Single nucleotide variants in UNC13C associated with neurodevelopmental disorders affect ethanol sensitivity in Drosophila.

UNC13s are presynaptic proteins essential for neurotransmitter release at chemical synapses. In this study, we present eleven patients from nine families with severe neurodevelopmental impairments, who carry rare, biallelic UNC13C single-nucleotide variants (SNVs). Six missense variants, each identified in compound heterozygosity in one of three of these patients, were introduced into the Drosophila melanogaster ortholog unc13 using a previously established CRISPR/Cas9-based method for rapid and scarless genomic modifications, hypothesising that they underlie the observed clinical manifestations. However, none of the introduced mutations influenced Mendelian ratios, negative geotaxis, or lifespan of the fruit flies. Interestingly, two variants located outside the gene regions encoding known UNC13C domains caused a decreased ethanol sensitivity in Drosophila, while the Thr1729Met substitution within the C1 domain resulted in increased ethanol sensitivity. Molecular dynamics simulations of the latter mutant gene product suggested that the altered protein conformation enhances exposure of the ethanol-binding site, thereby increasing sensitivity to ethanol. These findings reinforce previous evidence highlighting the critical role of the C1 domain in ethanol sensitivity. Given the involvement of the C1 domain in synaptic plasticity this result might implicate an influence of the Thr1729Met on synaptic function.