BackgroundThe expression of mutant HTT leads to many cellular alterations, including abnormal vesicle recycling, loss of signalling by brain-derived neurotrophic factor, excitotoxicity, perturbation of Ca2+ signalling, decreases in intracellular ATP, alterations of gene transcription, inhibition of protein clearance pathways, mitochondrial and metabolic disturbances, and ultimately cell death. While robust mammalian systems have been developed to model disease and extensive mechanistic insights have emerged, significant differences between rodent and human cells and between non-neuronal cells and neurons limit the utility of these models for accurately representing human disease. Human pluripotent stem cells can generate highly specified cell populations, including DARPP32-positive MSNs of the striatum, and provide a method for modelling HD in human neurons carrying the mutation. As it is caused by one single gene, HD is an ideal disorder for exploring the utility of modelling disease in induced pluripotent stem cells (iPSCs) through reprogramming adult cells from HD patients with known patterns of disease onset and duration.AimsGenerate iPSC lines from HD patients and controls and identify CAG-repeat expansion associated phenotypes.Methods/techniquesThrough the efforts of an international consortium effort, 14 lines were generated, differentiated into neuronal populations and assessed for CAG-repeat dependent outcome measures.Results/outcomesHD iPSC lines have reproducible CAG expansion–associated phenotypes upon differentiation, including CAG expansion-associated changes in gene expression patterns and alterations in electrophysiology, metabolism, cell adhesion, and ultimately an increased risk of cell death. While the lines with the longest repeats (HD180) showed a phenotype across all assays, those with shorter repeats (HD60) showed phenotypes in a specific sub set of assays. The most sensitive assay for establishing repeat dependent effects was found to be calcium responses to stress.ConclusionsThis HD iPSC collection represents a unique and well-characterised resource to elucidate disease mechanisms in HD and provides a novel human stem cell platform for screening new candidate therapeutics.FundingNIH, CHDI, CIRM.

Genome-scale functional genetic screens can identify key regulators of a phenotype of interest, such as determinants of protein expression or modification. Here, we present a rapid, high-throughput approach to phenotypic CRISPR-Cas9 screening. To study factors that modulate the display of CD47 on the cell surface, we processed an entire genome-wide screen containing more than 10 8 cells in under one hour and maintained high levels of cell viability using a highly scalable cell sorting technology. We robustly identified modulators of CD47 function including QPCTL, an enzyme required for formation of the pyroglutamyl modification at the N-terminus of this protein.