Huntingtin protein in health and Huntington's disease: Molecular mechanisms, pathology and therapeutic strategies

Huntington's Disease (HD) is a neurodegenerative, genetic disorder that affects the brain and is caused by the expansion of cytosine-adenine-guanine (CAG) trinucleotide in the huntingtin (HTT) gene exceeding 35 units. Further, the mutation occurs, which leads to the generation of mutant huntingtin (mHTT) protein, which is a toxic protein that damages the neurons and their functions, leading to disease progression. Phosphorylation, SUMOylation, O-GlcNAcylation, and ubiquitination are some of the post-translational modifications (PTMs) that affect the toxicity, location, and aggregation of this altered protein. The survival of neurons depends on autophagy, vesicle trafficking, transcriptional control, and mitochondrial function, all of which are disrupted by HTT. This protein tends to form aggregates, which disrupt vital neuronal functions and ultimately result in neuronal death, especially in the cortex and striatum. The three clinical manifestations of HD include mental health problems, cognitive impairment, and motor symptoms (bradykinesia, chorea). In this review, the HTT protein is examined, along with its normal functions, post-translational modifications, and role in HD pathogenesis. The therapeutic intervention under investigation includes PTM-targeted medications, which are those drugs that enhance neuroprotection and proteostasis, and gene silencing strategies such as antisense oligonucleotides and RNA interference. Disease models are being improved with several novel approaches, which include induced pluripotent stem cells (iPSCs) and CRISPR-based editing and preclinical models. By integrating these technologies, the mechanisms of the underlying disease have also been enhanced. The recent treatment approaches have also been explored by using molecular targets and diagnostic tools, including FANCD2 and FANCI-associated nuclease 1 (FAN1), which are genetic regulators of somatic CAG expansion; EPS8 dysregulation, which causes protein aggregation; and mismatch negativity (MMN), which is a brain response detected by EEG, a non-invasive biomarker for early cognitive impairment. These measures aim to slow down disease progression and improve the health and outcomes of patients.