Scientists discover rare genetic condition that attacks kids’ immune systems

Organoids Reveal A New Player In Huntington's Disease

Scientists have used an organoid model to gain new insights into Huntington's disease, a fatal genetic disorder that causes neurodenegeration that impairs movement and behavior. The disease is caused by mutations in the Huntingtin (HTT) gene. There are no good treatments for this disorder. Investigators have now found that a gene called CHCHD2 is also related to Huntington's; mutations in the HTT gene can reduce CHCD2 expression, which may present a new target option for treatments. The findings have been reported in Nature Communications.

Researchers turn to models when they want to learn more about the mechanisms underlying a physiological process or disease. But animals can only tell us so much about human conditions. One relatively recent creation that aims to bridge the gaps in research knowledge that arise from using animals, are organoids. These are simplified, miniaturized models of human organs that are created by genetically altering cells, so they take on the identity of human cell types that are specific to certain organs.

In this work, the scientists created brain organoids that carried mutations that are known to cause Huntington's. While repeats in the HTT gene don't cause disease if there are fewer than 35 repeats, a greater number of repeats has been linked to Huntington's. The mutations cause neuronal death in the brain. As the number of repeats increases, the severity of the disease tends to get worse, explained co-corresponding study author Dr. Jakob Metzger of Heinrich Heine University Düsseldorf (HHU).

The Huntington's cerebral organoid model showed that the expression of CHCHD2 was abnormally low throughout different stages of development. The CHCHD2 reduction slowed the metabolism of neurons. CHCHD2 normally keeps mitochondria healthy, and has been previously implicated as a player in Parkinson's. This is the first time it's been linked to Huntington's. The researchers also determined that when CHCHD2 function was restored, the metabolism of neurons returned to normal.

"That was surprising," noted study co-author Selene Lickfett, a graduate student at HHU. "It suggests in principle that this gene could be a target for future therapies."

This study has shown that mutations in HTT also seem to disrupt the development of the brain long before Huntington's symptoms arise. This emphasizes the importance of early diagnosis, Lickfett added.

The organoids showed that defects arose in neural progenitor cells before toxic aggregates that are a hallmark of Huntington's arise. The pathology may start in patients long before it is detectable in the clinic.

Huntington's is generally thought to progress as mature neurons degenerate. But there might be changes occurring far earlier, and it may be possible to develop treatments for earlier stages, the researchers suggested.

"Our genome editing strategies, in particular the removal of the CAG repeat region in the Huntington gene, showed great promise in reversing some observed developmental defects. This suggests a potential gene therapy approach," said co-corresponding study author Professor Alessandro Prigione of HHU.

Increasing CHCHD2 expression could be another option too, said Prigione, who also added that these findings could have implications for neurodegenerative disease in general. Mitochondrial defects may present new avenues for treating those diseases.

Sources: Max Delbrück Center for Molecular Medicine, Nature Communications

Mapping The Molecular Mayhem Caused By Huntington's Disease

Expansion microscopy reveals the structure of huntingtin aggregates inside the cell nucleus. Credit: © 2024 Korsten et al. Originally published in the Journal of Cell Biology. Https://doi.Org/10.1083/jcb.202307142

Researchers have uncovered a new mechanism by which Huntington's disease causes neuronal death.

They found that toxic protein aggregates from a mutated huntingtin protein can damage the nuclear envelope, leading to DNA damage and gene misregulation in neurons. This breakthrough could link similar mechanisms to other neurodegenerative diseases, suggesting a common pathway for neuronal damage and offering potential new targets for therapeutic intervention.

Toxic Protein Aggregates in Huntington's Disease

Scientists at Utrecht University in the Netherlands have identified a new way in which the toxic protein aggregates associated with Huntington's disease may damage nerve cells and cause them to die. The study, to be published today (August 16) in the Journal of Cell Biology (JCB), suggests that the aggregates can poke holes in the membrane that separates the nucleus from the rest of the cell, damaging the DNA inside the nucleus and changing the activity of neuronal genes.

Huntington's disease is a devastating neurogenerative disorder caused by a mutation in the HTT gene that results in cells producing abnormally large versions of the huntingtin protein. These expanded huntingtin proteins aggregate inside cells and damage them in various ways, although exactly how this results in the death of nerve cells remains uncertain.

Expansion microscopy shows thin fibrils emanating from a huntingtin aggregate (green) penetrating the protein meshwork (magenta) that underlies the nuclear envelope. Credit: © 2024 Korsten et al. Originally published in the Journal of Cell Biology. Https://doi.Org/10.1083/jcb.202307142 New Insights Into Neuronal Damage

Spearheaded by graduate student Giel Korsten from the group of Lukas Kapitein, the researchers discovered a major new way in which huntingtin aggregates damage cells when they examined neurons expressing the expanded version of the protein. The researchers found that many of the nerve cells had breaks in the membrane that separates the nucleus from the rest of the cell. This barrier, known as the nuclear envelope, protects and regulates the chromosomes inside the nucleus, allowing them to turn genes on and off as needed.

Kapitein and colleagues found that huntingtin aggregates inside the nucleus disrupt the protein meshwork that underlies and strengthens the nuclear envelope, making the membrane more likely to rupture. Using a specialized technique known as expansion microscopy to visualize the nuclear aggregates in high detail, the researchers saw that tiny fibrils stick out from the aggregates and poke through the meshwork underlying the nuclear envelope. The aggregates may also impair the cell's ability to reseal the envelope once it breaks, the researchers found.

Implications of Nuclear Envelope Ruptures

"We have discovered that the aggregates associated with Huntington's disease induce ruptures in the nuclear envelope that compromise its barrier function," Kapitein says.

Over time, these disruptions in the nuclear envelope likely lead to damage of the nerve cell's DNA and the misregulation of neuronal genes, cellular defects that have previously been linked to Huntington's disease pathology.

Broader Impact on Neurodegenerative Diseases

Kapitein notes that several other neurogenerative diseases, including certain types of amyotrophic lateral sclerosis and frontotemporal dementia, are associated with the formation of protein aggregates inside the cell nucleus. "We speculate that nuclear aggregate–induced ruptures in the nuclear envelope represent a common contributor to neurodegeneration that initiates a cascade of deregulated processes culminating in neuronal death and neuroinflammation," Kapitein says.

Reference: "Nuclear poly-glutamine aggregates rupture the nuclear envelope and hinder its repair" by Giel Korsten, Miriam Osinga, Robin A. Pelle, Albert K. Serweta , Baukje Hoogenberg, Harm H. Kampinga, Lukas C. Kapitein, 16 August 2024, Journal of Cell Biology.DOI: 10.1083/jcb.202307142

A New Culprit In Huntington's Disease

For the first time, researchers have implicated the gene CHCHD2 in Huntington's Disease (HD) -- an incurable genetic neurodegenerative disorder -- and identified the gene as a potentially new therapeutic target. In a brain organoid model of the disease, the researchers found that mutations in the Huntington gene HTT also affect CHCHD2, which is involved in maintaining the normal function of mitochondria. The study was published in "Nature Communications."

Six different labs at the Max Delbrück Center participated in the study, led by Dr. Jakob Metzger of the "Quantitative Stem Cell Biology" lab at the and the "Stem Cell Metabolism" lab of Professor Alessandro Prigione at Heinrich Heine University Düsseldorf (HHU). Each lab contributed their unique expertise on Huntington's disease, brain organoids, stem cell research and genome editing. "We were surprised to find that Huntington's disease can impair early brain development through defects associated with mitochondrial dysfunction," says Dr. Pawel Lisowski, co-lead author in the Metzger lab at the Max Delbrück Center.

Moreover, "the organoid model suggests that HTT mutations damage brain development even before clinical symptoms appear, highlighting the importance of detecting the late-onset neurodegenerative disease early," Selene Lickfett, co-lead author and a doctoral student in the Faculty of Mathematics and Natural Science in the lab of Prigione at HHU adds.

The unusual repetition of three letters

An organoid is a three-dimensional, organ-like structure that researchers grow in a laboratory from stem cells. Depending on the disease and research question, organoids can be grown from different types of tissue. Only a few millimeters in size, they serve as a model for how different cell types interact. No other bench-top model provides such a detailed look at the function of cells in the human body.

Huntington's disease is caused when the nucleotides Cytosine, Adenine and Guanine are repeated an excessive number of times in the in the Huntington gene HTT. People with 35 or less repeats are generally not at risk of developing the disease, while carrying 36 or more repeats has been associated with disease. The greater the number of repeats, the earlier the disease symptoms are likely to appear, explains Metzger, a senior author of the study. The mutations cause nerve cells in the brain to progressively die. Those affected, steadily lose muscle control and develop psychiatric symptoms such as impulsiveness, delusions and hallucinations. Huntington's disease affects approximately five to 10 in every 100,000 people worldwide. Existing therapies only treat the symptoms of the disease, they don't slow its progression or cure it.

The challenge of HTT gene editing

To study how mutations in the HTT gene affect early brain development, Lisowski, first used variants of the Cas9 gene editing technology and manipulation of DNA repair pathways to modify healthy induced pluripotent stem cells such that they carry a large number of CAG repeats. This was technically challenging because gene editing tools are not efficient in gene regions that contain sequence repeats, such as the CAG repeats in HTT, says Lisowski.

The genetically modified stem cells were then grown into brain organoids -- three-dimensional structures that resemble early-stage human brains. When the researchers analyzed gene expression profiles of the organoids at different stages of development, they noticed that the CHCHD2 gene was consistently under expressed, which reduced metabolism of neuronal cells. CHCHD2 is involved in ensuring the health of mitochondria -- the energy producing structures in cells. CHCHD2 has been implicated in Parkinson's disease, but never before in Huntington's.

They also found that when they restored the function of the CHCHD2 gene, they could reverse the effect on neuronal cells. "That was surprising," says Selene Lickfett. "It suggests in principle that this gene could be a target for future therapies."

Moreover, defects in neural progenitor cells and brain organoids occurred before potentially toxic aggregates of mutated Huntingtin protein had developed, adds Metzger, indicating that disease pathology in the brain may begin long before it is clinically evident.

"The prevalent view is that the disease progresses as a degeneration of mature neurons," says Prigione. "But if changes in the brain already develop early in life, then therapeutic strategies may have to focus on much earlier time-points."

Wide reaching implications

"Our genome editing strategies, in particular the removal of the CAG repeat region in the Huntington gene, showed great promise in reversing some of observed developmental defects. This suggests a potential gene therapy approach," says Prigione. Another potential approach could be therapies to increase CHCHD2 gene expression, he adds.

The findings may also have broader applications for other neurodegenerative diseases, Prigione adds. "Early treatments that reverse the mitochondrial phenotypes shown here could be a promising avenue for counteracting age-related diseases like Huntington's disease."

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