Patient-Derived iPSC Modeling Reveals Dysregulation of the Mucin-Type O-Glycan Biosynthesis-Related Transcriptional Program in TBX6-Associated Congenital Scoliosis

TBX6-associated congenital scoliosis (TACS) is a genetic subtype of congenital scoliosis caused by reduced TBX6 gene dosage, most commonly through compound inheritance of a rare loss-of-function allele and a hypomorphic T-C-A haplotype. However, the mechanisms linking TBX6 dysfunction to impaired human skeletal development remain unclear. Here, we established a patient-derived induced pluripotent stem cell (iPSC) model of TACS and recapitulated key stages of axial skeletal development through stepwise differentiation into paraxial mesoderm, nascent somites, sclerotome, and subsequently osteogenic and chondrogenic lineages. TACS-derived cells showed impaired differentiation, with reduced mineralized matrix deposition and diminished cartilage matrix formation. Bulk transcriptomic profiling revealed marked alterations in both lineages. Notably, mu-cin-type O-glycan biosynthesis was consistently enriched among downregulated genes in osteogenic and chondrogenic cells and was also significantly represented among genes commonly downregulated across both lineages. qRT-PCR further confirmed reduced expression of GALNT3, GALNT5, GALNT7, GALNT10, GCNT1, and ST6GALNAC2, which participate in distinct steps of mucin-type O-glycan biosynthesis. Consistent with these transcriptional alterations, Tn-antigen staining showed reduced Tn-associated O-GalNAc signals in TACS-derived osteogenic and chondrogenic cells. In silico promoter analysis identified candidate sequences matching the TBX6-binding motif within the upstream regions of these genes, providing potential regulatory regions for future experimental investigation. Collectively, these results establish a human iPSC model for TACS and identify coordinated dysregulation of a mucin-type O-glycan biosynthesis-related transcriptional program, accompanied by reduced Tn-associated O-GalNAc signals, as molecular features associated with impaired osteochondral differentiation in this model. These findings should be interpreted within the context of a single family-based iPSC model, and validation in additional TACS lines and isogenic TBX6-corrected models will be required.