Expression profile of the ADHD risk gene ADGRL3 during human neurodevelopment and the effects of genetic variation

Attention-deficit/hyperactivity disorder (ADHD) is one of the most common neurodevelopmental disorders, affecting ∼5.3 % children, with symptoms including hyperactivity, inattention and impulsivity. Moreover, ADHD persists into adulthood in ∼50 % cases, significantly affecting quality of life. Currently, the complex aetiology of ADHD remains unclear, although it has been shown that it is a highly heritable disorder. Single nucleotide polymorphisms (SNPs) in the adhesion G protein-coupled receptor isoform L3 gene ( ADGRL3 / LPHN3 ) have been consistently associated with ADHD development, symptom severity and treatment response, with the rs1397547 SNP previously shown associated with altered ADGRL3 transcription. Animal models have revealed an essential role of ADGRL3 in glutamatergic synapse development. However, ADGRL3 function has not been investigated in humans. We used human induced pluripotent stem cell (hiPSC)-derived cortical neurons to characterise ADGRL3 expression during human neurogenesis in vitro, and found that expression peaks early in neurodevelopment, consistent with in vivo data. ADGRL3 protein was found primarily expressed in glutamatergic neurons, and localised to growth cone-like structures, supporting a role in neurite outgrowth and glutamatergic synapse development. Lastly, using hiPSC-derived cortical neurons from healthy controls and ADHD patients, we investigated whether the rs1397547 SNP could affect ADGRL3 expression in cortical neurons. We found the rs1397547 SNP was associated with significantly increased ADGRL3 transcription in early neurodevelopmental stages. Moreover, single-cell RNA sequencing of maturing cortical neurons revealed a unique transcriptional profile in SNP carriers. Our results further implicate ADGRL3 in ADHD development and suggest that genetic variation may result in dysregulated glutamatergic neuron development.