An Inverse Agonist Ligand of the PTH Receptor Partially Rescues Skeletal Defects in a Mouse Model of Jansen's Metaphyseal Chondrodysplasia

Abstract

Jansen's metaphyseal chondrodysplasia (JMC) is a rare disease of bone and mineral ion physiology that is caused by activating mutations in PTHR1. Ligand‐independent signaling by the mutant receptors in cells of bone and kidney results in abnormal skeletal growth, excessive bone turnover, and chronic hypercalcemia and hyperphosphaturia. Clinical features further include short stature, limb deformities, nephrocalcinosis, and progressive losses in kidney function. There is no effective treatment option available for JMC. In previous cell‐based assays, we found that certain N‐terminally truncated PTH and PTHrP antagonist peptides function as inverse agonists and thus can reduce the high rates of basal cAMP signaling exhibited by the mutant PTHR1s of JMC in vitro. Here we explored whether one such inverse agonist ligand, [Leu11,dTrp12,Trp23,Tyr36]‐PTHrP(7‐36)NH2 (IA), can be effective in vivo and thus ameliorate the skeletal abnormalities that occur in transgenic mice expressing the PTHR1‐H223R allele of JMC in osteoblastic cells via the collagen‐1α1 promoter (C1HR mice). We observed that after 2 weeks of twice‐daily injection and relative to vehicle controls, the IA analog resulted in significant improvements in key skeletal parameters that characterize the C1HR mice, because it reduced the excess trabecular bone mass, bone marrow fibrosis, and levels of bone turnover markers in blood and urine. The overall findings provide proof‐of‐concept support for the notion that inverse agonist ligands targeted to the mutant PTHR1 variants of JMC can have efficacy in vivo. Further studies of such PTHR1 ligand analogs could help open paths toward the first treatment option for this debilitating skeletal disorder. © 2019 American Society for Bone and Mineral Research.

Mutations in G Protein–Coupled Receptors: Mechanisms, Pathophysiology and Potential Therapeutic Approaches

“…Besides small compounds, monoclonal antibodies that probably modulate the isomerization of the internal agonist of this receptor could also be suitable to suppress constitutive activity of the TSHR (Chen et al, 2018a). An inverse agonist approach has also been used to rescue the phenotype of mice expressing a constitutively active parathyroid hormone receptor (Noda et al, 2020). Viral genome-encoded GPCR can constitutively activate signaling cascades (see above).…”

Genetic causes of neonatal and infantile hypercalcaemia

“…Jansen’s metaphyseal chondrodysplasia (JMC, OMIM #156400) is an autosomal dominant disease characterised by short-limbed short stature, deformed, under-mineralised bones, chronic hypercalcaemia and hyperphosphaturia. Most patients have low or undetectable serum PTH levels and elevated serum markers of bone turnover [ 37 ].…”

“…All reported mutations occur in three residue positions His223, Thr410 and Ile458 (located in transmembrane helices 2, 6 and 7, respectively), which are located in a bundle at the cytoplasmic face of the receptor [ 39 , 41 ]. Structural models indicate these residues contribute to a network that controls the outward movements of the transmembrane helices, allowing formation of a cavity at which G proteins can engage with the cytoplasmic region of the receptor during activation [ 37 ]. Such a critical role for these residues in receptor activation is consistent with in vitro findings that JMC mutations are associated with ligand-independent constitutive activation of cAMP signalling [ 39 ].…”

“…There are currently no effective treatments for JMC, although the use of bisphosphonates in older children to reduce hypercalcaemia has been reported [ 42 , 46 ]. Recently, the effectiveness of an inverse agonist of the PTH1R was tested in a transgenic mouse model expressing the PTH1R-His223Arg mutation in osteoblast cells [ 37 ]. Mutant mice showed significant improvements in skeletal parameters including excess trabecular bone mass, bone marrow fibrosis and levels of bone turnover markers, although there was no effect on bone length [ 37 ].…”

Proceedings of the 2020 Rare Bone Disease Working Group

“…Jüppner and colleagues developed several PTH-PTHrP antagonist analogs that function as inverse agonists on the PTHR1-JMC mutants when tested in vitro using transfected cells. One of these inverse agonists, L11,dW12,W23,Y36-PTHrP(7-36), or PTH-IA, can partially reverse the skeletal defects induced in transgenic mice that express via the type I collagen promoter the most frequent JMC mutation, H223R-PTHR1 (Col1-H223R mice) 8 . PTH-IA thus effectively reduced the trabecular bone mass, the bone marrow fibrosis, the levels of bone turnover markers, and improved bone length in the Col1-H223R mice.…”