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. 2013 Nov 21;155(5):997-1007.
doi: 10.1016/j.cell.2013.10.020.

Coexpression networks implicate human midfetal deep cortical projection neurons in the pathogenesis of autism

A Jeremy Willsey  1 , Stephan J Sanders, Mingfeng Li, Shan Dong, Andrew T Tebbenkamp, Rebecca A Muhle, Steven K Reilly, Leon Lin, Sofia Fertuzinhos, Jeremy A Miller, Michael T Murtha, Candace Bichsel, Wei Niu, Justin Cotney, A Gulhan Ercan-Sencicek, Jake Gockley, Abha R Gupta, Wenqi Han, Xin He, Ellen J Hoffman, Lambertus Klei, Jing Lei, Wenzhong Liu, Li Liu, Cong Lu, Xuming Xu, Ying Zhu, Shrikant M Mane, Ed S Lein, Liping Wei, James P Noonan, Kathryn Roeder, Bernie Devlin, Nenad Sestan, Matthew W State
Affiliations

Coexpression networks implicate human midfetal deep cortical projection neurons in the pathogenesis of autism

A Jeremy Willsey et al. Cell. .

Abstract

Autism spectrum disorder (ASD) is a complex developmental syndrome of unknown etiology. Recent studies employing exome- and genome-wide sequencing have identified nine high-confidence ASD (hcASD) genes. Working from the hypothesis that ASD-associated mutations in these biologically pleiotropic genes will disrupt intersecting developmental processes to contribute to a common phenotype, we have attempted to identify time periods, brain regions, and cell types in which these genes converge. We have constructed coexpression networks based on the hcASD "seed" genes, leveraging a rich expression data set encompassing multiple human brain regions across human development and into adulthood. By assessing enrichment of an independent set of probable ASD (pASD) genes, derived from the same sequencing studies, we demonstrate a key point of convergence in midfetal layer 5/6 cortical projection neurons. This approach informs when, where, and in what cell types mutations in these specific genes may be productively studied to clarify ASD pathophysiology.

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Figures

Figure 1
Figure 1. Overview of Coexpression Analysis Workflow and Associated Data Sets
A) Nine hcASD genes derived from our data and additional published data sets. The number of independent de novo loss of function (LoF) mutations identified is indicated in parentheses. ANK2 is a novel ASD-associated gene. B) A comprehensive data set of spatiotemporal gene expression spanning human brain development (Kang et al., 2011) was used to perform coexpression analysis. This data set spans 12 brain regions during periods 1 and 2 (embryonic and early fetal development) and 16 regions from period 3 to period 15 (early fetal to late adulthood). Neocortical regions are in red. C) Periods of human brain development as defined by Kang et al. (2012). PCW, postconceptual weeks; M, postnatal months; Y, postnatal years. D) The hcASD genes (black) are used as “seeds” to build coexpression networks along spatial and temporal dimensions. E) Mean expression levels for two genes are plotted as a function of period of development (temporal axis) and region of the brain (spatial axis). The images illustrate that highly correlated genes have similar expression profiles across these dimensions. The Pearson’s correlation value quantifies the similarity between these profiles. F) Networks are interrogated for enrichment of an independent set of 122 probable ASD (pASD) genes (gray). G) Networks are tested for enrichment of layer and cell-type specific genes. FC, frontal cerebral wall; PC, parietal cerebral wall; TC, temporal cerebral wall; OC, occipital cerebral wall; HIP, hippocampal anlage (periods 1–2), hippocampus (periods 3–15); VF, ventral forebrain; MGE, medial ganglionic eminence; LGE, lateral ganglionic eminence; CGE, caudal ganglionic eminence; DIE, diencephalon; DTH, dorsal thalamus; URL, upper (rostral) rhombic lip; OFC, orbital prefrontal cortex; DFC, dorsal prefrontal cortex; VFC, ventral prefrontal cortex; MFC, medial prefrontal cortex; M1C, primary motor cortex; S1C, primary somatosensory cortex; IPC, posterior inferior parietal cortex; A1C, primary auditory cortex; STC, superior temporal cortex; ITC, inferior temporal cortex; V1C, primary visual cortex; AMY, amygdala; STR, striatum; MD, mediodorsal nucleus of the thalamus; CBC, cerebellar cortex. See also Figure S1 and Table S1.
Figure 2
Figure 2. Convergence in Prefrontal and Primary Motor-Somatosensory Cortex Regions During Midfetal Development
A) Hierarchical clustering of brain regions based on transcriptional similarity during fetal development (periods 3–7) divides the brain regions into four groups, demarcated by color. These clusters also reflect actual topographical proximity and functional segregation. B) To achieve spatiotemporal resolution, coexpression networks were formed from 52 subsets of the expression data based on 13 developmental stages (in three period windows) and four sets of brain regions (clusters shown in A). Each of the networks was tested for enrichment of 122 pASD genes. This heatmap shows the negative log10(p-value) (hypergeometric test) for enrichment in each network with developmental stages on the x-axis and brain regions on the y-axis. Networks that are not significant are in white; nominally significant networks are in light red; networks that are significant after correction for multiple comparisons are in red with the negative log10(p-value) noted. C) pASD gene enrichment is statistically significant, after correction for multiple comparisons, by hypergeometric test (gray line) in the given regions and time points. Black lines show p values estimated from the permutation test based on the number of pASD genes within the networks (corrected for multiple comparisons); the vertical red line shows the number of pASD genes observed for that specific network. pASD enrichment in period 3–5 (p=0.003), 4–6 (p-0.05), and 8–10 (p=0.04) networks remains significant, whereas the period 7–9 network (p=0.8) does not. PFC-MSC, prefrontal cortex and primary motor-somatosensory cortex; V1C-STC, V1C, ITC, IPC, A1C, STC. See also Figure S2 and Table S2.
Figure 3
Figure 3. The Period 3–5 and Period 4–6 PFC-MSC Networks
(A and B) The period 3–5 (A) and period 4–6 (B) coexpression networks are displayed with the force-directed layout function of Cytoscape using correlation as the edge weight (Cline et al., 2007). Gene coexpression analysis included the 20 genes best correlated with each hcASD gene. The hcASD seed genes are in black; pASD genes identified within the network are in gray; and top 20 co-expressed genes that are not pASD genes are in white. The lines (edges) reflect coexpression correlations ≥0.7 and the shade represents the strength of the correlation; positive correlations are in in red; negative correlations are in blue. (C and D) The pASD genes enriched within the networks in (A) and (B) represent those with the highest probability of being true ASD genes. The TADA score combines de novo mutation data with inherited variant data from trios, rare variant case-control data, and estimates of mutation rate in order to estimate the probability of ASD association for each gene (He et al., 2013). The histograms show the results of permutation tests (100,000 iterations each) assessing the combined TADA score in the period 3–5 (C) and period 4–6 (D) networks; the observed scores, shown by the vertical red lines, are highly significant (p=6.0×10−5 and p=2.7×10−4, respectively). See also Figure S3 and Tables S3 and S4.
Figure 4
Figure 4. The PFC-MSC Networks Show Enrichment for Markers of Deep Layer Projection Neurons
(A and B) To improve the spatial resolution of the coexpression analysis, the midfetal networks were assessed in an independent prenatal transcriptome (http://www.brainspan.org) data set comprised of microarray-based gene expression profiles of laser microdissected (LMD) human midfetal (period 4–6) brains. Each hcASD seed gene network was recreated within each layer from this LCM data and the significance of the observed connectivity (sum of correlations along network connections) for each layer was assessed by permutation test. The period 3–5 network (A) shows significant connectivity in the CPi region (inner cortical plate) corresponding to neocortical layers 5–6 (corrected p=2.7×10−4), whereas the the period 4–6 network (B) is not significantly connected in any layer. C) The period 3–5 PFC-MSC network is enriched for markers of deep layer cells in mouse cortex. At each postnatal day (P) indicated, differential gene expression analysis of RNA-seq data was performed on three cortical layers of two mouse cortices (1 male, 1 female) to identify genes exclusively differentially expressed in a particular layer (markers). The “superficial” layer corresponds to the human marginal zone and CPo (human layer 2 [L2] and L3), layer “four” corresponds to human CPo (human L4), and the “deep” layer corresponds to human CPi and subplate (human L5/L6 and subplate). From P4 to P10 (mid- to late fetal development), human orthologues of murine deep layer marker genes are significantly enriched in the period 3–5 network (hypergeometric test). From P14 to P180 (adulthood), there is insufficient resolution to differentiate layers and none are significantly enriched. (D and E) Enrichment of a set of previously published cell-type specific marker genes (Kang et al., 2011) is specific to cortical glutamatergic projection neurons (CPNs) in both midfetal networks (p=0.02 for period 3–5; p=0.01 for period 4–6). Markers of deep layer CPNs (L5/L6) are significantly enriched in both networks (p=0.02 for period 3–5; p=0.01 for period 4–6) whereas markers of superficial layer CPNs (L1 to L4) are not. All p values were assessed by permutation test with 100,000 iterations unless otherwise noted. See also Figure S4 and Tables S5 and S6.
Figure 5
Figure 5. hcASD Genes are Expressed in Midfetal Deep Layer Projection Neurons
A) A coronal tissue section through the prefrontal cortex (PFC) and striatum (STR) of a midfetal forebrain was Nissl stained to visualize cells in distinct developmental zones. The darkest labeled zones are denser with cells, namely the VZ (ventricular zone) and SZi (inner subventricular zone), CPi (inner cortical plate), and CPo (outer cortical plate). The corresponding adult zones are labeled on the right of the higher magnified boxed area. B) Tissue sections of PFC areas at approximately equivalent ages (18–21 PCW) were stained with either antibodies (as labeled on top of the first six columns; fluorescence in red, green, or blue; DAB in brown) or in situ hybridization probes (last column, SCN2A). Images in the top row were taken at the boundary between CPo and MZ (marginal zone), and images in the bottom row at the boundary between CPi and SP (subplate). Arrows indicate cells colabeled for an hcASD gene and a marker gene (SATB2 or FOXP2). SG, subpial granular zone; IZ, intermediate zone; SZo, outer subventricular zone; L, layer; WM, white matter; SE/EL, subependymal/ependymal layer.
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