Fig 2: PTHS-associated brain anomalies in TCF4-haploinsufficient mice. Hypoplastic corpus callosum in Tcf4-haploinsufficient PTHS patients at the age of 3 years (B) and 7 years (D). A, C Age-matched references. E Small hippocampus in PTHS patient at the age of 7 years. Tcf4-haploinsufficient mice (F’, G’) show agenesis of the corpus callosum and reduced hippocampal volume (H) compared to WT mice (F, G). Cortical diameter was determined along the rostro-caudal axis (I). Representative images of the cortical thickness of Tcf4ex4WT/WT mice (J, L) and Tcf4ex4lacZ/WT mice (K, M). Cortical diameter of Tcf4ex4lacZ/WT mice was significantly decreased compared to Tcf4ex4WT/WT mice in all analyzed cortical regions. The results are displayed as mean in microns ± SD. N–P Scale bars 500 μm. The DG area was measured in six representative slices spanning a distance of 240 μm. Abbreviations: PTLp, posterior parietal association area; SSp-bfd, primary somatosensory area-barrel field; SSp-tr, primary somatosensory area-trunk; Rost, rostral; Caud, caudal

Fig 3: TCF4 expression during hippocampal development. a–f Overview images of the hippocampal formation at different developmental stages. TCF4 is expressed throughout pre- and postnatal development of the hippocampus. g–i TCF4 expression in the CA1 region of the hippocampus. At E18.5, TCF4 is expressed in radially migrating pyramidal progenitor cells and in the forming pyramidal cell layer. Postnatally, TCF4 is expressed in the pyramidal cell layer and in scattered cells in the stratum oriens and radiatum. j–l At E18.5, TCF4 is expressed in dentate progenitor cells that migrate to the subpial zone (primary and secondary matrix) and in the forming pyramidal cell layer of the CA3 region. At postnatal stages P7 and P56, TCF4 is expressed in the pyramidal cell layer and in scattered cells of the strata oriens and radiatum. m–o At E18.5, TCF4 is expressed in dentate progenitor cells that accumulate in the hippocampal fissure (tertiary matrix). Postnatally, TCF4 is expressed in the granular cell layer of the dentate gyrus and in individual cells of the hilus. Abbreviations: 1ry, primary matrix; 2ry, secondary matrix; 3ry, tertiary matrix; ANE, ammonic neuroepithelium; CH, cortical hem; DG, dentate gyrus; DNE, dentate neuroepithelium; GCL, granular cell layer; H, hilus; LAC-MOL, stratum lacunosum-moleculare; MOL, molecular layer; OR, stratum oriens; PCL, pyramidal cell layer; RAD, stratum radiatum; SVZ, subventricular zone

Fig 4: Comparison of TCF4 protein expression at different stages of cortical and hippocampal development between Tcf4ex4WT/WT and Tcf4ex4lacZ/WT mice. A–D’ Immunostaining using the anti-TCF4 antibody on brain tissue of Tcf4ex4WT/WT and Tcf4exlacZ/WT animals during cortical development and in the adult. E–H’ Immunostaining using the anti-TCF4 antibody on brain tissue of Tcf4ex4WT/WT and Tcf4ex4lacZ/WT animals during hippocampus formation and in the adult. Abbreviations: ANE, ammonic neuroepithelium; CP, cortical plate; DG, dentate gyrus; DNE, dentate neuroepithelium; IZ, intermediate zone; MZ, marginal zone; PP, preplate; SP, subplate; SVZ, subventricular zone; VZ, ventricular zone; 1ry, primary matrix; 2ry, secondary matrix; 3ry, tertiary matrix

Fig 5: TCF4 expression during corticogenesis. (A–F) Heatmap converted TCF4 immunoreactivity at different stages of cortical development. TCF4 is expressed in all layers of the neocortex with particular high intensity in the expanding cortical plate during development (G–I”). TCF4 is expressed in Sox2-positive ventricular zone cells (G–G”), DCX-positive immature neurons (H–H”), and NeuN-positive neurons (I–I”). TCF4 co-stainings with layer V/VI neuron marker CTIP2 (J–J”) or layer II/III/IV neuron marker Cux1 (K-K”) at P7. TCF4 is expressed in layers II-VI but only individual TCF4-positive cells co-localize with the layer markers. TCF4 is expressed in GAD67-positive interneurons (L–L”). Abbreviations: CP, cortical plate; IZ, intermediate zone; MZ, marginal zone; PP, preplate; SP, subplate; SVZ, subventricular zone; VZ, ventricular zone