Chapter 32: Advances in Neurogenetics: Huntington Disease

HD is a universally penetrant, autosomal dominant condition resulting in progressive behavioral changes, cognitive decline, and involuntary movements known as chorea, typically manifesting around age 45 and culminating in death 10 to 15 years after symptom onset. The identification of the causative gene was a pioneering achievement in human genetics, spearheaded by figures like Nancy Wexler, who leveraged large Venezuelan pedigrees and early genomic methods like Restriction Fragment Length Polymorphisms (RFLPs) to establish linkage. This effort, utilizing positional cloning (reverse genetics), successfully mapped the disease locus to the short arm of human chromosome 4. The gene, designated HTT, contains an expanded tandem CAG trinucleotide repeat sequence (40 or more copies in affected individuals). This expansion translates into an increased tract of glutamine residues, classifying HD as a polyglutamine (polyQ) disorder driven by a toxic gain-of-function mutation. The resultant mutant huntingtin protein (mHTT) misfolds and aggregates, disrupting crucial cellular mechanisms, particularly in the brain’s striatum (caudate nucleus), leading to severe neuronal loss and associated motor and cognitive deficits. Key molecular disruptions caused by mHTT include the inhibition of histone acetyl transferases (leading to transcriptional shutdown), the collapse of protein folding and degradation systems (inhibiting chaperones and proteasomes), early synaptic dysfunction, and severe mitochondrial impairment (reducing ATP production and increasing oxidative stress). Advanced research utilizes transgenic animal models (mice and sheep) to test therapeutic strategies, including gene silencing techniques designed to reduce mHTT levels. Promising approaches involve using Antisense Oligonucleotides (ASOs) to degrade the mutant messenger RNA, employing Zinc-Finger Nucleases (ZFNs) to repress transcription, and applying the precise CRISPR-Cas9 gene editing system for allele-specific inactivation of the mutant HTT gene. HD also exhibits genetic anticipation and shares a critical mechanism of aggregate spreading via endocytosis with other major neurodegenerative diseases like Alzheimer disease and Parkinson disease. The application of presymptomatic testing for adults at risk remains highly regulated by strict ethical guidelines, especially concerning the testing of minors.