Table 3.

Evidence for distinct neurobiological mechanisms in ASD

Brain region and mechanism	Supporting evidence	Caveats and limitations	Treatment potential
Neocortex
Brain overgrowth	Multiple studies with large cohorts measuring brain size (MRI) and head circumference Mutations in genes controlling neurogenesis and growth (Fig. 2)	Small effect size (2 mm) Potential bias in measurements Incomplete understanding of biological mechanism. Evidence for both white and gray matter origin	Limited. Targeting key pathways potentially risks broad developmental problems Rapamycin treatment reversed macrocephaly in a mouse model184
Altered cortical cytoarchitecture (neuron size, number, positioning and/or orientation)	Cumulative evidence from >12 neuropathological studies Mutations in genes controlling neurogenesis, growth and neuronal migration (Fig. 2) Modeling in mice consistent with observed phenotypes (Table 1)	Small cohorts. No systematic assessment of the same brain regions and phenotypes Multiple phenotypes; none unique to ASD	Limited. Targeting key pathways potentially risks broad developmental problems
Neuronal morphogenesis	White matter reduction in neuropathological studies. Narrow minicolumns and altered connectivity in cortical circuits Mutations in genes controlling axon growth or guidance and dendrite arborization (Fig. 2)	Small cohorts. Limited number of studies	Limited. Targeting key pathways will probably lead to broad developmental problems Rapamycin successfully used in rescue experiments in mice184,240
Synaptogenesis	Increased layer-specific dendritic spine density in frontal (L2), parietal (L2) and temporal lobes (L2, L5)144,145 Mutations in genes converge in pathways regulating synaptogenesis (Fig. 2) Increased spines and upregulated spine dynamics in some mouse models192,193,283	Only two studies with small cohorts144,145 Unclear mechanism: both increase and decrease in synapse density reported in mouse models (Table 1)	Promising. Phenotypic reversal possible in postnatal periods IGF1 successfully used in rescue experiments in hiPSC100,102 PI3K antagonists rescue FXS-associated increased spine density in mice271
Synaptic dysfunction E/I imbalance	Decreased GABA receptor density and altered GAD1 and GAD2 levels. Functional imaging studies identify local hyperconnectivity and decreased long-range connections Mutations in genes converge in pathways regulating synaptic function (Fig. 2) Mouse models support disruption in E/I balance leads to ASD phenotypes (Table 1). Increasing E/I in prefrontal cortex using optogenetics leads to social deficits157	Poorly documented in neuropathological studies. Small cohorts Unclear mechanism: both increase and decrease in excitatory synaptic function reported with and without concomitant inhibitory compensation. Multiple molecular mechanisms leading to synaptic dysfunction, including altered translation, Ca2+ signaling and activity-dependent transcription (Fig. 2)	Promising. Phenotypic reversal possible in postnatal periods IGF1 rescues phenotypes in mouse models and hiPSC100,102,177 Positive allosteric modulators for GABAA receptor239, mGluR5 antagonists and agonists230,232,233, NMDAR partial agonist170,232, and blockers of NKCC1 cation-chloride cotransporter251 restored behavioral deficits in mice
Cerebellum
Purkinje cell (PC) loss and dysfunction	Reported decrease in PC size and number. Motor coordination problems in ASD PC-specific ablation of ASD risk gene Tsc1 in mice recapitulates core ASD phenotypes and PC degeneration183 Developmental cerebellar injury increases ASD risk206	Small cohorts. Limited number of studies. Gliosis observed in most Global gene expression profiles between cerebellums of control subjects and those with ASD very similar89 Limited knowledge of the role of the cerebellum in ASD behavioral domains	Promising. Postnatal cerebellar development increases therapeutic potential Rapamycin successfully used in rescue experiments in mice183
Widespread
Neuron-glia signaling	Reported increased microglia infiltration and astrogliosis in multiple brain regions (neuropathology and PET imaging) Post-mortem transcriptome identifies increased microglial and immune signature89,223 Role of microglia and astrocytes in regulating synapse formation, function and pruning. Disrupted neuron-microglia signaling in mice leads to social deficits225	Small cohorts. Limited number of studies Lack of genetic evidence suggests a reactive role Limited characterization in ASD mouse models	Untested, but promising Microglia- and astrocyte-specific rescue experiments in Rett mouse models rescues disease phenotypes284,285

Neuropathological and neuroimaging findings discussed here were recently reviewed¹⁰. See the section titled ‘Neurobiological models and mechanisms of ASD’ for additional references and detailed information on the genetic evidence and the function of specific genes involved in each biological process. hiPSC, human induced pluripotent stem cells; MRI, magnetic resonance imaging; PET, positron emission tomography; FXS, fragile X syndrome; IGF1, insulin-like growth factor 1; PC, Purkinje cell.