title: Functional characterization of rare FOXP2 variants in neurodevelopmental disorder.
abstract: BACKGROUND
  Heterozygous disruption ofFOXP2causes a rare form of speech and language impairment. Screens of theFOXP2sequence in individuals with speech/language-related disorders have identified several rare protein-altering variants, but their phenotypic relevance is often unclear.FOXP2encodes a transcription factor with a forkhead box DNA-binding domain, but little is known about the functions of protein regions outside this domain.
  
  
  METHODS
  We performed detailed functional analyses of seven rare FOXP2 variants found in affected cases, including three which have not been previously characterized, testing intracellular localization, transcriptional regulation, dimerization, and interaction with other proteins. To shed further light on molecular functions of FOXP2, we characterized the interaction between this transcription factor and co-repressor proteins of the C-terminal binding protein (CTBP) family. Finally, we analysed the functional significance of the polyglutamine tracts in FOXP2, since tract length variations have been reported in cases of neurodevelopmental disorder.
  
  
  RESULTS
  We confirmed etiological roles of multiple FOXP2 variants. Of three variants that have been suggested to cause speech/language disorder, but never before been characterized, only one showed functional effects. For the other two, we found no effects on protein function in any assays, suggesting that they are incidental to the phenotype. We identified a CTBP-binding region within the N-terminal portion of FOXP2. This region includes two amino acid substitutions that occurred on the human lineage following the split from chimpanzees. However, we did not observe any effects of these amino acid changes on CTBP binding or other core aspects of FOXP2 function. Finally, we found that FOXP2 variants with reduced polyglutamine tracts did not exhibit altered behaviour in cellular assays, indicating that such tracts are non-essential for core aspects of FOXP2 function, and that tract variation is unlikely to be a highly penetrant cause of speech/language disorder.
  
  
  CONCLUSIONS
  Our findings highlight the importance of functional characterization of novel rare variants in FOXP2 in assessing the contribution of such variants to speech/language disorder and provide further insights into the molecular function of the FOXP2 protein.
authors.affiliation: Language and Genetics Department, Max Planck Institute for Psycholinguistics, Wundtlaan 1, 6525 XD Nijmegen, the Netherlands.
authors.forename: Sara B
authors.initials: SB
authors.lastname: Estruch
authors.affiliation: Language and Genetics Department, Max Planck Institute for Psycholinguistics, Wundtlaan 1, 6525 XD Nijmegen, the Netherlands.
authors.forename: Sarah A
authors.initials: SA
authors.lastname: Graham
authors.affiliation: Language and Genetics Department, Max Planck Institute for Psycholinguistics, Wundtlaan 1, 6525 XD Nijmegen, the Netherlands.
authors.forename: Swathi M
authors.initials: SM
authors.lastname: Chinnappa
authors.affiliation: Language and Genetics Department, Max Planck Institute for Psycholinguistics, Wundtlaan 1, 6525 XD Nijmegen, the Netherlands.
authors.forename: Pelagia
authors.initials: P
authors.lastname: Deriziotis
authors.affiliation: Language and Genetics Department, Max Planck Institute for Psycholinguistics, Wundtlaan 1, 6525 XD Nijmegen, the Netherlands.|Donders Institute for Brain, Cognition and Behaviour, Radboud University, 6525 EN Nijmegen, the Netherlands.
authors.forename: Simon E
authors.initials: SE
authors.lastname: Fisher
keywords: Functional genetics; Language; Neuroscience; Speech; Transcription factor