Restoration of aspartoacylase activity in CNS neurons does not ameliorate motor deficits and demyelination in a model of Canavan disease

Abstract

Canavan disease is an early onset leukodystrophy associated with psychomotor retardation, seizures, and premature death. This disorder is caused by mutations in the gene encoding the enzyme aspartoacylase (ASPA). Normally, ASPA is enriched in oligodendrocytes and ASPA deficiency results in elevated levels of its substrate molecule, N-acetylaspartate (NAA), brain edema, and dysmyelination. Using adeno-associated virus, we permanently expressed ASPA in CNS neurons of the tremor rat, a genetic model of Canavan disease, and examined the efficacy of the treatment by monitoring NAA metabolism, myelination, motor behavior, and seizures. Assessment of ASPA protein and enzyme activity in whole brain hemispheres showed restoration to normal levels as long as 6 months after treatment. This finding correlated with a reduction of NAA levels, along with a rescue of the seizure phenotype. However, gross brain pathology, such as dilated ventricles and spongiform vacuolization, was unchanged. Moreover, hypomyelination and motor deficits were not resolved by ASPA gene transfer. Our data suggest that NAA-mediated neuronal hyperexcitation but not oligodendrocyte dysfunction can be compensated for by neuronal ASPA expression.

N-Acetylaspartate in the CNS: From neurodiagnostics to neurobiology

“…In one of these gene transfer studies in Tremor rats, NAA levels were reduced, and seizure activity was diminished, but brain vacuolation and dysmyelination were unaffected, suggesting that these pathological features of the disease are not mediated by excessive NAA concentrations (Klugmann et al, 2005). More importantly, no motor improvements were observed in the Tremor rats after gene transfer.…”

“…The most recent adenoviral gene transfer studies employed improved gene transfer technology using the recombinant adeno-associated virus serotype 2 vector (AAV-2) (Klugmann et al, 2005;McPhee et al, 2005). In one of these gene transfer studies in Tremor rats, NAA levels were reduced, and seizure activity was diminished, but brain vacuolation and dysmyelination were unaffected, suggesting that these pathological features of the disease are not mediated by excessive NAA concentrations (Klugmann et al, 2005).…”

Cell and Gene Therapies for Refractory Epilepsy

“…Thus, delivery of the ASPA gene by viral vectors to the brain ameliorated tonic convulsions and other behavioral defects in two independent studies [111,148]. However, a similar study using adeno associated virus-mediated overexpression of ASPA reported rescue of the seizure phenotype, but no effects on hypomyelination and motor defects, suggesting that this gene therapy approach reduced neuronal hyperexcitation, but not oligodendrocyte dysfunction [80].…”

Non-genetic therapeutic approaches to Canavan disease

“…However, it must be noted that in some cases the observed decrease in NAA and spongiform pathology is transient , while in others the effects are limited within a confined area surrounding the region of intracerebral injection (Leone et al, 2000;. Improved motor function was noted in certain cases (Ahmed et al, 2013;Leone et al, 2012;McPhee et al, 2005), but not in all (Klugmann et al, 2005). Moreover, where motor defects were improved, they did not disappear altogether and were shown in a longitudinal study to relapse later in life (Ahmed et al, 2013).…”

“…This approach has seen a degree of success in improving the major disease symptoms; however, progress is still to be made. The use of liposome encapsulated plasmids and adeno-associated viruses as vectors to insert the ASPA gene into CD mice, tremor mice and human CD subjects has resulted in enhanced ASPA production and activity and, in some cases, reduced CNS NAA levels and improved spongiform degeneration (Ahmed et al, 2013;Klugmann et al, 2005;Leone et al, 2000;Leone et al, 2012;McPhee et al, 2005). However, it must be noted that in some cases the observed decrease in NAA and spongiform pathology is transient , while in others the effects are limited within a confined area surrounding the region of intracerebral injection (Leone et al, 2000;.…”