Biochemical characterization of two (C300F, P425T) arylsulfatase a missense mutations

Metachromatic leukodystrophy: A single-center longitudinal study of 45 patients

In this study, we characterize the natural course of metachromatic leukodystrophy (MLD), explore intra/inter group differences, and identify biomarkers to monitor disease progression. This is a longitudinal observational study. Genotype and characteristics at disease onset were recorded. Time-to-event analyses were performed to assess time to major disease-related milestones in different subgroups. Longitudinal trajectories of nerve conduction velocities (NCV), brain MRI score, and brainstem auditory evoked responses (BAERs) were described. We recruited 22 late-infantile, 14 early-juvenile, 5 late-juvenile, and 4 adult MLD patients. Thirty-four were prospectively evaluated (median FU time 43 months). In late-infantile patients, the attainment of independent walking was associated with a later age at dysphagia. In early-juvenile, the presence of isolated cognitive impairment at onset was not a favorable prognostic factor. Late-infantile and early-juvenile subjects showed similar rapid loss of ambulation and onset of seizures, but late-infantile displayed earlier loss of trunk control, dysphagia, and death. We found significant differences in all major disease-related milestones (except death) between early-juvenile and late-juvenile patients. Late-juvenile and adult patients both presented with a predominant cognitive impairment, mild/no peripheral neuropathy, lower brain MRI score at plateau compared to LI/EJ, and later cerebellar involvement. NCV and BAER were consistently severely abnormal in late-infantile but not in older subjects, in whom both NCV and BAER were variably affected, with no deterioration over time in some cases. This study clarifies intra/inter group differences between MLD subtypes and provides additional indications regarding reliable clinical and instrumental tools to monitor disease progression and to serve as areference to evaluate the efficacy of future therapeutic interventions inthe different MLD variants.

Sixteen novel mutations in the arylsulfatase A gene causing metachromatic leukodystrophy

Metachromatic leukodystrophy (MLD) is a lysosomal storage disorder caused mainly by mutations in the arylsulfatase A (ARSA) gene. In this manuscript we report sixteen novel mutations identified in the ARSA gene of fifteen unrelated patients affected with MLD. Of these 16 mutations nine were missense mutations (p.L11Q, p.S44P, p.L81P, p.R84L, p.V177D, p.P284S, p.R288S, p.G301R, p.P425S), three were nonsense mutations (p.Q51X, p.Y149X, p.C156X), three were frame shift mutations (c.28delG, c.105C>A+106_124dup, c.189delC) and one was a splice-site mutation (c.1102-2A>G). In addition, three previously reported mutations were identified on an allelic background different from the one in the original reports. Two mutations, p.G309S and p.E312D, were identified on the background of the so-called pseudodeficiency (Pd) allele while previously they were reported alone. On the other hand, mutation p.R311X was identified in two unrelated patients not in cis with the Pd mutations, as previously reported.

Missense mutations as a cause of metachromatic leukodystrophy. Degradation of arylsulfatase A in the endoplasmic reticulum

Metachromatic leukodystrophy is a lysosomal storage disorder caused by a deficiency of arylsulfatase A (ASA). Biosynthesis studies of ASA with various structure-sensitive monoclonal antibodies reveal that some epitopes of the enzyme form within the first minutes of biosynthesis whereas other epitopes form later, between 10 and 25 min. When we investigated 12 various ASAs, with amino acid substitutions according to the missense mutations found in metachromatic leukodystrophy patients, immunoprecipitation with monoclonal antibodies revealed folding deficits in all 12 mutant ASA enzymes. Eleven of the 12 mutants show partial expression of the early epitopes, but only six of these show, in addition, incomplete expression of late epitopes. In none of the mutant enzymes were the late forming epitopes found in the absence of early epitopes. Thus, data from the wild-type and mutant enzymes indicate that the enzyme folds in a sequential manner and that the folding of early forming epitopes is a prerequisite for maturation of the late epitopes. All mutant enzymes in which the amino acid substitution prevents the expression of the late forming epitopes are retained in the endoplasmic reticulum (ER). In contrast, all mutants in which a single late epitope is at least partially expressed can leave the ER. Thus, irrespective of the missense mutation, the expression of epitopes forming late in biosynthesis correlates with the ability of the enzyme to leave the ER. The degradation of ER-retained enzymes can be reduced by inhibitors of the proteasome and ER alpha1,2-mannosidase I, indicating that all enzymes are degraded via the proteasome. Inhibition of degradation did not lead to an enhanced delivery from the ER for any of the mutant enzymes.

Oligomerization capacity of two arylsulfatase A mutants: C300F and P425T

Arylsulfatase A (ARSA) is a lysosomal enzyme implicated in most cases of metachromatic leukodystrophy (MLD). The quaternary structure of ARSA is pH-dependent: at neutral pH, ARSA is a homodimeric protein; at lysosomal (acidic) pH, ARSA is homo-octameric. This dimer-octamer transition seems to be of major importance for the stability of the enzyme in the lysosomal milieu. Sedimentation analysis was used to study the oligomerization capacity of C300F and P425T-substituted ARSA, two MLD-associated forms of the enzyme displaying reduced lysosomal half-lives. P425T-ARSA displays a modest reduction in its octamerization capacity. In contrast, the C300F mutation strongly interferes with the octamerization process of ARSA but not with its dimerization capacity. Interestingly, a major fraction of dimeric ARSA-C300F is composed of covalently linked ARSA molecules, through a thiol-cleavable bond that probably involves Cys414 residues from each monomer. Our data support the notion that the reduced lysosomal half-life of some mutated forms of ARSA is related to deficient octamerization.