Effects of a chemical chaperone on genetic mutations in alpha-galactosidase A in Korean patients with Fabry disease

Late-onset fabry disease due to the p.Phe113Leu variant: the first italian cluster of five families

BACKGROUND

The GLA c.337T > C (p.Phe113Leu) is a known pathogenic variant associated to late-onset Fabry disease phenotype with predominant cardiac manifestations. A founder effect was demonstrated in a large cohort in the Portuguese region of Guimarães. Herein we report an in-depth phenotype description of a cluster of five Southern Italy families.

METHODS

Family pedigrees of five index males with the p.Phe113Leu variant were obtained and all at-risk relatives underwent biochemical and genetical screening test. Carriers of GLA p.Phe113Leu variant underwent subsequent multidisciplinary clinical and instrumental evaluation.

RESULTS

Thirty-one (16 M, 15 F) individuals with p.Phe113Leu pathogenic variant were identified. Sixteen out of 31 patients (51.6%) had cardiac manifestations. Notably, myocardial fibrosis was found in 7/8 patients, of whom 2 were under 40 years. Stroke occurred in 4 patients. White matter lesions were detected in 12/19 patients and occurred in 2/10 of subjects under 40 years. Seven females complained of acroparesthesias. Renal involvement occurred in 10 patients. Angiokeratomas were evident in 9 subjects. Eyes, ear, gastrointestinal and pulmonary involvement occurred in the minority of subjects.

CONCLUSION

This study demonstrates that a cluster of subjects with p.Phe113Leu pathogenic variant is also present in Southern Italy. Disease manifestations are frequent in both sexes and may occur early in life. Cardiac involvement represents the core manifestation, but neurological and renal involvement is also frequent, suggesting that extra-cardiac complications deserve clinical attention.

Drug Repositioning for Fabry Disease: Acetylsalicylic Acid Potentiates the Stabilization of Lysosomal Alpha-Galactosidase by Pharmacological Chaperones

Fabry disease is caused by a deficiency of lysosomal alpha galactosidase and has a very large genotypic and phenotypic spectrum. Some patients who carry hypomorphic mutations can benefit from oral therapy with a pharmacological chaperone. The drug requires a very precise regimen because it is a reversible inhibitor of alpha-galactosidase. We looked for molecules that can potentiate this pharmacological chaperone, among drugs that have already been approved for other diseases. We tested candidate molecules in fibroblasts derived from a patient carrying a large deletion in the gene GLA, which were stably transfected with a plasmid expressing hypomorphic mutants. In our cell model, three drugs were able to potentiate the action of the pharmacological chaperone. We focused our attention on one of them, acetylsalicylic acid. We expect that acetylsalicylic acid can be used in synergy with the Fabry disease pharmacological chaperone and prolong its stabilizing effect on alpha-galactosidase.

Chaperone client proteins evolve slower than non-client proteins

Chaperones are important molecular machinery that assists proteins to attain their native three-dimensional structure crucial for function. Earlier studies using experimental evolution showed that chaperones impose a relaxation of sequence constraints on their "client" proteins, which may lead to the fixation of slightly deleterious mutations on the latter. However, we hypothesized that such a phenomenon might be harmful to the organism in a natural physiological condition. In this study, we investigated the evolutionary rates of chaperone client and non-client proteins in five model organisms from both prokaryotic and eukaryotic lineages. Our study reveals a slower evolutionary rate of chaperone client proteins in all five organisms. Additionally, the slower folding rate and lower aggregation propensity of chaperone client proteins reveal that the chaperone may play an essential role in rescuing the slightly disadvantageous effects due to random mutations and subsequent protein misfolding. However, the fixation of such mutations is less likely to be selected in the natural population.

Future clinical and biochemical predictions of Fabry disease in females by methylation studies of the GLA gene

Fabry disease is an X-linked lysosomal storage disorder caused by a deficiency of α-galactosidase A (α-gal A). The clinical variability of the phenotypes of Fabry disease in females is still poorly understood. The degree of aberrant methylation of non-mutated alleles is thought to have significant effects on X-chromosome inactivation (XCI). We previously reported that one heterozygous Fabry female showing classical phenotypes had complete methylation of the non-mutated allele of the GLA gene. In this report, we summarized 36 heterozygous females with a clinical severity score based on the FAbry STabilization indEX (FASTEX). We measured their α-gal A activity and plasma/ serum globotriaosylsphingosine (lyso-Gb3) accumulation and performed electron microscopy of skin biopsies. We analyzed the methylation-sensitive restriction enzyme sites throughout the GLA gene, including the 5’UTR, and found a single SacII site and multiple HhaI and HpaII sites aggregated in exon 1 and the 5’UTR. One HpaII sequence in exon 7 was also detected as a methylation-sensitive site. With methylation-sensitive restriction enzymes, methylated and non-methylated alleles could be separated, and the ratio of the methylation was quantified. We found a clear correlation between the severity of the phenotype and lyso-Gb3 accumulation for heterozygous Fabry disease in females. Methylation of the non-mutated allele was also proportionately correlated to the clinical severity score measured by FASTEX.

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We summarized 36 heterozygous Japanese Fabry females with their clinical severity score.

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We had detected methylation-sensitive restriction enzyme sites in exon 7 along with exon 1 and 5`UTR.

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A clear correlation of patients’ FASTEX scores, sphingolipids accumulations and dysmethylation of the GLA gene was detected.

Challenging popular tools for the annotation of genetic variations with a real case, pathogenic mutations of lysosomal alpha-galactosidase

Background

Severity gradation of missense mutations is a big challenge for exome annotation. Predictors of deleteriousness that are most frequently used to filter variants found by next generation sequencing, produce qualitative predictions, but also numerical scores. It has never been tested if these scores correlate with disease severity.

Results

wANNOVAR, a popular tool that can generate several different types of deleteriousness-prediction scores, was tested on Fabry disease. This pathology, which is caused by a deficit of lysosomal alpha-galactosidase, has a very large genotypic and phenotypic spectrum and offers the possibility of associating a quantitative measure of the damage caused by mutations to the functioning of the enzyme in the cells. Some predictors, and in particular VEST3 and PolyPhen2 provide scores that correlate with the severity of lysosomal alpha-galactosidase mutations in a statistically significant way.

Conclusions

Sorting disease mutations by severity is possible and offers advantages over binary classification. Dataset for testing and training in silico predictors can be obtained by transient transfection and evaluation of residual activity of mutants in cell extracts. This approach consents to quantitative data for severe, mild and non pathological variants.

Electronic supplementary material

The online version of this article (10.1186/s12859-018-2416-7) contains supplementary material, which is available to authorized users.

In Silico Analysis of Missense Mutations as a First Step in Functional Studies: Examples from Two Sphingolipidoses

In order to delineate a better approach to functional studies, we have selected 23 missense mutations distributed in different domains of two lysosomal enzymes, to be studied by in silico analysis. In silico analysis of mutations relies on computational modeling to predict their effects. Various computational platforms are currently available to check the probable causality of mutations encountered in patients at the protein and at the RNA levels. In this work we used four different platforms freely available online (Protein Variation Effect Analyzer- PROVEAN, PolyPhen-2, Swiss-model Expert Protein Analysis System—ExPASy, and SNAP2) to check amino acid substitutions and their effect at the protein level. The existence of functional studies, regarding the amino acid substitutions, led to the selection of the distinct protein mutants. Functional data were used to compare the results obtained with different bioinformatics tools. With the advent of next-generation sequencing, it is not feasible to carry out functional tests in all the variants detected. In silico analysis seems to be useful for the delineation of which mutants are worth studying through functional studies. Therefore, prediction of the mutation impact at the protein level, applying computational analysis, confers the means to rapidly provide a prognosis value to genotyping results, making it potentially valuable for patient care as well as research purposes. The present work points to the need to carry out functional studies in mutations that might look neutral. Moreover, it should be noted that single nucleotide polymorphisms (SNPs), occurring in coding and non-coding regions, may lead to RNA alterations and should be systematically verified. Functional studies can gain from a preliminary multi-step approach, such as the one proposed here.

Fluorescent probes for selective protein labeling in lysosomes: a case of α-galactosidase A

Fluorescence-based live-cell imaging (LCI) of lysosomal glycosidases is often hampered by unfavorable pH and redox conditions that reduce fluorescence output. Moreover, most lysosomal glycosidases are low-mass soluble proteins that do not allow for bulky fluorescent protein fusions. We selected α-galactosidase A (GALA) as a model lysosomal glycosidase involved in Anderson-Fabry disease (AFD) for the current LCI approach. Examination of the subcellular localization of AFD-causing mutants can reveal the mechanism underlying cellular trafficking deficits. To minimize genetic GALA modification, we employed a biarsenical labeling protocol with tetracysteine (TC-tag) detection. We tested the efficiency of halogen-substituted biarsenical probes to interact with C-terminally TC-tagged GALA peptide at pH 4.5 in vitro and identified F2FlAsH-EDT₂ as a superior detection reagent for GALA. This probe provides improved signal/noise ratio in labeled COS-7 cells transiently expressing TC-tagged GALA. The investigated fluorescence-based LCI technology of TC-tagged lysosomal protein using an improved biarsenical probe can be used to identify novel compounds that promote proper trafficking of mutant GALA to lysosomal compartments and rescue the mutant phenotype.-Bohl, C., Pomorski, A., Seemann, S., Knospe, A.-M., Zheng, C., Krężel, A., Rolfs, A., Lukas, J. Fluorescent probes for selective protein labeling in lysosomes: a case of α-galactosidase A.

Using CRISPR/Cas9-Mediated GLA Gene Knockout as an In Vitro Drug Screening Model for Fabry Disease

The CRISPR/Cas9 Genome-editing system has revealed promising potential for generating gene mutation, deletion, and correction in human cells. Application of this powerful tool in Fabry disease (FD), however, still needs to be explored. Enzyme replacement therapy (ERT), a regular administration of recombinant human α Gal A (rhα-GLA), is a currently available and effective treatment to clear the accumulated Gb3 in FD patients. However, the short half-life of rhα-GLA in human body limits its application. Moreover, lack of an appropriate in vitro disease model restricted the high-throughput screening of drugs for improving ERT efficacy. Therefore, it is worth establishing a large-expanded in vitro FD model for screening potential candidates, which can enhance and prolong ERT potency. Using CRISPR/Cas9-mediated gene knockout of GLA in HEK-293T cells, we generated GLA-null cells to investigate rhα-GLA cellular pharmacokinetics. The half-life of administrated rhα-GLA was around 24 h in GLA-null cells; co-administration of proteasome inhibitor MG132 and rhα-GLA significantly restored the GLA enzyme activity by two-fold compared with rhα-GLA alone. Furthermore, co-treatment of rhα-GLA/MG132 in patient-derived fibroblasts increased Gb3 clearance by 30%, compared with rhα-GLA treatment alone. Collectively, the CRISPR/Cas9-mediated GLA-knockout HEK-293T cells provide an in vitro FD model for evaluating the intracellular pharmacokinetics of the rhα-GLA as well as for screening candidates to prolong rhα-GLA potency. Using this model, we demonstrated that MG132 prolongs rhα-GLA half-life and enhanced Gb3 clearance, shedding light on the direction of enhancing ERT efficacy in FD treatment.

Glycomimetic-based pharmacological chaperones for lysosomal storage disorders: lessons from Gaucher, GM1-gangliosidosis and Fabry diseases

Lysosomal storage disorders (LSDs) are often caused by mutations that destabilize native folding and impair the trafficking of enzymes, leading to premature endoplasmic reticulum (ER)-associated degradation, deficiencies of specific hydrolytic functions and aberrant storage of metabolites in the lysosomes. Enzyme replacement therapy (ERT) and substrate reduction therapy (SRT) are available for a few of these conditions, but most remain orphan. A main difficulty is that virtually all LSDs involve neurological decline and neither proteins nor the current SRT drugs can cross the blood-brain barrier. Twenty years ago a new therapeutic paradigm better suited for neuropathic LSDs was launched, namely pharmacological chaperone (PC) therapy. PCs are small molecules capable of binding to the mutant protein at the ER, inducing proper folding, restoring trafficking and increasing enzyme activity and substrate processing in the lysosome. In many LSDs the mutated protein is a glycosidase and the accumulated substrate is an oligo- or polysaccharide or a glycoconjugate, e.g. a glycosphingolipid. Although it might appear counterintuitive, substrate analogues (glycomimetics) behaving as competitive glycosidase inhibitors are good candidates to perform PC tasks. The advancements in the knowledge of the molecular basis of LSDs, including enzyme structures, binding modes, trafficking pathways and substrate processing mechanisms, have been put forward to optimize PC selectivity and efficacy. Moreover, the chemical versatility of glycomimetics and the variety of structures at hand allow simultaneous optimization of chaperone and pharmacokinetic properties. In this Feature Article we review the advancements made in this field in the last few years and the future outlook through the lessons taught by three archetypical LSDs: Gaucher disease, GM1-gangliosidosis and Fabry disease.

The Large Phenotypic Spectrum of Fabry Disease Requires Graduated Diagnosis and Personalized Therapy: A Meta-Analysis Can Help to Differentiate Missense Mutations

Fabry disease is caused by mutations in the GLA gene and is characterized by a large genotypic and phenotypic spectrum. Missense mutations pose a special problem for graduating diagnosis and choosing a cost-effective therapy. Some mutants retain enzymatic activity, but are less stable than the wild type protein. These mutants can be stabilized by small molecules which are defined as pharmacological chaperones. The first chaperone to reach clinical trial is 1-deoxygalactonojirimycin, but others have been tested in vitro. Residual activity of GLA mutants has been measured in the presence or absence of pharmacological chaperones by several authors. Data obtained from transfected cells correlate with those obtained in cells derived from patients, regardless of whether 1-deoxygalactonojirimycin was present or not. The extent to which missense mutations respond to 1-deoxygalactonojirimycin is variable and a reference table of the results obtained by independent groups that is provided with this paper can facilitate the choice of eligible patients. A review of other pharmacological chaperones is provided as well. Frequent mutations can have residual activity as low as one-fourth of normal enzyme in vitro. The reference table with residual activity of the mutants facilitates the identification of non-pathological variants.

Identification of an Allosteric Binding Site on Human Lysosomal Alpha-Galactosidase Opens the Way to New Pharmacological Chaperones for Fabry Disease

Personalized therapies are required for Fabry disease due to its large phenotypic spectrum and numerous different genotypes. In principle, missense mutations that do not affect the active site could be rescued with pharmacological chaperones. At present pharmacological chaperones for Fabry disease bind the active site and couple a stabilizing effect, which is required, to an inhibitory effect, which is deleterious. By in silico docking we identified an allosteric hot-spot for ligand binding where a drug-like compound, 2,6-dithiopurine, binds preferentially. 2,6-dithiopurine stabilizes lysosomal alpha-galactosidase in vitro and rescues a mutant that is not responsive to a mono-therapy with previously described pharmacological chaperones, 1-deoxygalactonojirimycin and galactose in a cell based assay.

Carbohydrate-Processing Enzymes of the Lysosome: Diseases Caused by Misfolded Mutants and Sugar Mimetics as Correcting Pharmacological Chaperones

Lysosomal storage diseases are hereditary disorders caused by mutations on genes encoding for one of the more than fifty lysosomal enzymes involved in the highly ordered degradation cascades of glycans, glycoconjugates, and other complex biomolecules in the lysosome. Several of these metabolic disorders are associated with the absence or the lack of activity of carbohydrate-processing enzymes in this cell compartment. In a recently introduced therapy concept, for susceptible mutants, small substrate-related molecules (so-called pharmacological chaperones), such as reversible inhibitors of these enzymes, may serve as templates for the correct folding and transport of the respective protein mutant, thus improving its concentration and, consequently, its enzymatic activity in the lysosome. Carbohydrate-processing enzymes in the lysosome, related lysosomal diseases, and the scope and limitations of reported reversible inhibitors as pharmacological chaperones are discussed with a view to possibly extending and improving research efforts in this area of orphan diseases.

GLA variation p.E66Q identified as the genetic etiology of Fabry disease using exome sequencing

Fabry disease (FD) was an X-linked lysosomal storage disorder resulting from a deficiency in glycosphingolipid catabolism caused by mutations in the α-galactosidase A gene GLA. Variant FD patients did not present with classical symptoms during childhood and were undiagnosed or misdiagnosed with other kidney diseases, such as chronic glomerulonephritis (CGN). In this study, we utilized exome sequencing and Sanger sequencing identified the variation p.E66Q of GLA completely co-segregated with the disease phenotype in a Chinese family, which previously been diagnosed as possible CGN. Female patients exhibited preferential X-chromosome inactivation (XCI) of the normal p.E66 allele, as indicated by XCI analysis. By measuring α-Gal A activity, we found that male patients in the pedigree had just little enzymatic activity while female patients had residual enzymatic activity. These patients were diagnosed with renal variant FD in subsequent clinical review. Our results directly implicated the GLA mutation p.E66Q as the genetic etiology of the Chinese renal variant FD pedigree.

Characterization of early disease status in treatment-naive male paediatric patients with Fabry disease enrolled in a randomized clinical trial

Trial Design

This analysis characterizes the degree of early organ involvement in a cohort of oligo-symptomatic untreated young patients with Fabry disease enrolled in an ongoing randomized, open-label, parallel-group, phase 3B clinical trial.

Methods

Males aged 5–18 years with complete α-galactosidase A deficiency, without symptoms of major organ damage, were enrolled in a phase 3B trial evaluating two doses of agalsidase beta. Baseline disease characteristics of 31 eligible patients (median age 12 years) were studied, including cellular globotriaosylceramide (GL-3) accumulation in skin (n = 31) and kidney biopsy (n = 6; median age 15 years; range 13–17 years), renal function, and glycolipid levels (plasma, urine).

Results

Plasma and urinary GL-3 levels were abnormal in 25 of 30 and 31 of 31 patients, respectively. Plasma lyso-GL-3 was elevated in all patients. GL-3 accumulation was documented in superficial skin capillary endothelial cells (23/31 patients) and deep vessel endothelial cells (23/29 patients). The mean glomerular filtration rate (GFR), measured by plasma disappearance of iohexol, was 118.1 mL/min/1.73 m² (range 90.4–161.0 mL/min/1.73 m²) and the median urinary albumin/creatinine ratio was 10 mg/g (range 4.0–27.0 mg/g). On electron microscopy, renal biopsy revealed GL-3 accumulation in all glomerular cell types (podocytes and parietal, endothelial, and mesangial cells), as well as in peritubular capillary and non-capillary endothelial, interstitial, vascular smooth muscle, and distal tubules/collecting duct cells. Lesions indicative of early Fabry arteriopathy and segmental effacement of podocyte foot processes were found in all 6 patients.

Conclusions

These data reveal that in this small cohort of children with Fabry disease, histological evidence of GL-3 accumulation, and cellular and vascular injury are present in renal tissues at very early stages of the disease, and are noted before onset of microalbuminuria and development of clinically significant renal events (e.g. reduced GFR). These data give additional support to the consideration of early initiation of enzyme replacement therapy, potentially improving long-term outcome.

Trial Registration

ClinicalTrials.gov NCT00701415

Taming molecular flexibility to tackle rare diseases

Many mutations responsible of Fabry disease destabilize lysosomal alpha-galactosidase, but retain the enzymatic activity. These mutations are associated to a milder phenotype and are potentially curable with a pharmacological therapy either with chaperones or with drugs that modulate proteostasis. We demonstrate the effectiveness of molecular dynamics simulations to correlate the genotype to the severity of the disease. We studied the relation between protein flexibility and residual enzymatic activity of pathological missense mutants in the cell. We found that mutations occurring at flexible sites are likely to retain activity in vivo. The usefulness of molecular dynamics for diagnostic purposes is not limited to lysosomal galactosidase because destabilizing mutations are widely encountered in other proteins, too, and represent a large share of all the ones associated to human diseases.

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Residual alpha-galactosidase activity may relate to mild phenotype in Fabry disease.

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Molecular dynamics identifies flexible residues in lysosomal alpha-galactosidase.

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Mutations at flexible sites tend to maintain residual alpha-galactosidase activity.

Concise syntheses of potent chaperone drug candidates, N-octyl-4-epi-β-valinenamine (NOEV) and its 6-deoxy derivative, from (+)-proto-quercitol

Described are the efficient syntheses of β-galactose-type unsaturated carbasugar amine, N-octyl-4-epi-β-valienamine (1a, NOEV) and 6-deoxy NOEV (12), starting from (+)-proto-quercitol (2), which is readily provided by the bioconversion of myo-inositol. NOEV is a potent chemical chaperone drug candidate for G(M1)-gangliosidosis. An intermediate alkadiene benzoate was prepared from 2 in five steps, with the key step being a Wittig reaction with an enol ester. The 6-deoxy derivative 12 was conveniently synthesized from the versatile intermediate dibromo compound 6, which was also an intermediate in the synthesis of NOEV. Enzyme inhibition assays demonstrated that 12 possessed stronger inhibitory activity than the parent 1a, suggesting that the C-6 position of the 4-epi-β-valienamine-type inhibitor could have hydrophobic interactions at the β-galactosidase active site residues.

Functional analysis of variant lysosomal acid glycosidases of Anderson-Fabry and Pompe disease in a human embryonic kidney epithelial cell line (HEK 293 T)

The functional significance of missense mutations in genes encoding acid glycosidases of lysosomal storage disorders (LSDs) is not always clear. Here we describe a method of investigating functional properties of variant enzymes in vitro using a human embryonic kidney epithelial cell line. Site-directed mutagenesis was performed on the parental plasmids containing cDNA encoding for alpha-galactosidase A (α-Gal A) and acid maltase (α-Glu) to prepare plasmids encoding relevant point mutations. Mutant plasmids were transfected into HEK 293 T cells, and transient over-expression of variant enzymes was measured after 3 days. We have illustrated the method by examining enzymatic activities of four unknown α-Gal A and one α-Glu variants identified in our patients with Anderson-Fabry disease and Pompe diseases respectively. Comparison with control variants known to be either pathogenic or non-pathogenic together with over-expression of wild-type enzyme allowed determination of the pathogenicity of the mutation. One leader sequence novel variant of α-Gal A (p.A15T) was shown not to significantly reduce enzyme activity, whereas three other novel α-Gal A variants (p.D93Y, p.L372P and p.T410I) were shown to be pathogenic as they resulted in significant reduction of enzyme activity. A novel α-Glu variant (p.L72R) was shown to be pathogenic as this significantly reduced enzyme activity. Certain acid glycosidase variants that have been described in association with late-onset LSDs and which are known to have variable residual plasma and leukocyte enzyme activity in patients appear to show intermediate to low enzyme activity (p.N215S and p.Q279E α-Gal A respectively) in the over-expression system.

Pharmacological chaperone therapy for Fabry disease

Fabry disease is an inherited lysosomal storage disorder caused by deficient α-galactosidase A activity. Many missense mutations in Fabry disease often cause misfolded gene products, which leads to their retention in the endoplasmic reticulum by the quality control system; they are then removed by endoplasmic reticulum-associated degradation. We discovered that a potent α-galactosidase A inhibitor, 1-deoxygalactonojirimycin, acts as a pharmacological chaperone to facilitate the proper folding of the mutant enzyme by binding to its active site, thereby improving its stability and trafficking to the lysosomes in mammalian cells. The oral administration of 1-deoxygalactonojirimycin to transgenic mice expressing human mutant α-galactosidase A resulted in significant increases in α-galactosidase A activity in various organs, with concomitant reductions in globotriaosylceramide, which contributes to the pathology of Fabry disease. Seventy-eight missense mutations were found to be responsive to 1-deoxygalactonojirimycin. These data indicate that many patients with Fabry disease could potentially benefit from pharmacological chaperone therapy.

Therapy of Fabry disease with pharmacological chaperones: from in silico predictions to in vitro tests

Background

Fabry disease is a rare disorder caused by a large variety of mutations in the gene encoding lysosomal alpha-galactosidase. Many of these mutations are unique to individual families. Fabry disease can be treated with enzyme replacement therapy, but a promising novel strategy relies on small molecules, so called "pharmacological chaperones", which can be administered orally. Unfortunately only 42% of genotypes respond to pharmacological chaperones.

Results

A procedure to predict which genotypes responsive to pharmacological chaperones in Fabry disease has been recently proposed. The method uses a position-specific substitution matrix to score the mutations. Using this method, we have screened public databases for predictable responsive cases and selected nine representative mutations as yet untested with pharmacological chaperones. Mutant lysosomal alpha galactosidases were produced by site directed mutagenesis and expressed in mammalian cells. Seven out of nine mutations responded to pharmacological chaperones. Nineteen other mutations that were tested with pharmacological chaperones, but were not included in the training of the predictive method, were gathered from literature and analyzed in silico. In this set all five mutations predicted to be positive were responsive to pharmacological chaperones, bringing the percentage of responsive mutations among those predicted to be positive and not used to train the classifier to 86% (12/14). This figure differs significantly from the percentage of responsive cases observed among all the Fabry mutants tested so far.

Conclusions

In this paper we provide experimental support to an "in silico" method designed to predict missense mutations in the gene encoding lysosomal alpha galactosidase responsive to pharmacological chaperones. We demonstrated that responsive mutations can be predicted with a low percentage of false positive cases. Most of the mutations tested to validate the method were described in the literature as associated to classic or mild classic phenotype. The analysis can provide a guideline for the therapy with pharmacological chaperones supported by experimental results obtained in vitro. We are aware that our results were obtained in vitro and cannot be translated straightforwardly into benefit for patients, but need to be validated by clinical trials.

Identification and characterization of pharmacological chaperones to correct enzyme deficiencies in lysosomal storage disorders

Many human diseases result from mutations in specific genes. Once translated, the resulting aberrant proteins may be functionally competent and produced at near-normal levels. However, because of the mutations, the proteins are recognized by the quality control system of the endoplasmic reticulum and are not processed or trafficked correctly, ultimately leading to cellular dysfunction and disease. Pharmacological chaperones (PCs) are small molecules designed to mitigate this problem by selectively binding and stabilizing their target protein, thus reducing premature degradation, facilitating intracellular trafficking, and increasing cellular activity. Partial or complete restoration of normal function by PCs has been shown for numerous types of mutant proteins, including secreted proteins, transcription factors, ion channels, G protein-coupled receptors, and, importantly, lysosomal enzymes. Collectively, lysosomal storage disorders (LSDs) result from genetic mutations in the genes that encode specific lysosomal enzymes, leading to a deficiency in essential enzymatic activity and cellular accumulation of the respective substrate. To date, over 50 different LSDs have been identified, several of which are treated clinically with enzyme replacement therapy or substrate reduction therapy, although insufficiently in some cases. Importantly, a wide range of in vitro assays are now available to measure mutant lysosomal enzyme interaction with and stabilization by PCs, as well as subsequent increases in cellular enzyme levels and function. The application of these assays to the identification and characterization of candidate PCs for mutant lysosomal enzymes will be discussed in this review. In addition, considerations for the successful in vivo use and development of PCs to treat LSDs will be discussed.

Molecular mechanism for stabilization of a mutant α-galactosidase A involving M51I amino acid substitution by imino sugars

Small molecules including imino sugars are expected to act as chaperones for a mutant α-galactosidase A (GLA), which will be useful for pharmacological chaperone therapy for Fabry disease. However, there is little detailed information about the molecular mechanism. We paid attention to an M51I mutant GLA which had been reported to strongly react to an imino sugar. The predicted structural change caused by this amino acid substitution is very small and located on the surface of the molecule. We produced the mutant enzyme in yeast, and determined its enzymological characteristics. The enzymological parameter values are almost the same as those of the wild-type GLA, although the mutant enzyme is unstable not only under neutral pH conditions but also under acidic ones. Then, we directly examined the effect of imino sugars including 1-deoxygalactonojirimycin and galactostatin bisulfite on the purified mutant enzyme. The imino sugars apparently improved the stability of the mutant enzyme under both neutral and acidic pH conditions. The results of surface plasmon resonance biosensor assaying suggested that the imino sugars retained their binding activity as to the mutant enzyme under both neutral and acidic pH conditions. This information will facilitate improvement of pharmacological chaperone therapy for Fabry disease.

Enzyme replacement therapy for Fabry disease: some answers but more questions

Fabry disease (FD) is a multisystem, X-linked disorder of glycosphingolipid metabolism caused by enzyme deficiency of α-galactosidase A. Affected patients have symptoms including acroparesthesias, angiokeratomas, and hypohidrosis. More serious manifestations include debilitating pain and gastrointestinal symptoms, proteinuria and gradual deterioration of renal function leading to end-stage renal disease, hypertrophic cardiomyopathy, and stroke. Heterozygous females may have symptoms as severe as males with the classic phenotype. Before 2001, treatment of patients with FD was supportive. The successful development of enzyme replacement therapy (ERT) has been a great advancement in the treatment of patients with FD and can stabilize renal function and cardiac size, as well as improve pain and quality of life of patients with FD. In this review, we have provided a critical appraisal of the literature on the effects of ERT for FD. This analysis shows that data available on the treatment of FD are often derived from studies which are not controlled, rely on surrogate markers, and are of insufficient power to detect differences on hard clinical endpoints. Further studies of higher quality are needed to answer the questions that remain concerning the efficacy of ERT for FD.

Mutations of the GLA gene in Korean patients with Fabry disease and frequency of the E66Q allele as a functional variant in Korean newborns

Fabry disease is caused by an alpha-galactosidase A (GLA) deficiency. In this study, we identified 28 unrelated Korean families with Fabry disease with 25 distinct mutations in the GLA gene including six novel mutations (p.W47X, p.C90X, p.D61EfsX32, IVS4(-11)T>A, p.D322E and p.W349). Notably, five subjects from four unrelated families carried the p.E66Q variant, previously known as a pathogenic mutation in atypical Fabry disease. Among these patients, only one had proteinuria and two had hypertrophic cardiomyopathy without any other systemic manifestation of Fabry disease. Substantial residual GLA activity was shown both in the leukocytes of p.E66Q patients (19.0-30.3% of normal activity) and in transiently overexpressed COS-7 cells (43.8 + or - 3.03% of normal activity). Although GLA harboring p.E66Q is unstable at neutral pH, the enzyme is efficiently expressed in the lysosomes of COS-7 cells. The location of p.E66 is distant from both the active site and the dimer interface, and has a more accessible surface area than have other mutations of atypical Fabry disease. In addition, the allele frequency of p.E66Q determined in 833 unrelated Korean individuals was remarkably high at 1.046% (95% confidence interval, 0.458-1.634%). These results indicate that p.E66Q is a functional polymorphism rather than a pathogenic mutation.

Improving binding specificity of pharmacological chaperones that target mutant superoxide dismutase-1 linked to familial amyotrophic lateral sclerosis using computational methods

We recently described a set of drug-like molecules obtained from an in silico screen that stabilize mutant superoxide dismutase-1 (SOD-1) linked to familial amyotrophic lateral sclerosis (ALS) against unfolding and aggregation but exhibited poor binding specificity toward SOD-1 in presence of blood plasma. A reasonable but not a conclusive model for the binding of these molecules was proposed on the basis of restricted docking calculations and site-directed mutagenesis of key residues at the dimer interface. A set of hydrogen bonding constraints obtained from these experiments were used to guide docking calculations with compound library around the dimer interface. A series of chemically unrelated hits were predicted, which were experimentally tested for their ability to block aggregation. At least six of the new molecules exhibited high specificity of binding toward SOD-1 in the presence of blood plasma. These molecules represent a new class of molecules for further development into clinical candidates.

Synthesis of lipophilic 1-deoxygalactonojirimycin derivatives as D-galactosidase inhibitors

N-Alkylation at the ring nitrogen of the D-galactosidase inhibitor 1-deoxygalactonojirimycin with a functionalised C ₆alkyl chain followed by modification with different aromatic substituents provided lipophilic 1-deoxygalactonojirimycin derivatives which exhibit inhibitory properties against β-glycosidases from E. coli and Agrobacterium sp. as well as green coffee bean α-galactosidase. In preliminary studies, these compounds also showed potential as chemical chaperones for GM1-gangliosidosis related β-galactosidase mutants.