Targeted silencing of GNAS in a human model of osteoprogenitor cells results in the deregulation of the osteogenic differentiation program.

Introduction: The dysregulation of cell fate toward osteoprecursor cells associated with most GNAS-based disorders may lead to episodic de novo extraskeletal or ectopic bone formation in subcutaneous tissues. The bony lesion distribution suggests the involvement of abnormal differentiation of mesenchymal stem cells (MSCs) and/or more committed precursor cells. Data from transgenic mice support the concept that GNAS is a crucial factor in regulating lineage switching between osteoblasts (OBs) and adipocyte fates. The mosaic nature of heterotopic bone lesions suggests that GNAS genetic defects provide a sensitized background for ectopic osteodifferentiation, but the underlying molecular mechanism remains largely unknown. Methods: The effect of GNAS silencing in the presence and/or absence of osteoblastic stimuli was evaluated in the human L88/5 MSC line during osteodifferentiation. A comparison of the data obtained with data coming from a bony lesion from a GNAS-mutated patient was also provided. Results: Our study adds some dowels to the current fragmented notions about the role of GNAS during osteoblastic differentiation, such as the premature transition of immature OBs into osteocytes and the characterization of the differences in the deposed bone matrix. Conclusion: We demonstrated that our cell model partially replicates the in vivo behavior results, resulting in an applicable human model to elucidate the pathophysiology of ectopic bone formation in GNAS-based disorders.

Novel Pathogenetic Variants in PTHLH and TRPS1 Genes Causing Syndromic Brachydactyly.

Skeletal disorders, including both isolated and syndromic brachydactyly type E, derive from genetic defects affecting the fine tuning of the network of pathways involved in skeletogenesis and growth-plate development. Alterations of different genes of this network may result in overlapping phenotypes, as exemplified by disorders due to the impairment of the parathyroid hormone/parathyroid hormone-related protein pathway, and obtaining a correct diagnosis is sometimes challenging without a genetic confirmation. Five patients with Albright's hereditary osteodystrophy (AHO)-like skeletal malformations without a clear clinical diagnosis were analyzed by whole-exome sequencing (WES) and novel potentially pathogenic variants in parathyroid hormone like hormone (PTHLH) (BDE with short stature [BDE2]) and TRPS1 (tricho-rhino-phalangeal syndrome [TRPS]) were discovered. The pathogenic impact of these variants was confirmed by in vitro functional studies. This study expands the spectrum of genetic defects associated with BDE2 and TRPS and demonstrates the pathogenicity of TRPS1 missense variants located outside both the nuclear localization signal and the GATA ((A/T)GATA(A/G)-binding zinc-containing domain) and Ikaros-like binding domains. Unfortunately, we could not find distinctive phenotypic features that might have led to an earlier clinical diagnosis, further highlighting the high degree of overlap among skeletal syndromes associated with brachydactyly and AHO-like features, and the need for a close interdisciplinary workout in these rare patients. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

2q37 Deletions in Patients With an Albright Hereditary Osteodystrophy Phenotype and PTH Resistance.

Pseudohypoparathyroidism (PHP) is a rare endocrine disorder derived from the defective activation of the cAMP pathway by the parathyroid hormone secondary to GNAS molecular defects. PHP subtypes are defined by the presence/absence of specific clinical/biochemical features. PHP1A is characterized by resistance to multiple hormones with features of Albright hereditary osteodystrophy (AHO), while pseudopseudohypoparathyroidism (PPHP) is characterized by AHO in the absence of PTH resistance. Small subsets of PHP and PPHP patients without known molecular defects have been re-diagnosed as being affected by the brachydactyly-mental retardation syndrome (BDMR), also known as the AHO-like syndrome. This study aimed to analyse 24 PHP1A and 51 PPHP patients without a molecular diagnosis for the presence of BDMR-associated 2q37 deletions to improve the differential diagnosis and to identify features that might help to avoid a misdiagnosis. Molecular investigations identified 4 deletions in 4 unrelated patients. The affected patients showed a combination of the most pathognomonic AHO features. Of note, 3 of the patients also displayed mild PTH resistance, and none of the patients developed ectopic ossifications. Our work confirmed the rarity of the misdiagnosis of BDMR in PHP patients through the identification of 4 patients bearing a 2q37 deletion in a cohort of 73 PHP patients (5.3%). Three patients with the deletion presented a PHP1A phenotype in the absence of any BDMR-specific findings. Further studies on larger case series are needed to elucidate the overlap between these clinical entities and to allow the early identification of patients.