Synapses are essential components of neurons and allow information to travel coordinately throughout the nervous system to adjust behavior to environmental stimuli and to control body functions, memories, and emotions. Thus, optimal synaptic communication is required for proper brain physiology, and slight perturbations of synapse function can lead to brain disorders. In fact, increasing evidence has demonstrated the relevance of synapse dysfunction as a major determinant of many neurological diseases. This notion has led to the concept of synaptopathies as brain diseases with synapse defects as shared pathogenic features. In this review, which was initiated at the 13th International Society for Neurochemistry Advanced School, we discuss basic concepts of synapse structure and function, and provide a critical view of how aberrant synapse physiology may contribute to neurodevelopmental disorders (autism, Down syndrome, startle disease, and epilepsy) as well as neurodegenerative disorders (Alzheimer and Parkinson disease). We finally discuss the appropriateness and potential implications of gathering synapse diseases under a single term. Understanding common causes and intrinsic differences in diseaseassociated synaptic dysfunction could offer novel clues toward synapse-based therapeutic intervention for neurological and neuropsychiatric disorders.

The human dopaminergic system is vital for a broad range of neurological processes, including the control of voluntary movement. Here we report a proband presenting with clinical features of dopamine deficiency: severe infantile parkinsonism-dystonia, characterised by frequent oculogyric crises, dysautonomia and global neurodevelopmental impairment. CSF neurotransmitter analysis was unexpectedly normal. Triome whole-genome sequencing revealed a homozygous variant (c.110C>A, (p.T37K)) in DRD1, encoding the most abundant dopamine receptor (D1) in the central nervous system, most highly expressed in the striatum. This variant was absent from gnomAD, with a CADD score of 27.5. Using an in vitro heterologous expression system, we determined that DRD1-T37K results in loss of protein function. Structure-function modelling studies predicted reduced substrate binding, which was confirmed in vitro. Exposure of mutant protein to the selective D1 agonist Chloro APB resulted in significantly reduced cyclic AMP levels. Numerous D1 agonists failed to rescue the cellular defect, reflected clinically in the patient, who had no benefit from dopaminergic therapy. Our study identifies DRD1 as a new disease-associated gene, suggesting a crucial role for the D1 receptor in motor control.

Objective: Birk-Landau-Perez syndrome is a genetic disorder caused by biallelic pathogenic variants in SLC30A9 presenting with a complex movement disorder, developmental regression, oculomotor abnormalities and renal impairment. So far it has been reported in only two families. We describe the clinical phenotype of eight further individuals from four unrelated families with SLC30A9-related disease. Method: Following detailed clinical phenotyping, one family underwent research wholegenome sequencing (WGS), one research whole-exome sequencing (WES) and two diagnostic WGS. Variants of interest were assessed for pathogenicity using in silico prediction tools, homology modelling and where relevant, sequencing of cDNA for splicing impact. Results: In two unrelated families of Pakistani origin (one consanguineous, one not), the same homozygous missense variant in SLC30A9 (c.1253G>T, p.Gly418Val) was identified. Family 1 included two affected brothers, and Family 2 one affected boy. In Family 3, also consanguineous, there were four affected siblings homozygous for the variant c.1049delCAG, pAla350del. The fourth family was non-consanguineous: the one affected individual was compound heterozygous for c.1083dup, p.Val362Cysfs*5 and c.1413A>G, p.Ser471=. Despite phenotypic variability between the four families, all affected patients manifested with a progressive hyperkinetic movement disorder, associated with oculomotor apraxia and ptosis. None had evidence of severe renal impairment. For the novel missense variant, the conformation of the loop domain and packing of transmembrane helices are likely to be disrupted based on structure modelling. Its presence in two unrelated Pakistani families suggests a possible founder variant. For the synonymous variant p.Ser471=, an impact on splicing was confirmed through cDNA analysis. Discussion: Pathogenic variants in SLC30A9 cause a progressive autosomal recessive neurological syndrome associated with a complex hyperkinetic movement disorder. Our report highlights the expanding disease phenotype, which can present with a wider spectrum of severity than has previously been recognised.