Role of Ser-65 in the activity of alpha-galactosidase A: characterization of a point mutation (S65T) detected in a patient with Fabry disease

Identification of c.146G > A mutation in a Fabry patient and its correction by customized Cas9 base editors in vitro

Fabry disease (FD) is a rare X-linked lysosomal storage disorder caused by mutations in the GLA gene, leading to reduced α-galactosidase (α-Gal A) activity. Current treatments, like enzyme replacement, have limitations affecting efficacy and patient outcomes. CRISPR/Cas9 genome editing tools may offer the potential to develop therapeutic strategy via correcting GLA mutations. In this study, we diagnosed a female FD patient with a missense mutation in exon 1 of the GLA gene (c.146G > A, p.R49H). Bioinformatic predictions and biochemical analyses in GLA-knockout cells revealed that this mutation significantly reduced α-Gal A stability and activity, confirming its pathogenicity. To correct this, we used adenine base editing. The mutation, along with a nearby bystander A, was efficiently edited by the traditional N-terminal adenine base editor. To avoid unwanted bystander editing, we developed a series of domain-inlaid base editors with the aim of narrowing editing window. The most effective variant, with deaminase inserted between the 947th and 948th residues of the RUVC3 domain, was further optimized by modifying linker rigidity. These adjustments shifted the editing window, eliminating bystander editing. Our findings clarify the pathogenic nature of a novel GLA mutation and demonstrate the potential of a customized base editor for therapeutic application in FD.

Enzyme Replacement Therapy for FABRY Disease: Possible Strategies to Improve Its Efficacy

Enzyme replacement therapy is the only therapeutic option for Fabry patients with completely absent AGAL activity. However, the treatment has side effects, is costly, and requires conspicuous amounts of recombinant human protein (rh-AGAL). Thus, its optimization would benefit patients and welfare/health services (i.e., society at large). In this brief report, we describe preliminary results paving the way for two possible approaches: i. the combination of enzyme replacement therapy with pharmacological chaperones; and ii. the identification of AGAL interactors as possible therapeutic targets on which to act. We first showed that galactose, a low-affinity pharmacological chaperone, can prolong AGAL half-life in patient-derived cells treated with rh-AGAL. Then, we analyzed the interactomes of intracellular AGAL on patient-derived AGAL-defective fibroblasts treated with the two rh-AGALs approved for therapeutic purposes and compared the obtained interactomes to the one associated with endogenously produced AGAL (data available as PXD039168 on ProteomeXchange). Common interactors were aggregated and screened for sensitivity to known drugs. Such an interactor-drug list represents a starting point to deeply screen approved drugs and identify those that can affect (positively or negatively) enzyme replacement therapy.

Abnormal Pre-mRNA Splicing in Exonic Fabry Disease-Causing GLA Mutations

Fabry disease (FD) is a rare X-linked disease due to a multiverse of disrupting mutations within the GLA gene encoding lysosomal α-galactosidase A (AGAL). Absent AGAL activity causes the accumulation of complex glycosphingolipids inside of lysosomes in a variety of cell types and results in a progressive multisystem disease. Known disease-associated point mutations in protein-coding gene regions usually cause translational perturbations and result in premature chain termination, punctual amino acid sequence alterations or overall altered sequence alterations downstream of the mutation site. However, nucleotide exchanges at the border between introns and exons can affect splicing behavior and lead to abnormal pre-mRNA processing. Prediction with the Human Splicing Finder (HSF) revealed an indication of a significant change in splicing-relevant information for some known FD-associated GLA mutations. To experimentally determine the extent of the change, we made use of a minigene reporter assay and verified alternative splicing events for the exonic mutations c.194G>T and c.358C>G, which led to the usage of alternative donor splice sites at exon 1 and exon 2, respectively. In addition, the mutations c.548G>T and c.638A>T led to significant exon 4 skipping. We conclude that splicing phenotype analysis should be employed in the in vitro analysis of exonic GLA gene mutations, since abnormal splicing may result in a reduction of enzyme activity and alter the amenability for treatment with pharmacological chaperone (PC).

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.

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.

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.

Carboxyl-terminal truncations alter the activity of the human α-galactosidase A

Fabry disease is an X-linked inborn error of glycolipid metabolism caused by deficiency of the human lysosomal enzyme, α-galactosidase A (αGal), leading to strokes, myocardial infarctions, and terminal renal failure, often leading to death in the fourth or fifth decade of life. The enzyme is responsible for the hydrolysis of terminal α-galactoside linkages in various glycolipids. Enzyme replacement therapy (ERT) has been approved for the treatment of Fabry disease, but adverse reactions, including immune reactions, make it desirable to generate improved methods for ERT. One approach to circumvent these adverse reactions is the development of derivatives of the enzyme with more activity per mg. It was previously reported that carboxyl-terminal deletions of 2 to 10 amino acids led to increased activity of about 2 to 6-fold. However, this data was qualitative or semi-quantitative and relied on comparison of the amounts of mRNA present in Northern blots with αGal enzyme activity using a transient expression system in COS-1 cells. Here we follow up on this report by constructing and purifying mutant enzymes with deletions of 2, 4, 6, 8, and 10 C-terminal amino acids (Δ2, Δ4, Δ6, Δ8, Δ10) for unambiguous quantitative enzyme assays. The results reported here show that the k_cat/K_m approximately doubles with deletions of 2, 4, 6 and 10 amino acids (0.8 to 1.7-fold effect) while a deletion of 8 amino acids decreases the k_cat/K_m (7.2-fold effect). These results indicate that the mutated enzymes with increased activity constructed here would be expected to have a greater therapeutic effect on a per mg basis, and could therefore reduce the likelihood of adverse infusion related reactions in Fabry patients receiving ERT treatment. These results also illustrate the principle that in vitro mutagenesis can be used to generate αGal derivatives with improved enzyme activity.

Alteration of proteomic profiles in PBMC isolated from patients with Fabry disease: preliminary findings

Fabry disease (FD) is an X-linked progressive multisystem disease due to mutations in the gene encoding the lysosomal enzyme α-galactosidase A (α-GalA). The deficiency in α-GalA activity leads to an intra-lysosomal accumulation of neutral glycosphingolipids, mainly globotriaosylceramide (Gb3), in various organs and systems. Enzyme replacement therapy is available and alternative therapeutic approaches are being explored. No diagnostic test, other than sequencing of the α-galactosidase A gene, is available, no biomarker has been proven useful to screen for and predict the disease, and underlying mechanisms are still elusive. The aim of this study is to identify FD specific biomarkers and to better understand the pathophysiological changes that occur over time in FD. We compared peripheral blood mononuclear cells (PBMC) from FD patients (n = 8) with control PBMC from healthy individuals (n = 6), by two-dimensional electrophoresis (2DE) and the detected differentially expressed proteins were then subjected to matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS). In FD patients we identified, among the down-regulated proteins, Calnexin, Rho GDP-dissociation inhibitor 2, Rho GDP-dissociation inhibitor 1, Chloride intracellular channel protein 1; on the other hand γ-enolase, 14-3-3 protein theta, 14-3-3 protein zeta/delta, and galectin-1 were identified as up-regulated proteins. Calnexin and Rho GDP-dissociation inhibitor-1,2 are related to protein folding, signal transduction and cell proliferation. This is the first time that γ-enolase and galectin-1 are described to be up-regulated in Fabry patients. Levels of γ-enolase increase dramatically in cardiovascular accidents and cerebral trauma, whereas galectins are regulators of acute and chronic inflammation. These findings may improve our understanding of the molecular mechanisms underlying the pathology and provide new insight and knowledge for future studies in this field.

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.

Fabry disease - current treatment and new drug development

Fabry disease is a rare inherited lysosomal storage disorder caused by a partial or complete deficiency of α-galactosidase A (GLA), resulting in the storage of excess cellular glycosphingolipids. Enzyme replacement therapy is available for the treatment of Fabry disease, but it is a costly, intravenous treatment. Alternative therapeutic approaches, including small molecule chaperone therapy, are currently being explored. High throughput screening (HTS) technologies can be utilized to discover other small molecule compounds, including non-inhibitory chaperones, enzyme activators, molecules that reduce GLA substrate, and molecules that activate GLA gene promoters. This review outlines the current therapeutic approaches, emerging treatment strategies, and the process of drug discovery and development for Fabry disease.

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.

Synthesis and characterization of a new fluorogenic substrate for alpha-galactosidase

Alpha-galactosidase A hydrolyzes the terminal alpha-galactosyl moieties from glycolipids and glycoproteins in lysosomes. Mutations in alpha-galactosidase cause lysosomal accumulation of the glycosphingolipid, globotriaosylceramide, which leads to Fabry disease. Small-molecule chaperones that bind to mutant enzyme proteins and correct their misfolding and mistrafficking have emerged as a potential therapy for Fabry disease. We have synthesized a red fluorogenic substrate, resorufinyl alpha-D-galactopyranoside, for a new alpha-galactosidase enzyme assay. This assay can be measured continuously at lower pH values, without the addition of a stop solution, due to the relatively low pK(a) of resorufin (approximately 6). In addition, the assay emits red fluorescence, which can significantly reduce interferences due to compound fluorescence and dust/lint as compared to blue fluorescence. Therefore, this new red fluorogenic substrate and the resulting enzyme assay can be used in high-throughput screening to identify small-molecule chaperones for Fabry disease.

Active-site-specific chaperone therapy for Fabry disease. Yin and Yang of enzyme inhibitors

Protein misfolding is recognized as an important pathophysiological cause of protein deficiency in many genetic disorders. Inherited mutations can disrupt native protein folding, thereby producing proteins with misfolded conformations. These misfolded proteins are consequently retained and degraded by endoplasmic reticulum-associated degradation, although they would otherwise be catalytically fully or partially active. Active-site directed competitive inhibitors are often effective active-site-specific chaperones when they are used at subinhibitory concentrations. Active-site-specific chaperones act as a folding template in the endoplasmic reticulum to facilitate folding of mutant proteins, thereby accelerating their smooth escape from the endoplasmic reticulum-associated degradation to maintain a higher level of residual enzyme activity. In Fabry disease, degradation of mutant lysosomal alpha-galactosidase A caused by a large set of missense mutations was demonstrated to occur within the endoplasmic reticulum-associated degradation as a result of the misfolding of mutant proteins. 1-Deoxygalactonojirimycin is one of the most potent inhibitors of alpha-galactosidase A. It has also been shown to be the most effective active-site-specific chaperone at increasing residual enzyme activity in cultured fibroblasts and lymphoblasts established from Fabry patients with a variety of missense mutations. Oral administration of 1-deoxygalactonojirimycin to transgenic mice expressing human R301Q alpha-galactosidase A yielded higher alpha-galactosidase A activity in major tissues. These results indicate that 1-deoxygalactonojirimycin could be of therapeutic benefit to Fabry patients with a variety of missense mutations, and that the active-site-specific chaperone approach using functional small molecules may be broadly applicable to other lysosomal storage disorders and other protein deficiencies.

Mutant alpha-galactosidase A enzymes identified in Fabry disease patients with residual enzyme activity: biochemical characterization and restoration of normal intracellular processing by 1-deoxygalactonojirimycin

Fabry disease is a lysosomal storage disorder caused by the deficiency of alpha-Gal A (alpha-galactosidase A) activity. In order to understand the molecular mechanism underlying alpha-Gal A deficiency in Fabry disease patients with residual enzyme activity, enzymes with different missense mutations were purified from transfected COS-7 cells and the biochemical properties were characterized. The mutant enzymes detected in variant patients (A20P, E66Q, M72V, I91T, R112H, F113L, N215S, Q279E, M296I, M296V and R301Q), and those found mostly in mild classic patients (A97V, A156V, L166V and R356W) appeared to have normal K(m) and V(max) values. The degradation of all mutants (except E59K) was partially inhibited by treatment with kifunensine, a selective inhibitor of ER (endoplasmic reticulum) alpha-mannosidase I. Metabolic labelling and subcellular fractionation studies in COS-7 cells expressing the L166V and R301Q alpha-Gal A mutants indicated that the mutant protein was retained in the ER and degraded without processing. Addition of DGJ (1-deoxygalactonojirimycin) to the culture medium of COS-7 cells transfected with a large set of missense mutant alpha-Gal A cDNAs effectively increased both enzyme activity and protein yield. DGJ was capable of normalizing intracellular processing of mutant alpha-Gal A found in both classic (L166V) and variant (R301Q) Fabry disease patients. In addition, the residual enzyme activity in fibroblasts or lymphoblasts from both classic and variant hemizygous Fabry disease patients carrying a variety of missense mutations could be substantially increased by cultivation of the cells with DGJ. These results indicate that a large proportion of mutant enzymes in patients with residual enzyme activity are kinetically active. Excessive degradation in the ER could be responsible for the deficiency of enzyme activity in vivo, and the DGJ approach may be broadly applicable to Fabry disease patients with missense mutations.

4-Phenylbutyrate rescues trafficking incompetent mutant alpha-galactosidase A without restoring its functionality

Fabry disease is a lysosomal storage disorder caused by deficiency of alpha-galactosidase A. Most mutant enzyme is catalytically active but due to misfolding retained in the endoplasmic reticulum. We have tested 4-phenylbutyrate for its potential to rescue various trafficking incompetent mutant alpha-galactosidase A. Although we found that the trafficking blockade for endoplasmic reticulum-retained mutant alpha-Gal A was released, neither a mature enzyme was detectable in transgenic mice fibroblasts nor a reversal of lysosomal Gb3 storage in fibroblasts from Fabry patients could be observed. Because of lack of functionality of rescued mutant alpha-galactosidase A, 4-phenylbutyrate seems to be of limited use as a chemical chaperone for Fabry disease.

Hydrophilic iminosugar active-site-specific chaperones increase residual glucocerebrosidase activity in fibroblasts from Gaucher patients

Gaucher disease is an autosomal recessive lysosomal storage disorder caused by the deficient activity of glucocerebrosidase. Accumulation of glucosylceramide, primarily in the lysosomes of cells of the reticuloendothelial system, leads to hepatosplenomegaly, anemia and skeletal lesions in type I disease, and neurologic manifestations in types II and III disease. We report herein the identification of hydrophilic active-site-specific chaperones that are capable of increasing glucocerebrosidase activity in the cultured fibroblasts of Gaucher patients. Screening of a variety of natural and synthetic alkaloid compounds showed isofagomine, N-dodecyl deoxynojirimycin, calystegines A3, B1, B2 and C1, and 1,5-dideoxy-1,5-iminoxylitol to be potent inhibitors of glucocerebrosidase. Among them, isofagomine was the most potent inhibitor of glucocerebrosidase in vitro, and the most effective active-site-specific chaperone capable of increasing residual glucocerebrosidase activity in fibroblasts established from Gaucher patients with the most prevalent Gaucher disease-causing mutation (N370S). Intracellular enzyme activity increased approximately two-fold after cells had been incubated with isofagomine, and the increase in glucocerebrosidase activity was both dose-dependent and time-dependent. Western blotting demonstrated that there was a substantial increase in glucocerebrosidase protein in cells after isofagomine treatment. Immunocytochemistry revealed an improvement in the glucocerebrosidase trafficking pattern, which overlaps that of lysosome-associated membrane protein 2 in Gaucher fibroblasts cultivated with isofagomine, suggesting that the transport of mutant glucocerebrosidase is at least partially improved in the presence of isofagomine. The hydrophilic active-site-specific chaperones are less toxic to cultured cells. These results indicate that these hydrophilic small molecules are suitable candidates for further drug development for the treatment of Gaucher disease.

High incidence of later-onset fabry disease revealed by newborn screening

The classic phenotype of Fabry disease, X-linked alpha -galactosidase A (alpha -Gal A) deficiency, has an estimated incidence of approximately 1 in 50,000 males. The recent recognition of later-onset variants suggested that this treatable lysosomal disease is more frequent. To determine the disease incidence, we undertook newborn screening by assaying the alpha-Gal A activity in blood spots from 37,104 consecutive Italian male neonates. Enzyme-deficient infants were retested, and "doubly screened-positive" infants and their relatives were diagnostically confirmed by enzyme and mutation analyses. Twelve (0.03%) neonates had deficient alpha-Gal A activities and specific mutations, including four novel missense mutations (M51I, E66G, A73V, and R118C), three missense mutations (F113L, A143T, and N215S) identified previously in later-onset patients, and one splicing defect (IVS5(+1G-->T)) reported in a patient with the classic phenotype. Molecular modeling and in vitro overexpression of the missense mutations demonstrated structures and residual activities, which were rescued/enhanced by an alpha-Gal A-specific pharmacologic chaperone, consistent with mutations that cause the later-onset phenotype. Family studies revealed undiagnosed Fabry disease in affected individuals. In this population, the incidence of alpha-Gal A deficiency was 1 in approximately 3,100, with an 11 : 1 ratio of patients with the later-onset : classic phenotypes. If only known disease-causing mutations were included, the incidence would be 1 in approximately 4,600, with a 7 : 1 ratio of patients with the later-onset : classic phenotypes. These results suggest that the later-onset phenotype of Fabry disease is underdiagnosed among males with cardiac, cerebrovascular, and/or renal disease. Recognition of these patients would permit family screening and earlier therapeutic intervention. However, the higher incidence of the later-onset phenotype in patients raises ethical issues related to when screening should be performed--in the neonatal period or at early maturity, perhaps in conjunction with screening for other treatable adult-onset disorders.

Pharmacological chaperone corrects lysosomal storage in Fabry disease caused by trafficking-incompetent variants

Fabry disease is a lysosomal storage disorder caused by deficiency of alpha-galactosidase A (alpha-Gal A) resulting in lysosomal accumulation of glycosphingolipid globotriosylceramide Gb3. Misfolded alpha-Gal A variants can have residual enzyme activity but are unstable. Their lysosomal trafficking is impaired because they are retained in the endoplasmic reticulum (ER) by quality control. Subinhibitory doses of the competitive inhibitor of alpha-Gal A, 1-deoxygalactonojirimycin (DGJ), stabilize mutant alpha-Gal A in vitro and correct the trafficking defect. We showed by immunolabeling that the chaperone-like action of DGJ significantly reduces the lysosomal Gb3 storage in human Fabry fibroblasts harboring the novel mutations T194I and V390fsX8. The specificity of the DGJ effect was proven by RNA interference. Electron microscopic morphometry demonstrated a reduction of large-size, disease-associated lysosomes and loss of characteristic multilamellar lysosomal inclusions on DGJ treatment. In addition, the pre-Golgi intermediates were decreased. However, the rough ER was not different between DGJ-treated and untreated cells. Pulse-chase experiments revealed that DGJ treatment resulted in maturation and stabilization of mutant alpha-Gal A. Genes involved in cell stress signaling, heat shock response, unfolded protein response, and ER-associated degradation show no apparent difference in expression between untreated and DGJ-treated fibroblasts. The DGJ treatment has no apparent cytotoxic effects. Thus our data show the usefulness of a pharmacological chaperone for correction of the lysosomal storage in Fabry fibroblasts harboring different mutations with residual enzyme activity. Pharmacological chaperones acting on misfolded, unstable mutant proteins that exhibit residual biological activity offer a convenient and cost-efficient therapeutic strategy.

Efficient and rapid purification of recombinant human alpha-galactosidase A by affinity column chromatography

The lysosomal enzyme alpha-galactosidase A (alpha-Gal A) metabolizes neutral glycosphingolipids that possess alpha-galactoside residues at the non-reducing terminus, and inherited defects in the activity of alpha-Gal A lead to Fabry disease. We describe here an efficient and rapid purification procedure for recombinant alpha-Gal A by sequential Concanavalin A (Con A)-Sepharose and immobilized thio-alpha-galactoside (thio-Gal) agarose column chromatography. Optimal elution conditions for both columns were obtained using overexpressed human alpha-Gal A. We recommend the use of a mixture of 0.9 M methyl alpha-mannoside and 0.9 M methyl alpha-glucoside in 0.1 M acetate buffer (pH 6.0) with 0.1 M NaCl for the maximum recovery of glycoproteins with multiple high-mannose type sugar chains from Con A column chromatography, and that the Con A column should not be reused for the purification of glycoproteins that are used for structural studies. Binding of the enzyme to the thio-Gal column requires acidic condition at pH 4.8. A galactose-containing buffer (25 mM citrate-phosphate buffer, pH 5.5, with 0.1 M galactose, and 0.1 M NaCl) was used to elute alpha-Gal A. This procedure is especially useful for the purification of mutant forms of alpha-Gal A, which are not stable under conventional purification techniques. A protocol that purifies an intracellular mutant alpha-Gal A (M279I) expressed in COS-7 cells within 6h at 62% overall yield is presented.

The molecular defect leading to Fabry disease: structure of human alpha-galactosidase

Fabry disease is an X-linked lysosomal storage disease afflicting 1 in 40,000 males with chronic pain, vascular degeneration, cardiac impairment, and other symptoms. Deficiency in the lysosomal enzyme alpha-galactosidase (alpha-GAL) causes an accumulation of its substrate, which ultimately leads to Fabry disease symptoms. Here, we present the structure of the human alpha-GAL glycoprotein determined by X-ray crystallography. The structure is a homodimer with each monomer containing a (beta/alpha)8 domain with the active site and an antiparallel beta domain. N-linked carbohydrate appears at six sites in the glycoprotein dimer, revealing the basis for lysosomal transport via the mannose-6-phosphate receptor. To understand how the enzyme cleaves galactose from glycoproteins and glycolipids, we also determined the structure of the complex of alpha-GAL with its catalytic product. The catalytic mechanism of the enzyme is revealed by the location of two aspartic acid residues (D170 and D231), which act as a nucleophile and an acid/base, respectively. As a point mutation in alpha-GAL can lead to Fabry disease, we have catalogued and plotted the locations of 245 missense and nonsense mutations in the three-dimensional structure. The structure of human alpha-GAL brings Fabry disease into the realm of molecular diseases, where insights into the structural basis of the disease phenotypes might help guide the clinical treatment of patients.

Analysis of splice-site mutations of the alpha-galactosidase A gene in Fabry disease

Fabry disease is an X-linked disease caused by a defective lysosomal enzyme, alpha-galactosidase A, and characterized by skin lesions and multiorgan involvement, including kidney, heart, and the central nervous system. Currently more than 200 genotypes have been identified, including several aberrant splicing. However, most of the mutation analyses were performed using genomic sequencing only, and therefore some of the splicing mutations were misclassified as missense mutations. In order to predict the splicing event caused by each mutation, we conducted a literature search for all published mutations located near the splice sites, including exonic point mutations, and performed a splice-site score (SSS) analysis. The literature search identified 13 donor-site mutations, including four exonic mutations (S65T, D183S, K213N, and M267I), located at the end of exons 1, 3, 4, and 5, respectively, six acceptor-site mutations, and one new exon creation. All mutated splice sites, except for the one associated with the new exon creation, had a lower SSS than their respective natural sites. Cryptic or newly created sites were identified with SSS from 0.09 to 1.0. The predictions, based on SSS analysis, are in agreement with all six mutations with known cDNA sequence from the literature, including five mutations with exon skipping and one mutation with creation of a new acceptor site. For the S65T genotype, we performed reverse transcription-polymerase chain reaction (RT-PCR) analysis using RNA isolated from the whole-blood sample. We verified that a weak cryptic site (SSS = 0.09) 14 nucleotides downstream was activated and resulted in an insertion of 14 bp and a frameshift stop at codon 106. This change is more consistent with the clinical presentation of the patient, the classical Fabry disease, than the amino acid substitution (S65T), which does not affect the enzyme function. In conclusion, the SSS analysis is very useful for predicting splicing events and genotype/phenotype correlation in Fabry disease. As different mechanisms may be involved in pre-mRNA splicing, it is important to obtain cDNA sequencing for molecular diagnosis.