7598 A Humanized PTH Receptor Mouse Model Of Eiken Syndrome: Delayed Bone Mineralization Caused By A Receptor C-tail Truncation

Abstract Disclosure: J. Höppner: None. P. Hanna: None. M.N. Wein: None. H. Jueppner: None. T.J. Gardella: None. R. Civitelli: None. I.A. Portales-Castillo: None. Abstract The parathyroid hormone receptor-1 (PTH1R) plays a key role in bone development and acts in the growth plates in response to PTHrP ligand. Eiken syndrome is characterized by a delay in bone mineralization and is caused by homozygous PTH1R mutations. One such mutation, R485X, truncates the receptor's C-tail and removes serine phosphorylation sites involved in βarrestin binding. In HEK293 cells, R485X-hPTH1R exhibits deficient interaction with βarrestin and increased cAMP signaling both basally and in response to PTHrP (Portales-Castillo et al., Comms. Biology 2023). To understand how the R485X mutation causes delayed bone mineralization, we generated humanized R485X-hPTH1R knock-in mice. Homozygous hPTH1RR485X/R485X mice closely recapitulate the delayed bone mineralization seen in Eiken patients, as skeletal whole mount preparations from neonatal mutant mice showed reduced Alizarin red staining compared to WT controls, and metatarsal explants from the mutant mice showed a pronounced absence of mineralized bone. Tails of hPTH1RR485X/R485X mice were ∼50% shorter than those of WT littermates, and H&E-stained sections revealed only proliferative chondrocytes in the growth plates of mutant mice. This delay in growth plate chondrocyte maturation in hPTH1RR485X/R485X mice resolved with age, although older long bones and tails remained smaller in size than WT controls (Höppner et al. Presented at ASBMR 2023).Based on our in vitro findings and initial mouse characterization, we reasoned that the phenotype of Eiken mice may be explained by increased PTHrP/PTH1R signaling in growth plate chondrocyte. The class IIa histone deacetylase HDAC4 suppresses chondrocyte maturation downstream of PTH1R signaling. Therefore, we generated compound mutant mice bearing both Hdac4 deletion and the mutant R485X PTH1R allele. Indeed, Hdac4 deletion largely rescued the mineralization defect apparent in metatarsals of P1 hPTH1RR485X/R485X mice. We then assessed the contribution of endogenous PTHrP to the Eiken-like phenotype by generating hPTH1RR485X/R485X/PTHrPflox/+/Col2-Cre(tg) mice. Remarkably, these mice exhibit approximately the same tail lengths as hPTH1R-WT littermate controls, indicating that reduced PTHrP production can rescue the effects of homozygosity for R485X-PTH1R. In line, vertebral growth plates of the rescued mice exhibited normal zones of chondrocytes, including hypertrophic cells. The overall results support a disease mechanism for Eiken syndrome by which excess cAMP signaling by PTH1R-R485X, as induced in part by endogenous PTHrP, results in delayed chondrocyte differentiation and bone mineralization. Further studies employing βarr1/2-KO mice may help shed light on the specific roles of βarrestins in growth plate development by PTH1R in Eiken syndrome. Presentation: 6/3/2024