Over the past 6 months we have turned a corner in our studies of the genetic basis of schizophrenia and autism. For years the field of psychiatric genetics has struggled: family and twin studies demonstrated that these disorders were heritable, but findings from small studies reporting specific risk genes could not be replicated. With larger samples and better tools, we have gone from famine to feast, with almost too many genetic findings to follow up. A new report has just described 13 new genetic findings associated with schizophrenia, resulting in over 100 common variants now identified as risk factors.1
The problem now is not how to find the genetic variation associated with mental disorders, but how to understand these genetic changes and, specifically, link them to changes in brain circuits. Adult-derived stem cells that can be differentiated into neurons are an obvious tool. Either the person’s own cells can be used or standard stem cells can be engineered to have the same genetic variations associated with autism or schizophrenia. An extraordinary new method called CRISPR uses an enzyme from bacteria found in yogurt to edit DNA so that a genetic variant can be inserted quickly, accurately, and inexpensively.2 Turning stem cells into neurons with a risk gene is now straightforward.
This approach, sometimes called “disease in a dish,” has already been used for Timothy syndrome, a rare form of autism caused by a single gene. Neurons grown in vitro from children with Timothy syndrome show abnormal dendrites and unusual dendritic responses to activation, possibly revealing a mechanism of the disorder.3 Stem cells differentiated into neurons are now being tested for a number of other neurodevelopmental disorders for which we know the genetic cause. Complex disorders, with multiple genes involved, will be more of a challenge but with CRISPR for DNA editing, what seemed impossible only last year begins to look plausible.
But growing neurons in this way may not be enough. If the neurodevelopmental disease is due to a dysfunctional circuit we need to study neurons as they develop in an environment where they can wire and fire with other cells. A new paper by Madeline Lancaster and her colleagues in Vienna has just described a cerebral “organoid” – a sort of budding brain growing in a dish.4 The organoid is built from human stem cells, following the steps required for human brain development in utero. To be clear, this is not a whole brain, it is a 3-dimensional in vitro culture system in which stem cells are induced to form many of the different kinds of cells and tissues found in the human cortex and other areas. What is amazing – eclipsing earlier “disease-in-a-dish” discoveries – is that, over weeks and months, these cells organize themselves according to the architecture that we see in a functioning human brain. But this is not a functioning brain -- the full organoid is smaller than a kernel of corn and it lacks a circulatory system or the complex circuitry needed for function.
Lancaster and her colleagues used these cerebral organoids to study a rare genetic disease called microcephaly , in which the brain does not develop to its full size. Attempts to model this in rodents have not been successful, but in the organoid system they showed that cultures built with stem cells from infants with microcephaly differentiate prematurely rather than continuing to divide, providing a potential explanation for how the genetic mutation leads to the disease. Microcephaly might seem a long way from the needs of people with serious mental illness. After all, for psychotic disorders, mood disorders, OCD, and autism, we are worried about brain circuits not brain size. The question now is whether this approach can help us understand how the genes and environmental factors that confer risk for brain disorders lead to changes in brain circuits?
Engineers like to say that you don’t understand something until you can build it. We are a long way from building even the simplest invertebrate brain, but we are making progress identifying the factors important for brain development.
By assembling some of the components – first cells, then circuits, then organoids – we will begin to understand much more about normal and abnormal development. For neurodevelopmental disorders, organoids may seem like science fiction, but these in vitro cell culture systems could prove to be an important milestone on the road to a cure.
1Genome-wide association analysis identifies 13 new risk loci for schizophrenia.
Ripke S, O'Dushlaine C, Chambert K, Moran JL, Kähler AK, Akterin S, Bergen SE, Collins AL, Crowley JJ, Fromer M, Kim Y, Lee SH, Magnusson PK, Sanchez N, Stahl EA, Williams S, Wray NR, Xia K, Bettella F, Borglum AD, Bulik-Sullivan BK, Cormican P, Craddock N, de Leeuw C, Durmishi N, Gill M, Golimbet V, Hamshere ML, Holmans P, Hougaard DM, Kendler KS, Lin K, Morris DW, Mors O, Mortensen PB, Neale BM, O'Neill FA, Owen MJ, Milovancevic MP, Posthuma D, Powell J, Richards AL, Riley BP, Ruderfer D, Rujescu D, Sigurdsson E, Silagadze T, Smit AB, Stefansson H, Steinberg S, Suvisaari J, Tosato S, Verhage M, Walters JT; Multicenter Genetic Studies of Schizophrenia Consortium, Levinson DF, Gejman PV, Kendler KS, Laurent C, Mowry BJ, O'Donovan MC, Owen MJ, Pulver AE, Riley BP, Schwab SG, Wildenauer DB, Dudbridge F, Holmans P, Shi J, Albus M, Alexander M, Campion D, Cohen D, Dikeos D, Duan J, Eichhammer P, Godard S, Hansen M, Lerer FB, Liang KY, Maier W, Mallet J, Nertney DA, Nestadt G, Norton N, O'Neill FA, Papadimitriou GN, Ribble R, Sanders AR, Silverman JM, Walsh D, Williams NM, Wormley B; Psychosis Endophenotypes International Consortium, Arranz MJ, Bakker S, Bender S, Bramon E, Collier D, Crespo-Facorro B, Hall J, Iyegbe C, Jablensky A, Kahn RS, Kalaydjieva L, Lawrie S, Lewis CM, Lin K, Linszen DH, Mata I, McIntosh A, Murray RM, Ophoff RA, Powell J, Rujescu D, Van Os J, Walshe M, Weisbrod M, Wiersma D; Wellcome Trust Case Control Consortium 2; Management Committee:, Donnelly P, Barroso I, Blackwell JM, Bramon E, Brown MA, Casas JP, Corvin AP, Deloukas P, Duncanson A, Jankowski J, Markus HS, Mathew CG, Palmer CN, Plomin R, Rautanen A, Sawcer SJ, Trembath RC, Viswanathan AC, Wood NW; Data and Analysis Group:, Spencer CC, Band G, Bellenguez C, Freeman C, Hellenthal G, Giannoulatou E, Pirinen M, Pearson RD, Strange A, Su Z, Vukcevic D, Donnelly P; DNA, Genotyping, Data QC and Informatics Group:, Langford C, Hunt SE, Edkins S, Gwilliam R, Blackburn H, Bumpstead SJ, Dronov S, Gillman M, Gray E, Hammond N, Jayakumar A, McCann OT, Liddle J, Potter SC, Ravindrarajah R, Ricketts M, Tashakkori-Ghanbaria A, Waller MJ, Weston P, Widaa S, Whittaker P, Barroso I, Deloukas P; Publications Committee:, Mathew CG, Blackwell JM, Brown MA, Corvin AP, McCarthy MI, Spencer CC, Bramon E, Corvin AP, O'Donovan MC, Stefansson K, Scolnick E, Purcell S, McCarroll SA, Sklar P, Hultman CM, Sullivan PF. Nat Genet. 2013 Aug 25. doi: 10.1038/ng.2742. [Epub ahead of print] PMID: 23974872
2Multiplex genome engineering using CRISPR/Cas systems. Cong L, Ran FA, Cox D, Lin S, Barretto R, Habib N, Hsu PD, Wu X, Jiang W, Marraffini LA, Zhang F. Science. 2013 Feb 15;339(6121):819-23. doi: 10.1126/science.1231143. Epub 2013 Jan 3.PMID:23287718
3Timothy syndrome is associated with activity-dependent dendritic retraction in rodent and human neurons. Krey JF, Paşca SP, Shcheglovitov A, Yazawa M, Schwemberger R, Rasmusson R, Dolmetsch RE. Nat Neurosci. 2013 Feb;16(2):201-9. doi: 10.1038/nn.3307. Epub 2013 Jan 13. PMID:23313911
4Cerebral organoids model human brain development and microcephaly.
Lancaster MA, Renner M, Martin CA, Wenzel D, Bicknell LS, Hurles ME, Homfray T, Penninger JM, Jackson AP, Knoblich JA. Nature. 2013 Aug 28. doi: 10.1038/nature12517. [Epub ahead of print] PMID:23995685