Huntington’s Disease: Progress towards a Cure
1. Introduction
Expansions of the CAG repeat in the HTT gene can lead to a range of Huntington’s disease-like phenotypes. HTT alleles with 40 or more CAG units cause Huntington’s disease full penetrance. Because the CAG sequence is unstable in gametes, the repeat size can increase from one generation to the next. Repeats of ~27-35 confer an intermediate risk for an HD-like condition known as ‘adult-onset distal spinal muscular atrophy’. Although people with repeats in this range might or might not develop the disease, their symptoms are usually much milder than what is seen with Huntington’s disease. These patients experience muscle atrophy, cramping, and twitching, especially in the lower limbs, late in life, usually after the age of 50. More substantial expansions of the CAG repeat may lead to an adult-onset atypical glutamine disease, which is characterized by choreiform movement and psychiatric disturbances. An extremely rare juvenile-onset form of Huntington’s disease can appear at any time in a span ranging from infancy to late adolescence and is probably driven by a pathology unique to patients with these early-onset phenotypes.
Huntington’s disease, also known as Huntington’s chorea, is a devastating disease that presents with a combination of motor, cognitive, and psychiatric symptoms. Symptoms usually appear when a person is between 30 and 50 years old and progress gradually as more nerve cells in the brain succumb to the effects of the Huntington gene. The protein made by the gene, called huntingtin or Htt, is essential to human survival, but the mutant, expanded Htt protein kills neurons, mostly in the striatum and cortex. Neurons with the mutant gene also exhibit defects in transcription, axonal transport, DNA repair, mitochondrial function, and other cellular processes. Despite extensive examination, no single mechanism can account for the extreme effect the mutant gene has on neuronal cells in the striatum and certain parts of the cortex.
2. Understanding Huntington’s Disease
Molecularly, the mutation consists of an expansion of the CAG triplet (glutamine, Q) in the first exon of the huntingtin gene, which changes the huntingtin protein (mHTT). If the affected individual carries repeats that are greater than 35 CAGs, he usually will develop the disease symptoms. The triplet effect is associated with the protein expansion and, essentially, altitude in the repeat sequences correlates with earlier onset and increased probability for signs of the disease to occur. Protein function is not well understood and there are controversies about HD sign appearance mechanism. The predicted huntingtin is ubiquitous, but cells from different tissues may present different mHTT amounts, suggesting different susceptibility degrees to the signs of the disease. At first stage, the huntingtin aggregates are found in the nucleus and subsequently, they migrate to the cytoplasm.
Huntington’s disease (HD) is a hereditary neurodegenerative disorder that impacts humans. Risk to develop the disease is genetically inherited and the cause of the disease is mutations in the huntingtin gene (HTT). The disease, whose typical onset is at around 40 years of age, has as principal signs motor, cognitive and neuropsychiatric symptoms. Motor disturbance is the first selective sign of the disease affecting the balance, chorea and some incoordination of movements. After onset, patients typically die of the disease in about 10 to 20 years. The prevalence rate is around 5-7 cases per a population of 100,000 individuals, the occurrence is similar between males and females, life expectancy after onset is about 20 years, and there are no significant differences in the global distribution of HD.
3. Current Treatments and Management
Dimerization of HTT proteins is one of the steps in the preparation of inclusions that condense from the nontoxic soluble monomer. Once formed, inclusions sequester and accumulate essential cellular proteins into them, creating a toxic environment. Inclusions themselves may be toxic. When the mutant HTT is actively being transcribed into an mRNA, inclusions may likewise sequester mRNA, blocking its translation and capacity to produce functional HTT. Such events are likely to occur in the brain cells. Therefore, treatments aimed at delaying or preventing the formation of inclusions, or at dispersing the proteins aggregated in them, or at both may be of benefit to patients.
Presently, there are a number of treatments aimed at relieving certain aspects of HD. Several medications are available to treat symptoms and complications, including physical hyperactivity, psychiatric troubles, and abnormal movements. Nutritional aspects are also discussed, either to ensure body weight when it’s low or to help prevent obesity, a frequent complication of HD, when there are swallowing or feeding troubles. Nonetheless, these treatments lie far from addressing the cause of the disease. However, the molecular mechanism of neurodegeneration in HD has been studied in detail, suggesting a possible way to develop an intervention targeting WI. Independent of the genetic cause, all forms of HD show common molecular characteristics: Dimerization of HTT inclusions and block of vital genes and energy-induced gene (ERG).
4. Promising Research and Advances
These findings and the ongoing research into what causes HD, the identification of what hallmarks the progression of the disease, and the development of a number of molecules that either delay or treat the symptoms of the disease encourage the science community to find and develop more molecules that can speed up the delivery of molecules across the brain and spinal cord, that can slow down the disease progression while simultaneously by themselves developing mechanisms to screen, combine, and test them on the long run in people alongside the molecules already proved safe in people in separate studies aimed at preventing the onset of the disease in people who carry the gene mutation responsible for HD.
Research efforts to date have produced a number of advances. The gene involved in HD (HTT) was discovered in 1993, and this in turn led to the development of a meaningful genetic test that could predict which people would develop HD. Since then, advances in tracking the disease have facilitated the identification of who is at risk of developing symptoms of HD, allowing physicians to enroll people who are at risk for the disease in studies aimed at preventing the onset of the disease. This in turn has led to the discovery of molecules, including the first occurring naturally, that in animal models can prevent, delay, or treat the symptoms of the disease. It is because of these advances that now more than fifty approaches to stopping the molecular processes that characterize HD, which includes ongoing research led by investigators in Huntington’s Disease Youth Organization-led Initiatives in HD (H.D. H.)
5. Future Prospects and Challenges
Adenosinergic-mediated posttranscriptional pathophysiology has been widely presumed in investigations into HD. Every effort has been made for 30 years to avoid spurious pharmacological consequences by agents that specifically block transcription (actinomycin D) or translation (e.g. tunicamycin). It is thought that higher adenosine accumulation is responsible for post-transcriptionally blocking a compensatory response caused by a mutated entity in HD mice before their strains exhibited phenotypes that were observed with the R6/2 mutation in people. Quantitatively, pre-symptomatic HD individuals were already co-localized at 30 years of age in a single pathological pathway with R6/2, R6/1, and N171-82Q mice. Within 5 years of having a CAG expansion of the requisite length in a given gene, the onset of symptoms and the genotype-size dependent penetrating is based upon the product of each of their respective expression ratios, and not the sum that was previously discussed. This is based on a modestly more complex formula but has been explained with more clarity in some detail to answer an appropriate query of this debate. Also, DNA repair responses can be informed that, in each species and AI with the same genotype, age in the same degree and in the same order of magnitude as the products of each expression ratio with each age of the individual. By informing the growth of other pathophysiological mechanisms, inhibiting adenosine accumulation and A2a receptor activity can be reinforced to reduce its contribution.
Development of an HD therapy will depend on considerable knowledge, generating a strong rationale from which to find appropriate known drugs for combination therapy or generate new ones. For this, the numbers of HD mice need to be increased, with even higher order complexity to possibly include humans. Eventually, HD exosomes will have to be evaluated. We need a new technology to substantially expand the number of potential steps for drug discovery while also decreasing the cost. Pharmacogenomics has been suggested for this, matching drugs to genes whose pathways and proteins are targeted. This is applicable to purified mRNAs from individual patient cells, and patients will be treated with a high combination of supplements. The pathways that are best changed from existing HD transgenic mice studies can also amplify rare transcriptomes, change protein, and identify rare transcripts. It allows therapeutic testing on HD exosomes.
