ClinVar

In 4 affected individuals from 3 unrelated consanguineous families with sacral agenesis with vertebral anomalies (SAVA; 615709), Postma et al. (2014) identified a homozygous c.796A-G transition in the T gene, resulting in a his171-to-arg (H171R) substitution at a highly conserved residue in a region of the T-box domain that forms the dimerization interface. The mutation, which was found by homozygosity mapping followed by candidate gene sequencing, segregated with the disorder in the families. It was not found in 600 control chromosomes, or in the dbSNP, 1000 Genomes Project or the Exome Variant Server databases. In vitro functional expression studies in a cell line with chondrogenic potential showed that the mutant protein had about 50% loss of DNA-binding activity compared to wildtype. In a reporter construct, the mutant protein showed a complete loss of function on a T-box half site, while it only showed a significant decrease on a full consensus T-site. These findings suggested that the binding of the mutant to the T-half site is thermodynamically unfavorable, whereas a homodimer may be more stable. Overexpression of the mutant protein in embryonic cells resulted in an increase in alkaline phosphatase, suggesting a misdirection towards the endoderm lineage in vitro. The mutation also caused an increase in cell growth and interfered with the normal expression of genes involved in ossification, notochord maintenance, and axial mesoderm development.