The highly conserved TBX1 transcription factor is essential for normal development and mutations are associated with conotruncal anomaly face syndrome, DiGeorge syndrome, tetralogy of Fallot, and velocardiofacial syndrome. Tbx1 homozygous null mutant mice die neonatally with truncas arteriosis and other cardiac developmental defects, cleft palate and other craniofacial defects, thymic defects, otic vesicle hypoplasia and other ear defects, and other defects stemming from defective development of the pharyngeal pouches. Mice heterozygous for null alleles have also been found to have abnormal aortic arch morphology and some neonatal lethality. The nmf219 point mutation in Tbx1 is not a null allele but rather a single amino acid substitution of glycine in place of aspartic acid at amino acid 212 in the T-Box DNA binding domain. TBX1 protein expression is found in homozygotes and appears reasonably similar to that of wildtype controls by immunohistochemistry of P3 and P15 inner ears. This substitution does not cause homozygous lethality or the severe cardiovascular, thymic and craniofacial defects that other Tbx1 mutations cause, but still causes developmental defects of the stria vascularis and semicircular canals, resulting in complete deafness and the circling and head tossing phenotypes indicative of a vestibular defect. Homozygotes as young as 4 weeks of age displayed no detectable ABR at 100 dB SPL at 8, 16, or 32 kHz.

The underlying developmental failure stems in part from the failure of epithelial cells to become mature marginal cells such that at P3 there are few or no marginal cells interdigitating with intermediate cells at the stria vascularis. This may result from failure of mutant TBX1 to induce expression of Esrrb in the inner ear, which is diminished compared with controls at E16.5 and P3. At the earliest timepoint assessed, E14.5, defects are found and paintfills of inner ears at E14.5 and E16.5 found that there is a single fused ampulla connecting the anterior and lateral canals, the fusion plate of the posterior semicircular canal fails to resorb, the posterior canal ampulla is not formed, and the posterior semicircular canal is fused with the common crus. As a result, at 7 days of age the stria vascularis is underdeveloped, the scala media small, and Reissner’s membrane is displaced, and at 5 months of age the stria vascularis is still severely underdeveloped, the scala media extremely reduced, Reissner’s membrane is collapsed, there is degeneration of the organ of Corti and loss of spiral ganglion neurons. in situ hybridization and RNA-Seq have identified an array of transcripts with upregulated or downregulated expression relative to controls. (Tian and Johnson, 2019.)