Missense mutations as a cause of metachromatic leukodystrophy

Abstract

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 α1,2‐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.

Clinical Applications Involving CNS Gene Transfer

“…Since sphingolipids play a key role in myelin synthesis, the disease affects both branches of the nervous systems: the peripheral and the central. MLD is caused by a deficiency in the enzyme, arylsulfatase A (ARSA) (Poeppel et al, 2005). It has been demonstrated that the enzyme activity in the majority of cases declines to the level of 10% or lower compared to that of unaffected controls.…”

Developing therapeutic approaches for metachromatic leukodystrophy

“…Though several missense mutations have been identified, few of them have been biochemically characterized 10,43. Most missense mutations result in misfolded mutant ASA that is early selected for the ER-associated degradation (ERAD) pathway and targeted to the ubiquitin-proteasome system 44. Therefore, small amounts of partially folded and/or misfolded mutant ASA pass the ER post-translational quality control and ultimately reach the lysosomal compartment, resulting in the residual ASA enzymatic activity 45.…”

inPHAP: Interactive visualization of genotype and phased haplotype data

“…The disease is caused by several missense mutations on Chromosome 22, causing defects of the enzyme arylsulfatase A ( ARSA ) [29]. One of the SNPs with dbSNP ID that is the supposedly responsible mutation for MLD, is a C → G substitution, leading to an amino acid change of T h r e o n i n e → S e r i n e in the corresponding protein ARSA .…”