Synostosis of joints caused by mutant FBN2 is linked to the abnormalities and misdifferentiation of articular surface cells

Purpose

FBN2, a high-confidence effector gene for osteoarthritis (OA), was investigated for its potential role in synostosis of joints (SJ) because several OA-related genes are known to cause SJ.

Methods

We analyzed variants in OA-related genes using exome sequencing data from Chinese-Han participants with radioulnar synostosis (RUS). Variants were classified following American College of Medical Genetics and Genomics/Association for Molecular Pathology guidelines. Fbn2 knockout mice were generated via CRISPR/Cas9 and evaluated through radiological and histopathological analyses at multiple developmental stages, with complementary cellular and molecular studies.

Results

We identified 15 rare, damaging FBN2 variants in unrelated RUS families, including 7 likely pathogenic variants (4 null variants). Fbn2 knockout mice (both homozygous and heterozygous) exhibited SJ phenotypes. Unlike previously reported SJ mechanisms involving failed interzone formation, Fbn2-related SJ occurred after normal interzone formation. Mutant mice showed significant alterations in extracellular matrix composition and volume within articular surface cells. We proposed that these extracellular matrix changes mediated the transdifferentiation of articular surface cells into osteoblasts, which ultimately developed into bones over time.

Conclusion

We identified FBN2 pathogenic variants that caused SJ in humans and mice. SJ caused by mutant FBN2 is linked to the abnormalities and misdifferentiation of articular surface cells.