Constitutive Hedgehog Signaling in PTHrP+ Resting Chondrocytes Facilitates Enchondromatous Lesions

Objectives: Enchondromas – benign cartilage tumors arising in proximity to developing growth plates – occur in more than 3% of the population and are also observed within the cranial base. A key etiological factor in enchondroma formation is abnormal activation of Hedgehog signaling and disruption of Ihh-PTHrP feedback regulation. We investigated a murine model simulating constitutively activated Hedgehog signaling in PTHrP+ resting chondrocytes to better understand this mechanism.
Methods: We utilized a tamoxifen-inducible PTHrP-creER line with Patched-1 (Ptch1) floxed and tdTomato reporter alleles to specifically activate Hedgehog signaling in PTHrP+ resting chondrocytes and visualize their descendants within the growth plate. A cohort of 24 experimental (Ptch1-deficient, cKO) and 20 control (Ptch1-sufficient, Cont) mice received tamoxifen at postnatal day (P) 6 and were chased up to 3 months. Distal femurs were analyzed at five times points.
Results: Lineage-marked chondrocyte number across growth plates remained similar between the two groups up to P14, but drastically increased in Ptch1-cKO at later time points, with a peak difference occurring at P28. Similar patterns were observed for parameters including number of lineage-marked columns and column width. Notably, formation of large enchondroma-like lesions was visible in all Ptch1-cKO samples at P21 and beyond, with no such lesions observed in controls.
Conclusions: Constitutive activation of Hedgehog signaling via conditional deletion of Ptch1 in PTHrP+ resting chondrocytes is sufficient to produce enchondroma-like lesions within developing growth plates. In this model, a marked increase in Hedgehog-activated PTHrP+ resting chondrocytes occurred between P14 and P28, suggesting “hijacking” of normal chondrocytes. As PTHrP+ resting chondrocytes represent skeletal stem cells within the growth plate, we propose “runaway” PTHrP-Ihh feedback regulation in these stem cells plays a causative role in tumorigenesis. Identification of critical regulators in this process may offer therapeutic targets for prevention of cartilage tumors within the cranial base and elsewhere.