Objectives Given the role of growth and differentiation factor 5 (GDF5) in knee development and osteoarthritis risk, we sought to characterise knee defects resulting from Gdf5 loss of function and how its regulatory regions control knee formation and morphology.
Methods The brachypodism (bp) mouse line, which harbours an inactivating mutation in Gdf5, was used to survey how Gdf5 loss of function impacts knee morphology, while two transgenic Gdf5 reporter bacterial artificial chromosome mouse lines were used to assess the spatiotemporal activity and function of Gdf5 regulatory sequences in the context of clinically relevant knee anatomical features.
Results Knees from homozygous bp mice (bp/bp) exhibit underdeveloped femoral condyles and tibial plateaus, no cruciate ligaments, and poorly developed menisci. Secondary ossification is also delayed in the distal femur and proximal tibia. bp/bp mice have significantly narrower femoral condyles, femoral notches and tibial plateaus, and curvier medial femoral condyles, shallower trochlea, steeper lateral tibial slopes and smaller tibial spines. Regulatory sequences upstream from Gdf5 were weakly active in the prenatal knee, while downstream regulatory sequences were active throughout life. Importantly, downstream but not upstream Gdf5 regulatory sequences fully restored all the key morphological features disrupted in the bp/bp mice.
Conclusions Knee morphology is profoundly affected by Gdf5 absence, and downstream regulatory sequences mediate its effects by controlling Gdf5 expression in knee tissues. This downstream region contains numerous enhancers harbouring human variants that span the osteoarthritis association interval. We posit that subtle alterations to morphology driven by changes in downstream regulatory sequence underlie this locus’ role in osteoarthritis risk.
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SKP and AMK contributed equally.
Handling editor Josef S Smolen
Contributors TDC conceived and oversaw the project. HC and MS designed BAC transgenic mice. MS, HC and TDC performed mouse rescue experiments. SKP, JC, MY, and ZL performed mouse breeding and collected bp and BAC-LacZ tissue samples. SKP performed histology and lacZ staining. AMK performed all morphometric and statistical analyses on bp and BAC-rescue mice. SKP, AMK and TDC wrote the manuscript, with input from VR and all other authors.
Funding This work was funded in part by the Milton Fund and Dean’s Competitive Fund (Harvard University), and grants from DOD (W81XWH-13-1-0244), NIAMS (R01AR064227 and R01AR065462), and the Department of Orthopaedic Surgery (Boston Children’s Hospital).
Competing interests None declared.
Ethics approval Institutional Animal Care and Use Committee approvals were obtained before initiating this study.
Provenance and peer review Not commissioned; externally peer reviewed.
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