Due to the interaction-mediated inhibitory effects of mutHTT on HAT enzymes, certain inhibitors of histone deacetylases (HDAC), in particular HDAC4, have been examined for their protective effects in some models of HD. The findings showed that these inhibitors were able to reduce the aggregation of mutHTT and also rescue the neuronal and corticostriatal synaptic function (36,37). Notably, acetylation of mutHTT marks it for ubiquitinylation and subsequent proteosomal degradation and there is a general decline in chromosomal and protein acetylation in HD. Thus, inhibition of HDACs, which sustains an elevated level of protein acetylation, can lead to an increased acetylation status of mutHTT (38). Inhibitors of other deacetylase enzymes such as sirtuin 1, and selisistat are shown to curtail the mutHTT-induced pathology in several model systems (39) and proved to be safe and tolerable in recent phase 1B clinical trials (40). Promotion of the proteolytic breakdown of mHTT through activation of the ubiquitin- proteasome- autophagy system is another pharmacological approach that is being explored (41). Thus, promoting autophagy by inhibiting mTOR with rapamycin, was shown to improve phenotypes in HD models in Drosophila and mouse (38) and similar effects were observed with other autophagy-promoting agents (42). Thus, enhancing autophagy to degrade mutHTT is a viable and important strategy towards HD therapy (Fig. 2).

Considering that selective modulation of phosphorylation of serine residues can be exploited to modulate mutHTT activity, small molecule kinase inhibitors are being tested, even though their selectivity is being investigated (43). Inasmuch as improper folding and aggregation of mutHTT is central to the pathogenesis of HD, attempts are being made to devise cell-permeable chaperones, such as TCP1-ring complex and ApiCCT1 to selectively prevent the aggregation of mutHTT and associated toxicity in neuronal cells (44,45).

Another important HD pathology-associated change is in the cyclic AMP (cAMP) signaling (46) and aberrant transcription of genes regulated by the cAMP response element (CRE) (47). Inhibition of phosphodiesterase (PDE) 10A, which regulates cAMP and cyclic guanosine monophosphate signaling, and is mostly expressed in the MSNs of striatum (48) is shown to be beneficial against HD, via restoration of CRE-mediated gene expression (49). Speciffically, PDE10A inhibitor-based clinical trials in HD patients are currently addressing the efficacy and motor functional endpoints (50).

An important signaling pathway that is hyperactive and contributes to the pathology of HD is MAPK signaling (51). Specifically, overactive c-Jun N-terminal kinase likely leads to dysregulated axonal transport (52) and hyperactive p38 may cause NMDA receptor-mediated excitotoxicity (53). Thus, the overexpression of MKP-1, a negative modulator of MAPKs, was shown to prevent against mutHTT-mediated neuronal dysfunction in several models of HD (54). Similarly, inhibition of MLK-2 was retarded mutHTT mediated-toxicity (55). NMDA receptor-mediated excitotoxicity, has been suspected to be an important contributor to HD pathogenesis and quinolinic acid, an endogenous degradation product of tryptophan, is a known NMDA receptor agonist. In the pathway of tryptophan catabolism, kynurenine monooxygenase (KMO) activity determines the balance between the neuroprotective kynurenic acid and neurotoxic quinolinic acid. Post-mortem examination of brains from HD patients revealed that there is an increase in quinolinic acid and decrease in kynurenic acid. Treatment of HD animal models with an inhibitor of KMO led to elevated kynurenic acid, as well as improved survival and striatal neuron function. Recent studies reported an improved KMO inhibitor, CHDI-340246, which acts only peripherally and elevates kynurenine and kynurenic acid in HD rodent and non-human primate models, and protects from neuronal loss and dysfunction (50,56).

Reducing the content of mutHTT by inhibiting gene transcription, mRNA translation or promoting the breakdown of mRNA coding for HTT, may reduce any associated downstream damaging effects of mutHTT, which otherwise lead to the pathogenesis of HD. However, considering that loss of HTT protein, even conditionally, led to neurodegeneration, caution must be exercised to employ procedures that suppress HTT completely. It is more prudent to selectively target HTT genes that harbor excessive CAG repeats, and not normal HTT gene (Figs. 1 and 2).

Inhibition of transcription by zinc finger proteins (ZFPs) were used to reduce the transcription from the HTT gene. ZFPs can be designed to allow specific binding to selected DNA sequences, and are fused to a transcriptional repressor domain, in order that the gene to which these ZFPs bind, is not expressed, and thus the corresponding protein production is blocked (57). Using ZFPs it has been observed that the proximity of the CAG repeat to the 5′ end of the HTT gene confers selectivity over other genes containing poly-CAG sequences for targeting with viral vectors for delivering the ZFPs (58). Such as approach has been used successfully utilised in mouse model of HD with the resultant decease in pathological motor manifestations (59,60). ZFPs have been employed to deliver DNA nucleases to the target sequences, in a way that excessive CAG repeats are excised from HTT genes, thus raising the prospect of gene therapy for HD (61).

Another approach to lower the expression of mutHTT is to target the corresponding mRNA with specific antisense oligonucleotides (ASOs), which are single-stranded DNA oligonucleotides, and bind to complimentary mRNA sequences via base-pairing and lead to the degradation of the mRNA by RNAse H. Previous findings have shown that intraventricular infusion of mutHTT targeting chemically modified ASOs in three separate HD mouse models was successful in reducing HTT mRNA by 60% and HTT protein by >80% reduction, in a dose-dependent manner. These changes were accompanied by delayed mutHTT aggregation and improved motor performance on a rotarod test. This ASO-induced restoration of normal functionality was sustained even after the infused ASOs were removed, indicating that there was a restoration and recovery of the neurons rendered dysfunctional due to mutHTT (62–64). An advantage of use of ASO is its broad distribution into different brain regions following intraventricular infusion. Inasmuch as mutHTT synthesis is rather ubiquitous, a wider distribution of ASOs is useful in targeting mutHTT expression and thus curtailing its deleterious effects (62). Intrathecal infusion of ASOs for 21 days in non-human primates led to a sustained decrease by approximately 50% in mutHTT mRNA levels in frontal cortex, occipital cortex (68%), and spinal cord, indicating the possible application of ASOs for human situation (62).

Recent advancements in gene silencing efficiency as well as sustained long-term effects of RNAi agents have overtaken the ZFPs and ASOs, despite their therapeutic potential (65). RNAi techniques were successfully employed to reduce HTT mRNA and protein in in vitro models of HD (66). Subsequent, in vivo studies in HD transgenic mice employing AAV-based delivery of shRNA targeting HTT via single bilateral injections into the striatum, revealed a significant reduction in mutHTT mRNA and protein levels, mutHTT aggregates and marked improvement in behavioral and motor performance parameters (67). This study was followed by several other in vivo studies using RNAi approach with improvements in many other HD-associated pathologies (68). Although many of these initial studies employed pre-symptomatic animal models of HD, subsequent studies showed that RNAi approaches also reduced the number of mutHTT inclusions and significantly improved striatal functionality and motor performance (69). Despite the success of these gene expression approaches, clinical application of these methods is not yet feasible and much refinement needs to be attained in these technologies.

In addition to the abovementioned approaches, stem cell-based therapies, in particular, using the patient-specific iPSCs are being developed and are promising to combat HD (Fig. 2) (70).