Advances in the Genetics of Dup15q Syndrome: Insights Gained from Induced Pluripotent Stem Cells

Background: Duplications of maternal chromosome 15q11.2-q13 are one of the most common cytogenetic anomalies associated with autism spectrum disorders (ASD).  There are two major structural versions of this copy number variant:  isodicentric chromosome 15 [idic(15)] and interstitial duplication of chromosome 15 [int.dup(15)].  Both structural versions of 15q11.2-q13 duplication syndrome result in a disorder characterized by hypotonia, developmental delay, epilepsy, and ASD.

Objectives: To develop human induced pluripotent stem cell (iPSC) models of 15q11.2-q13 duplication syndrome in order to 1) understand the local gene dysregulation underlying the disorder, 2) understand the impact of 15q11.2-q13 duplications on global gene expression in human iPSC-derived neurons,  3) to identify cellular phenotypes associated with the disorder, and 4) to test potential therapeutic approaches to restore normal 15q gene expression. 

Methods: iPSCs with idic (15), maternal and paternal int.dup(15), and maternal interstitial triplication of chromosome 15 were generated from patient fibroblast or cord blood samples using either retrovirus, lentivirus, episomal, or sendai virus reprogramming methods.  iPSCs were differentiated into forebrain cortical neurons using an embryoid body-based or monolayer protocol.  RT-qPCR and RNA-Seq were used to quantify local and global gene expression.  Microscopy and electrophysiology were used to investigate cellular phenotypes.

Results: We have generated iPSCs from five different patients with duplications of chromosome 15q11.2-q13, and have differentiated them into forebrain cortical neurons.   We compared 15q gene expression between the different iPSC lines as well as from their neuronal derivatives.  We found that gene expression closely followed copy number in iPSCs, but deviated somewhat from copy number in iPSC-derived neurons.  We identified global transcriptional changes between idic(15) and neurotypical iPSC-derived neurons that suggest defects in neuronal development pathways and upregulation of genes involved in protein degradation.  Early electrophysiology experiments and examination of dendritic spine morphology corroborate a defect in neuronal development.  

 Conclusions: Human iPSCs derived from chromosome 15q11.2-q13 individuals provide an attractive model to study gene expression and cellular phenotypes of neurodevelopmental disorders, including ASD.  They can be used to gain important insight into neuronal development.