Jansen metaphyseal chondrodysplasia: analysis of craniofacial manifestations

Jansen metaphyseal chondrodysplasia (JMC) is an ultra-rare disorder caused by constitutive activation of parathyroid hormone type 1 receptor (PTH1R). We sought to characterize the craniofacial phenotype of patients with the disease. Six patients with genetically confirmed JMC underwent comprehensive craniofacial phenotyping revealing a distinct facial appearance that prompted a cephalometric analysis demonstrating a pattern of mandibular retrognathia. Oral examination was notable for flat and shallow palate, delayed eruption pattern, and impacted maxillary teeth. Subclinical and/or mild hearing loss was noted in 4 of 5 patients studied. The most common etiology was conductive, likely due to overcrowding of epitympanum which impedes the normal vibration of ossicles to sound. Paranasal sinus obliteration was noted in 5 of 6 patients. Computed tomography (CT) scan evaluation of craniofacial bones revealed bilaterally symmetric expansile lesions with predominant involvement of neural crest cell (NCC)-derived bones. Bilateral narrowing of facial nerve canals, particularly at the labyrinthine segment, was seen in 5 of 6 patients when compared to age-matched controls; 1 patient presented with progressive facial nerve palsy. Sagittal suture craniosynostosis was present in 5 of 6 patients-one of whom had a history of cranial reconstruction for pansynostosis in infancy. All patients demonstrated a significant degree of upper airway stenosis, as well as a more anterior hyoid bone displacement. Two patients had a diagnosis of obstructive sleep apnea. 18F-NaF Positron-emission tomography (PET)-CT revealed increased uptake associated with the skull base and gnathic bones in all patients. In conclusion, this first detailed systematic evaluation of the craniofacial phenotype of patients with JMC demonstrates a distinct and pronounced phenotype that predominantly affects the NCC-derived cranial bones indicating a critical role of PTH1R signaling in their development. These affects can result in significant disease-related morbidity, include hearing loss, nerve compression, craniosynostosis, dentoskeletal malocclusion, and airway compromise; all of which require close monitoring.