Childhood syndromes disturbing language development are common and display high degrees of heritability. In most cases, the underlying genetic architecture is likely to be complex, involving multiple chromosomal loci and substantial heterogeneity, which makes it difficult to track down the crucial genomic risk factors. Investigation of rare Mendelian phenotypes offers a complementary route for unravelling key neurogenetic pathways. The value of this approach is illustrated by the discovery that heterozygous FOXP2 (where FOX is forkhead box) mutations cause an unusual monogenic disorder, characterized by problems with articulating speech along with deficits in expressive and receptive language. FOXP2 encodes a regulatory protein, belonging to the forkhead box family of transcription factors, known to play important roles in modulating gene expression in development and disease. Functional genetics using human neuronal models suggest that the different FOXP2 isoforms generated by alternative splicing have distinct properties and may act to regulate each other's activity. Such investigations have also analysed the missense and nonsense mutations found in cases of speech and language disorder, showing that they alter intracellular localization, DNA binding and transactivation capacity of the mutated proteins. Moreover, in the brains of mutant mice, aetiological mutations have been found to disrupt the synaptic plasticity of Foxp2-expressing circuitry. Finally, although mutations of FOXP2 itself are rare, the downstream networks which it regulates in the brain appear to be broadly implicated in typical forms of language impairment. Thus, through ongoing identification of regulated targets and interacting co-factors, this gene is providing the first molecular entry points into neural mechanisms that go awry in language-related disorders.

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