An in vivo analysis of autism risk genes
CRISPR targeting in vivo, especially in mammals, can be difficult and time consuming when attempting to determine the effects of a single gene. However, such studies may be required to identify pathological gene variants with effects in specific cells along a developmental trajectory. To study the function of genes implicated in autism spectrum disorders (ASDs), Jin et al. applied a gene-editing and single-cell–sequencing system, Perturb-Seq, to knock out 35 ASD candidate genes in multiple mice embryos (see the Perspective by Treutlein and Camp). This method identified networks of gene expression in neuronal and glial cells that suggest new functions in ASD-related genes.
Human genetic studies have revealed long lists of genes and loci associated with risk for many diseases and disorders, but to systematically evaluate their phenotypic effects remains challenging. Without any a priori knowledge, these risk genes could affect any cellular processes in any cell type or tissue, which creates an enormous search space for identifying possible downstream effects. New high-throughput approaches are needed to functionally dissect these large gene sets across a spectrum of cell types in vivo.
Analysis of trio-based whole-exome sequencing has implicated a large number of de novo loss-of-function variants that contribute to autism spectrum disorder and developmental delay (ASD/ND) risk. Such de novo variants often have large effect sizes, thus providing a key entry point for mechanistic studies. We have developed in vivo Perturb-Seq to allow simultaneous assessment of the individual phenotypes of a panel of such risk genes in the context of the developing mouse brain.
Using CRISPR-Cas9, we introduced frameshift mutations in 35 ASD/ND risk genes in pools, within the developing mouse neocortex in utero, followed by single-cell transcriptomic analysis of perturbed cells from the early postnatal brain. We analyzed five broad cell classes—cortical projection neurons, cortical inhibitory neurons, astrocytes, oligodendrocytes, and microglia—and selected cells that had received only single perturbations. Using weighted gene correlation network analysis, we identified 14 covarying gene modules that represent transcriptional programs expressed in different classes of cortical cells.
These modules included both those affecting common biological processes across multiple cell subsets and others representing cell type–specific features restricted to certain subsets. We estimated the effect size of each perturbation on each of the 14 gene modules by fitting a joint linear regression model, estimating how module gene expression in cells from each perturbation group deviated from their expression level in internal control cells. Perturbations in nine ASD/ND genes had significant effects across five modules across four cell classes, including cortical projection neurons, cortical inhibitory neurons, astrocytes, and oligodendrocytes. Some of these results were validated by using a single-perturbation model as well as a germline-modified mutant mouse model.
To establish whether the perturbation-associated gene modules identified in the mouse cerebral cortex are relevant to human biology and ASD/ND pathology, we performed co-analyses of data from ASD and control human brains and human cerebral organoids. Both gene expression and gene covariation (“modularity”) of several of the gene modules identified in the mouse Perturb-Seq analysis are conserved in human brain tissue. Comparison with single-cell data from ASD patients showed overlap in both affected cell types and transcriptomic phenotypes.
In vivo Perturb-Seq can serve as a scalable tool for systems genetic studies of large gene panels to reveal their cell-intrinsic functions at single-cell resolution in complex tissues. In this work, we demonstrated the application of in vivo Perturb-Seq to ASD/ND risk genes in the developing brain. This method can be applied across diverse diseases and tissues in the intact organism.
De novo risk genes in this study were chosen from Satterstrom et al. (2018), and co-analysis with ASD patient data at bottom right is from Velmeshev et al. (2019); full citations for both are included in the full article online.
The number of disease risk genes and loci identified through human genetic studies far outstrips the capacity to systematically study their functions. We applied a scalable genetic screening approach, in vivo Perturb-Seq, to functionally evaluate 35 autism spectrum disorder/neurodevelopmental delay (ASD/ND) de novo loss-of-function risk genes. Using CRISPR-Cas9, we introduced frameshift mutations in these risk genes in pools, within the developing mouse brain in utero, followed by single-cell RNA-sequencing of perturbed cells in the postnatal brain. We identified cell type–specific and evolutionarily conserved gene modules from both neuronal and glial cell classes. Recurrent gene modules and cell types are affected across this cohort of perturbations, representing key cellular effects across sets of ASD/ND risk genes. In vivo Perturb-Seq allows us to investigate how diverse mutations affect cell types and states in the developing organism.