N-Acetylaspartate Synthase Deficiency Corrects the Myelin Phenotype in a Canavan Disease Mouse Model But Does Not Affect Survival Time

Abstract

Canavan disease (CD) is a severe, lethal leukodystrophy caused by deficiency in aspartoacylase (ASPA), which hydrolyzesN-acetylaspartate (NAA). In the brains of CD patients, NAA accumulates to high millimolar concentrations. The pathology of the disease is characterized by loss of oligodendrocytes and spongy myelin degeneration in the CNS. Whether accumulating NAA, absence of NAA-derived acetate, or absence of any unknown functions of the ASPA enzyme is responsible for the pathology of the disease is not fully understood. We generated ASPA-deficient (Aspanur7/nur7) mice that are also deficient for NAA synthase Nat8L (Nat8L−/−/Aspanur7/nur7). These mice have no detectable NAA. Nevertheless, they exhibited normal myelin content, myelin sphingolipid composition, and full reversal of spongy myelin and axonal degeneration. Surprisingly, although pathology was fully reversed, the survival time of the mice was not prolonged. In contrast, Aspanur7/nur7mice with only one intact Nat8L allele accumulated less NAA, developed a less severe pathology, phenotypic improvements, and, importantly, an almost normal survival time. Therefore, inhibition of NAA synthase is a promising therapeutic option for CD. The reduced survival rate of Nat8L−/−/Aspanur7/nur7mice, however, indicates that complete inhibition of NAA synthase may bear unforeseeable risks for the patient. Furthermore, we demonstrate that acetate derived from NAA is not essential for myelin lipid synthesis and that loss of NAA-derived acetate does not cause the myelin phenotype of Aspanur7/nur7mice. Our data clearly support the hypothesis that NAA accumulation is the major factor in the development of CD. SIGNIFICANCE STATEMENTCanavan disease (CD) is a severe, lethal leukodystrophy caused by aspartoacylase deficiency leading to accumulation of its substrate,N-acetylaspartate (NAA), and spongy myelin degeneration. In our study, we found that deletion of the NAA synthase gene Nat8L in a CD mouse model prevents myelin degeneration. This shows that accumulation of NAA to toxic levels causes the spongy myelin degeneration and excludes that a deficiency of NAA degradation products is responsible for the disease. This conclusion also suggests that inhibition of NAA synthase may be a therapeutic option for CD. The observed sudden premature death of Nat8L-deficient mice, however, also indicates that complete inhibition of NAA synthase may bear unforeseeable risks for patients.