The complexity of the human brain has made it difficult to study many brain disorders in model organisms, and highlights the need for an in vitro model of human brain development. We have developed a human pluripotent stem cell-derived 3D organoid culture system, termed cerebral organoid, which develops various discrete though interdependent brain regions. These include cerebral cortex containing progenitor populations that organize and produce mature cortical neuron subtypes. Furthermore, cerebral organoids recapitulate features of human cortical development, namely characteristic progenitor zone organization with abundant outer radial glial stem cells. Finally, we use RNAi and patient-specific iPS cells to model microcephaly, a disorder that has been difficult to recapitulate in mice. We demonstrate premature neuronal differentiation in patient organoids, a defect that could explain the disease phenotype. Our data demonstrate that 3D organoids can recapitulate development and disease of even this most complex human tissue.

SummaryDNA is strictly compartmentalised within the nucleus to prevent autoimmunity1; despite this cGAS, a cytosolic sensor of dsDNA, is activated in autoinflammatory disorders and by DNA damage2–6. Precisely how cellular DNA gains access to the cytoplasm remains to be determined. Here, we report that cGAS localises to micronuclei arising from genome instability in a model of monogenic autoinflammation, after exogenous DNA damage and spontaneously in human cancer cells. These micronuclei occur after mis-segregation of DNA during cell division and consist of chromatin surrounded by their own nuclear membrane. Breakdown of the micronuclear envelope, a process associated with chromothripsis7, leads to rapid accumulation of cGAS, providing a mechanism by which self-DNA becomes exposed to the cytosol. cGAS binds to and is activated by chromatin and, consistent with a mitotic origin, micronuclei formation and the proinflammatory response following DNA-damage are cell-cycle dependent. Furthermore, by combining live-cell laser microdissection with single cell transcriptomics, we establish that induction of interferon stimulated gene expression occurs in micronucleated cells. We therefore conclude that micronuclei represent an important source of immunostimulatory DNA. As micronuclei formed from lagging chromosomes also activate this pathway, cGAS recognition of micronuclei may act as a cell-intrinsic immune surveillance mechanism detecting a range of neoplasia-inducing processes.

Aicardi-Goutières syndrome (AGS) presents as a severe neurological brain disease and is a genetic mimic of the sequelae of transplacentally acquired viral infection. Evidence exists for a perturbation of innate immunity as a primary pathogenic event in the disease phenotype. Here, we show that TREX1, encoding the major mammalian 3' --> 5' DNA exonuclease, is the AGS1 gene, and AGS-causing mutations result in abrogation of TREX1 enzyme activity. Similar loss of function in the Trex1(-/-) mouse leads to an inflammatory phenotype. Our findings suggest an unanticipated role for TREX1 in processing or clearing anomalous DNA structures, failure of which results in the triggering of an abnormal innate immune response.