Induced pluripotent stem cells from Huntington’s disease patients: a promising approach to define and correct disease-related alterations

Adult somatic cells such as skin or blood cells from either health donors or patients can be reprogrammed into induced pluripotent stem cells (iPSCs). Given their unlimited self-renewal and differentiation capacities, iPSCs are an invaluable resource to generate terminally differentiated cells. Thus, iPSCs can facilitate the study of human diseases and drug screening, holding great promise for regenerative medicine. Another significant advantage of iPSC disease-modeling is that normal and mutant proteins are expressed at endogenous levels. In addition, subtle phenotypes and the effects of genetic background variations can be assessed by comparison between iPSC lines obtained from different patients and healthy donors as well as isogenic lines, in which disease-related mutations are corrected. As with other multiple diseases, iPSCs derived from Huntington’s disease (HD) patients provide an opportunity to define disease-related changes and possible interventions to correct these alterations. HD is an autosomal dominant neurodegenerative disorder characterized by cognitive deficits, psychosis and motor dysfunction (Finkbeiner, 2011). HD is caused by mutations that extend the cytosine-adenine-guanine (CAG) trinucleotide repeat in the exon 1 of the huntingtin (HTT) gene. 34 CAG repeats or fewer do not result in HD symptoms. Alleles containing 35–39 CAG repeats produce incomplete penetrance, as individuals harboring these alleles may or may not develop the disease. However, > 39 CAG repeats is considered fully penetrant as individuals with these alleles will eventually develop HD symptoms. In this regard, the length of the CAG repeats correlates with the disease progression and longer CAG stretches predict younger HD onset (Finkbeiner, 2011). Expanded CAG mutations result in an unstable polyglutamine (polyQ) stretch in HTT protein, leading to its aberrant aggregation. As such, the accumulation of mutant HTT aggregates is one of the hallmarks of the disease. Although HTT is ubiquitously expressed, gamma-aminobutyric acid (GABA)ergic medium spiny neurons of the striatum undergo the greatest neurodegeneration in HD patients (Finkbeiner, 2011). Extensive data indicate that mutant HTT aggregation is toxic and contributes to neurodegeneration (Koyuncu et al., 2017). However, the molecular mechanisms by which these inclusions induce neuronal dysfunction remain unsolved. For instance, polyQ-expanded aggregates may collapse distinct proteostasis(definition) nodes such as protein clearance mechanisms (i.e., autophagy(definition) or the ubiquitin proteasome system) or the chaperone network (Koyuncu et al., 2017). Moreover, aberrant aggregates could sequester signalling and regulatory components such as transcription factors or physically obstruct neuronal extensions. Besides aggregates, growing evidence indicates that intermediate species called “oligomers” formed during the aggregation or disaggregation process also contribute to neurotoxicity. In this regard, the initial formation of polyQ-expanded HTT inclusions has been proposed to have a protective role. These aggregates could form to sequester highly toxic oligomers of mutant HTT, reducing the amount of soluble oligomeric intermediates (Arrasate et al., 2004). However, mutant HTT inclusions may eventually sequester other proteins, contributing to neurodegeneration. iPSCs derived from HD patients express significant amounts of mutant HTT protein (Koyuncu et al., 2018). Whether HD-iPSCs exhibit toxic soluble oligomers of mutant HTT is unknown. However, HD-iPSCs do not exhibit increased cellular death, higher sensitivity to cellular stressors or defects in GABAergic neuronal differentiation (Koyuncu et al., 2018), suggesting that these cells have increased mechanisms to either avoid the generation of toxic oligomers or eliminate them. Nevertheless, extensive evidence indicates that HD-iPSCs suppress the accumulation of mutant HTT aggregates (Koyuncu et al., 2018), which are important determinants of cellular viability and function. These findings suggest a rejuvenation process during cell reprogramming that rewires the ability to maintain proteostasis of mutant HTT, resulting in iPSCs with increased mechanisms to prevent aberrant aggregation. For this reason, HD-iPSCs have been used to define anti-aggregation mechanisms, which can be then mimicked in differentiated neurons to suppress polyQ-expanded HTT aggregation (Koyuncu et al., 2018). For instance, this research led to identify novel activators of mutant HTT degradation as well as inhibitors of aggregation (Koyuncu et al., 2017). On the other hand, the rejuvenation step also represents an important limitation for HD disease modeling. Although HD-iPSCs can terminally differentiate into striatal neurons, these cells do not exhibit mutant HTT aggregates (Koyuncu et al., 2018). The lack of polyQ-expanded HTT aggregates in these cells could reflect the long period of time before aggregates accumulate in the neurons of HD patients. In support of this hypothesis, HD-neurons derived from iPSCs do not present polyQ-expanded aggregates at 12 weeks after transplantation into HD rat models, whereas they accumulate aggregates after 33 weeks. Notably, recent advances have provided a novel tool to circumvent this limitation and bypass the induction of pluripotency. In particular, this approach allows for direct conversion of fibroblasts from HD patients into neurons that recapitulate age-associated aggregation of mutant HTT (Victor et al., 2018). Besides aggregation of mutant HTT, loss of normal HTT function could also contribute to HD (Saudou and Humbert, 2016). In these lines, downregulation of wild-type HTT levels induces HD-related changes such as progressive neurodegeneration and motor dysfunction or aggravate these changes in HD models. Moreover, overexpression of wild-type HTT improves brain cell survival and ameliorates the deleterious effects of the mutant protein (Saudou and Humbert, 2016). Although differentiated cells from