Can Gene Therapy Help Treat Brain Diseases?

Post by D. Chloe Chung

What is gene therapy?

The goal of gene therapy is to alleviate disease symptoms and ultimately cure diseases by correcting abnormal gene expression. This can be done by introducing genetic materials that express exogenous genes or suppress the expression level of endogenous genes in an effort to modify gene expression levels. Gene therapy can be also designed to directly edit gene mutations present in patients. Over the past years, development of novel gene-editing tools has resulted in improved efficacy of gene delivery to the brain. With this exciting technical advancement, gene therapies have recently gained more attention as a potential therapeutic strategy for neurodevelopmental and neurodegenerative diseases, especially for those caused by genetic mutations that disrupt the body’s usual patterns of gene expression.

Different strategies behind gene therapy

There are several ways to design gene therapy to correct aberrant gene expression in neurological diseases. One of them is to express exogenous proteins that can restore the function of faulty endogenous proteins. For this purpose, the adeno-associated virus (AAV) is a preferred viral vector because of its relative safety as well as its long-term gene expression which reduces the need for repeated administration. After being introduced into cells, AAVs can express genes that they carry by using the transcriptional and translational machinery in the host cells. In this way, AAVs produce proteins that can make up for the loss of gene function. The virus engineering field continues to refine structures of AAVs to increase their safety as well as effectiveness in delivery to the central nervous system.

DNA-editing is also an appealing approach for gene therapy as it can directly fix disease-causing mutations and modify gene expression level. The newest advancement in DNA-editing tools involves the synthetic CRISPR system (an abbreviation for ‘clustered regularly interspaced short palindromic repeats’) in which a customized guide RNA can bring a DNA-cutting enzyme (e.g., Cas9) to the specific site within the gene of interest. Once the enzyme cuts out a few base pairs from the gene, the gene sequence will eventually shift and create a premature stop codon). The cellular mechanism takes this stop codon as a sign to degrade messenger RNAs transcribed by this gene, ultimately silencing the expression of the target gene. This genetic tool has great potential to treat neurological diseases as it can be used to inactivate or activate genes of interest, or to edit precise bases within the gene and correct pathogenic gene mutations.

In addition to DNA-editing tools, the RNA-based therapy utilizing antisense oligonucleotides (ASOs) has gained much attention for its potential efficacy. ASOs are short DNA or RNA fragments that can bind to messenger RNAs based on the complementary sequence, subsequently changing the RNA expression level. As such, ASOs can be synthesized to target messenger RNAs transcribed from the disease gene of interest in hopes to regulate the protein level that could play crucial roles in neurological diseases.

How is gene therapy treatment for neurological diseases going?

In 2017, a groundbreaking study published in The New England Journal of Medicine reported the successful usage of gene therapy in young children with spinal muscular atrophy (SMA) type 1, a devastating neuromuscular disease characterized by motor neuron degeneration and progressive muscle loss. In SMA, a defective gene SMN1 reduces the amount of functional SMN1 protein, so researchers treated SMA patients with a one-time blood infusion of AAV that can express the SMN1 gene and restore protein expression level. Excitingly, most of the patients who received this gene therapy showed drastic improvement in their survival and motor functions that lasted through the 2-year follow-up assessment. Some of the patients were even able to walk with no assistance, which is a striking development considering that many SMA patients need wheelchair assistance and die at a very young age.

As of August 2021, a medication designed to increase the level of SMN protein is the only disease-modifying gene therapy approved by the US Food and Drug Administration (FDA) to treat neurological symptoms. Yet, in addition to this medication, numerous preclinical and clinical studies are actively investigating the safety and efficacy of gene therapy for a wider range of neurological diseases. For example, ASOs targeting the gene that makes tau, a protein that becomes abnormally aggregated in Alzheimer’s disease, have been tested in mouse models. After demonstrating their ability to reduce tau pathology and subsequently rescue behavioral deficits in a mouse model, the ASO-mediated tau-targeting gene therapy is being tested in a clinical trial for Alzheimer’s disease patients.

Similarly, ASO-based gene therapy has been investigated for its potential benefits in patients of Huntington’s disease, which is caused by an abnormal trinucleotide expansion in the huntingtin gene. The idea behind this therapy is to use ASOs either to globally target the total huntingtin gene level or to specifically reduce the function of the mutant allele. Based on promising results from preclinical studies, several ASOs entered clinical trials to evaluate their safety and efficacy for Huntington’s disease patients. Unfortunately, however, a couple of these clinical trials have been recently discontinued due to lack of evidence of anticipated benefits for patients.

What are the challenges and what is the hope for the future?

Perhaps the biggest challenge in utilizing gene therapy to treat neurological diseases is safety. For example, when genetic materials are being delivered at a high dosage, they can cause toxicity in patients. Also, undesirable immune responses can occur upon introduction of genetic materials. As these scenarios can lead to fatal consequences, safety is always the first aspect to consider and monitor when designing gene therapy and testing it in clinical trials. Moreover, off-target effects – unwanted changes in genes that were not the target of gene therapy – are also a potential concern. This can happen when ASOs or gene-editing tools target alternative genes based on partially complementary sequences. How to avoid off-target effects can be a major challenge when designing the reagents.

Despite these many challenges, biomedical techniques that improve our ability to utilize gene therapy to effectively treat neurological diseases continue to advance. For instance, delivery of AAVs into the central nervous system used to be highly challenging in the past, since it is not feasible to directly inject AAVs into the brain. At the same time, the blood-brain barrier firmly isolates the brain from the periphery, making it difficult for locally delivered reagents (e.g., injection into blood vessels) to bypass this physical barrier and reach the brain. Yet, characterization of various AAVs and persistent efforts in virus engineering allowed for development of a specific AAV that can be successfully and non-invasively (e.g., without surgeries) delivered to the central nervous system. This type of AAV was also used in designing the effective gene therapy for SMA patients as described above, demonstrating that technological advancement can lead to breakthrough therapeutic strategies. With rapidly evolving gene-editing tools, it's an exciting time for the development of gene therapy for neurological diseases.

References

Sun & Roy. Gene-based therapies for neurodegenerative diseases. Nature Neuroscience (2020). Access the original scientific publication here.

Martier & Konstantinova. Gene therapy for neurodegenerative diseases: slowing down the ticking clock. Frontiers in Neuroscience (2020). Access the original scientific publication here.

Bennett et al. Antisense oligonucleotide therapies for neurodegenerative diseases. Annual Review of Neuroscience (2020). Access the original scientific publication here.

Mendell et al. Single-dose gene-replacement therapy for spinal muscular atrophy. The New England Journal of Medicine (2017). Access the original scientific publication here.

DeVos et al. Tau reduction prevents neuronal loss and reverses pathological tau deposition and seeding in mice with tauopathy. Science Translational Medicine (2017). Access the original scientific publication here.

Kordasiewicz et al. Sustained therapeutic reversal of Huntington’s disease by transient repression of huntingtin synthesis. Neuron (2013). Access the original scientific publication here.

Southwell et al. In vivo evaluation of candidate allele-specific mutant huntingtin gene silencing antisense oligonucleotides. Molecular Therapy (2014). Access the original scientific publication here.

Kwon. Failure of genetic therapies for Huntington’s devastates community. Nature News (2021). Access the news article here.

Hudry & Vandenberghe. Therapeutic AAV Gene Transfer to the Nervous System: A Clinical Reality. Neuron (2019). Access the news article here.

Doudna. The promise and challenge of therapeutic genome editing. Nature (2020).Access the news article here.

Common Brain Circuits in Patients with Depression

Post by Leanna Kalinowski

What's the science?

The mapping of psychiatric symptoms onto specific brain circuits is typically based on a correlation between the symptoms and brain activity, leading to difficulties in attempting to causally translate this information into effective treatments for psychotic disorders. Three modalities, brain lesions, transcranial magnetic stimulation (TMS), and deep brain stimulation (DBS), have been used to link depression symptoms to specific brain circuits based on the location of lesions or stimulation sites that affect symptom severity. However, it is unclear whether these three modalities converge on the same brain circuit or therapeutic target. This week in Nature Human Behaviour, Siddiqi and colleagues analyzed datasets of patients with depression symptoms to determine whether brain lesions, TMS, and DBS sites converge on the same brain circuits.

How did they do it?

The authors examined 14 datasets that included magnetic resonance imaging or computed tomography scans of 461 brain lesions, 151 TMS sites, and 101 DBS sites, in addition to scores on a continuous scale from a validated depression questionnaire for each patient. They then mapped each lesion or stimulation site onto a brain circuit using a normative human connectome database based on data from 1,000 healthy subjects. This method was used to create a circuit map of each patient’s lesion or stimulation site. Then, to determine whether the three modalities converge on the same circuit, the circuit maps were compared to each other by computing correlations between the depression score and brain lesion/stimulation site.

The authors also compared circuit maps derived from patients with major depressive disorder with those derived from patients with other disorders to determine whether this circuit is associated with depression severity irrespective of baseline diagnosis. Finally, they extended this approach to additional datasets of patients with brain lesions or DBS sites associated with motor symptoms of Parkinson’s disease to determine whether this approach is relevant beyond depression.

What did they find?

First, the authors found that brain lesion and stimulation sites that modulate depressive symptoms are connected to a similar circuit, providing evidence that these three modalities converge on common brain circuitry. This convergent circuit included brain regions and circuits previously implicated in depression. Next, they identified similar depression circuits in patients with major depressive disorder, penetrating brain injury, stroke, epilepsy, and Parkinson’s disease. This indicates that depression symptoms map to a common circuitry regardless of baseline diagnosis. Finally, they found that this approach could be extended beyond depression by demonstrating that brain lesion and stimulation sites associated with Parkinson’s disease symptoms map onto similar circuits.

What's the impact?

This study demonstrates that brain lesions, TMS, and DBS all converge on common brain circuitry, representing potentially improved therapeutic targets for depression symptoms regardless of diagnosis. The methods used to determine these common circuits generalize to Parkinson’s disease, indicating that this approach may also be used to identify brain circuits involved in other neuropsychiatric diseases. Further work is needed to determine whether this approach provides improved therapeutic targets in patients with neuropsychiatric diseases.

Siddiqi et al. Brain stimulation and brain lesions converge on common causal circuits in neuropsychiatric disease. Nature Human Behaviour (2021). Access the original scientific publication here.

Dopamine Controls How New Information Updates Reactivated Memories

 Post by Leanna Kalinowski

What's the science?

When new memories are acquired, they are initially unstable. The process of memory consolidation is required to stabilize new memories. Consolidated memories are long-lasting, however when reactivated during memory recall, these memories can become destabilized once again. Memory reconsolidation is the process of restabilizing memories that were destabilized during the recall process. While it is known that memory reconsolidation is a protein-synthesis-dependent process and can be impacted by dopamine receptor blockade, its biological role is not fully understood. This week in PNAS, Gonzalez and colleagues tested whether hippocampal dopamine D1/D5 receptors control whether new memories are linked to old ones through reconsolidation or whether they are consolidated as independent traces.

How did they do it?

Rats underwent a novel recognition task, a common test of episodic memory (memory of events) that capitalizes on rodents’ innate preferences for novelty by measuring the amount of time spent interacting with a previously encountered versus novel object. In this experiment, rats first underwent a training session where they were exposed to two different but behaviorally equivalent novel objects (objects A and B). 24 hours later, they underwent a reactivation session where they were exposed to a familiar object from the training session (object A) and a novel object (object C) to destabilize the memory. 24 hours after this, memory retention was tested by exposing the rats to one of three test sessions: 1) exposure to an object from both the training and reactivation sessions (object A) and a novel object (object D), 2) exposure to an object from only the training session (object B) and object D, or 3) exposure to an object from only the reactivation session (object C) and object D. Time spent interacting with each object was measured.

Twenty minutes before the reactivation session, rats received an infusion of either saline or a dopamine D1/D5 receptor antagonist into the CA1 region of the hippocampus to test whether dopamine D1/D5 receptor blockade impacts memory destabilization. Five minutes after the reactivation session, rats were then given one of four substances into the CA1 to pharmacologically dissociate consolidation and reconsolidation: saline, to serve as a control; anisomycin, which inhibits consolidation and reconsolidation by blocking protein synthesis; AIP, which inhibits consolidation by blocking CaMKII activity; or ZIP, which inhibits reconsolidation by blocking PKMζ activity.

What did they find?

First, the researchers found that rats given saline after reactivation were able to discriminate familiar objects from the novel object regardless of their treatment before reactivation, showing that dopamine inhibition has no effect on memory retention. Next, they found that dopamine inhibition is necessary for memory destabilization but does not protect against amnesia induced by anisomycin.

Finally, they found that the memory of a new object during recall of an old object can be formed by either consolidation or reconsolidation mechanisms, depending on the activation state of hippocampal D1/D5 receptors. The first mechanism, which was activated when dopamine receptors were inhibited, leads to new memories being formed through consolidation. This requires activation of CaMKII and would be employed when new experiences share little similarity with old ones. The other mechanism, which was activated when dopamine receptors were activated, leads to new memories being formed through reconsolidation. This requires activation of PKMζ and would be employed when information from novel and past events overlap. The latter mechanism is essential for the construction of schemas, which are networks of related knowledge that help animals to rapidly incorporate new information into related representations to preserve its relevance.

What's the impact?

This study shows that, depending on the activation of hippocampal dopamine D1/D5 receptors, the memory of a novel object that was presented during recall of a familiar object can be formed by either consolidation or reconsolidation; however, only reconsolidation can link memories of the two objects together. Given that learning seldom occurs in a cognitive vacuum, these findings have broad implications for how new memories are linked with old ones during the reconsolidation process. They also highlight the importance of considering these mechanisms during reconsolidation-based psychotherapy, since recall of a single memory during these sessions can make other memories within the schema susceptible to modification.

Gonzalez et al. Dopamine controls whether new declarative information updates reactivated memories through reconsolidation. PNAS (2021). Access the original scientific publication here.