The translational genetics of ADHD and related phenotypes in model organisms.

Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent neurodevelopmental disorder resulting from the interaction between genetic and environmental risk factors. It is well known that ADHD co-occurs frequently with other psychiatric disorders due, in part, to shared genetics factors. Although many studies have contributed to delineate the genetic landscape of psychiatric disorders, their specific molecular underpinnings are still not fully understood. The use of animal models can help us to understand the role of specific genes and environmental stimuli-induced epigenetic modifications in the pathogenesis of ADHD and its comorbidities. The aim of this review is to provide an overview on the functional work performed in rodents, zebrafish and fruit fly and highlight the generated insights into the biology of ADHD, with a special focus on genetics and epigenetics. We also describe the behavioral tests that are available to study ADHD-relevant phenotypes and comorbid traits in these models. Furthermore, we have searched for new models to study ADHD and its comorbidities, which can be useful to test potential pharmacological treatments.

Investigating cytosolic 5'-nucleotidase II family genes as candidates for neuropsychiatric disorders in Drosophila (114/150 chr).

Cytosolic 5'-nucleotidases II (cNT5-II) are an evolutionary conserved family of 5'-nucleotidases that catalyze the intracellular hydrolysis of nucleotides. In humans, the family is encoded by five genes, namely NT5C2, NT5DC1, NT5DC2, NT5DC3, and NT5DC4. While very little is known about the role of these genes in the nervous system, several of them have been associated with neuropsychiatric disorders. Here, we tested whether manipulating neuronal expression of cNT5-II orthologues affects neuropsychiatric disorders-related phenotypes in the model organism Drosophila melanogaster. We investigated the brain expression of Drosophila orthologues of cNT5-II family (dNT5A-CG2277, dNT5B-CG32549, and dNT5C-CG1814) using quantitative real-time polymerase chain reaction (qRT-PCR). Using the UAS/Gal4 system, we also manipulated the expression of these genes specifically in neurons. The knockdown was subjected to neuropsychiatric disorder-relevant behavioral assays, namely light-off jump reflex habituation and locomotor activity, and sleep was measured. In addition, neuromuscular junction synaptic morphology was assessed. We found that dNT5A, dNT5B, and dNT5C were all expressed in the brain. dNT5C was particularly enriched in the brain, especially at pharate and adult stages. Pan-neuronal knockdown of dNT5A and dNT5C showed impaired habituation learning. Knockdown of each of the genes also consistently led to mildly reduced activity and/or increased sleep. None of the knockdown models displayed significant alterations in synaptic morphology. In conclusion, in addition to genetic associations with psychiatric disorders in humans, altered expression of cNT5-II genes in the Drosophila nervous system plays a role in disease-relevant behaviors.

Contribution of Intellectual Disability-Related Genes to ADHD Risk and to Locomotor Activity in Drosophila.

OBJECTIVE: Attention deficit hyperactivity disorder (ADHD) is a common, highly heritable neuropsychiatric disorder. ADHD often co-occurs with intellectual disability, and shared overlapping genetics have been suggested. The aim of this study was to identify novel ADHD genes by investigating whether genes carrying rare mutations linked to intellectual disability contribute to ADHD risk through common genetic variants. Validation and characterization of candidates were performed using Drosophila melanogaster. METHODS: Common genetic variants in a diagnostic gene panel of 396 autosomal intellectual disability genes were tested for association with ADHD risk through gene set and gene-wide analyses, using ADHD meta-analytic data from the Psychiatric Genomics Consortium for discovery (N=19,210) and ADHD data from the Lundbeck Foundation Initiative for Integrative Psychiatric Research for replication (N=37,076). The significant genes were functionally validated and characterized in Drosophila by assessing locomotor activity and sleep upon knockdown of those genes in brain circuits. RESULTS: The intellectual disability gene set was significantly associated with ADHD risk in the discovery and replication data sets. The three genes most consistently associated were MEF2C, ST3GAL3, and TRAPPC9. Performing functional characterization of the two evolutionarily conserved genes in Drosophila melanogaster, the authors found that their knockdown in dopaminergic (dMEF2) and circadian neurons (dTRAPPC9) resulted in increased locomotor activity and reduced sleep, concordant with the human phenotype. CONCLUSIONS: This study reveals that a large set of intellectual disability-related genes contribute to ADHD risk through effects of common alleles. Utilizing this continuity, the authors identified TRAPPC9, MEF2C, and ST3GAL3 as novel ADHD candidate genes. Characterization in Drosophila suggests that TRAPPC9 and MEF2C contribute to ADHD-related behavior through distinct neural substrates.

Habituation Learning Is a Widely Affected Mechanism in Drosophila Models of Intellectual Disability and Autism Spectrum Disorders.

BACKGROUND: Although habituation is one of the most ancient and fundamental forms of learning, its regulators and its relevance for human disease are poorly understood. METHODS: We manipulated the orthologs of 286 genes implicated in intellectual disability (ID) with or without comorbid autism spectrum disorder (ASD) specifically in Drosophila neurons, and we tested these models in light-off jump habituation. We dissected neuronal substrates underlying the identified habituation deficits and integrated genotype-phenotype annotations, gene ontologies, and interaction networks to determine the clinical features and molecular processes that are associated with habituation deficits. RESULTS: We identified >100 genes required for habituation learning. For 93 of these genes, a role in habituation learning was previously unknown. These genes characterize ID disorders with macrocephaly and/or overgrowth and comorbid ASD. Moreover, individuals with ASD from the Simons Simplex Collection carrying damaging de novo mutations in these genes exhibit increased aberrant behaviors associated with inappropriate, stereotypic speech. At the molecular level, ID genes required for normal habituation are enriched in synaptic function and converge on Ras/mitogen-activated protein kinase (Ras/MAPK) signaling. Both increased Ras/MAPK signaling in gamma-aminobutyric acidergic (GABAergic) neurons and decreased Ras/MAPK signaling in cholinergic neurons specifically inhibit the adaptive habituation response. CONCLUSIONS: Our work supports the relevance of habituation learning to ASD, identifies an unprecedented number of novel habituation players, supports an emerging role for inhibitory neurons in habituation, and reveals an opposing, circuit-level-based mechanism for Ras/MAPK signaling. These findings establish habituation as a possible, widely applicable functional readout and target for pharmacologic intervention in ID/ASD.