Spinal muscular atrophy (SMA) is a severe neurological disease caused by reduced survival of motor neuron (SMN) protein levels. There is presently no cure for the condition, although current therapies can alleviate symptoms. In the search for better SMA treatment strategies scientists at DZNE and the Dresden University of Technology investigated previously unnoticed abnormalities in embryonic development.
Using what they claim is the first patient-derived induced pluripotent stem cell (iPSC) model and a spinal cord organoid (SCO) system, the team showed that SMA SCOs exhibited abnormal development, including defective neural stem cell (NSC) and progenitor cell specification. Their findings pointed to the possibility that early neurodevelopmental defects may underlie later motor neuron degeneration.
In their published paper in Cell Reports Medicine (“Isogenic patient-derived organoids reveal early neurodevelopmental defects in spinal muscular atrophy initiation”), research lead Natalia Rodriguez-Muela, PhD, and colleagues suggested their results may indicate that “… postnatal SMN-increasing interventions might not completely amend SMA pathology in all patients.” The investigators say the new platform also provides a precise system to functionally investigate alternative molecular pathways involved in SMA genetics and pathology. Rodriguez-Muela is a research group leader at DZNE’s Dresden site and the Center for Regenerative Therapies Dresden (CRTD) of Dresden University of Technology.
Accumulating evidence suggests that neurodegenerative diseases (NDs) may have a developmental component, the authors wrote. Yet, while diseases such as familial amyotrophic lateral sclerosis, Alzheimer’s disease, Parkinson’s disease, or Huntington’s disease have been linked to causal gene mutations or deletions, disease pathology often appears only decades after birth, and this has led to a lack of developmental studies on such disorders. However, the team continued, “Recent evidence hints at neurodevelopmental alterations that could revolutionize how these NDs are studied, diagnosed, and treated.”
In SMA, neurons in the spinal cord degenerate, leading to paralysis and muscle wasting. The disease usually manifests in childhood and can lead to premature death. SMA is caused by mutations or deletions in the SMN1 gene, which encodes the SMN protein. These mutations result in a deficiency of SMN, which is critical for neurons involved in motor control.
More recently, medical treatments have been available to address the protein deficiency by means of gene therapy. Intervention can begin within a few days after birth. However, while this approach can alleviate disease symptoms, experience to date indicates that it provides no cure.
Studies by the DZNE-led team have now suggested broadening the perspective in the search for better therapies. “The current perception of SMA focuses on the disease after birth, when the basic framework of the nervous system is mostly formed,” said Rodríguez-Muela. “This view ignores that phenomena relevant to the disease could occur much before, when the nervous system is still developing.” As the investigators commented in their paper, “We hypothesized that MN degeneration in SMA is imprinted during early development.”
Rodríguez-Muela continued, “In fact, our studies suggest that SMA is associated with anomalies in the embryonic development not known until now. We therefore believe that there is a hitherto unrecognized prelude to this disease, and that interventions are needed that go beyond existing therapies.”
For their reported research, Rodríguez-Muela and colleagues created organoids that recapitulate key features of both spinal cord and muscle tissue. These complex, tiny samples of artificially generated tissue, each of them about the size of a grain of rice, were grown from human-induced pluripotent stem cells derived from skin cells of individuals affected by SMA.
“It is the first time that organoids of this complexity have been generated for studying SMA”, Rodríguez-Muela said. “Although these are model systems that have certain limitations, they come quite close to the real situation, because they comprise a diversity of cell types and tissue structures that occur in the human body.”
The scientists were able to study different developmental stages as the organoids matured over time. “The earliest phase we can emulate with our organoid model corresponds to that of a human embryo a few weeks old. However, we only replicate the spinal cord and muscle tissue. Starting from the early developmental phase, we can go up to the situation after birth, in particular as it is observed in patients with SMA”, Rodríguez-Muela explained.
The scientists found significant differences between organoids exhibiting SMA pathology, and healthy specimens. Specifically, stem cells in SMA organoids tended to develop prematurely into spinal cord neurons. In addition, there was a distortion in the cell population, in that there were fewer neurons than normal, which also were highly vulnerable, and more muscle cells derived from the stem cells. “We investigated SMN’s functional role in early development, finding that SMA SCOs exhibited faulty and delayed growth compared to their isogenic or healthy counterparts, with defective neural stem cell (NSC) and progenitor cell specification,” they wrote. “Longitudinal single-cell RNA sequencing (scRNA-seq) and targeted transcriptomic analysis indicated differential cell distributions in SMA SCOs, with a bias of neuromesodermal progenitors (NMPs) toward muscle cell identity. In support of their findings in organoids Rodríguez-Muela and coworkers also observed similar effects in mouse embryos with SMA-like pathology.
These tissue cultures also yielded another important result. “When we corrected the genetic defect associated with SMA, we still observed developmental abnormalities, although to a lesser extent”, said Rodríguez-Muela. “This suggests that restoring the gene, as current therapies kind of do, is most likely not enough to completely amend SMA pathology. This is in line with clinical experience to date. Thus, I believe, we need to address the developmental abnormalities, if we want to improve treatment for SMA.”
“Overall, our new platform indicates an early developmental role for SMN, suggesting that postnatal SMN restoration might not fully correct pathological phenotypes in all patients with SMA,” the team stated. “These insights constitute a foundation for future studies to uncover molecular events governing early developmental defects preceding SMA progression and selective MN loss.”
Rodríguez-Muela suspects that the cause for the observed developmental defects could lie in impaired gene regulation. “It may not only be a question of whether the gene producing the SMN protein is defective or not. Perhaps it is also relevant, if the deficiency of this protein impacts other genes critical for the embryo’s early development. There could be a regulatory effect. The fact is that we still don’t know, but it is a plausible possibility,” she pointed out. “I believe that this idea should be explored further. In the long term, this may lead to improved therapies that combine existing approaches with drugs targeting gene regulation. That is, they would have to act on what is called ‘epigenetics.’ In order to minimize the developmental abnormalities, such treatment would most likely need to be applied in early pregnancy. If prenatal testing indicates SMA, this could be a therapeutic option.”
