucleotide (AAAT)n polymorphism is in intron 1. Linkage disequilibrium between the (AAAT)n, TaqI, and PstI polymorphic sites has been established. Adapted with permission from Parkinson DB, Thakker RV. A donor splice site mutation in the parathyroid hormone gene is associated with autosomal recessive hypoparathyroidism. Nat Genet. 1992;1(2):149– 52. Copyright © 1992, Springer Nature. (ref 24).
Autosomal dominant hypoparathyroidism due to heterozygous PTH mutations has been reported in two unrelated families. In one family, a heterozygous single base substitution (T→C) in exon 2 of the PTH gene, which resulted in a p.Cys18Arg missense substitution within the hydrophobic core of the signal peptide was shown to impair processing of the mutant pre- pro- PTH to pro- PTH in vitro. However, secretion of the preproPTH pep tide harbouring the p.Cys18Arg mutation could be enhanced by the addition of 4- phenylbutyric acid, which is a pharmacological chaperone. In another family, a heterozygous single base sub stitution (T→A) in exon 2 of the PTH gene, which resulted in a p.Met14Lys missense substitution in the signal peptide has been reported. This mutation was predicted to impair cleavage of pre- pro- PTH, and in vitro studies showed intracellular retention of the mutant pre- pro- PTH peptide, which could be partially rescued by the addition of 4- phenylbutyric acid.
Autosomal recessive hypoparathyroidism due to homozygous PTH mutations that impaired processing of the PTH peptide have been identified in three unrelated families. In one such family a donor splice- site mutation at the exon 2- intron 2 boundary has been identified. This mutation involved a single base transition (g→c) at position 1 of intron 2 and an assessment of the effects of this alteration in the invariant gt dinucleotide of the 5’ donor splice- site consensus on mRNA processing revealed that the mutation resulted in exon skipping, in which exon 2 of the PTH gene was lost and exon 1 was spliced to exon 3. The lack of exon 2 would lead to a loss of the initiation codon (ATG) and the signal peptide sequence, which are required respectively for the commencement of PTH mRNA translation and for the translocation of the PTH peptide. In another family with autosomal recessive hypopara thyroidism a homozygous single base substitution (T→C) in exon 2, which resulted in a p.Ser23Pro missense substitution in the signal peptide was detected. This mutation leads to the introduction of a mutant proline residue at the – 3 position of the pre- pro- PTH protein cleavage site and is predicted to disrupt cleavage of the mu tant pre- pro- PTH protein. In the third unrelated family with autosomal recessive hypoparathyroidism, a homozygous single base substitution (C→A) in exon 2, which resulted in a stop codon (p.Ser23Stop) was detected. This stop codon was predicted to be introduced at the pre- pro- PTH protein cleavage site, and would lead to a truncated and inactive PTH peptide. A homozygous arginine- to- cysteine mutation has also been identified at codon 25, (p.Arg25Cys), of the mature PTH(1- 84) peptide in a family with hypocalcaemia. The plasma PTH levels of affected family members varied from low- normal to markedly elevated, depending on the type of PTH assay used. In contrast to previously re ported PTH gene mutations, which affect secretion of PTH, the p.Arg25Cys missense substitution was shown to diminish the binding of the mutant PTH peptide with the PTH/ PTHrP receptor. Moreover, the PTH p.Arg25Cys mutation interfered with PTH immunoassays that utilized antibodies affinity- purified using PTH 1- 34 and 13- 34 fragments, thereby explaining why some assays were unable to detect the mutant PTH peptide
