Genetic Factors of Specific Language Impairments

The term "specific language impairment" (SLI) is used for a primary, specific developmental disorder of language without any obvious reason, e. g., hearing disorders or other developmental delay. Family aggregation and twin studies have demonstrated that SLI is a highly heritable disorder. Most pedigrees do not show Mendelian inheritance and are not consistent with a single gene defect. Over the past years, evidence had been gathered that the etiology of SLI is multifactorial and that the phenotype is influenced by numerous of genes which interact both with one another and with the environment, as it is the case with many other complex traits. Up to today, linkage analyses of SLI have identified 4 gene regions (SLI1-SLI4). CNTNAP2 on chromosome 7 which plays a role in the neuronal development of the human cortex was identified as the first gene with an association to SLI. Polymorphisms of this gene showed significant associations with poor non-word repetition as a marker of phonological short-term memory. In the present review, we describe different types of methods and study designs to provide evidence for the genetic influence of specific language impairment. Several reasons are discussed to show that SLI is not a distinct disorder, but rather the extreme end of a normal distribution of language ability, likely to be influenced by multiple genetic and environmental influences of small effect.