Unsuspected osteochondroma-like outgrowths in the cranial base of Hereditary Multiple Exostoses patients and modeling and treatment with a BMP antagonist in mice

Hereditary Multiple Exostoses (HME) is a rare pediatric disorder caused by loss-of-function mutations in the genes encoding the heparan sulfate (HS)-synthesizing enzymes EXT1 or EXT2. HME is characterized by formation of cartilaginous outgrowths—called osteochondromas- next to the growth plates of many axial and appendicular skeletal elements. Surprisingly, it is not known whether such tumors also form in endochondral elements of the craniofacial skeleton. Here, we carried out a retrospective analysis of cervical spine MRI and CT scans from 50 consecutive HME patients that included cranial skeletal images. Interestingly, nearly half of the patients displayed moderate defects or osteochondroma-like outgrowths in the cranial base and specifically in the clivus. In good correlation, osteochondromas developed in the cranial base of mutant Ext1f/f;Col2-CreER or Ext1f/f;Aggrecan-CreER mouse models of HME along the synchondrosis growth plates. Osteochondroma formation was preceded by phenotypic alteration of cells at the chondro-perichondrial boundary and was accompanied by ectopic expression of major cartilage matrix genes -collagen 2 and collagen X- within the growing ectopic masses. Because chondrogenesis requires bone morphogenetic protein (BMP) signaling, we asked whether osteochondroma formation could be blocked by a BMP signaling antagonist. Systemic administration with LDN-193189 effectively inhibited osteochondroma growth in conditional Ext1-mutant mice. In vitro studies with mouse embryo chondrogenic cells clarified the mechanisms of LDN-193189 action that turned out to include decreases in canonical BMP signaling pSMAD1/5/8 effectors but interestingly, concurrent increases in such anti-chondrogenic mechanisms as pERK1/2 and Chordin, Fgf9 and Fgf18 expression. Our study is the first to reveal that the cranial base can be affected in patients with HME and that osteochondroma formation is amenable to therapeutic drug intervention.
Author summary The rare musculoskeletal pediatric disorder Hereditary Multiple Exostoses (HME) is characterized by benign cartilaginous outgrowths called osteochondromas that cause limb and axial skeletal problems and can also transform into life threatening chondrosarcomas. Surprisingly, no report has yet identified osteochondromas in the craniofacial skeleton though it contains several endochondral structures, including the cranial base. We report here for the first time that the cranial base of many HME patients does in fact exhibit defects and osteochondroma-like outgrowths and that similar lesions form in the cranial base of mouse models of HME. Of similar major importance is our novel demonstration that osteochondroma formation in mutant mice can be effectively inhibited by systemic treatment with a drug that blocks BMP signaling, an essential pro-chondrogenic pathway. Our study thus represents a major step ahead in understanding the pathogenesis and complexities of HME and provides proof-of-principle evidence that osteochondroma formation is amenable to drug-based treatment.