Screening for pharmacological chaperones in Fabry disease

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.

Enzyme Replacement Therapy for Genetic Disorders Associated with Enzyme Deficiency

Mutations in human genes might lead to loss of functional proteins, causing diseases. Among these genetic disorders, a large class is associated with the deficiency in metabolic enzymes, resulting in both an increase in the concentration of substrates and a loss in the metabolites produced by the catalyzed reactions. The identification of therapeutic actions based on small molecules represents a challenge to medicinal chemists because the target is missing. Alternative approaches are biology-based, ranging from gene and stem cell therapy, CRISPR/Cas9 technology, distinct types of RNAs, and enzyme replacement therapy (ERT). This review will focus on the latter approach that since the 1990s has been successfully applied to cure many rare diseases, most of them being lysosomal storage diseases or metabolic diseases. So far, a dozen enzymes have been approved by FDA/EMA for lysosome storage disorders and only a few for metabolic diseases. Enzymes for replacement therapy are mainly produced in mammalian cells and some in plant cells and yeasts and are further processed to obtain active, highly bioavailable, less degradable products. Issues still under investigation for the increase in ERT efficacy are the optimization of enzymes interaction with cell membrane and internalization, the reduction in immunogenicity, and the overcoming of blood-brain barrier limitations when neuronal cells need to be targeted. Overall, ERT has demonstrated its efficacy and safety in the treatment of many genetic rare diseases, both saving newborn lives and improving patients' life quality, and represents a very successful example of targeted biologics.

Misfolding of Lysosomal α-Galactosidase a in a Fly Model and Its Alleviation by the Pharmacological Chaperone Migalastat

Fabry disease, an X-linked recessive lysosomal disease, results from mutations in the GLA gene encoding lysosomal α-galactosidase A (α-Gal A). Due to these mutations, there is accumulation of globotriaosylceramide (GL-3) in plasma and in a wide range of cells throughout the body. Like other lysosomal enzymes, α-Gal A is synthesized on endoplasmic reticulum (ER) bound polyribosomes, and upon entry into the ER it undergoes glycosylation and folding. It was previously suggested that α-Gal A variants are recognized as misfolded in the ER and undergo ER-associated degradation (ERAD). In the present study, we used Drosophila melanogaster to model misfolding of α-Gal A mutants. We did so by creating transgenic flies expressing mutant α-Gal A variants and assessing development of ER stress, activation of the ER stress response and their relief with a known α-Gal A chaperone, migalastat. Our results showed that the A156V and the A285D α-Gal A mutants underwent ER retention, which led to activation of unfolded protein response (UPR) and ERAD. UPR could be alleviated by migalastat. When expressed in the fly’s dopaminergic cells, misfolding of α-Gal A and UPR activation led to death of these cells and to a shorter life span, which could be improved, in a mutation-dependent manner, by migalastat.

Screening for Fabry Disease in patients with unexplained left ventricular hypertrophy

Fabry Disease (FD) is a systemic disorder that can result in cardiovascular, renal, and neurovascular disease leading to reduced life expectancy. FD should be considered in the differential of all patients with unexplained left ventricular hypertrophy (LVH). We therefore performed a prospective screening study in Edmonton and Hong Kong using Dried Blood Spot (DBS) testing on patients with undiagnosed LVH. Participants found to have unexplained LVH on echocardiography were invited to participate and subsequently subjected to DBS testing. DBS testing was used to measure α-galactosidase (α-GAL) enzyme activity and for mutation analysis of the α-galactosidase (GLA) gene, both of which are required to make a diagnosis of FD. DBS testing was performed as a screening tool on patients (n = 266) in Edmonton and Hong Kong, allowing for detection of five patients with FD (2% prevalence of FD) and one patient with hydroxychloroquine-induced phenocopy. Left ventricular mass index (LVMI) by GLA genotype showed a higher LVMI in patients with IVS4 + 919G > A mutations compared to those without the mutation. Two patients were initiated on ERT and hydroxychloroquine was discontinued in the patient with a phenocopy of FD. Overall, we detected FD in 2% of our screening cohort using DBS testing as an effective and easy to administer screening tool in patients with unexplained LVH. Utilizing DBS testing to screen for FD in patients with otherwise undiagnosed LVH is clinically important due to the availability of effective therapies and the value of cascade screening in extended families.

Protein Misfolding Diseases and Therapeutic Approaches

Protein folding is the process by which a polypeptide chain acquires its functional, native 3D structure. Protein misfolding, on the other hand, is a process in which protein fails to fold into its native functional conformation. This misfolding of proteins may lead to precipitation of a number of serious diseases such as Cystic Fibrosis (CF), Alzheimer's Disease (AD), Parkinson's Disease (PD), and Amyotrophic Lateral Sclerosis (ALS) etc. Protein Quality-control (PQC) systems, consisting of molecular chaperones, proteases and regulatory factors, help in protein folding and prevent its aggregation. At the same time, PQC systems also do sorting and removal of improperly folded polypeptides. Among the major types of PQC systems involved in protein homeostasis are cytosolic, Endoplasmic Reticulum (ER) and mitochondrial ones. The cytosol PQC system includes a large number of component chaperones, such as Nascent-polypeptide-associated Complex (NAC), Hsp40, Hsp70, prefoldin and T Complex Protein-1 (TCP-1) Ring Complex (TRiC). Protein misfolding diseases caused due to defective cytosolic PQC system include diseases involving keratin/collagen proteins, cardiomyopathies, phenylketonuria, PD and ALS. The components of PQC system of Endoplasmic Reticulum (ER) include Binding immunoglobulin Protein (BiP), Calnexin (CNX), Calreticulin (CRT), Glucose-regulated Protein GRP94, the thiol-disulphide oxidoreductases, Protein Disulphide Isomerase (PDI) and ERp57. ER-linked misfolding diseases include CF and Familial Neurohypophyseal Diabetes Insipidus (FNDI). The components of mitochondrial PQC system include mitochondrial chaperones such as the Hsp70, the Hsp60/Hsp10 and a set of proteases having AAA+ domains similar to the proteasome that are situated in the matrix or the inner membrane. Protein misfolding diseases caused due to defective mitochondrial PQC system include medium-chain acyl-CoA dehydrogenase (MCAD)/Short-chain Acyl-CoA Dehydrogenase (SCAD) deficiency diseases, hereditary spastic paraplegia. Among therapeutic approaches towards the treatment of various protein misfolding diseases, chaperones have been suggested as potential therapeutic molecules for target based treatment. Chaperones have been advantageous because of their efficient entry and distribution inside the cells, including specific cellular compartments, in therapeutic concentrations. Based on the chemical nature of the chaperones used for therapeutic purposes, molecular, chemical and pharmacological classes of chaperones have been discussed.

Correlation between GLA variants and alpha-Galactosidase A profile in dried blood spot: an observational study in Brazilian patients

BACKGROUND

Fabry disease is a rare X-linked inherited disorder caused by deficiency of α-Galactosidase A. Hundreds of mutations and non-coding haplotypes in the GLA gene have been described; however, many are variants of unknown significance, prompting doubts about the diagnosis and treatment. The α-Galactosidase A enzymatic activity in dried blood spot (DBS) samples are widely used for screening purposes; however, even when values below the normal are found, new tests are required to confirm the diagnosis. Here we describe an analysis of GLA variants and their correlation with DBS α-Galactosidase A enzymatic activity in a large Brazilian population with Fabry disease symptoms.

RESULTS

We analyzed GLA variants by DNA sequencing of 803 male patients with suspected Fabry disease or belonging to high-risk populations; in 179 individuals, 58 different exonic variants were detected. From these, 50 are variants described as pathogenic and eight described as variants of unknown significance. The other individuals presented complex non-coding haplotypes or had no variants. Interestingly, the enzymatic activity in DBS was different among pathogenic variants and the other genotypes, including variants of unknown significance; the first presented mean of 12% of residual activity, while the others presented levels above 70% of the activity found in healthy controls.

CONCLUSION

The activity of α-Galactosidase A in DBS was markedly reduced in males with known pathogenic variants when compared with subjects presenting variants of unknown significance, non-coding haplotypes, or without variants, indicating a possible non-pathogenic potential of these latter genotypes. These findings bring a better understanding about the biochemical results of α-Galactosidase A in DBS samples, as well as the possible non-pathogenic potential of non-coding haplotypes and variants of unknown significance in GLA gene. These results certainly will help clinicians to decide about the treatment of patients carrying variants in the gene causing this rare but life-threatening disease.

Pre-diagnosing and managing patients with GM1 gangliosidosis and related disorders by the evaluation of GM1 ganglioside content

GM1 ganglioside, a monosialic glycosphingolipid and a crucial component of plasma membranes, accumulates in lysosomal storage disorders, primarily in GM1 gangliosidosis. The development of biomarkers for simplifying diagnosis, monitoring disease progression and evaluating drug therapies is an important objective in research into neurodegenerative lysosomal disorders. With this in mind, we established fluorescent imaging and flow-cytometric methods to track changes in GM1 ganglioside levels in patients with GM1 gangliosidosis and in control cells. We also evaluated GM1 ganglioside content in patients’ cells treated with the commercially available Miglustat, a substrate inhibitor potentially suitable for the treatment of late-onset GM1 gangliosidosis. The flow-cytometric method proved to be sensitive, unbiased, and rapid in determining variations in GM1 ganglioside content in human lymphocytes derived from small amounts of fresh blood. We detected a strong correlation between GM1 ganglioside content and the clinical severity of GM1 gangliosidosis. We confirm the ability of Miglustat to act as a substrate reduction agent in the patients’ treated cells. As well as being suitable for diagnosing and managing patients with GM1 gangliosidosis this method could be useful in the diagnosis and management of other lysosomal diseases, such as galactosialidosis, Type C Niemann-Pick, and any other disease with pathologic variations of GM1 ganglioside.

Inter-assay variability influences migalastat amenability assessments among Fabry disease variants

Fabry disease is a lysosomal storage disorder caused by mutations in the GLA gene that encodes for the lysosomal enzyme α-galactosidase A (α-Gal A). Reduced or absent α-Gal A activity leads to substrate accumulation and deleterious effects in multiple organs. Migalastat is a pharmacological chaperone that may stabilize the enzyme in specific GLA variants, considered amenable, assisting enzyme trafficking to lysosomes and thus increasing enzyme activity. Using a good laboratory practice (GLP)-validated human embryonic kidney cell (HEK)-based (GLP-HEK) amenability assay established during the clinical development of migalastat, approximately one-third of GLA variants are reported to be amenable to migalastat. On the basis of this biochemical amenability, migalastat is approved for use in patients with specific GLA variants. In this study, the reproducibility of the amenability assay was assessed by evaluation of 59 GLA variants for α-Gal A activity in the presence and absence of migalastat. As for the GLP-HEK assay, variants were considered amenable when there was both an absolute increase in enzyme activity of ≥3% wild-type and a relative increase in enzyme activity ≥1.2 fold over baseline following incubation with migalastat. Six of the 59 variants tested here did not match the classification of amenability reported using the GLP-HEK assay. Linear regression and Bland-Altman analyses, comparing data from all variants with and without migalastat, provided additional evidence for a lack of assay reproducibility. Data from the GLP-HEK assay (and the resulting classification of amenability) can determine treatment strategy and, ultimately, patient outcomes, so discrepancies between amenability assay data could be a cause for concern for physicians managing patients with Fabry disease.

In Vitro Enzyme Measurement to Test Pharmacological Chaperone Responsiveness in Fabry and Pompe Disease

The use of personalized medicine to treat rare monogenic diseases like lysosomal storage disorders (LSDs) is challenged by complex clinical trial designs, high costs, and low patient numbers. Hundreds of mutant alleles are implicated in most of the LSDs. The diseases are typically classified into 2 to 3 different clinical types according to severity. Moreover, molecular characterization of the genotype can help predict clinical outcomes and inform patient care. Therefore, we developed a simple cell culture assay based on HEK293H cells heterologously over-expressing the mutations identified in Fabry and Pompe disease. A similar assay has recently been introduced as a preclinical test to identify amenable mutations for Pharmacological Chaperone Therapy (PCT) in Fabry disease. This manuscript describes an amended cell culture assay which enables rapid phenotypic assessment of allelic variants in Fabry and Pompe disease to identify eligible patients for PCT and may aid in the development of novel pharmacochaperones.

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.

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.

The validation of pharmacogenetics for the identification of Fabry patients to be treated with migalastat

Purpose:

Fabry disease is an X-linked lysosomal storage disorder caused by mutations in the α-galactosidase A gene. Migalastat, a pharmacological chaperone, binds to specific mutant forms of α-galactosidase A to restore lysosomal activity.

Methods:

A pharmacogenetic assay was used to identify the α-galactosidase A mutant forms amenable to migalastat. Six hundred Fabry disease–causing mutations were expressed in HEK-293 (HEK) cells; increases in α-galactosidase A activity were measured by a good laboratory practice (GLP)-validated assay (GLP HEK/Migalastat Amenability Assay). The predictive value of the assay was assessed based on pharmacodynamic responses to migalastat in phase II and III clinical studies.

Results:

Comparison of the GLP HEK assay results in in vivo white blood cell α-galactosidase A responses to migalastat in male patients showed high sensitivity, specificity, and positive and negative predictive values (≥0.875). GLP HEK assay results were also predictive of decreases in kidney globotriaosylceramide in males and plasma globotriaosylsphingosine in males and females. The clinical study subset of amenable mutations (n = 51) was representative of all 268 amenable mutations identified by the GLP HEK assay.

Conclusion:

The GLP HEK assay is a clinically validated method of identifying male and female Fabry patients for treatment with migalastat.

Genet Med19 4, 430–438.

Alpha-Galactosidase A p.A143T, a non-Fabry disease-causing variant

BACKGROUND

Fabry disease (FD) is an X-linked multisystemic disorder with a heterogeneous phenotype. Especially atypical or late-onset type 2 phenotypes present a therapeutical dilemma.

METHODS

To determine the clinical impact of the alpha-Galactosidase A (GLA) p.A143T/ c.427G > A variation, we retrospectively analyzed 25 p.A143T patients in comparison to 58 FD patients with other missense mutations.

RESULTS

p.A143T patients suffering from stroke/ transient ischemic attacks had slightly decreased residual GLA activities, and/or increased lyso-Gb3 levels, suspecting FD. However, most male p.A143T patients presented with significant residual GLA activity (~50 % of reference), which was associated with normal lyso-Gb3 levels. Additionally, p.A143T patients showed less severe FD-typical symptoms and absent FD-typical renal and cardiac involvement in comparison to FD patients with other missense mutations. Two tested female p.A143T patients with stroke/TIA did not show skewed X chromosome inactivation. No accumulation of neurologic events in family members of p.A143T patients with stroke/transient ischemic attacks was observed.

CONCLUSIONS

We conclude that GLA p.A143T seems to be most likely a neutral variant or a possible modifier instead of a disease-causing mutation. Therefore, we suggest that p.A143T patients with stroke/transient ischemic attacks of unknown etiology should be further evaluated, since the diagnosis of FD is not probable and subsequent ERT or chaperone treatment should not be an unreflected option.

Coformulation of a Novel Human α-Galactosidase A With the Pharmacological Chaperone AT1001 Leads to Improved Substrate Reduction in Fabry Mice

Fabry disease is an X-linked lysosomal storage disorder caused by mutations in the gene that encodes α-galactosidase A and is characterized by pathological accumulation of globotriaosylceramide and globotriaosylsphingosine. Earlier, the authors demonstrated that oral coadministration of the pharmacological chaperone AT1001 (migalastat HCl; 1-deoxygalactonojirimycin HCl) prior to intravenous administration of enzyme replacement therapy improved the pharmacological properties of the enzyme. In this study, the authors investigated the effects of coformulating AT1001 with a proprietary recombinant human α-galactosidase A (ATB100) into a single intravenous formulation. AT1001 increased the physical stability and reduced aggregation of ATB100 at neutral pH in vitro, and increased the potency for ATB100-mediated globotriaosylceramide reduction in cultured Fabry fibroblasts. In Fabry mice, AT1001 coformulation increased the total exposure of active enzyme, and increased ATB100 levels in cardiomyocytes, cardiac vascular endothelial cells, renal distal tubular epithelial cells, and glomerular cells, cell types that do not show substantial uptake with enzyme replacement therapy alone. Notably, AT1001 coformulation also leads to greater tissue globotriaosylceramide reduction when compared with ATB100 alone, which was positively correlated with reductions in plasma globotriaosylsphingosine. Collectively, these data indicate that intravenous administration of ATB100 coformulated with AT1001 may provide an improved therapy for Fabry disease and thus warrants further investigation.

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.

Unfolding the Therapeutic Potential of Chemical Chaperones for Age-related Macular Degeneration

Recent studies suggest that pathological processes involved in age-related macular degeneration (AMD) might induce endoplasmic reticulum (ER) stress. Growing evidence demonstrates the ability of chemical chaperones to decrease ER stress and ameliorate ER stress-related disease phenotypes, suggesting that the field of chaperone therapy might hold novel treatments for AMD. In this review, we examine the evidence suggesting a role for ER stress in AMD. Furthermore, we discuss the use of chaperone therapy for the treatment of ER stress-associated diseases, including other neurodegenerative diseases and retinopathies. Finally, we examine strategies for identifying potential chaperone compounds and for experimentally demonstrating chaperone activity in in vitro and in vivo models of human disease.

A Phase 2 study of migalastat hydrochloride in females with Fabry disease: selection of population, safety and pharmacodynamic effects

BACKGROUND

Fabry disease (FD) is a genetic disorder resulting from deficiency of the lysosomal enzyme α-galactosidase A (α-Gal A) which leads to globotriaosylceramide (GL-3) accumulation in multiple tissues. We report on the safety and pharmacodynamics of migalastat hydrochloride, an investigational pharmacological chaperone given orally every other day (QOD) to females with FD.

METHODS

This was an open-label, uncontrolled, Phase 2 study of 12 weeks with extension to 48 weeks in nine females with FD. Doses of 50mg, 150 mg and 250 mg were given QOD. At multiple time points, α-Gal A activity and GL-3 levels were quantified in blood cells, kidney and skin. GL-3 levels were also evaluated through skin and renal histology. Each individual GLA mutation was retrospectively categorized as being amenable or not to migalastat HCl based on an in vitro α-Gal A transfection assay developed in human embryonic kidney (HEK)-293 cells.

RESULTS

Migalastat HCl was generally well tolerated. Patients with amenable mutations seem to demonstrate greater pharmacodynamic response to migalastat HCl compared to patients with non-amenable mutations. The greatest declines in urine GL-3 were observed in the three patients with amenable GLA mutations that were treated with 150 or 250 mg migalastat HCl QOD. Additionally, these three patients all demonstrated decreases in GL-3 inclusions in kidney peri-tubular capillaries.

CONCLUSIONS

Migalastat HCl is a candidate oral pharmacological chaperone that provides a potential novel genotype-specific treatment for FD. Treatment resulted in GL-3 substrate decrease in female patients with amenable GLA mutations. Phase 3 studies are ongoing.

Migalastat HCl reduces globotriaosylsphingosine (lyso-Gb3) in Fabry transgenic mice and in the plasma of Fabry patients

Fabry disease (FD) results from mutations in the gene (GLA) that encodes the lysosomal enzyme α-galactosidase A (α-Gal A), and involves pathological accumulation of globotriaosylceramide (GL-3) and globotriaosylsphingosine (lyso-Gb₃). Migalastat hydrochloride (GR181413A) is a pharmacological chaperone that selectively binds, stabilizes, and increases cellular levels of α-Gal A. Oral administration of migalastat HCl reduces tissue GL-3 in Fabry transgenic mice, and in urine and kidneys of some FD patients. A liquid chromatography-tandem mass spectrometry method was developed to measure lyso-Gb₃ in mouse tissues and human plasma. Oral administration of migalastat HCl to transgenic mice reduced elevated lyso-Gb₃ levels up to 64%, 59%, and 81% in kidney, heart, and skin, respectively, generally equal to or greater than observed for GL-3. Furthermore, baseline plasma lyso-Gb₃ levels were markedly elevated in six male FD patients enrolled in Phase 2 studies. Oral administration of migalastat HCl (150 mg QOD) reduced urine GL-3 and plasma lyso-Gb₃ in three subjects (range: 15% to 46% within 48 weeks of treatment). In contrast, three showed no reductions in either substrate. These results suggest that measurement of tissue and/or plasma lyso-Gb₃ is feasible and may be warranted in future studies of migalastat HCl or other new potential therapies for FD.

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.

Endoplasmic Reticulum Stress and Unfolded Protein Response Pathways: Potential for Treating Age-related Retinal Degeneration

Accumulation of misfolded proteins in the endoplasmic reticulum (ER) and their aggregation impair normal cellular function and can be toxic, leading to cell death. Prolonged expression of misfolded proteins triggers ER stress, which initiates a cascade of reactions called the unfolded protein response (UPR). Protein misfolding is the basis for a variety of disorders known as ER storage or conformational diseases. There are an increasing number of eye disorders associated with misfolded proteins and pathologic ER responses, including retinitis pigmentosa (RP). Herein we review the basic cellular and molecular biology of UPR with focus on pathways that could be potential targets for treating retinal degenerative diseases.

A pharmacogenetic approach to identify mutant forms of α-galactosidase A that respond to a pharmacological chaperone for Fabry disease

Fabry disease is caused by mutations in the gene (GLA) that encodes α-galactosidase A (α-Gal A). The iminosugar AT1001 (GR181413A, migalastat hydrochloride, 1-deoxygalactonojirimycin) is a pharmacological chaperone that selectively binds and stabilizes α-Gal A, increasing total cellular levels and activity for some mutant forms (defined as “responsive”). In this study, we developed a cell-based assay in cultured HEK-293 cells to identify mutant forms of α-Gal A that are responsive to AT1001. Concentration-dependent increases in α-Gal A activity in response to AT1001 were shown for 49 (60%) of 81 mutant forms. The responses of α-Gal A mutant forms were generally consistent with the responses observed in male Fabry patient-derived lymphoblasts. Importantly, the HEK-293 cell responses of 19 α-Gal A mutant forms to a clinically achievable concentration of AT1001 (10 µM) were generally consistent with observed increases in α-Gal A activity in peripheral blood mononuclear cells from male Fabry patients orally administered AT1001 during Phase 2 clinical studies. This indicates that the cell-based responses can identify mutant forms of α-Gal A that are likely to respond to AT1001 in vivo. Thus, the HEK-293 cell-based assay may be a useful aid in the identification of Fabry patients with AT1001-responsive mutant forms. Hum Mutat 32:1–13, 2011. © 2011 Wiley-Liss, Inc.

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.

High throughput screening for inhibitors of alpha-galactosidase

Fabry disease is a rare X-linked lysosomal storage disorder caused by a deficiency in α-galactosidase A (GLA), which catalyzes the hydrolysis of terminal α-galactosyl groups from glycosphingolipids, such as globotriaosylceramide (Gb3). Many of the mutations in the GLA gene are missense alterations that cause misfolding, decreased stability, and/or mistrafficking of this protein. Small molecule compounds that correct the misfolding and mistrafficking, or activate the mutant enzyme, may be useful in the treatment of Fabry disease. We have screened a library of approximately 230,000 compounds using preparations of human recombinant protein and purified coffee bean enzyme in an effort to find activators and inhibitors of this enzyme. Lansoprazole was identified as a small molecule inhibitor of GLA derived from coffee beans (IC₅₀ = 6.4 μM), but no inhibitors or activators were identified for the human enzyme. The screening results indicate that human GLA is a difficult target for small molecule inhibition or activation.

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.

Functional studies of new GLA gene mutations leading to conformational Fabry disease

Fabry Disease (FD) is an X-linked multisystemic lysosomal disorder caused by mutations of alpha-galactosidase (GLA) gene. Only a few of the 450 genetic lesions identified so far have been characterised by in vitro expression studies. Thus the significance of newly identified GLA nucleotide variants in FD patients which lead to alpha-galactosidase (GAL-A) amino acid substitutions or intronic changes can be uncertain. We identified three GLA mutations, c.155G>A (p.C52Y), c.548G>C (p.G183A), c.647A>G (p.Y216C) in as many individuals (two male; one female) and performed in vitro expression studies and Western blot analysis in order to clarify their functional effects. Reduced GAL-A activity and normal or partially reduced mutant proteins were present in all overexpressed mutant systems in which three-dimensional structural analysis showed that the active site was not directly involved. We hypothesize that the three new mutations affect the GAL-A protein, leading to conformational FD. When mutant proteins overexpressed in COS-1 cells and in patients' lymphocytes were tested in the presence of the 1-deoxygalactonojirimicin (DGJ) chaperone, the p.G183A and p.Y216C systems showed increased GAL-A enzyme activities and protein stabilisation while p.C52Y was not responsive. We underline that genetic, biochemical and functional studies are helpful in clarifying the consequences of the missense genetic lesions detected in FD. ERT is the elective therapy for Fabry patients, but it is not always possible to issue the enzyme's active form in all involved organs. Our study endorses the hypothesis that an active site-specific chemical chaperone, which could be administered orally, might be effective in treating GAL-A conformational defects.

The pharmacological chaperone 1-deoxygalactonojirimycin reduces tissue globotriaosylceramide levels in a mouse model of Fabry disease

Fabry disease is an X-linked lysosomal storage disorder caused by a deficiency in alpha-galactosidase A (alpha-Gal A) activity and subsequent accumulation of the substrate globotriaosylceramide (GL-3), which contributes to disease pathology. The pharmacological chaperone (PC) DGJ (1-deoxygalactonojirimycin) binds and stabilizes alpha-Gal A, increasing enzyme levels in cultured cells and in vivo. The ability of DGJ to reduce GL-3 in vivo was investigated using transgenic (Tg) mice that express a mutant form of human alpha-Gal A (R301Q) on a knockout background (Tg/KO), which leads to GL-3 accumulation in disease-relevant tissues. Four-week daily oral administration of DGJ to Tg/KO mice resulted in significant and dose-dependent increases in alpha-Gal A activity, with concomitant GL-3 reduction in skin, heart, kidney, brain, and plasma; 24-week administration resulted in even greater reductions. Compared to daily administration, less frequent DGJ administration, including repeated cycles of 4 days with DGJ followed by 3 days without or every other day with DGJ, resulted in even greater GL-3 reductions that were comparable to those obtained with Fabrazyme. Collectively, these data indicate that oral administration of DGJ increases mutant alpha-Gal A activity and reduces GL-3 in disease-relevant tissues in Tg/KO mice, and thus merits further evaluation as a treatment for Fabry disease.

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.

The pharmacological chaperone 1-deoxygalactonojirimycin increases alpha-galactosidase A levels in Fabry patient cell lines

Fabry disease is an X-linked lysosomal storage disorder caused by mutations in the gene encoding alpha-galactosidase A (alpha-Gal A), with consequent accumulation of its major glycosphingolipid substrate, globotriaosylceramide (GL-3). Over 500 Fabry mutations have been reported; approximately 60% are missense. The iminosugar 1-deoxygalactonojirimycin (DGJ, migalastat hydrochloride, AT1001) is a pharmacological chaperone that selectively binds alpha-Gal A, increasing physical stability, lysosomal trafficking, and cellular activity. To identify DGJ-responsive mutant forms of alpha-Gal A, the effect of DGJ incubation on alpha-Gal A levels was assessed in cultured lymphoblasts from males with Fabry disease representing 75 different missense mutations, one insertion, and one splice-site mutation. Baseline alpha-Gal A levels ranged from 0 to 52% of normal. Increases in alpha-Gal A levels (1.5- to 28-fold) after continuous DGJ incubation for 5 days were seen for 49 different missense mutant forms with varying EC(50) values (820 nmol/L to >1 mmol/L). Amino acid substitutions in responsive forms were located throughout both structural domains of the enzyme. Half of the missense mutant forms associated with classic (early-onset) Fabry disease and a majority (90%) associated with later-onset Fabry disease were responsive. In cultured fibroblasts from males with Fabry disease, the responses to DGJ were comparable to those of lymphoblasts with the same mutation. Importantly, elevated GL-3 levels in responsive Fabry fibroblasts were reduced after DGJ incubation, indicating that increased mutant alpha-Gal A levels can reduce accumulated substrate. These data indicate that DGJ merits further evaluation as a treatment for patients with Fabry disease with various missense mutations.

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

Fabry disease is an X-linked inborn error of glycosphingolipid catabolism that results from mutations in the gene encoding the alpha-galactosidase A (GLA) enzyme. We have identified 15 distinct mutations in the GLA gene in 13 unrelated patients with classic Fabry disease and 2 unrelated patients with atypical Fabry disease. Two of the identified mutations were novel (i.e., the D231G missense mutation and the L268delfsX1 deletion mutation). This study evaluated the effects of the chemical chaperones 1-deoxygalactonojirimycin (DGJ) on the function of GLA in vitro, in cells containing missense mutations in the GLA gene. Nine missense and a nonsense mutations, including one novel mutation were cloned into mammalian expression vectors. After transient expression in COS-7 cells, GLA enzyme activity and protein expression were analyzed using fluorescence spectrophotometry and Western blot analysis, respectively. DGJ enhanced GLA enzyme activity in the M42V, I91T, R112C and F113L mutants. Interestingly, the I91T and F113L mutations are associated with the atypical form of Fabry disease. However, DGJ treatment did not have any significant effect on the GLA enzyme activity and protein expression of other mutants, including C142W, D231G, D266N, and S297F. Of note, GLA enzyme activity was not detected in the novel mutant (i.e., D231G), although protein expression was similar to the wild type. In the absence of DGJ, the E66Q mutant had wild-type levels of GLA protein expression and approximately 40% GLA activity, indicating that E66Q is either a mild mutation or a functional single nucleotide polymorphism (SNP). Thus, the results of this study suggest that the chemical chaperone DGJ enhances GLA enzyme activity and protein expression in milder mutations associated with the atypical form of Fabry disease.

Preclinical efficacy and safety of 1-deoxygalactonojirimycin in mice for Fabry disease

Fabry disease is an inborn error of glycosphingolipid metabolism caused by deficiency of alpha-galactosidase A (alpha-Gal A) activity. It has been shown that protein misfolding is primarily responsible for the enzyme deficiency in a large proportion of mutations identified in Fabry patients with residual enzyme activity, and 1-deoxygalactonojirimycin (DGJ) can effectively increase the residual enzyme activity in cultured patient's cells. Herein, we demonstrate the preclinical efficacy and safety of DGJ in transgenic mice that express human mutant alpha-Gal A activity. alpha-Gal A activity in heart, kidney, spleen, and liver was increased dose- and time-dependently. The mutant alpha-Gal A was increased in cardiomyocytes and distal convoluted tubules of the transgenic mice in a null background after 2 weeks of DGJ treatment. Globotriaosylceramide storage was remarkably reduced in kidney of mice after a 4-week treatment at a dosage of approximately 3 mg/kg body weight/day. The half-life of DGJ was less than 1 day in all major issues and that of the enzyme synthesized during the DGJ treatment period was approximately 4 days. No abnormality of blood chemistry and pathological tissue damage was found in mice treated with DGJ at approximately 30 mg/kg body weight/day for 9 weeks. Furthermore, no change was observed in appearance, growth, fertility, and life span in mice during a 2-year period of continuous administration of DGJ at the effective dosage. These preclinical results indicate that DGJ is effective in restoring mutant enzyme activity in tissues and reversing substrate storage in kidney and is well tolerated in mice.

Fabry's disease

Fabry's disease is an X-linked lysosomal storage disorder caused by abnormalities in the GLA gene, which leads to a deficiency in alpha-galactosidase A. The consequent abnormal accumulation of glycosphingolipids results in several clinical signs and symptoms and substantial morbidity and mortality. This review covers all basic aspects of the disease such as epidemiology, pathophysiology, clinical presentation by systems, diagnosis, management, prevention, and repercussions on quality of life. With the development of enzyme replacement therapy in the past few years, early initiation of treatment is key for improvement in major affected organs with decreased cardiac mass and stabilisation of kidney function, and improvement in neuropathic pain, sweating, gastrointestinal symptoms, hearing loss, and pulmonary symptoms. However, treatment of individual symptoms in addition to enzyme replacement therapy seems to be needed for many patients, especially those who have had some degree of organ dysfunction. Additional data are needed to document long-term treatment outcomes.

Prediction of response of mutated alpha-galactosidase A to a pharmacological chaperone

OBJECTIVE

To examine the relationship between types and locations of mutations of the enzyme alpha-galactosidase (Gal) A in Fabry disease and the response to the pharmacological chaperone 1-deoxygalactonojirimycin (DGJ).

METHODS

T cells grown from normal individuals or from patients with Fabry disease were tested for response to treatment with DGJ by increased activity of alpha-Gal A.

RESULTS

Cells from normal controls responded with a 28% increase in alpha-Gal A activity, whereas response in Fabry individuals was mutation dependent ranging from no increase to fully normal activity. Nine truncation mutations (all nonresponsive) and 31 missense mutations were tested. Three groups of missense mutations were categorized: responders with activity more than 25% of normal, nonresponders, with less than 7% and an intermediate response group. In normal cells and in responders an increase in the mature lysosomal form of alpha-Gal A was observed after DGJ treatment. Nonresponders showed little or no protein with or without DGJ. The intermediate response group showed an increase in band intensity but incomplete processing of the enzyme to the mature form.

CONCLUSION

Mapping the missense mutations to the structure of alpha-Gal A identified several factors that may influence response. Mutations in regions that are not in alpha-helix or beta-sheets, neither involved in disulfide bonds nor with an identified functional or structural role were more likely to respond. Predictability is, however, not precise and testing of each mutation for response to pharmacological chaperone therapy is necessary for Fabry disease and related lysosomal storage disorders.

Unbalanced GLA mRNAs ratio quantified by real-time PCR in Fabry patients' fibroblasts results in Fabry disease

Total or partial deficiency of the human lysosomal hydrolase alpha-galactosidase A is responsible for Fabry disease, the X-linked inborn error of glycosphingolipid metabolism. Together with the predominant alpha-galactosidase A gene mRNA product encoding the lysosomal enzyme, a weakly regulated alternatively spliced alpha-galactosidase A transcript is expressed in normal tissues, but its overexpression, due to the intronic g.9331G>A mutation, leads to the cardiac variant. We report the molecular characterization of five Fabry patients including two siblings. Sequencing analysis of the alpha-galactosidase A gene coding region and intron/exon boundaries identified the new c.124A>G (p.M42V) genetic lesion as well as a known deletion in three patients, whereas in the two remaining patients, no mutations were identified. To evaluate possible alpha-galactosidase A gene transcription alterations, both predominant and alternatively spliced mRNAs were quantified by absolute real-time PCR on total RNA preparations from the patients' fibroblasts. An impressive reduction in the predominant alpha-galactosidase A transcript was detected in the last patients (Pt 4 and Pt 5). However, the alternatively spliced mRNA was dramatically overexpressed in one of them, carrying a new intronic lesion (g.9273C>T). These findings strongly suggest a correlation between this new intronic mutation and the unbalanced alpha-galactosidase A mRNAs ratio, which could therefore be responsible for the reduced enzyme activity causing Fabry disease. The real-time assay developed here to investigate the two alpha-galactosidase A mRNAs might play a crucial role in revealing possible genetic lesions and in confirming the pathogenetic mechanisms underlying 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.