Cutting Out a Cure for Huntington’s Disease
Alison Bond, Junjie Liu, and Anna Tidswell
Illustrations by: Molly Berinato
How far would you go to save your own life? Would you undergo highly experimental treatments? Would you be willing to alter your own DNA -- the very thing that makes you you? This might seem like a far-fetched hypothetical dilemma, but for hundreds of thousands of people affected by Huntington’s disease (HD) across the globe, this fanciful hypothetical is becoming a present-day reality. To date, the only treatments available for HD relieve symptoms; but, they are unable to target the root of the disease. However, new research about CRISPR/Cas9 technology may provide a cure for this devastating disease. This controversial Nobel prize-winning mechanism is progressing towards clinical trials for the treatment of genetic diseases.
Huntington’s disease is a neurodegenerative disease characterized by motor, cognitive, behavioral, and psychiatric symptoms. The average age of onset is 40 years, although symptoms can appear any time from early childhood to one’s elder years [1]. Among these symptoms, one of the most recognizable is a movement disorder, chorea, which presents as erratic fidgeting and twitching [2]. Although deficits in motor functionality are typically the most prominent symptoms of HD, cognitive and neuropsychiatric symptoms also develop as a result of the disease. In most cases, cognitive impairments that affect executive functioning (i.e. processing speed, problem solving, planning, organization), attention deficits, and memory retrieval precede the onset of motor symptoms [2]. Furthermore, HD has a high rate of comorbidity with depression, anxiety, and obsessive compulsive disorder (OCD), which exacerbate the cognitive decline associated with HD [2].
Within the human genome, Huntington’s arises from a dominantly inherited mutation in the Huntingtin gene (also called the HTT gene), located on chromosome 4 [3]. The HTT gene is expected to code for a protein involved in many aspects of cell development and function [4]. However, the proper functionality of this protein can be impaired by a mutation in the coding region of the HTT gene. The coding region of a gene is the sequence of DNA that codes for the production of a protein. In order to do this, cells transcribe DNA into mRNA before using this mRNA transcript as a guide to assemble a protein. This process can be compared to the process of baking a cake: transcribing the DNA into mRNA is equivalent to writing down the recipe while putting the ingredients together, and baking the cake is analogous to translating mRNA to a protein. Within the coding region of the HTT gene, there is a consecutive stretch of repeats of the CAG codon (or three nucleotides that code for a specific amino acid—in this case, glutamine) which is found in the huntingtin protein [3]. In those with HD, an insertional mutation results in the addition of too many CAG repeats, leading to the production of a mutated protein; therefore, HD is classified as a CAG trinucleotide repeat disease [1]. Not only does having too many CAG repeats predict development of HD, but the number of additional CAG codons is correlated with the age of onset of the disease [1]. As a result, the primary objective of genetic testing for HD predictors is to assess the amount of CAG repeats a patient has in their HTT gene. Most individuals without HD have 26 repeats or fewer [1]. Those with a count of 27 to 35 repeats will not develop the disease themselves, but can possibly pass the disease to offspring, depending on their partner’s genetic predisposition. Having 36 to 39 repeats yields ambiguous results, meaning that HD may or may not develop; it is past 40 repeats when the disease generally develops. In general, higher numbers of repeats indicate earlier onset of Huntington’s [1].
Now that we have discussed the symptomatic presentation and genetic basis of HD, it is essential to understand the neuroscience behind Huntington’s. HD is neurodegenerative, meaning that it causes neuronal cell death, which leads to “shrinkage” of the brain (atrophy) [3]. While HD can progress to affect several brain regions, the basal ganglia system is the most directly impacted. The basal ganglia are made up of nuclei—interconnected clusters of neurons that work together and are highly involved in motor control [5]. In regulating movement, the basal ganglia system must communicate with the rest of the brain by receiving and sending information; this means that the system acts as a loop. It takes the information in, processes it, and then sends appropriate responses to other brain regions. When considering motor control, we can think about our ability to both initiate and inhibit movements. Within the basal ganglia, these oppositional functions of excitation and inhibition are processed by two different pathways, termed the direct and indirect pathways, respectively [5]. In addition to bearing opposing effects on movement, the neurons that primarily make up each pathway differ slightly in terms of their receptors [6]. There is a loss of neurons in both pathways as a result of HD; however, cells of the indirect pathway are most affected. This means the ability to inhibit movement is greatly impaired, leading to many of the motor symptoms observed in those with HD.
Needless to say, individuals with Huntington’s disease experience a range of symptoms that affect their day-to-day lives. Think about your typical morning routine. In the morning, you might wake up and stretch by extending your arms in front of you. Hopefully, your hands and arms are generally still, with minimal involuntary movement. If you are actively exhibiting symptoms of HD, your arms might jerk and flail around with very little control or effort. This movement disorder is the aforementioned chorea, which results from a loss of neurons that contribute to the basal ganglia system [2]. Because the transmittance of too much dopamine (a neurotransmitter) can produce uncontrollable movement, medications that target the production of dopamine can be used to treat chorea [7]. Prior to 2017, there was only one FDA-approved medication available to treat chorea in individuals with Huntington’s disease: Tetrabenazine. Unfortunately, Tetrabenazine is quickly broken down by the body and requires multiple doses throughout the day [7]. Researchers with the Huntington Study Group (HSG)— a research group dedicated to finding treatments for HD— strived to find a drug that offered the same benefits as Tetrabenazine, but would not degrade as quickly . In 2016, a groundbreaking study conducted by multiple HSG research centers determined Deutetrabenazine to be a new and effective drug for treating chorea in HD [7]. Deutetrabenazine has a longer half-life, meaning one dose of the drug will take longer to metabolize upon consumption, thus eliminating the frequent doses required of Tetrabenazine. In the end, regaining control of one’s movement with medications of this nature can significantly enhance the quality of life for an individual with HD.
Now, imagine you are getting ready to have breakfast before the start of your day. You take a massive spoonful of your cereal without realizing that it is simply too large to swallow. As a result, you choke and cough. You cannot get the food down, and you inhale it into your lungs [8]. This scenario, though simple, can help illustrate an incredibly deadly symptom of HD: aspiration. Individuals with HD inhale food and saliva into their lungs, potentially causing pneumonia or other dangerous side effects [8]. HD can also affect the ability to estimate the size of one’s bite. According to a sixteen year study that tracked individuals with HD-induced dysphagia (difficulty swallowing), the majority of HD patients had difficulties regulating the ingestion of excessively large quantities of liquid or solid food [9]. Several approaches can be taken to prevent harm, but none of them completely alleviate the problem. Speech language pathologists recommend coughing after swallowing large bites of food to confirm that the food has not travelled into the individual’s windpipe [10]. Additionally, physical supports like back braces and leg weights can also help to stabilize the body and position of the esophagus [9]. Furthermore, changing the texture of liquids and foods that will be consumed can help slow the eating or drinking process down for those with HD [9]. As HD progresses, a feeding tube may be required to sustain nutrition and hydration.
Some of the most devastating effects of HD are cognitive and psychological. As individuals progress to the point of persistent physical symptoms of HD, they may also experience a cognitive decline in a form of dementia known as “frank dementia.” This type of dementia is classified by memory issues that result in a decreased ability to solve complex problems and process new information [2]. These symptoms, however, are often complicated by a lack of awareness on the part of the individual, making it difficult for family members to help them, let alone for those with HD to recognize their own mental decline. Even the most common class of medications for dementia, known as cholinesterase inhibitors, are not largely effective for individuals with HD [2]. This means that support should be offered in the form of cues and additional time for completing tasks. It is undeniable that to make the most immediate impact on individuals with HD, scientists and health care providers must direct further research efforts towards effective drugs and treatments that can alleviate symptoms until a cure is found. Or might that cure have already been found with CRISPR/Cas9?
CRISPR/Cas9 is a natural defense mechanism that bacteria use to defend against other viruses [13]. After being infected by a virus, bacteria remember viruses by incorporating some of the viral DNA into their own genome, in what is termed “the CRISPR array” [14]. Bacteria then produce CRISPR-derived RNAs (crRNA), which bind and work with an enzyme called Cas9. The crRNA helps guide the Cas9 enzyme to viral DNA through complementary base pairings (molecular interactions between nucleic acids), while the Cas9 enzyme does the bulk of the workload by cutting up the DNA, allowing for an enhanced immune response [13, 14].
Scientists have long been able to accurately design nucleic acids in laboratories, which gives us some control over CRISPR/Cas9. By designing guide-RNAs (gRNA) similar to bacterial crRNAs, researchers can mimic natural processes, allowing the Cas9 cutting site to be chosen; this means that the CRISPR/Cas9 system can be used to cut genes at specific locations [15,16]. Many scientists believe the answer to treating and potentially curing diseases deemed incurable, such as HD, lies within this remarkable system.
So how can CRISPR/Cas9 be engineered to help treat HD? One method is to directly cut up the mutant HTT gene itself. Think of a computer’s many electrical cords, which provide the electricity to power the computer. By cutting one of the cords, you can reduce or destroy the function of the computer. Scientists can use this train of thought to disrupt the mutant codons in the HTT gene by designing an sgRNA that targets the increased number of CAG repeats responsible for HD. In order to insert CRISPR/Cas9 into the brain, scientists must package the genes for the Cas9 enzyme and the gRNA into an Adeno-Associated Virus (AAV) vector: a type of virus manipulated to deliver genetic material for gene therapy [17]. Normally, viruses are seen as harmful pathogens, but AAV vectors have been deemed safe and effective gene delivery vehicles. After the direct injection of AAV vectors into the brain, the AAV delivers genetic material into the cell. The cell transcribes this genetic material into RNAs, with some RNAs translated into proteins like Cas9 [17].
Finally, the customized gRNA and Cas9 enzyme join to start the process. The gRNA helps Cas9 locate the HTT gene, driving Cas9 to cut the gene and create a double-stranded break in the DNA. As the cell frantically tries to repair this lesion, it is bound to make mistakes, leaving the repaired HTT gene slightly different from the original. The next time this HTT gene is transcribed, the cell rightfully recognizes mistakes in the mRNA and degrades the RNA before mutant HTT proteins can be produced [18]. By using CRISPR/Cas9 to cut mutant HTT genes in 4-week-old mice, scientists were able to reduce the prevalence of mutant HTT proteins, improve life span, reduce dystonia, and improve motor function [19].
Compared to pharmaceutical approaches that treat symptoms on a short-term basis, gene-editing provides an unparalleled long-term solution. There is no need for regular dosage of pharmacological agents following CRISPR/Cas9 implementation because gene-editing is believed to be permanent, especially for the non-dividing cells of the brain. However, CRISPR/Cas9 treatments are far from perfect. Genetic editing increased lifespans of the mentioned mice subjects, but they all still died prematurely [19]. CRISPR/Cas9 treatments can accidentally target cellular processes that mediate important functions by cutting up important genes. Since DNA is only made of four different “letters,” there are bound to be repeats or similarities in the genetic code and, subsequently, unintended mistakes by CRISPR/Cas9. To improve CRISPR gene therapy, researchers continue to meticulously engineer Cas enzymes, gRNAs, and viral vectors to improve safety and effectiveness, while minimizing the treatment’s risks. As CRISPR systems rapidly improve, we may soon see clinical trials of using CRISPR/Cas9 to treat Huntington’s disease in humans. Ultimately, we hope to one day be able to edit nucleic acids to treat Huntington’s disease and other neurodegenerative diseases without unintended repercussions.
Research is rapidly advancing in the field of neurodegenerative diseases, but more work is required to find a safe and effective cure for Huntington’s disease. A myriad of drugs have been developed to treat HD symptoms, though these only act as short-term solutions. Nonetheless, scientists are determined to find a permanent treatment. While current research regarding CRISPR/Cas9 shows promise as a long-term treatment, more time is needed to reduce both inefficiencies of CRISPR/Cas9 editing and address ethical considerations before scientists can conduct successful clinical trials. Ideally, the implementation of CRISPR/Cas9 in individuals with HD would result in a complete cure, rather than a reduction in symptoms, which would largely eliminate the need for prescriptions. CRISPR/Cas9 may provide hope and a cure for those living with incurable, neurodegenerative diseases in the form of a microscopic pair of scissors.
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