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.
Keywords: Alternative transcription; Non-syndromic intellectual disability; Pitt-Hopkins syndrome; TCF4.
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