Positional cloning by fast-track SNP-mapping in Drosophila melanogaster

Genetic background mutations drive neural circuit hyperconnectivity in a fragile X syndrome model

BACKGROUND

Neural circuits are initially assembled during development when neurons synapse with potential partners and later refined as appropriate connections stabilize into mature synapses while inappropriate contacts are eliminated. Disruptions to this synaptogenic process impair connectivity optimization and can cause neurodevelopmental disorders. Intellectual disability (ID) and autism spectrum disorder (ASD) are often characterized by synaptic overgrowth, with the maintenance of immature or inappropriate synapses. Such synaptogenic defects can occur through mutation of a single gene, such as fragile X mental retardation protein (FMRP) loss causing the neurodevelopmental disorder fragile X syndrome (FXS). FXS represents the leading heritable cause of ID and ASD, but many other genes that play roles in ID and ASD have yet to be identified.

RESULTS

In a Drosophila FXS disease model, one dfmr150M null mutant stock exhibits previously unreported axonal overgrowths at developmental and mature stages in the giant fiber (GF) escape circuit. These excess axon projections contain both chemical and electrical synapse markers, indicating mixed synaptic connections. Extensive analyses show these supernumerary synapses connect known GF circuit neurons, rather than new, inappropriate partners, indicating hyperconnectivity within the circuit. Despite the striking similarities to well-characterized FXS synaptic defects, this new GF circuit hyperconnectivity phenotype is driven by genetic background mutations in this dfmr150M stock. Similar GF circuit synaptic overgrowth is not observed in independent dfmr1 null alleles. Bulked segregant analysis (BSA) was combined with whole genome sequencing (WGS) to identify the quantitative trait loci (QTL) linked to neural circuit hyperconnectivity. The results reveal 8 QTL associated with inappropriate synapse formation and maintenance in the dfmr150M mutant background.

CONCLUSIONS

Synaptogenesis is a complex, precisely orchestrated neurodevelopmental process with a large cohort of gene products coordinating the connectivity, synaptic strength, and excitatory/inhibitory balance between neuronal partners. This work identifies a number of genetic regions that contain mutations disrupting proper synaptogenesis within a particularly well-mapped neural circuit. These QTL regions contain potential new genes involved in synapse formation and refinement. Given the similarity of the synaptic overgrowth phenotype to known ID and ASD inherited conditions, identifying these genes should increase our understanding of these devastating neurodevelopmental disease states.

A simple TALEN-based protocol for efficient genome-editing in Drosophila

Drosophila is a well-established genetic model organism: thousands of point mutations, deficiencies or transposon insertions are available from stock centres. However, to date, it is still difficult to modify a specific gene locus in a defined manner. A potential solution is the application of transcription activator-like effector nucleases (TALENs), which have been used successfully to mutate genes in various model organisms. TALENs are constructed by fusion of TALE proteins to the endonuclease FokI, resulting in artificial, sequence-specific endonucleases. They induce double strand breaks, which are either repaired by error-prone non-homologous end joining (NHEJ) or homology directed repair (HDR). We developed a simple TALEN-based protocol to mutate any gene of interest in Drosophila within approximately 2 months. We inject mRNA coding for two TALEN pairs targeting the same gene into embryos, employ T7 endonuclease I screening of pooled F1 flies to identify mutations and generate a stable mutant stock in the F3 generation. We illustrate the efficacy of our strategy by mutating CG11617, a previously uncharacterized putative transcription factor with an unknown function in Drosophila. This demonstrates that TALENs are a reliable and efficient strategy to mutate any gene of interest in Drosophila.

Genetic screens to identify new Notch pathway mutants in Drosophila

Notch signaling controls a wide range of developmental processes, including proliferation, apoptosis, and cell fate specification during both development and adult tissue homeostasis. The functional versatility of the Notch signaling pathway is tightly linked with the complexity of its regulation in different cellular contexts. To unravel the complexity of Notch signaling, it is important to identify the different components of the Notch signaling pathway. A powerful strategy to accomplish this task is based on genetic screens. Given that the developmental context of signaling is important, these screens should be customized to specific cell populations or tissues. Here, I describe how to perform F1 clonal forward genetic screens in Drosophila to identify novel components of the Notch signaling pathway. These screens combine a classical EMS (ethyl methanesulfonate) chemical mutagenesis protocol along with clonal analysis via FRT-mediated mitotic recombination. These F1 clonal screens allow rapid phenotypic screening within clones of mutant cells induced at specific developmental stages and in tissues of interest, bypassing the pleiotropic effects of isolated mutations. More importantly, since EMS mutations have been notoriously difficult to map to specific genes in the past, I briefly discuss mapping methods that allow rapid identification of the causative mutations.