Functional Diversity of Human Basic Helix-Loop-Helix Transcription Factor TCF4 Isoforms Generated by Alternative 5′ Exon Usage and Splicing

Abstract

BackgroundTranscription factor 4 (TCF4 alias ITF2, E2-2, ME2 or SEF2) is a ubiquitous class A basic helix-loop-helix protein that binds to E-box DNA sequences (CANNTG). While involved in the development and functioning of many different cell types, recent studies point to important roles for TCF4 in the nervous system. Specifically, human TCF4 gene is implicated in susceptibility to schizophrenia and TCF4 haploinsufficiency is the cause of the Pitt-Hopkins mental retardation syndrome. However, the structure, expression and coding potential of the human TCF4 gene have not been described in detail.Principal FindingsIn the present study we used human tissue samples to characterize human TCF4 gene structure and TCF4 expression at mRNA and protein level. We report that although widely expressed, human TCF4 mRNA expression is particularly high in the brain. We demonstrate that usage of numerous 5′ exons of the human TCF4 gene potentially yields in TCF4 protein isoforms with 18 different N-termini. In addition, the diversity of isoforms is increased by alternative splicing of several internal exons. For functional characterization of TCF4 isoforms, we overexpressed individual isoforms in cultured human cells. Our analysis revealed that subcellular distribution of TCF4 isoforms is differentially regulated: Some isoforms contain a bipartite nuclear localization signal and are exclusively nuclear, whereas distribution of other isoforms relies on heterodimerization partners. Furthermore, the ability of different TCF4 isoforms to regulate E-box controlled reporter gene transcription is varied depending on whether one or both of the two TCF4 transcription activation domains are present in the protein. Both TCF4 activation domains are able to activate transcription independently, but act synergistically in combination.ConclusionsAltogether, in this study we have described the inter-tissue variability of TCF4 expression in human and provided evidence about the functional diversity of the alternative TCF4 protein isoforms.

Transcription factor 4 (TCF4) and schizophrenia: integrating the animal and the human perspective

“…Given the enormous complexity of TCF4 splice variants and biochemical properties of different PTHS associated mutations [10,11], it is still possible that dominant-negative effects beyond dosage effects contribute to the severity of the neurodevelopmental disturbances in humans. In a transgenic mouse model (Tcf4tg) with slightly elevated expression of Tcf4 in the forebrain and displaying cognitive and sensorimotor deficits were observed supporting the critical gene dosage sensitivity [112].…”

“…TCF4 may thus exert pleiotropic functions depending on its dimerization partner(s) at a given developmental stage and in a particular cell type. In consequence, TCF4 functions have been shown to be modulated by spatiotemporal expression patterns of its various interaction partners, differences in DNA-binding specificities, by post-translational modifications and associated co-factors [10][11][12][13].…”

“…72). More than 18 coding splice variants with alternative N-termini have been described in humans [10] and the Ensembl genome browser (www.ensembl.org, rel. 72) lists 43 potentially protein encoding variants with the majority including the bHLH domain.…”

“…72) lists 43 potentially protein encoding variants with the majority including the bHLH domain. Moreover, Sepp and colleagues [10] subgrouped 22 exons that are alternatively spliced particularly in the 5' region with multiple alternative transcription initiation sites. Up to date, two putative antisense and one micro RNA transcripts (miR4529) have been annotated on the opposite strand within the human TCF4 locus (none so far in the mouse) potentially indicating regulation at the RNA level of increased complexity in the human genome.…”

Knockdown of the schizophrenia susceptibility gene TCF4 alters gene expression and proliferation of progenitor cells from the developing human neocortex

“…1 TCF4 is highly expressed in the fetal as well as adult human brain 12,13 and is known to dimerize with several bHLH factors that are important for neural development. [14][15][16] Knockout of the TCF4 gene has been reported to affect the differentiation of specific neuronal populations in the mouse hindbrain.…”