Lowering Fbn1 levels may increase bone length

TENDON-DEPENDENT CONTROL OF LONGITUDINAL BONE GROWTH

“Skeletal abnormalities caused by disproportioned bone overgrowth (LBO), are a common trait in Marfan syndrome (MFS), a connective tissue disease caused by mutations in the extracellular matrix (ECM) protein and TGFβ regulator fibrillin-1 (Fbn1). The cause of LBO in MFS is unknown and therapies are not available. Fibrillin-1 hypomorphic mouse model (Fbn1mgR/mgR) faithfully replicates MFS skeletal manifestations including elongated bones however, its early demise due aortic rupture limit the magnitude of LBO investigation.

To circumvent Fbn1mgR/mgR lethality and investigate the contribution of specific skeletal tissues to LBO, Fbn1 gene expression was targeted in developing limbs by crossing Fbn1Lox/Lox mice with Prx1-Cre, in or bone with Osx-Cre, in cartilage and perichondrium with Col2-Cre, in skeletal muscles with Mef2c-Cre, and ligaments and tendons with Scx-Cre. Bones length of Fbn1 conditional mice KO was measured and relevant histological, cellular and biomechanical parameters were assessed.

Fbn1Prx1−/+ and Fbn1Prx1−/− mice had longer limbs bones compared to WT mice and amount of fibrillin-1 in the limb matrix was inversely proportional to bone length. Interestingly, Fbn1 gene targeting in ligaments/tendons resulted in LBO, altered tissues’ mechanics and TGFβ-induced switch of tendon stem cells to chondrocytes. Gene targeting in other limb’s anatomical locations did not result in LBO thus ruling out the participation of surrounding tissues to this bone phenotype.

Fbn1 gene inactivation in ligament/tendon is associated with increased local TGFβ, altered biomechanical properties and LBO. As previously reported, ligaments/tendons respond to changes in mechanical load by increasing the levels and/or the activity of TGF-β while bones undergo morphological adaptation in response to muscle loads transmitted by tendons. We hypothesize that dysregulation of local TGFβ signaling and altered biomechanical properties of fibrillin-1 deficient ligaments/tendons affect endochondral ossification by improper load transmission to bone. By showing ligament/tendon-dependent regulation of postnatal longitudinal bone growth this study provides a paradigm-shift in tendon biology and it shades a new light on LBO pathophysiology in MFS, thus providing the bases for new pharmacological interventions for this and related skeletal conditions.”

So lower levels of Fbn1 means longer bone length and FBN1 deficient tendons and ligaments alter endochdondral ossification by altering load transmission to bone.  We can alter load transmission without altering FBN1.

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