SCFJFK regulates osteoblast differentiation through targeting RUNX2.

RUNX2 (Runt-related transcription factor 2) is a master regulator of osteogenesis, and its genetic mutations are associated with ~65% cases of cleidocranial dysplasia (CCD), an autosomal dominant abnormal bone development disorder in humans with defective intramembranous bone formation. However, how these mutations affect bone formation remains to be investigated. Here, we report that RUNX2 interacts with the F-box protein JFK and is destabilized by the SKP1-CUL1-JFK E3 ubiquitin ligase complex (SCFJFK). We find that several CCD-associated mutations of RUNX2 acquire an increased affinity toward SCFJFK thus an accelerated proteasomal degradation. We demonstrate that SCFJFK-mediated RUNX2 degradation suppresses osteoblast differentiation. Consistently, Jfk-null mice exhibit increased trabecular bone volume and bone mineral density and are resistant to Runx2 haploinsufficiency-induced CCD-like syndrome. Interestingly, RUNX2 binds to the JFK promoter and represses its transcription, establishing a feedback loop to promote osteoblast differentiation, and remarkably, the CCD-associated RUNX2 mutants also display an impaired transcription repression of JFK. Our study demonstrates SCFJFK as an E3 ligase for RUNX2 and uncovers a feedback regulatory loop between SCFJFK and RUNX2 that is implemented in bone development and implicated in CCD, supporting the pursuit of JFK as a potential target to ameliorate CCD-like syndrome.