T-Box Brain Transcription Factor 1 (TBR1) plays essential roles in brain development, mediating neuronal migration, fate specification, and axon tract formation. While heterozygous loss-of-function and missense TBR1 mutations are associated with neurodevelopmental conditions, the effects of these heterogeneous mutations on brain development have yet to be fully explored. We characterized multiple mouse lines carrying Tbr1 mutations differing by type and exonic location, including the previously generated Tbr1 exon 2-3 knock-out (KO) line, and we analyzed male and female mice at neonatal and adult stages. The frameshift patient mutation A136PfsX80 (A136fs) caused reduced TBR1 protein in cortex similar to Tbr1 KO, while the missense patient mutation K228E caused significant TBR1 upregulation. Analysis of cortical layer formation found similar defects between KO and A136fs homozygotes in their CUX1+ and CTIP2+ layer positions, while K228E homozygosity produced layering defects distinct from these mutants. Meanwhile, the examination of cortical apoptosis found extensive cell death in KO homozygotes but limited cell death in A136fs or K228E homozygotes. Despite their discordant cortical phenotypes, these Tbr1 mutations produced several congruent phenotypes, including anterior commissure reduction in heterozygotes, which was previously observed in humans with TBR1 mutations. These results indicate that patient-specific Tbr1 mutant mice will be valuable translational models for pinpointing shared and distinct etiologies among patients with TBR1-related developmental conditions.SIGNIFICANCE STATEMENT Mutations of the TBR1 gene increase the likelihood of neurodevelopmental conditions such as intellectual disability and autism. Therefore, the study of TBR1 can offer insights into the biological mechanisms underlying these conditions, which affect millions worldwide. To improve the modeling of TBR1-related conditions over current Tbr1 knock-out mice, we created mouse lines carrying Tbr1 mutations identical to those found in human patients. Mice with one mutant Tbr1 copy show reduced amygdalar connections regardless of mutation type, suggesting a core biomarker for TBR1-related disorders. In mice with two mutant Tbr1 copies, brain phenotypes diverge by mutation type, suggesting differences in Tbr1 gene functionality in different patients. These mouse models will serve as valuable tools for understanding genotype-phenotype relationships among patients with neurodevelopmental conditions.

Genetic risk factors for autism spectrum disorder (ASD) have yet to be fully elucidated. Postzygotic mosaic mutations (PMMs) have been implicated in several neurodevelopmental disorders and overgrowth syndromes. By leveraging whole-exome sequencing data on a large family-based ASD cohort, the Simons Simplex Collection, we systematically evaluated the potential role of PMMs in autism risk. Initial re-evaluation of published single-nucleotide variant (SNV) de novo mutations showed evidence consistent with putative PMMs for 11% of mutations. We developed a robust and sensitive SNV PMM calling approach integrating complementary callers, logistic regression modeling, and additional heuristics. In our high-confidence call set, we identified 470 PMMs in children, increasing the proportion of mosaic SNVs to 22%. Probands have a significant burden of synonymous PMMs and these mutations are enriched for computationally predicted impacts on splicing. Evidence of increased missense PMM burden was not seen in the full cohort. However, missense burden signal increased in subcohorts of families where probands lacked nonsynonymous germline mutations, especially in genes intolerant to mutations. Parental mosaic mutations that were transmitted account for 6.8% of the presumed de novo mutations in the children. PMMs were identified in previously implicated high-confidence neurodevelopmental disorder risk genes, such as CHD2, CTNNB1, SCN2A, and SYNGAP1, as well as candidate risk genes with predicted functions in chromatin remodeling or neurodevelopment, including ACTL6B, BAZ2B, COL5A3, SSRP1, and UNC79. We estimate that PMMs potentially contribute risk to 3%-4% of simplex ASD case subjects and future studies of PMMs in ASD and related disorders are warranted.