Friday, August 26, 2016

Pitt-Hopkins syndrome

Sweatt JD. Pitt-Hopkins Syndrome: intellectual disability due to loss of
TCF4-regulated gene transcription. Exp Mol Med. 2013 May 3;45:e21.

TCF4 (transcription factor 4; E2-2, ITF2) is a transcription factor that when haplo-insufficient causes Pitt-Hopkins Syndrome (PTHS), an autism-spectrum disorder that is associated with pervasive developmental delay and severe intellectual disability. The TCF4 gene is also a risk factor with highly significant linkage to schizophrenia, presumably via overexpression of the TCF4 gene product in the central nervous system. This review will present an overview of the clinical manifestations of PTHS and relate those clinical attributes to the underlying molecular genetics of TCF4. In order to provide a molecular biological context for the loss of function of TCF4 in PTHS, the review will also present a brief overview of the basic biochemistry of TCF4-mediated regulation of cellular and neuronal gene expression. In the final section of this review, I will discuss and speculate upon possible roles for the TCF4 transcription factor in neuronal function and comment upon how understanding these roles may give new insights into the molecular neurobiology of human cognition.

Sepp M, Pruunsild P, Timmusk T. Pitt-Hopkins syndrome-associated mutations in
TCF4 lead to variable impairment of the transcription factor function ranging
from hypomorphic to dominant-negative effects. Hum Mol Genet. 2012 Jul

Transcription factor TCF4 (alias ITF2, SEF2 or E2-2) is a broadly expressed basic helix-loop-helix (bHLH) protein that functions as a homo- or heterodimer. Missense, nonsense, frame-shift and splice-site mutations as well as translocations and large deletions encompassing TCF4 gene cause Pitt-Hopkins syndrome (PTHS), a rare developmental disorder characterized by severe motor and mental retardation, typical facial features and breathing anomalies. Irrespective of the mutation, TCF4 haploinsufficiency has been proposed as an underlying mechanism for PTHS. We have recently demonstrated that human TCF4 gene is transcribed using numerous 5' exons. Here, we re-evaluated the impact of all the published PTHS-associated mutations, taking into account the diversity of TCF4 isoforms, and assessed how the reading frame elongating and missense mutations affect TCF4 functions. Our analysis revealed that not all deletions and truncating mutations in TCF4 result in complete loss-of-function and the impact of reading frame elongating and missense mutations ranges from subtle deficiencies to dominant-negative effects. We show that (i) missense mutations in TCF4 bHLH domain and the reading frame elongating mutation damage DNA-binding and transactivation ability in a manner dependent on dimer context (homodimer versus heterodimer with ASCL1 or NEUROD2); (ii) the elongating mutation and the missense mutation at the dimer interface of the HLH domain destabilize the protein; and (iii) missense mutations outside of the bHLH domain cause no major functional deficiencies. We conclude that different PTHS-associated mutations impair the functions of TCF4 by diverse mechanisms and to a varying extent, possibly contributing to the phenotypic variability of PTHS patients.

Forrest MP, Hill MJ, Quantock AJ, Martin-Rendon E, Blake DJ. The emerging
roles of TCF4 in disease and development. Trends Mol Med. 2014 Jun;20(6):322-31.

Genome-wide association studies have identified common variants in transcription factor 4 (TCF4) as susceptibility loci for schizophrenia, Fuchs' endothelial corneal dystrophy, and primary sclerosing cholangitis. By contrast, rare TCF4 mutations cause Pitt-Hopkins syndrome, a disorder characterized by intellectual disability and developmental delay, and have also been described in patients with other neurodevelopmental disorders. TCF4 therefore sits at the nexus between common and rare disorders. TCF4 interacts with other basic helix-loop-helix proteins, forming transcriptional networks that regulate the differentiation of several distinct cell types. Here, we review the role of TCF4 in these seemingly diverse disorders and discuss recent data implicating TCF4 as an important regulator of neurodevelopment and epithelial-mesenchymal transition.

Forrest M, Chapman RM, Doyle AM, Tinsley CL, Waite A, Blake DJ. Functional
analysis of TCF4 missense mutations that cause Pitt-Hopkins syndrome. Hum Mutat.
2012 Dec;33(12):1676-86.

Pitt-Hopkins syndrome (PTHS) is a rare developmental disorder associated with severe mental retardation, facial abnormalities, and intermittent hyperventilation. Autosomal dominant PTHS is caused by mutations in the transcription factor 4 (TCF4) gene, whereas NRXN1 and CNTNAP2 mutations are associated with autosomal recessive PTHS. To determine the impact of missense mutations on TCF4 function, we tested a panel of PTHS-associated mutations using a range of quantitative techniques. Mutations in the basic helix-loop-helix (bHLH) domain of TCF4 alter the subnuclear localization of the mutant protein and can attenuate homo- and heterodimer formation in homogenous time-resolved fluorescence (HTRF) assays. By contrast, mutations proximal to the bHLH domain do not alter the location of TCF4 or impair heterodimer formation. In addition, we show that TCF4 can transactivate the NRXN1β and CNTNAP2 promoters in luciferase assays. Here we find variable, context-specific deficits in the ability of the different PTHS-associated TCF4 mutants to transactivate these promoters when coexpressed with different bHLH transcription factors. These data demonstrate that PTHS-associated missense mutations can have multiple effects on the function of the protein, and suggest that TCF4 may modulate the expression of NRXN1 and CNTNAP2 thereby defining a regulatory network in PTHS.

Inspired by a patient

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