Use of Human Induced Pluripotent Stem Cells as a Model to Study Primary Microcephaly

Abstract

The aim of this study was to use human induced pluripotent stem cells (hiPSC) to understand key steps of neuronal differentiation and to model the human brain developmental disorder, primary microcephaly (MCPH). MCPH is a genetically and clinically heterogeneous group of autosomal recessive disorders characterized by significant reduction of brain volume associated with intellectual disabilities. Since available animal models are inadequate for fully understanding the pathology of this disorder, the objective was to create humanized models of MCPH. This was achieved by inducing cellular reprogramming of dermal fibroblasts from healthy and MCPH patients into pluripotent stem cells, and driving them towards neuronal differentiation to study neural proliferation and differentiation in the correct genetic context. hiPSCs were created from fibroblasts obtained from one unaffected individual (control) and one microcephaly patient having mutations in MCPH1. These were differentiated into neural stem cells and cortical neurons following embryoid body methodology of differentiation. Noggin was used for neural induction to obtain a forebrain identity. Patient iPSC-derived neural rosettes showed reduced neural induction, reduced neural proliferation and increased cell death as compared to control. They also showed higher proportion of simplified rosettes as compared to control. Interestingly, some of the patient cells which were unable to form a rosette and had an apoptotic centre, showed presence of neurites indicating premature differentiation, while no such neurites were observed in the control rosettes. Most importantly, when immunostained for cortical layer specific markers, neurons obtained from patient iPSC showed comparable expression of Cux1, while Ctip2 expression was reduced and there was no expression of Tbr1 as compared to control. During development, normal brain size is dependent on the number of neural progenitors and neurons generated. Since patient-derived neural stem cells showed lower neural induction, reduced proliferation, increased cell death of neural progenitors as well as premature differentiation, and absence of a subset of neurons, it is likely that these factors together or partly could contribute to the reduced brain size seen in microcephaly patients