GENETIC DISSECTION OF SEVERITY AND ONSET MODULATORS FOR ALZHEIMER’S PATHOLOGY IN DOWN SYNDROME USING CELLULAR SYSTEMS

Abstract

Trisomy of APP is responsible for extremely high incidence and early onset of AD pathology in Down syndrome (DS) brains. However, other genes on human chr21 likely modulate the age of onset, severity and modality of the clinical picture, as DS individuals have later or absent onset of clinical AD, and less intracerebral haemorrhage pathology, than euploid individuals with the familial early onset AD caused by a duplication of APP gene (dupAPP). Our aim is to identify such modulator genes on chr21 using cellular models. Neurons derived from isogenic hiPSCs we generated from a mosaic DS individual, unpublished iPSCs from segmental trisomy 21 DS and non-DS individuals, as well as from DS individuals with extremely early or late dementia onset are used in 2D, 3D and cerebral organoid paradigms. Optogenetic stimulation of Channelrhodopsin-2 engineered hiPSCs is used to determine the neuronal activity-dependent cellular phenotype modulation. CRISPR/Cas9 mediated reduction from trisomy to disomy for individual selected candidate genes on chr21 are performed on isogenic T21 iPSCs. We detect T21-caused neurodevelopmental delay, increased β-amyloid and phospho-Tau presence, complex mitochondrial dysfunction, accelerated DNA damage and abnormal endosomes, and some of these phenotypes exacerbate in 3D and cerebral organoid paradigms, and/or can be reproduced in primary human neuroectodermal cells expanded and differentiated in vitro. Cellular phenotypes relevant for AD pathology caused by trisomy21 can be reproduced and quantitatively assessed in iPSC-derived and primary NSC-derived neurons and differences sharpened by the use of cerebral organoid technology. Isogenic iPSCs allow detection of subtle changes in phenotypes and evaluation of single gene's trisomy contribution using CRISPR/Cas9 editing. Segmental trisomy 21 iPSCs allow for assignment of phenotype effects to a region of chromosome 21, and can faster eliminate some candidate genes.