The functional consequences of mis-sense mutations affecting an intra-molecular salt bridge in arylsulphatase A

A natural variant of arylsulfatase from Kluyveromyces lactis shows no formylglycine modification and has no enzyme activity

Kluyveromyces lactis is a common fungal microorganism used for the production of enzyme preparations such as β-galactosidases (native) or chymosin (recombinant). It is generally important that enzyme preparations have no unwanted side activities. In the case of β-galactosidase preparations produced from K. lactis, an unwanted side activity could be the presence of arylsulfatase (EC 3.1.6.1). Due to the action of arylsulfatase, an unpleasant "cowshed-like" off-flavor would occur in the final product. The best choice to avoid this is to use a yeast strain without this activity. Interestingly, we found that certain natural K. lactis strains express arylsulfatases, which only differ in one amino acid at position 139. The result of this difference is that K. lactis DSM 70799 (expressing R139 variant) shows no arylsulfatase activity, unlike K. lactis GG799 (expressing S139 variant). After recombinant production of both variants in Escherichia coli, the R139 variant remains inactive, whereas the S139 variant showed full activity. Mass spectrometric analyses showed that the important posttranslational modification of C56 to formylglycine was not found in the R139 variant. By contrast, the C56 residue of the S139 variant was modified. We further investigated the packing and secondary structure of the arylsulfatase variants using optical spectroscopy, including fluorescence and circular dichroism. We found out that the inactive R139 variant exhibits a different structure regarding folding and packing compared to the active S139 variant. The importance of the amino acid residue 139 was documented further by the construction of 18 more variants, whereof only ten showed activity but always reduced compared to the native S139 variant.

Four novel ARSA gene mutations with pathogenic impacts on metachromatic leukodystrophy: a bioinformatics approach to predict pathogenic mutations

Metachromatic leukodystrophy (MLD) disorder is a rare lysosomal storage disorder that leads to severe neurological symptoms and an early death. MLD occurs due to the deficiency of enzyme arylsulfatase A (ARSA) in leukocytes, and patients with MLD excrete sulfatide in their urine. In this study, the ARSA gene in 12 non-consanguineous MLD patients and 40 healthy individuals was examined using polymerase chain reaction sequencing. Furthermore, the structural and functional effects of new mutations on ARSA were analyzed using SIFT (sorting intolerant from tolerant), I-Mutant 2, and PolyPhen bioinformatics software. Here, 4 new pathogenic homozygous mutations c.585G>T, c.661T>A, c.849C>G, and c.911A>G were detected. The consequence of this study has extended the genotypic spectrum of MLD patients, paving way to a more effective method for carrier detection and genetic counseling.

Effects of glycosylation and pH conditions in the dynamics of human arylsulfatase A

Arylsulfatase A (ARSA) is a lysosomal sulfatase that catalyzes the hydrolysis of cerebroside sulfate. Its deficiency results in Metachromatic Leukodystrophy, whereas a minor condition called ARSA pseudodeficiency occurs in healthy individuals, which has been associated with the substitution of the glycosylated Asn350 by a Ser and with the loss of the polyadenylation signal. In this work, we have investigated ARSA dynamics employing molecular dynamics simulations in response to (1) different pH's, as, beyond its natural lysossomal environment, it has been recently identified in cytoplasmatic medium and (2) glycan occupancies, including its normal glycosylation state, presenting three high mannose-type oligosaccharides. Accordingly, four systems were studied considering ARSA under different conditions: (1) nonglycosylated at pH ∼ 7 (ARSApH7); (2) non-glycosylated at pH ∼ 5 (ARSApH5); (3) triple glycosylated at pH ∼ 5 (ARSAglyc,pH5); and (4) ARSA-N350S mutant at pH ∼ 5 (ARSAN350S,pH5). Lowering pH and increasing glycosylation was found to reduce the flexibility of the enzyme. In addition, at acidic pH, the glycosylated enzyme presented a higher secondary conformational stability when compared to its nonglycosylated counterpart, supporting experimental findings on triple glycosylation as the essential state of ARSA. The N350S mutant exhibited a consistent degree of unfolding, which may be related to its in vitro reduced stability. Finally, the obtained data are discussed in the search for structural evidences able to contribute to the understanding of biological activity of ARSA and molecular etiology of ARSA pseudodeficiency, as determined by ARSA-N350S in the absence of polyadenylation defect.

Metachromatic leukodystrophy: genetics, pathogenesis and therapeutic options

Metachromatic leukodystrophy is a lysosomal storage disease caused by the deficiency of arylsulphatase A (ASA). This leads to storage of the membrane lipid sulphatide, which is abundant in myelin. A pathological hallmark of the disease is demyelination, causing various and ultimately lethal neurological symptoms. Today more than 110 mutations in the ASA gene have been identified, of which only three are frequent. Patients homozygous for alleles, which do not allow for the synthesis of functional ASA always suffer from the severe form of the disease, whereas alleles allowing the expression of residual enzyme activity are associated with the later onset juvenile or adult forms of metachromatic leukodystrophy. In addition, there are other as yet unknown genetic or epigenetic factors modifying the phenotype substantially. ASA-deficient mice have been generated as a model of metachromatic leukodystrophy. These mice store sulphatide and show progressive neurological symptoms, but do not demyelinate. This animal model was recently improved using a transgenic approach, which generated mice in which sulphatide synthesis in myelin-producing cells is enhanced. This new animal model reflects the pathological characteristics of the human disease. ASA-deficient mice have been used in various therapeutic trials involving enzyme replacement, haematopoietic stem-cell-based gene therapy and direct injections of ASA-expressing viral vectors into the brain. These animal studies have paved the way for future clinical studies of enzyme replacement and gene therapy.

CONCLUSION

For many years this devastating disorder was considered untreatable and the outlook for patients was poor. Within a comparatively short period of time since the ASA gene was cloned in 1989, genetic and biochemical studies and data generated from newly developed animal models have led to the first clinical trials. It is hoped that these developments will prove beneficial for patients.

Rethinking peptide supply to MHC class I molecules

The notion that peptides bound to MHC class I molecules are derived mainly from newly synthesized proteins that are defective, and are therefore targeted for immediate degradation, has gained wide acceptance. This model, still entirely hypothetical, has strong intuitive appeal and is consistent with some experimental results, but it is strained by other findings, as well as by established and emerging concepts in protein quality control. While not discounting defectiveness as a driving force for the processing of some proteins, we propose that MHC-class-I-restricted epitopes are derived mainly from nascent proteins that are accessed by the degradation machinery prior to any assessment of fitness, and we outline one way in which this could be accomplished.

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.

What can cell biology tell us about heterogeneity in lysosomal storage diseases?

Lysosomal storage diseases are clinically heterogeneous with respect to their age of onset, progression of symptoms and the particular organs involved. Varying levels of residual enzyme activity, associated with different defective alleles that cause the respective diseases, are responsible in part for this clinical heterogeneity. In general, the higher the residual enzyme activity, the milder the phenotype. Enzyme activity in severe forms of disease is frequently zero, and in mild forms usually does not exceed approximately 5%. However, the correlation is not so strict as to allow prediction of the phenotype of individual patients. The molecular basis of the different levels of enzyme activity can only be revealed by biochemical investigations of the defective lysosomal proteins. Null alleles may be due to splice-site mutations or deletions. In the case of missense mutations, enzymes frequently fold incorrectly and are retained in the endoplasmic reticulum and subsequently degraded. As these enzymes do not reach the lysosome, they do not provide any functional residual activity. Residual enzyme activity is only observed in cases where the defective enzyme reaches the lysosome and has retained enzymatic activity. Patients carrying the same mutant alleles still show considerable phenotypic variability due to modifying genes and epigenetic factors. None of these has so far been elucidated. However, there are some indications that differences in splicing-factor machinery may influence the phenotypic expression of splice-site mutations and that hormonal modulation of secondary microglial activation in lipidosis may also influence the disease course.

CONCLUSION

Phenotypic variability is a frequent phenomenon in lysosomal storage diseases. Residual enzyme activity has been identified as one of the factors influencing the clinical outcome of disease; however, it is obvious that other genetic and epigenetic factors also affect phenotypic variability, particularly in patients with late onset disease.