An isogenic human iPSC model unravels neurodevelopmental abnormalities in SMA

Abstract

Whether neurodevelopmental defects underlie the selective neuronal death that characterizes neurodegenerative diseases is becoming an intriguing question. To address it, we focused on the motor neuron (MN) disease Spinal Muscular Atrophy (SMA), caused by reduced levels of the ubiquitous protein SMN. Taking advantage of the first isogenic human induced pluripotent stem cell-derived SMA model that we have generated and a spinal cord organoid system, here we report that the relative and temporal expression of early neural progenitor and MN markers is altered in SMA. Furthermore, the corrected isogenic controls only partially reverse these abnormalities. These findings raise the relevant clinical implication that SMN-increasing treatments might not fully amend SMA pathological phenotypes. The approach we have taken demonstrates that the discovery of new disease mechanisms is greatly improved by using human isogenic models. Moreover, our study implies that SMA has a developmental component that might trigger the MN degeneration.

Spinal Cord Organoids to Study Motor Neuron Development and Disease

“…These in vitro methods can also be used as powerful platforms for drug testing that may improve the reliability of preclinical trials and decrease dependence on animal experimentation. Moreover, by utilizing differentiation protocols that generate MNs via guiding hiPSCs specification through neural progenitor cell fates, neurodegenerative diseases (NDs) with potentially developmental underlying pathologies can be studied with no ethical hurdles, as has been recently reported for Huntington’s disease [ 124 ] and SMA [ 90 , 108 ]. Additionally, MN co-cultures with other cell types, such as skeletal muscle and microglia, further enable the study of the muscular and inflammatory aspects of MNDs, with a reported critical contribution to disease progression or even initiation [ 125 , 126 , 127 ].…”

“…In the first study that used SCOs to model SMA, alterations in MNs biogenesis was not observed [ 90 ]. However, by generating the first CRISPR/Cas9-mediated isogenic cohort of SMA corrected hiPSC lines, and following a physiologically relevant SCO model that our group has developed, we recently discovered that markers of NSCs, early pMN and spMNs are significantly reduced and expressed in a temporally abnormal fashion [ 108 ]. In particular, a marked decrease in the expression levels of SOX2 and NESTIN brought forth developmental alterations in NSCs/NPCs as potential initial drivers of SMA pathology [ 108 ] ( Table 2 ).…”

“…However, by generating the first CRISPR/Cas9-mediated isogenic cohort of SMA corrected hiPSC lines, and following a physiologically relevant SCO model that our group has developed, we recently discovered that markers of NSCs, early pMN and spMNs are significantly reduced and expressed in a temporally abnormal fashion [ 108 ]. In particular, a marked decrease in the expression levels of SOX2 and NESTIN brought forth developmental alterations in NSCs/NPCs as potential initial drivers of SMA pathology [ 108 ] ( Table 2 ). While organoid-based studies for SMA are still very few, future research will likely benefit from employing these models to dissect the developmental vs. post-mitotic contributors of premature MN degeneration in SMA and to systematically characterize the cell-autonomous and non-autonomous mechanisms of disease, their order of appearance and the crosstalk between them.…”

“…Additionally, the responsiveness of hiPSC lines to small molecules supplemented into the culture medium also varies, which greatly affects the outcome of the patterning protocols [ 49 , 79 , 102 ]. Furthermore, since not all organoid modeling studies are based on the hiPSC lines of isogenic background, unknown mutations in the genome of the somatic cell donor may impact the dynamics of organoid development or the appearance and degree of disease phenotypes [ 108 ] ( Figure 3 A). Subsequently, questions and criticism about the standardization of such organoid approaches for modeling neuromuscular development and disease need to be addressed.…”