Disease-modifying treatment approaches in Huntington disease : Past and future

Huntington disease (HD) is the most frequent monogenetic neurodegenerative disease and can be unequivocally diagnosed even in the preclinical stage, at least in all individuals in whom the CAG expansion mutation in the huntingtin gene (HTT) is in the range of full penetrance. Therefore, important preconditions for an intervention early in the disease process are met, rendering modification of the course of the disease in a clinically meaningful way possible. In this respect, HD can be viewed as a model disorder for exploring neuroprotective treatment approaches. In the past emphasis was placed on the compensation of a suspected neurotransmitter deficit (GABA) analogous to Parkinson's disease and on classical neuroprotective strategies to influence hypothetical common pathways in neurodegenerative diseases (e.g., excitotoxicity, mitochondrial dysfunction, oxidative stress). With the discovery of the causative HTT mutation in 1993, therapeutic research increasingly focused on intervening as proximally as possible in the chain of pathophysiological events. Currently, an important point of intervention is the HTT mRNA with the aim of reducing the continued production of mutant huntingtin gene products and thus relieving the body of their detrimental actions. To this end, various treatment modalities (single-stranded DNA and RNA, divalent RNA and zinc finger repressor complexes, orally available splice modulators) were developed and are currently in clinical trials (phases I-III) or in late stages of preclinical development. In addition, there is the notion that it may be possible to modify the length of the somatically unstable CAG mutation, i.e. its increase in the brain during the lifetime, thereby slowing the progression of HD.