Nur7Is a Nonsense Mutation in the Mouse Aspartoacylase Gene That Causes Spongy Degeneration of the CNS

Abstract

Aspartoacylase (ASPA) is an oligodendrocyte-restricted enzyme that catalyzes the hydrolysis of neuronally derived N-acetylaspartate (NAA) to acetate and aspartic acid. ASPA deficiency leads to the fatal childhood autosomal recessive leukodystrophy Canavan disease (CD). Here we demonstrate that the previously described ENU-induced nur7 mouse mutant is caused by a nonsense mutation, Q193X, in the Aspa gene (Aspa nur7 ). Homozygous Aspa nur7nur7 mice do not express detectable Aspa protein and display an early-onset spongy degeneration of CNS myelin with increased NAA levels similar to that observed in CD patients. In addition, CNS regions rich in neuronal cell bodies also display vacuolization. Interestingly, distinct myelin rich areas, such as the corpus callosum, optic nerve, and spinal cord white matter appear normal in Aspa nur7/nur7 mice. Reduced cerebroside synthesis has been demonstrated in CD patients and animal models. To determine the potential relevance of this observation in disease pathogenesis, we generated Aspa nur7/nur7 mice that were heterozygous for a null allele of the gene that encodes the enzyme UDP-galactose:ceramide galactosyltransferase (Cgt), which is responsible for catalyzing the synthesis of the abundant myelin galactolipids. Despite reduced amounts of cerebrosides, the Aspa nur7/nur7 ;Cgt ϩ/ Ϫ mice were not more severely affected than the Aspa nur7 mutants, suggesting that diminished cerebroside synthesis is not a major contributing factor in disease pathogenesis. Furthermore, we found that myelin degeneration leads to significant axonal loss in the cerebellum of older Aspa nur7 mutants. This finding suggests that axonal pathology caused by CNS myelin defects may underlie the neurological disabilities that CD patients develop at late stages of the disease.

Aspartoacylase Supports Oxidative Energy Metabolism during Myelination

“…(Mheta and Namboodiri, 1995). However, a genetically engineered reduction in myelin cerebroside synthesis does not exacerbate the phenotype of nur7 mice (Traka et al, 2008), suggesting that reduced lipid synthesis is not solely responsible for gross pathology. Given the strong association between fluctuations in NAA and neuronal oxidative metabolism, we hypothesized that the loss of ASPA function would impact negatively not only on developmental myelination, but on metabolic energy Figure 4 Developmental myelination.…”

“…The nur7 mouse has a point mutation in exon 4 of the aspa gene and presents a progressive pathology from 2 to 3 weeks of age onwards that includes oligodendrocyte loss and spongiform degeneration (Traka et al, 2008), which coincides with normal developmental increases in ASPA activity (Bhakoo et al, 2001) and expression (Kirmani et al, 2003). Nur7 mice do not generate any detectable ASPA protein and present with chronically elevated NAA.…”

“…We next employed design-based stereology to generate spatiotemporal estimates of late stage oligodendrocyte numbers at 2, 4, and 8 weeks of age to highlight possible oligodendrocyte loss relative to the accumulation of NAA. Large tracts of subcortical white matter remain unaffected until relatively late in life in the nur7 mouse (Traka et al, 2008), and so two general regions of the brain were distinguished for sampling purposes, namely, the neuron-rich neocortex (cortex; Figure 2A) and white matter-rich areas of the corpus callosum and internal capsule (collectively termed subcortical white matter; Figure 2B). At 2 weeks of age, the nur7 cortex contained an average of 3.1 Â 10 5 oligodendrocytes per hemisphere, which was not significantly different from wild-type controls (P = 0.995; Figure 2A).…”

Making the White Matter Matters: Progress in Understanding Canavan’s Disease and Therapeutic Interventions Through Eight Decades

“…Studies on the Nur7 KO mouse (Traka et al 2008) suggest that spongy degeneration is not dependent on disrupted myelin synthesis. Even though Nur7 mice are heterozygous for a null allele of a galactolipid-synthesizing enzyme which could further reduce brain cerebroside content, they do not show more severe myelin pathology implicating additional mechanisms in the pathophysiology of aspartoacylase deficiency (Madhavarao et al 2009).…”

“…Newer studies have created a knock-in model (Mersmann et al 2011) and a model with a single-point mutation in Nur7 (Traka et al 2008) making the development of therapeutic modalities much easier. Though currently the patients are mostly supported by palliative measures, existing treatment modules focus on different aspects of the disease phenotypes in CD and are described below.…”