Gene Therapy Validated In Human SCA3 Stem Cells

Written by Dr. Marija Cvetanovic Edited by Dr. Sriram Jayabal

Research group in Michigan report the creation of the first NIH-approved human cell model that mirrors SCA3 disease features – cellular defects that, after gene therapy, show improvement

Spinocerebellar ataxia type 3 (SCA3) is a dominantly-inherited, late onset genetic disease that affects multiple brain regions. Affected individuals suffer from several symptoms, with impaired movement coordination being the most debilitating. SCA3 is caused by a mutation in the Ataxin-3 (ATXN3) gene. In unaffected individuals, ATXN3 typically has anywhere from 12 to 44 repeats of the genetic code “CAG;” however, in some people’s genetic code, the number of CAG repeats can become abnormally high. If this “repeat expansion” mutation causes the ATXN3 gene to have more than 56 CAG repeats, the person develops SCA3. Cells use repeating CAG sequences in their genome to make proteins with long tracts of the amino acid glutamine. In SCA3 cells, these “polyglutamine” (polyQ) tracts are abnormally long in the ATXN3 protein, which makes the protein more prone to form clumps (or “aggregates”) in the cell. The presence of these protein clumps in the cells of the brain is one of the hallmarks of SCA3.

Despite knowing the genetic cause of SCA3, it is still not known how this mutation affects cells on the molecular level. Having said that, several cellular and animal models have been developed in the past two decades to help study these underlying mechanisms. SCA3 models have not only helped to  our increased understanding of the disease’s progression at all levels – molecular, cellular, tissue, and behavioral – but also helped move us closer to therapeutic interventions. For instance, recent studies using SCA3 mouse models have established that targeting ATXN3 with a form of gene therapy known as antisense oligonucleotide (ASO) treatment could very well be an effective strategy for improving the lives of patients. ATXN3-targeting ASOs cause the cells of the brain to produce less of the mutant ATXN3 protein and, when given to SCA3 mice, improved their motor function. These results strongly support the potential use of ASOs to treat SCA3. Still, it is important to see if this finding can be repeated in human neurons (a step that is needed to bring us closer to ASO clinical trials).

Female scientist in a while lab coat busy at work, we are looking at her from behind through some glass bottles
Image of a research scientist hard at work in the lab. Image courtesy of pxfuel.

Previous experience from unsuccessful clinical trials highlight the importance of determining the similarities and differences between humans and mice when it comes to disease. For instance, the SCA3 mutation does not naturally occur in mice; therefore, modeling SCA3 with mice usually requires additional genetic manipulation, which could lead to unexpected effects that we do not typically see in patients. In addition, we may miss important determinants of SCA3 pathology due to the inherent differences between humans and mice. For example, proteins that help contribute to SCA3 in human patients may simply not be present in mouse neurons (and vice versa). Because of such species differences, the therapeutic interventions that are effective in mice are not always as effective in humans.

SCA3 human neurons can help bridge the gap between rodent models and human patients, acting as a clinically relevant tool for looking into disease mechanisms and testing new therapies. Because we cannot remove a portion of an SCA3 patient’s brain to study the disease, these neurons must be created in a lab. Human neurons can be generated from induced pluripotent stem cells (iPSCs) or from human embryonic stem cells (hESCs). Induced pluripotent stem cells (iPSCs) are made from adult cells (usually blood or skin cells) that are reprogrammed to return to an embryo-like form (known as the “pluripotent” state). Just like during normal development, iPSCs can create many different types of cells, including neurons. One problem with this approach is that the process of reprograming can potentially change these cells in way that could affect how the disease presents. To avoid this issue, researchers can also create human neurons from human embryonic stem cells (hESCs), which are derived from embryos and are, therefore, naturally pluripotent. Because hESCs do not require reprograming, they are more likely to accurately model disease. However, they are more difficult to obtain and work with. The researchers in this study, led by Lauren Moore in Dr. Hank Paulson’s lab at the University of Michigan, used hESCs to generate the first ever National Institutes of Health (NIH)-approved SCA3 model using human cells.

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Snapshot: What are stem cells?

Embryonic and adult stem cells

Stem cells are cells that provide new cells during growth, and replace cells that are damaged or lost during life. They have the following two important properties that enable them to do this:

  1. The ability to develop (differentiate) into many other, different cell types, for example brain cells, heart cells or liver cells.
  2. The capacity to replicate (self-renewal) and generate more of these important cells.

Mammals have two types of stem cells; embryonic stem cells (ESCs) and adult stem cells. ESCs are derived from an early-stage embryo, while adult stem cells are found throughout the body after development. The main difference between these cells is their ability in the number and type of different cell types they can become. Embryonic stem cells are pluripotent, meaning that they can become all cell types. Adult stem cells are multipotent and can only develop into a limited set of cell types.

magnified image of stem cells with a blue tint
Human embryonic stem cells, image courtesy of WikiMedia.

Induced pluripotent stem cells

In 2006, the lab of Shinya Yamanaka showed that mouse skin cells could be converted into stem cells by using four specific factors, the Yamanaka factors (1). These cells were named “induced pluripotent stem cells (iPSC). For this important finding Yamanaka was awarded the 2012 Nobel Prize together with Sir John Gurdon. One year later a similar strategy was used to successfully reprogram human skin cells into human iPSCs (2). Nowadays, these iPSCs are an important tool in biomedical research and used for disease modelling, to study disease mechanisms, for drug development and cell replacement studies. To model a disease, a skin biopsy or a urine sample from a patient can be used to generate patient-specific iPSCs. Another option is to modify iPSCs using CRISPR/Cas9. Using defined protocols, these iPSCs can be converted into, for example, brain cells or even mini brains (organoids), which can be used to study a disease. Furthermore, when differences (phenotypes) are found in these cultures, this can be used to screen for drugs that reverse this particular aspect of the disease.

Stem cell-based therapies for SCA

A last aspect is that these iPSCs can be used to replace damaged or lost cells. Since the first stem cell-based clinical trials to replace brain cells that are lost in Parkinson Disease, stem cell replacement therapy has evolved and numerous clinical trials were initiated. In the spinocerebellar ataxia (SCA) field, the first preclinical experiment of embryonic transplants into a mouse model of SCA1 showed a positive effect on animal behaviour and brain pathology (4). Although these first preclinical experiments in SCA disease models are positive, an iPSC-based therapy for SCAs is far from a clinical application.

If you would like to learn more about stem cells, take a look at these resources by the National Ataxia Foundation and National Institutes of Health.

Snapshot written by Dr. Ronald A.M. Buijsen and edited by Frida Niss.

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Early Cerebellum Development Abnormality in Adult-Onset Spinocerebellar Ataxia Type 1

Written by Dr. Vitaliy V Bondar  Edited by Dr. Chandrakanth Edamakanti

Researchers for the first time identified that spinocerebellar ataxia type 1 (SCA1) may have roots in early cerebellar circuit malfunction.

Cartoon of a neuron
Artist representation of a neuron. Image courtesy of Pixabay

Since the discovery of the cause of SCA1, researchers have wondered: why does it take three to four decades of life for symptoms to reveal themselves? This late stage disease progression is surprising, given that early molecular changes are observed in many SCA1 animal models. Furthermore, this is true for many other neurodegenerative diseases (i.e., that molecular changes precede symptoms). Studying and understanding this delay in symptom onset may reveal potential treatment options to mitigate and slow down the progression of the disease.

The cerebellum is one of the most important brain regions for SCA1 research because it is responsible for the fine movement control that SCA1 patients have difficulty with. Moreover, the cerebellum is the brain region that degenerates the earliest in SCA1. Given that SCA1 symptoms strike late in adulthood, many scientists thought that there would not be any cellular changes during the cerebellum’s development (that is, early in SCA1 patients’ lives). However, Chandrakanth Edamakanti, a postdoctoral scientist in Puneet Opal’s laboratory at Northwestern University, has recently demonstrated that the stem cells in the cerebellum behave differently in SCA1. These stem cells, which exist in the cerebellum for the first three weeks after birth, help to complete cerebellar development by adding new neurons and supporting cells (known as glia). Dr. Edamakanti and colleagues have shown that, in SCA1, this process is disturbed, which likely contributes to Purkinje cell toxicity at later ages. This represents the first cellular and anatomical difference that has been seen in neurons prior to degeneration in SCA1. Other neurodegenerative diseases, including Alzheimer’s, Huntington’s and Parkinson’s, may also stem from such developmental defects that set the stage for later disease vulnerability.

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