Our finding that activating DDR variants are a cause
Our finding that activating DDR2 variants are a cause of this disease suggested that the ABL inhibitor dasatinib, a leukemia drug that also inhibits DDR2, could be used for treatment of affected individuals.10, 11 To examine the effect of dasatinib on p.Leu610Pro- and p.Tyr740Cys-induced autophosphorylation, we cultured fibroblasts from affected individuals and controls as described above. When the jnk inhibitor were 80%–90% confluent, the medium was replaced with serum-free DMEM. After 16 hours, cells were either left untreated or treated with 0.05 or 0.1μM dasatinib (#S1021, Selleckchem); they were then harvested after 6 hours as described above. Immunoblot analysis determined the presence of phospho-Tyr740-DDR2. At both concentrations, dasatinib abolished the observed autophosphorylation of DDR2 (Figure4C and Figures S22 and S23), providing invitro support for experimental treatment of affected individuals. Penttinen syndrome, associated with activating mutations in PDGFRB, and Warburg-Cinottisyndrome have many similarities, such as lipodystrophy, subcutaneous-tissue wasting and accompanying hypertrophic lesions, and marked acro-osteolysis. Of note, PDGF-targeted therapy has been effective in three reported individuals with germline activating PDGFRB mutations.13, 14 DDR2 is an important regulator of bone growth and resorption, both as a promoter of osteoblastogenesis and as an inhibitor of osteoclastogenesis.15, 16 DDR2 has been suggested as a therapeutic target for osteoporosis. In addition, DDR2-collagen interaction stimulates the secretion of lysyl oxidase, which cross-links collagen fibers in the ECM.3, 17 Why activating mutations in DDR2 might be associated with osteolysis in the individuals described here remains to be elucidated. However, bi-allelic loss-of-function variants in DDR2 cause spondylometaepiphyseal dysplasia accompanied by short limbs and abnormal (premature) calcifications (SMED-SL [MIM: 271665]).18, 19 This developmental disorder is associated with decreased bone formation but not increased bone destruction (or osteolysis). No skin or eye changes have been reported.18, 19, 20 The individuals described in this report had normal or tall stature and normal limb lengths, indicating normal developmental bone growth. They also had an acquired arthropathy with osteolysis that was associated with flexion contractures. Premature calcifications (as seen in SMED-SL) were not observed in any of the individuals with Warburg-Cinotti syndrome. Thus, the phenotype of the individuals reported here with gain-of-function DDR2 variants was distinct from that seen in SMED-SL, which is associated with loss-of-function DDR2 variants. The individuals described here experienced corneal vascularization in early adult life. DDR2 is able to both drive and prevent angiogenesis under different conditions.21, 22, 23 The primary effect seems to be angiogenesis stimulation, but a rebound effect or overcompensation can counterbalance this. In several of the individuals, limbal stem-cell deficiency (LSCD) was detected at an early age. The corneal epithelium undergoes constant shedding and regeneration to maintain optic clarity. The surrounding limbus forms a barrier to protect the cornea from neovascularization. In LSCD this barrier is disrupted, and the corneal epithelium might be replaced with conjunctival cells, similar to what we observed in these individuals. Although symptoms of LSCD often include signs of inflammation (redness and irritation), corneal neovascularization might also develop in the absence of such signs. One example is in aniridia caused by haploinsufficiency of PAX6.24, 25 The individuals described here had multiple skin problems, including thin skin, chronic ulcers, and a tendency to form keloid-like lesions. In mice, the highest levels of phosphorylated DDR2 were found in the lungs, ovaries, and skin. DDR2 is thought to be both a marker and key regulator of the epithelial-mesenchymal transition. DDR2 signaling is important for wound healing through multiple mechanisms: chemotactic migration to the wounded area, proliferation, synthesis and remodeling of the wound matrix by collagen cross-linking, and finally, fibroblast-mediated contraction of the healing wound. Inhibition of DDR2 activity has been postulated as a therapeutic option to improve wound healing and reduce keloid formation, which is supported by our data.