To Örebro University

Background: Heterozygous loss-of-function mutations in the aggrecan gene (ACAN) cause autosomal dominant short stature with advanced bone age, early-onset osteoarthritis and intervertebral disc disease (SSOAOD; OMIM#165800). ACAN mutations is a relatively common finding in idiopathic short stature (ISS) and has been reported to be the cause of growth failure in approximately 2% of children with ISS. However, the underlying cellular and molecular mechanisms by which ACAN mutations cause growth failure in SSOAOD have not been elucidated.

Objective: To investigate the underlying cellular and molecular mechanisms of growth failure using a mouse model of SSOAOD.

Methods: Cartilage matrix deficiency mouse (Acan cmd) has a naturally occurring 7 bp micro-deletion in aggrecan gene. Heterozygous Acancmd and wild-type (WT) male and female mice were assessed for skeletal and body growth at 1,3,6,12 and 24 weeks of age. Histomorphometric analysis was performed on Masson-Trichrome stained proximal tibial and distal femoral growth plates. Cell proliferation was assessed by EdU incorporation. Quantification of percentage matrix area was performed using Image J. Single-cell RNA sequencing was carried out on chondro-cytes isolated from 18 day old WT and Acan cmd female mice according to 3’ gene expression protocol (10X Genomics).

Results: Heterozygous Acancmd mice were born at a normal size and similar to humans with SSOAOD but showed decreased postnatal growth resulting in a gradually worsening dwarfism with reduced total body length and tibial and femoral lengths (p<0.0001). In the growth plates, chondrocytes were found to be more tightly packed with reduced matrix area (p<0.0001) and increased column density in Acan cmd mice compared to WT mice. Growth plate height (p<0.0001), heights of the individual zones (p<0.001), the number of resting zone chondrocytes (p<0.01), proliferative cells per column (p<0.0001), and the size of terminal hypertrophic chondrocytes (p<0.001) were slightly reduced in both male and female Acan cmd mice, especially at 1 week of age. Interestingly, chondrocyte proliferation was similar in Acan cmd and WT mice at all time-points assessed (p=0.90). Female Acan cmd mice exhibited a more pronounced phenotype than male mice.

Conclusions: Similar to children with heterozygous ACAN mutations, heterozygous Acancmd mice exhibit a growth pattern with postnatal growth failure resulting in adult short stature. The growth failure is primarily caused by decreased matrix production and hypertrophic cell size, whereas chondrocyte proliferation is normal. Single-cell RNA sequencing of growth plate chondrocytes is ongoing and will identify the underlying pathogenic mechanisms and might also identify compensatory mechanisms limiting the effects of aggrecan haploinsufficiency.