Functional Genomics of Epilepsy and Associated Neurodevelopmental Disorders Using Simple Animal Models: From Genes, Molecules to Brain Networks

Abstract

The genetic diagnosis of patients with seizure disorders has been improved significantly by the development of affordable next-generation sequencing technologies. Indeed, in the last 20 years, dozens of causative genes and thousands of associated variants have been described and, for many patients, are now considered responsible for their disease. However, the functional consequences of these mutations are often not studied in vivo, despite such studies being central to understanding pathogenic mechanisms and identifying novel therapeutic avenues. One main roadblock to functionally characterizing pathogenic mutations is generating and characterizing in vivo mammalian models carrying clinically relevant variants in specific genes identified in patients. Although the emergence of new mutagenesis techniques facilitates the production of rodent mutants, the fact that early development occurs internally hampers the investigation of gene function during neurodevelopment. In this context, functional genomics studies using simple animal models such as flies or fish are advantageous since they open a dynamic window of investigation throughout embryonic development. In this review, we will summarize how the use of simple animal models can fill the gap between genetic diagnosis and functional and phenotypic correlates of gene function in vivo. In particular, we will discuss how these simple animals offer the possibility to study gene function at multiple scales, from molecular function (i.e., ion channel activity), to cellular circuit and brain network dynamics. As a result, simple model systems offer alternative avenues of investigation to model aspects of the disease phenotype not currently possible in rodents, which can help to unravel the pathogenic substratum in vivo.

Keywords: Drosophila; brain disorder; epilepsy; neurodevelopmental disorder; zebrafish.

FIGURE 1

Epilepsy genes. This figure illustrates the functional classes of the most commonly identified genetic mutations in children with DDEs. These affect a broad range of neuronal functions, ranging from gene expression and intracellular signaling to neurotransmission.

FIGURE 2

Recording whole-brain dynamics at single-cell resolution in zebrafish models of neurodevelopmental disorders. (A) Larval zebrafish at 7 days post fertilization are freely behaving and have all the major anatomical subdivisions of the vertebrate brain (left). Transgenic lines expressing genetically encoded calcium indicators in neurons can be used to record neuronal function through fluorescence signals. Because of their small size, the whole brain can be captured at single cell resolution (top). This allows recording of whole brain dynamics alongside single-cell behavior (bottom). (B) Zebrafish larvae can be embedded in transparent agarose, allowing in vivo imaging using fluorescence microscopy (shown here is a two-photon microscopy setup). Depending on the experimental paradigm, behavioral output can further be tracked using recordings of tail movements in tail free set ups. This allows e.g., linking of convulsive movements and brain hypersynchrony to identify epileptic seizures in the zebrafish.