Gene Therapy for Huntington’s Disease - Gene Delivery and Therapy for Neurological Disorders - page 133

Htt mRNA expression levels in the putamen by 45 % relative to
control vectors at 6 weeks following bilateral stereotaxic injections.
Transgene expression was also observed in other regions of the basal
ganglia which project to and receive projections from the putamen.
Partial Htt suppression was well tolerated over the 6-week duration
of the study and did not result in neuronal loss or inflammatory
responses. In addition, there was no evidence of motor deficits.
Although partial reduction of wild-type Htt expression by AAV-
miRNA has demonstrated safety in the primate putamen, it is
uncertain how well these effects can translate to human patients
and whether or not long-term gene suppression may negatively
impact neurons.
In all, RNAi-based gene therapy has shown much promise as a
potential treatment strategy for HD. Future studies should aim for
a successful translation of RNAi to non-human primate genetic
models of HD to provide strong preclinical support for the thera-
peutic approach.

Although partial knockdown of wild-type HTT has been safely
demonstrated in the preclinical setting, its effects remain largely
unknown and there is little control over the level of gene suppres-
sion. Allele-specific silencing of HTT is favored over nonallele-
specific targeting strategies due to the retention of wild-type protein
expression. As mentioned previously, the polyglutamine expansion
is the feature which most distinguishes mutant HTT gene from its
wild-type counterpart. However, RNAi molecules are unable to tar-
get this region because they are too short in length. Instead,
researchers are now investigating alternative allele-selective targets
such as HD-linked single nucleotide polymorphisms (SNPs) [ 90 ]
and deletion polymorphisms [ 91 ]. Many SNPs have been located in
the mutant HTT gene [ 92 , 93 ], with some occurring at high fre-
quencies among individuals with HD. It has been suggested that as
few as five siRNAs targeting common SNPs are sufficient for treat-
ing ~75 % of HD patients [ 94 ], but the development of treatment
agents on this basis may be complicated by various factors. It is
currently unclear whether RNAi molecules targeting these SNP
sites can discriminate between the mutant and wild-type alleles and
possess adequate silencing activity for mHtt suppression. Regardless
of the outcome, the ~25 % of patients who do not fit the criteria for
the five siRNAs may inevitably require individualized treatments.

1.3.4 Allele-Specific
Targeting

The requirement for long-term correction of the pathological effects
caused by expression of mHtt in HD makes gene therapy a promis-
ing treatment modality for HD. Neuroprotective and/or neurore-
generative approaches based on overexpression of neurotrophic
factors important in mediating the survival of the susceptible MSNs
have shown some promise. Gene silencing and intrabody strategies
provide a rational approach for disease correction by mitigating the

1.4 Summary