Forkhead transcription factor FOXO3a levels are increased in Huntington disease because of overactivated positive autofeedback loop.

Huntington disease (HD) is a fatal autosomal dominant neurodegenerative disorder caused by an increased number of CAG repeats in the HTT gene coding for huntingtin. Decreased neurotrophic support and increased mitochondrial and excitotoxic stress have been reported in HD striatal and cortical neurons. The members of the class O forkhead (FOXO) transcription factor family, including FOXO3a, act as sensor proteins that are activated upon decreased survival signals and/or increased cellular stress. Using immunocytochemical screening in mouse striatal Hdh(7/7) (wild type), Hdh(7/109) (heterozygous for HD mutation), and Hdh(109/109) (homozygous for HD mutation) cells, we identified FOXO3a as a differentially regulated transcription factor in HD. We report increased nuclear FOXO3a levels in mutant Hdh cells. Additionally, we show that treatment with mitochondrial toxin 3-nitropropionic acid results in enhanced nuclear localization of FOXO3a in wild type Hdh(7/7) cells and in rat primary cortical neurons. Furthermore, mRNA levels of Foxo3a are increased in mutant Hdh cells compared with wild type cells and in 3-nitropropionic acid-treated primary neurons compared with untreated neurons. A similar increase was observed in the cortex of R6/2 mice and HD patient post-mortem caudate tissue compared with controls. Using chromatin immunoprecipitation and reporter assays, we demonstrate that FOXO3a regulates its own transcription by binding to the conserved response element in Foxo3a promoter. Altogether, the findings of this study suggest that FOXO3a levels are increased in HD cells as a result of overactive positive feedback loop.

Partial deletion of TCF4 in three generation family with non-syndromic intellectual disability, without features of Pitt-Hopkins syndrome.

Mutations in TCF4 (basic helix-loop-helix transcription factor 4), a gene with complex organization and multiple transcription initiation sites, are usually associated with Pitt-Hopkins syndrome (PTHS). However, a translocation encompassing the 5' end of TCF4 and several point mutations have been linked to non-syndromic intellectual disability (NSID). Here we describe a family with autosomal dominantly inherited NSID in seven relatives with a partial deletion of TCF4, disrupting the 5' end of the gene, predicted to result in the reduction of the number of mRNAs that can be produced by alternative transcription initiation. Functional studies indicate that it leads to reduced levels of transcripts coding for TCF4 protein isoforms with a nuclear localization signal, which may be relevant to the phenotype. The findings in our family support the notion that the position of the mutation in TCF4 is relevant to the phenotype, with those mutations in the 5' region, cassette exons and regions not affecting the important functional domains being linked to NSID rather than PTHS. We suggest that screening for mutations in TCF4 could be considered in the investigation of NSID.

Functional diversity of human basic helix-loop-helix transcription factor TCF4 isoforms generated by alternative 5' exon usage and splicing.

BACKGROUND: Transcription 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 FINDINGS: In 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. CONCLUSIONS: Altogether, 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.