Gene Therapy for Epilepsies - Gene Delivery and Therapy for Neurological Disorders - page 237

The first studies targeted the most obvious candidate, the
GABAergic system, which has a primary inhibitory role on neuro-
transmission. With the demonstration by Loscher et al. that trans-
plantation of fetal GABAergic neurons into the substantia nigra
(SN) conferred significant, albeit transient, seizure suppression in a
rat kindling model [ 4 ], most of the subsequent studies focused on
ex vivo-engineered GABA-producing cells. Transplantation of
genetically engineered cells expressing GAD (glutamic acid decar-
boxylase, the enzyme that catalyzes the synthesis of GABA) into
the SN, piriform cortex, and dentate gyrus showed anticonvulsant
effects in both kindling and kainic acid seizure models (Table1).
Moreover, transplantation of these GAD-overexpressing cells into
the SN of spontaneously seizing animals suppressed spontaneous
seizures [ 5 , 6 ]. Despite the promise of therapeutic efficacy, the
advance of cell transplantation approach is severely limited by the
poor survival of transplanted cells [ 5 ]. Recently, strong inflamma-
tory immune responses followed by graft rejection were reported
when engineered GAD67-expressing cells were transplanted into
the SN of kindled rats [ 7 ].
Others have used a different approach, targeting the GABA
receptor subunits rather than direct modulation of GABA levels.
An initial attempt by Xiao et al. (1997) using AAV2-mediated gene
transfer to upregulate or knockdown the GABAR-ʱ1 subunit in
the rat inferior colliculus showed increased seizure sensitivity with
GABAR-ʱ1 knockdown but no effect with overexpression [ 8 ].
Almost a decade later, Raol et al. (2006) designed an AAV2 con-
struct coding for GABAR-ʱ1 subunit under the control of the ʱ4
subunit (GABRA4) promoter, which is upregulated after status
epilepticus [ 9 ]. Intrahippocampal injection of this vector led to an
increased expression of the ʱ1 subunit at 1-2 weeks after status
epilepticus and resulted in a marked protection against recurrent
spontaneous seizures [ 10 ].
Another logical approach for epilepsy gene therapy is to decrease
glutamatergic hyperexcitability by manipulating the N -methyl-d-
aspartic acid (NMDA) receptors, which are key receptors in excit-
atory neurotransmission and important for the propagation of
seizures. Repeated intracerebroventricular (i.c.v.) injection of an
antisense oligodeoxynucleotide to the NMDA receptor subunit
NR1 suppressed seizure behaviors in genetically epilepsy-prone rats
and protected cortical neurons from excitotoxicity in vitro [ 11 ]. In
a study by Haberman et al. (2002), AAV-mediated delivery of anti-
sense RNA to NR1 under the control of the cytomegalovirus
(CMV) constitutive promoter into the rat inferior colliculus reduced
seizure sensitivity. However, an opposite effect was observed if the
antisense NR1 expression was driven by a tetracycline-off regulat-
able promoter [ 12 ]. The authors suggested that the divergent effect
on seizure sensitivity was likely due to promoter-related tropic differ-
ences as the two constructs transduced distinct neuronal populations.