Gene Therapy for Huntington’s Disease - Gene Delivery and Therapy for Neurological Disorders - page 127

prion diseases [ 57 ]. Intrabodies which bind to the mHtt protein
have produced promising but variable results in animal models of
HD depending on the epitope targeted.
The expanded polyglutamine tract represents a logical target
for selective therapy because it is the discriminating feature between
mHtt and wild-type Htt proteins. Unfortunately, intrabodies
directed against the polyglutamine expansion have been shown to
exacerbate mHtt toxicity and aggregation in a cell culture model of
HD [ 58 ]. C4 is an intrabody which binds to the first 17 amino
acids in the N-terminus of Htt exon 1. Early intrastriatal injection
with AAV-C4 delayed the formation of mHtt aggregates and
reduced their size in R6/1 transgenic mice [ 59 ]. The delivery of
C4 during later stages of disease progression also proved to be
beneficial albeit less efficacious. However, blockade of the
N-terminus by intrabodies may lead to nuclear accumulation of
mHtt and undermine this treatment approach [ 60 ].
Intrabodies directed against regions flanking the polygluta-
mine tract have been shown to promote mHtt degradation despite
targeting epitopes shared with the wild-type protein. One such
intrabody is scFV-EM48, a single-chain variable fragment derived
from a monoclonal EM48 antibody whose immunoreactivity to
Htt is augmented by polyglutamine expansion [ 61 ]. Adenoviral
vector-mediated expression of scFV-EM48 in the striatum of R6/2
and N171-82Q mice reduced the formation of neuropil aggre-
gates, possibly through enhancing degradation of mHtt. In addi-
tion, significant improvements in stride length and rotarod
performance were observed in N171-82Q mice treated with scFV-
EM48, but there was no change in intranuclear inclusion forma-
tion, body weight, or longevity.
Recognized by the intrabody Happ1, the proline-rich region
of mHtt is another candidate epitope because it is a site of abnor-
mal protein-protein interactions [ 62 ]. Happ1 can selectively
increase mHtt turnover in vitro [ 63 ] and attenuate HD phenotype
in several mouse models [ 64 ]. Co-injection of AAV-Happ1 with
mHtt-expressing lentivirus into the mouse striatum resulted in
reduced striatal neurodegeneration and mHtt aggregation.
Bilateral intrastriatal injections of AAV-Happ1 in the BACHD,
YAC128, and N171-82Q transgenic models restored motor func-
tion to near-normal levels, while significant motor improvements
were observed in R6/2 mice. Furthermore, the treatment enhanced
the performance of YAC128 mice in spatial and cortical learning
tasks. As for neuropathological changes, AAV-Happ1 prevented
ventricular enlargement in the R6/2, BACHD, and YAC128 mod-
els. Positive effects on overall health such as increased body weight
and lifespan were observed in N171-82Q mice but not the more
severe R6/2 model.
Altogether, highly specific intrabodies targeting selective regions
along the Htt protein demonstrate potential as novel therapeutic