Growth failure in aggrecan deficiency is due to decreased extracellular matrix and impaired growth plate chondrocyte hypertrophy

Heterozygous loss-of-function mutations in the aggrecan (ACAN) gene cause autosomal dominant short stature often associated with advanced bone age, early-onset osteoarthritis and intervertebral disc disease (SSOAOD). These mutations are relatively common in patients with idiopathic short stature. However, the pathogenic mechanism of growth failure in this condition is not fully understood. Here, we studied the heterozygous cartilage matrix deficiency mouse (Acan^+/-), which harbors a 7 bp microdeletion in aggrecan and develops postnatal growth cessation despite being born of normal size. Using detailed histomorphometric analysis, we found that the growth failure was primarily due to decreased extracellular matrix and impaired chondrocyte hypertrophy, whereas proliferation was largely unaffected. Furthermore, single-cell transcriptomic profiling revealed decreased total Acan mRNA expression in the Acan^+/- chondrocytes. Notably, Akt signalling, which is important for hypertrophic differentiation was suppressed in Acan^+/- pre-hypertrophic and hypertrophic chondrocytes. The decreased Akt signalling was associated with increased expression of calcium-calmodulin dependent protein kinase 1D (Camk1D), which negatively regulates Akt signalling, thereby providing a potential mechanism for the impaired hypertrophic differentiation. These findings reveal key cellular and molecular causes of growth failure in aggrecan deficiency and suggest that boosting proteoglycan expression and Akt signalling may help restore growth.