Genome sequencing reveals high diagnostic yield in children with severe sporadic developmental language disorder.

A developmental language disorder (DLD) is a neurobiological condition characterized by impaired language development despite adequate linguistic input and normal intelligence. The role of monogenic causes in the pathogenesis of DLD is largely unknown and diagnostic genetic testing is rarely offered. This study assessed the diagnostic yield of genome sequencing in severe DLD and investigated the underlying genetic foundations. We designed a prospective cohort study involving trio-based genome sequencing in 25 individuals (aged 3-25 years) with severe DLD, no intellectual disability (non-verbal IQ ≥ 70), no autism spectrum disorder diagnosis and no first-degree family history of DLD or neurodevelopmental disorders. (Likely) pathogenic variants were found in nine of 25 individuals, reflecting a molecular diagnostic yield of 36%. The genetic causes were diverse and involved seven autosomal dominant disorders, one autosomal recessive disorder, and one sex chromosome aneuploidy. In seven individuals, the disease-causing variant occurred de novo. All identified (likely) pathogenic variants were located in genes or loci that have been previously implicated in neurodevelopmental disorders with variable clinical presentations. Importantly, the genetic diagnosis provided actionable insights for counselling and clinical follow-up for all nine individuals. This study reveals a high prevalence of rare (mainly de novo) genetic variants in individuals with severe and sporadic DLD, and demonstrates extensive molecular overlap with other neurodevelopmental disorders. Our results support the implementation of routine trio-based genetic testing in individuals with severe and sporadic DLD.