The parathyroid hormone receptor 1 (PTH1R) transmits stimuli provided by parathyroid hormone (PTH) and PTH-related protein (PTHrP) and thus plays key roles in calcium and phosphate homeostasis as well as skeletal development. Variants in PTH1R have been linked to several conditions including Jansen's metaphyseal chondrodysplasia, Blomstrand chondrodysplasia, Primary Failure of Tooth Eruption and Eiken syndrome. Here, we report a novel skeletal phenotype identified in two unrelated families associated with PTH1R variants. The clinical features include brachydactyly type E (BDE), mild short stature, and dental anomalies. A novel heterozygous PTH1R substitution (p.E469K) was identified in affected members of Family 1, while the affected individual from Family 2 had a previously described heterozygous de novo substitution (p.E465K); these two mutated sites lie within helix 8 of the PTH1R. Cell-based assays revealed reduced cell surface expression, as well as impaired basal and PTH- or PTHrP-induced cAMP signaling responses for both mutants, as compared to WT-PTH1R. Introduction of the p.E469K substitution into humanized PTH1R mice resulted in mildly increased mineralization of bones in the paws as well as shortening of long bones. Our findings demonstrate a new skeletal phenotype associated with PTH1R variants and suggest that helix 8 of the receptor contributes to PTH1R expression and/or signaling during bone development. The parathyroid hormone receptor 1 (PTH1R) is essential for calcium signaling and bone development. Changes in the PTH1R gene and/or protein affect its ability to function properly thus leading to disease. In a large family and an unrelated case, we identified changes in the PTH1R gene as the cause of short fingers and toes (Brachydactyly type E; BDE), short stature, and dental abnormalities, which is novel for PTH1R variants. Through experiments in cells and mice, we showed that the altered PTH1R protein affects cellular signaling and function, and leads to abnormal bone development. This work improves the understanding of PTH1R-related signaling and disease.

The parathyroid hormone receptor type 1 (PTH1R) is a G protein-coupled receptor that mediates the actions of parathyroid hormone (PTH) in the regulation of blood calcium levels, as well as PTH-related protein (PTHrP) in the regulation of skeletal development. Severe loss-of-function homozygous mutations in PTH1R are incompatible with life as in Blomstrand's lethal chondrodysplasia, characterized by accelerated growth plate ossification. More recently, homozygous mutations located in the transmembrane helices, extracellular domains and C-tail of the PTH1R were identified in patients with milder conditions characterized by variable degrees of skeletal and mineral abnormalities. These include delayed ossification in Eiken syndrome, hypocalcemia in a pseudohypoparathyroidism-like disorder, and non-syndromic primary failure of tooth eruption; which is usually caused by heterozygous PTH1R mutations. Recent detailed pharmacologic characterization of these PTH1R mutants has revealed new insights into how even subtle perturbations in PTH1R function can result in disease.

We report on 2 patients of East African ancestry with the same novel homozygous variant in the parathyroid hormone receptor type 1 (PTH1R). Both patients shared skeletal features, including brachydactyly, extensive metacarpal pseudo-epiphyses, elongated cone-shaped epiphyses, ischiopubic hypoplasia, and deficient sacral ossification, suggestive of Eiken syndrome. Strikingly, both patients exhibited clinically manifest parathyroid hormone (PTH) resistance with hypocalcemia and elevated serum phosphate levels. These laboratory and clinical abnormalities initially suggested pseudohypoparathyroidism, which is typically associated with GNAS abnormalities. In both patients, however, a homozygous novel PTH1R variant was identified (c.710 T > A; p.IIe237Asn, p.I237N) that is located in the second transmembrane helical domain. Previously, others have reported a patient with a nearby PTH1R mutation (D241E) who presented with similar clinical features (eg, delayed bone mineralization as well as clinical PTH resistance). Functional analysis of the effects of both novel PTH1R variants (I237N- and D241E-PTH1R) in HEK293 reporter cells transfected with plasmid DNA encoding the wild-type or mutant PTH1Rs demonstrated increased basal cAMP signaling for both variants, with relative blunting of responses to both PTH and PTH-related peptide (PTHrP) ligands. The clinical presentation of PTH resistance and delayed bone mineralization combined with the functional properties of the mutant PTH1Rs suggest that this form of Eiken syndrome results from alterations in PTH1R-mediated signaling in response to both canonical ligands, PTH and PTHrP. Eiken syndrome is rare genetic disorder of skeletal development, 7 due to alterations in the gene for parathyroid hormone receptor type 1 (PTH1R). This receptor can bind 2 hormones: parathyroid hormone (PTH), the body’s main regulator of the level of calcium in blood, and PTH-related peptide (PTHrP), that regulates bone development. We report 2 new cases of Eiken syndrome sharing the same not previously reported change in PTH1R. The patients had typical findings in the skeleton reported in previous cases, but with some variation in the features. Unlike previously reported cases, the 2 patients we describe had low blood calcium levels causing symptoms. We wanted to explore how this new mutation affects the function of the receptor, particularly how it might affect the signals generated when the receptor binds to its 2 t hormones, PTH and PTHrP. We genetically reprogrammed a cell line with the new mutation, and tested those cells’ responses to stimulation by the hormones. We showed that the altered receptor appears unable to bind both hormones in a stable fashion, explaining why the patients showed changes both in the skeleton (due to altered PTHrP signaling) and in the blood level of calcium (due to altered PTH signaling). In primary hypoparathyroidism with hypocalcemia and hyperphosphatemia, deficient parathyroid hormone (PTH) secretion most commonly occurs from surgical excision of, or damage to, the parathyroid glands. The term idiopathic hypoparathyroidism describes isolated cases when a cause is not obvious, and there is no family history. However, hypoparathyroidism is also a feature common to a variety of hereditable syndromes that may present de novo. Familial isolated hypoparathyroidism may show autosomal dominant, autosomal recessive, or X-linked inheritance. Genes involved include PTH, SOX3, CASR, GNA11 and GCM2. Parathyroid hypoplasia is a frequent feature of 22q11.2 deletion syndrome with involvement of the TBX1 gene. The Hypoparathyroidism, Nerve Deafness, and Renal Dysplasia syndrome is due to haploinsufficiency of the GATA3 gene. Antibodies against parathyroid tissue are found in isolated hypoparathyroidism or combined with other endocrine deficiencies. Antibodies against the CASR occur in type 1 autoimmune polyglandular syndrome, due to mutations of the AIRE gene, or in acquired hypoparathyroidism. Disorders characterized by end-organ resistance to PTH are described collectively by the term pseudohypoparathyroidism (PHP), and PHP1A and PHP1B are caused by maternally-inherited changes at the imprinted GNAS complex gene that encodes the Gsα protein. Deleterious mutations of the PTH1R gene show resistance to PTH and PTHrP and present as Blomstrand lethal chondrodysplasia, Eiken syndrome, endochondromatosis, and primary failure of tooth eruption. Calcium and vitamin D are the standard therapy for the management of hypoparathyroidism, with hormone replacement [recombinant human PTH(1-84)] therapy recently becoming an option. Calcilytics, PTH analogs, and orally active small molecule PTH1R agonists may, in the future, join the treatment armamentarium. For complete coverage of all related areas of Endocrinology, please visit our on-line FREE web-text, WWW.ENDOTEXT.ORG.

Mutations in PTH1R are associated with Jansen-type metaphyseal chondrodysplasia (JMC), Blomstrand osteochondrodysplasia (BOCD), Eiken syndrome, enchondroma, and primary failure of tooth eruption (PFE). Inheritance of the PTH1R gene can be either autosomal dominant or autosomal recessive, indicating the complexity of the gene. Our objective was to identify the phenotypic differences in members of a family with a novel PTH1R mutation. The proband was a 13-year, 6-month-old girl presenting with short stature, abnormal tooth eruption, skeletal dysplasia, and midface hypoplasia. The brother and father of the proband presented with short stature and abnormal tooth eruption. High-throughput sequencing was performed on the proband, and the variant was confirmed in the proband and other family members by Sanger sequencing. Amino acid sequence alignment was performed using ClustalX software. Three-dimensional structures were analyzed and displayed using the I-TASSER website and PyMOL software. High-throughput genome sequencing and Sanger sequencing validation showed that the proband, her father, and her brother all carried the PTH1R (NM_000316) c.1393G>A (p.E465K) mutation. The c.1393G>A (p.E465K) mutation was novel, as it has not been reported in the literature database. According to the American College of Medical Genetics and Genomics (ACMG) guidelines, the p.E465K variant was considered to have uncertain significance. Biological information analysis demonstrated that this identified variant was highly conserved and highly likely pathogenic. We identified a novel heterozygous mutation in the PTH1R gene leading to clinical manifestations with incomplete penetrance that expands the spectrum of known PTH1R mutations.

The parathyroid hormone receptor type 1 (PTH1R) is a G protein-coupled receptor that plays key roles in regulating calcium homeostasis and skeletal development via binding the ligands, PTH and PTH-related protein (PTHrP), respectively. Eiken syndrome is a rare disease of delayed bone mineralization caused by homozygous PTH1R mutations. Of the three mutations identified so far, R485X, truncates the PTH1R C-terminal tail, while E35K and Y134S alter residues in the receptor's amino-terminal extracellular domain. Here, using a variety of cell-based assays, we show that R485X increases the receptor's basal rate of cAMP signaling and decreases its capacity to recruit β-arrestin2 upon ligand stimulation. The E35K and Y134S mutations each weaken the binding of PTHrP leading to impaired β-arrestin2 recruitment and desensitization of cAMP signaling response to PTHrP but not PTH. Our findings support a critical role for interaction with β-arrestin in the mechanism by which the PTH1R regulates bone formation.