Characterization of two alpha-galactosidase mutants (Q279E and R301Q) found in an atypical variant of Fabry disease

Fabry Disease and the Heart: A Comprehensive Review

Fabry disease (FD) is an X-linked lysosomal storage disorder caused by mutations of the GLA gene that result in a deficiency of the enzymatic activity of α-galactosidase A and consequent accumulation of glycosphingolipids in body fluids and lysosomes of the cells throughout the body. GB3 accumulation occurs in virtually all cardiac cells (cardiomyocytes, conduction system cells, fibroblasts, and endothelial and smooth muscle vascular cells), ultimately leading to ventricular hypertrophy and fibrosis, heart failure, valve disease, angina, dysrhythmias, cardiac conduction abnormalities, and sudden death. Despite available therapies and supportive treatment, cardiac involvement carries a major prognostic impact, representing the main cause of death in FD. In the last years, knowledge has substantially evolved on the pathophysiological mechanisms leading to cardiac damage, the natural history of cardiac manifestations, the late-onset phenotypes with predominant cardiac involvement, the early markers of cardiac damage, the role of multimodality cardiac imaging on the diagnosis, management and follow-up of Fabry patients, and the cardiac efficacy of available therapies. Herein, we provide a comprehensive and integrated review on the cardiac involvement of FD, at the pathophysiological, anatomopathological, laboratory, imaging, and clinical levels, as well as on the diagnosis and management of cardiac manifestations, their supportive treatment, and the cardiac efficacy of specific therapies, such as enzyme replacement therapy and migalastat.

Ion channels and pain in Fabry disease

Fabry disease (FD) is a progressive, X-linked inherited disorder of glycosphingolipid metabolism due to deficient or absent lysosomal α-galactosidase A (α-Gal A) activity which results in progressive accumulation of globotriaosylceramide (Gb3) and related metabolites. One prominent feature of Fabry disease is neuropathic pain. Accumulation of Gb3 has been documented in dorsal root ganglia (DRG) as well as other neurons, and has lately been associated with the mechanism of pain though the pathophysiology is still unclear. Small fiber (SF) neuropathy in FD differs from other entities in several aspects related to the perception of pain, alteration of fibers as well as drug therapies used in the practice with patients, with therapies far from satisfying. In order to develop better treatments, more information on the underlying mechanisms of pain is needed. Research in neuropathy has gained momentum from the development of preclinical models where different aspects of pain can be modelled and further analyzed. This review aims at describing the different in vitro and FD animal models that have been used so far, as well as some of the insights gained from their use. We focus especially in recent findings associated with ion channel alterations -that apart from the vascular alterations-, could provide targets for improved therapies in pain.

The Ckd. Qld fabRy Epidemiology (aCQuiRE) study protocol: identifying the prevalence of Fabry disease amongst patients with kidney disease in Queensland, Australia

Background

Fabry disease (FD) is a rare, lysosomal storage disorder caused by the absence or deficiency of the enzyme alpha-galactosidase A (α-Gal A) that leads to the abnormal accumulation of the lipid globotriaosylceramide (GB3) in a variety of cell types and tissues throughout the body. FD has an x-linked inheritance pattern. Previously thought to be only carriers, females can also experience FD symptomatology. Symptoms vary in type and severity from patient to patient and tend to increase in severity with age. FD symptoms are non-specific and may be shared with those of other diseases. Misdiagnoses and diagnostic delays are common, often resulting in progressive, irreversible tissue damage. The estimated prevalence of FD in the general population is 1:40,000 to 1:117,000 individuals. However, it is estimated that the prevalence of FD in the dialysis population is 0.12 to 0.7%. Little is known about the prevalence of FD in the broader Chronic Kidney Disease (CKD) population.

Methods

This is an epidemiological study of the prevalence of FD in CKD patents identified from the public renal speciality practices in Queensland, Australia. A cascade approach to screening is being employed with dried blood spot testing for blood levels of alpha-galactosidase A (Alpha-Gal), with follow-up testing for patients with abnormal results by plasma levels of globotriaosylsphingosine (Lyso-GB3) for females and non-definitive cases in males. A diagnosis of FD is confirmed through genetic testing of the GLA gene in cases suspected of having FD based upon Alpha-Gal and Lyso-GB3 testing.

Discussion

Expected outcomes of this study include more information about the prevalence of FD at all stages of CKD, including for both males and females. The study may also provide information about common characteristics of FD to assist with diagnosis and optimal management/treatment. Screening is also available for family members of diagnosed patients, with potential for early diagnosis of FD and intervention for those individuals.

Trial registration

Queensland Health Database of Research Activity (DORA, https://dora.health.qld.gov.au) pj09946 (Registered 3rd July 2017).

Rapid, proteomic urine assay for monitoring progressive organ disease in Fabry disease

BACKGROUND

Fabry disease is a progressive multisystemic disease, which affects the kidney and cardiovascular systems. Various treatments exist but decisions on how and when to treat are contentious. The current marker for monitoring treatment is plasma globotriaosylsphingosine (lyso-Gb3), but it is not informative about the underlying and developing disease pathology.

METHODS

We have created a urine proteomic assay containing a panel of biomarkers designed to measure disease-related pathology which include the inflammatory system, lysosome, heart, kidney, endothelium and cardiovascular system. Using a targeted proteomic-based approach, a series of 40 proteins for organ systems affected in Fabry disease were multiplexed into a single 10 min multiple reaction monitoring Liquid Chromatography Tandem Mass Spectrometry (LC-MS/MS) assay and using only 1 mL of urine.

RESULTS

Six urinary proteins were elevated in the early-stage/asymptomatic Fabry group compared with controls including albumin, uromodulin, α1-antitrypsin, glycogen phosphorylase brain form, endothelial protein receptor C and intracellular adhesion molecule 1. Albumin demonstrated an increase in urine and could indicate presymptomatic disease. The only protein elevated in the early-stage/asymptomatic patients that continued to increase with progressive multiorgan involvement was glycogen phosphorylase brain form. Podocalyxin, fibroblast growth factor 23, cubulin and Alpha-1-Microglobulin/Bikunin Precursor (AMBP) were elevated only in disease groups involving kidney disease. Nephrin, a podocyte-specific protein, was elevated in all symptomatic groups. Prosaposin was increased in all symptomatic groups and showed greater specificity (p<0.025-0.0002) according to disease severity.

CONCLUSION

This work indicates that protein biomarkers could be helpful and used in conjunction with plasma lyso-Gb3 for monitoring of therapy or disease progression in patients with Fabry disease.

Efficacy of the pharmacologic chaperone migalastat in a subset of male patients with the classic phenotype of Fabry disease and migalastat-amenable variants: data from the phase 3 randomized, multicenter, double-blind clinical trial and extension study

PURPOSE

Outcomes in patients with Fabry disease receiving migalastat during the phase 3 FACETS trial (NCT00925301) were evaluated by phenotype.

METHODS

Data were evaluated in two subgroups of patients with migalastat-amenable GLA variants: "classic phenotype" (n = 14; males with residual peripheral blood mononuclear cell α-galactosidase A <3% normal and multiorgan system involvement) and "other patients" (n = 36; males not meeting classic phenotype criteria and all females). Endpoints included estimated glomerular filtration rate (eGFR), left ventricular mass index (LVMi), Gastrointestinal Symptoms Rating Scale diarrhea subscale (GSRS-D), renal peritubular capillary (PTC) globotriaosylceramide (GL-3) inclusions, and plasma globotriaosylsphingosine (lyso-Gb₃).

RESULTS

Baseline measures in the classic phenotype patients suggested a more severe phenotype. At month 24, mean (SD) annualized change in eGFR_CKD-EPI with migalastat was -0.3 (3.76) mL/min/1.73 m² in the classic phenotype subgroup; changes in LVMi, GSRS-D, and lyso-Gb₃ were -16.7 (18.64) g/m², -0.9 (1.66), and -36.8 (35.78) nmol/L, respectively. At month 6, mean PTC GL-3 inclusions decreased with migalastat (-0.8) and increased with placebo (0.3); switching from placebo to migalastat, PTC inclusions decreased by -0.7. Numerically smaller changes in these endpoints were observed in the other patients.

CONCLUSION

Migalastat provided clinical benefit to patients with Fabry disease and amenable variants, regardless of disease severity.

Fabry's disease: an example of cardiorenal syndrome type 5

Cardiorenal syndrome type 5 (CRS-5) includes conditions where there is a simultaneous involvement of the heart and kidney from a systemic disorder. This is a bilateral organ cross talk. Fabry's disease (FD) is a devastating progressive inborn error of metabolism with lysosomal glycosphingolipid deposition in variety of cell types, capillary endothelial cells, renal, cardiac and nerve cells. Basic effect is absent or deficient activity of lysosomal exoglycohydrolase a-galactosidase A. Renal involvement consists of proteinuria, isosthenuria, altered tubular function, presenting in second or third decade leading to azotemia and end-stage renal disease in third to fifth decade mainly due to irreversible changes to glomerular, tubular and vascular structures, especially highlighted by podocytes foot process effacement. Cardiac involvement consists of left ventricular hypertrophy, right ventricular hypertrophy, arrhythmias (sinus node and conduction system impairment), diastolic dysfunction, myocardial ischemia, infarction, transmural replacement fibrosis, congestive heart failure and cardiac death. Management of FD is based on enzymatic replacement therapy and control of renal (with anti-proteinuric agents such as angiotensin-converting enzyme inhibitors-and/or angiotensin II receptor blockers), brain (coated aspirin, clopidogrel and statin to prevent strokes) and heart complications (calcium channel blockers for ischemic cardiomyopathy, warfarin and amiodarone or cardioverter device for arrhythmias).

Docking study and biological evaluation of pyrrolidine-based iminosugars as pharmacological chaperones for Gaucher disease

α-1-C-Alkylated 1,4-dideoxy-1,4-imino-d-arabinitols (DAB) derivatives as pharmacological chaperones for Gaucher disease.

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.

Docking and SAR studies of calystegines: binding orientation and influence on pharmacological chaperone effects for Gaucher's disease

We report on the identification of the required configuration and binding orientation of nor-tropane alkaloid calystegines against β-glucocerebrosidase. Calystegine B2 is a potent competitive inhibitor of human lysosomal β-glucocerebrosidase with Ki value of 3.3 μM. A molecular docking study revealed that calystegine B2 had a favorable van der Waals interactions (Phe128, Trp179, and Phe246) and the hydrogen bonding (Glu235, Glu340, Asp127, Trp179, Asn234, Trp381 and Asn396) was similar to that of isofagomine. All calystegine isomers bound into the same active site as calystegine B2 and the essential hydrogen bonds formed to Asp127, Glu235 and Glu340 were maintained. However, their binding orientations were obviously different. Calystegine A3 bound to β-glucocerebrosidase with the same orientations as calystegine B2 (Type 1), while calystegine B3 and B4 had different binding orientations (Type 2). It is noteworthy that Type 1 orientated calystegines B2 and A3 effectively stabilized β-glucocerebrosidase, and consequently increased intracellular β-glucocerebrosidase activities in N370S fibroblasts, while Type 2 orientated calystegines B3 and B4 could not keep the enzyme activity. These results clearly indicate that the binding orientations of calystegines are changed by the configuration of the hydroxyl groups on the nor-tropane ring and the suitable binding orientation is a requirement for achieving a strong affinity to β-glucocerebrosidase.

Intrafamilial phenotypic variability in four families with Anderson-Fabry disease

We analysed the clinical history of 16 hemizygous males affected by Anderson-Fabry Disease, from four families, to verify their intrafamilial phenotypic variability. Seven male patients, ranging from 26 to 61 years of age, died, whereas nine (age range 23-55) are alive. Eleven patients have undergone enzyme replacement therapy (ERT) for a period of 5-10 years. We have found a wide range of intrafamilial phenotypic variability in these families, both in terms of target-organs and severity of the disease. Overall, our findings confirm previous data from the literature showing a high degree of intrafamilial phenotypic variability in patients carrying the same mutation. Furthermore, our results underscore the difficulty in giving accurate prognostic information to patients during genetic counselling, both in terms of rate of disease progression and involvement of different organs, when such prognosis is solely based on the patient's family history.

Functional characterisation of alpha-galactosidase a mutations as a basis for a new classification system in fabry disease

Fabry disease (FD) is an X-linked hereditary defect of glycosphingolipid storage caused by mutations in the gene encoding the lysosomal hydrolase α-galactosidase A (GLA, α-gal A). To date, over 400 mutations causing amino acid substitutions have been described. Most of these mutations are related to the classical Fabry phenotype. Generally in lysosomal storage disorders a reliable genotype/phenotype correlation is difficult to achieve, especially in FD with its X-linked mode of inheritance. In order to predict the metabolic consequence of a given mutation, we combined in vitro enzyme activity with in vivo biomarker data. Furthermore, we used the pharmacological chaperone (PC) 1-deoxygalactonojirimycin (DGJ) as a tool to analyse the influence of individual mutations on subcellular organelle-trafficking and stability. We analysed a significant number of mutations and correlated the obtained properties to the clinical manifestation related to the mutation in order to improve our knowledge of the identity of functional relevant amino acids. Additionally, we illustrate the consequences of different mutations on plasma lyso-globotriaosylsphingosine (lyso-Gb3) accumulation in the patients' plasma, a biomarker proven to reflect the impaired substrate clearance caused by specific mutations. The established system enables us to provide information for the clinical relevance of PC therapy for a given mutant. Finally, in order to generate reliable predictions of mutant GLA defects we compared the different data sets to reveal the most coherent system to reflect the clinical situation.

Fabry disease is caused by a single gene deficiency. It is the second most common lysosomal storage disorder and the result is a build-up of glycosphingolipids in different areas of the body (kidneys, intestine, etc). It is an important consideration for clinicians in the diagnosing of stroke, kidney and cardiovascular diseases. Many symptoms of Fabry are seen in other diseases as well (both inherited and non- inherited), which makes diagnosis difficult. We observed numerous novel mutations in patients that displayed a monosymptomatic, however life-threatening course of Fabry disease. This prompted us to study and characterise those mutations with regard to their biochemical and clinical consequences. Overall, 171 Fabry mutations were considered in an overexpression system for the prediction of the clinical course of Fabry disease. Furthermore, we highlight the usefulness of the in vitro system that we developed which will help with therapeutical decisions, by testing the responsiveness of mutant enzymes to the pharmacological chaperone DGJ. This work aims to draw the attention of clinicians and researchers to milder forms of Fabry disease which might at first appear unrelated to this clinically heterogenous disease.

Chaperone therapy update: Fabry disease, GM1-gangliosidosis and Gaucher disease

Chaperone therapy is a newly developed molecular therapeutic approach to lysosomal diseases, a group of human genetic diseases causing severe brain damage. Based on early molecular studies during the last decade of the 20th century and early years of the 21st century, mainly on Fabry disease and GM1-gangliosidosis, we found some mutant enzyme proteins were unstable in the cell, and unable to express catalytic activities. Subsequently galactose and other active-site binding substrate analogs were found stabilized and enhance the mutant enzyme activity in culture cells. We concluded that the mutant misfolding enzyme protein and substrate analog competitive inhibitor (chemical chaperone) form a stable complex to be transported to the lysosome, to restore the catalytic activity of mutant enzyme after spontaneous dissociation under the acidic condition. This gene mutation-specific molecular interaction is a paradoxical phenomenon that an enzyme inhibitor in vitro serves as an enzyme stabilizer in situ. First we developed a commercially available compound 1-deoxygalactonojirimycin (DGJ) for Fabry disease, and confirmed the above molecular phenomenon. Currently DGJ has become a new candidate of oral medicine for Fabry disease, generalized vasculopathy involving the kidneys, heart and central nervous system in the middle age. This drug development has reached the phase 3 of human clinical study. Then we found two valienamine derivatives, N-octyl-4-epi-β-valienamine (NOEV) and N-octyl-β-valienamine (NOV), as promising therapeutic agents for human β-galactosidase deficiency disorders (GM1-gangliosidosis and Morquio B disease) and β-glucosidase deficiency disorders (phenotypic variations of Gaucher disease), respectively. Originally NOEV and NOV had been discovered as competitive inhibitors, and then their paradoxical bioactivities as chaperones were confirmed in cultured fibroblasts from patients with these disorders. Subsequently GM1-gangliosidosis model mice have been used for confirmation of clinical effectiveness, adverse effects and pharmacokinetic studies. Orally administered NOEV entered the brain through the blood-brain barrier, enhanced β-galactosidase activity, reduced substrate storage, and improved neurological deterioration clinically. Computational analysis revealed pH-dependent enzyme-chaperone interactions. Our recent study indicated chaperone activity of a new DGJ derivative, MTD118, for β-galactosidase complementary to NOEV. NOV also showed the chaperone effect toward several β-glucosidase gene mutants in Gaucher disease. Furthermore a commercial expectorant drug ambroxol was found to be a chaperone for β-glucosidase. A few Gaucher patients responded to this drug with remarkable improvement of oculomotor dysfunction and myoclonus. We hope chaperone therapy will become available for some patients with Fabry disease, GM1-gangliosidosis, Gaucher disease, and other lysosomal storage diseases particularly with central nervous system involvement.

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.

Fabry disease

Fabry disease (FD) is a progressive, X-linked inherited disorder of glycosphingolipid metabolism due to deficient or absent lysosomal α-galactosidase A activity. FD is pan-ethnic and the reported annual incidence of 1 in 100,000 may underestimate the true prevalence of the disease. Classically affected hemizygous males, with no residual α-galactosidase A activity may display all the characteristic neurological (pain), cutaneous (angiokeratoma), renal (proteinuria, kidney failure), cardiovascular (cardiomyopathy, arrhythmia), cochleo-vestibular and cerebrovascular (transient ischemic attacks, strokes) signs of the disease while heterozygous females have symptoms ranging from very mild to severe. Deficient activity of lysosomal α-galactosidase A results in progressive accumulation of globotriaosylceramide within lysosomes, believed to trigger a cascade of cellular events. Demonstration of marked α-galactosidase A deficiency is the definitive method for the diagnosis of hemizygous males. Enzyme analysis may occasionnally help to detect heterozygotes but is often inconclusive due to random X-chromosomal inactivation so that molecular testing (genotyping) of females is mandatory. In childhood, other possible causes of pain such as rheumatoid arthritis and 'growing pains' must be ruled out. In adulthood, multiple sclerosis is sometimes considered. Prenatal diagnosis, available by determination of enzyme activity or DNA testing in chorionic villi or cultured amniotic cells is, for ethical reasons, only considered in male fetuses. Pre-implantation diagnosis is possible. The existence of atypical variants and the availability of a specific therapy singularly complicate genetic counseling. A disease-specific therapeutic option - enzyme replacement therapy using recombinant human α-galactosidase A - has been recently introduced and its long term outcome is currently still being investigated. Conventional management consists of pain relief with analgesic drugs, nephroprotection (angiotensin converting enzyme inhibitors and angiotensin receptors blockers) and antiarrhythmic agents, whereas dialysis or renal transplantation are available for patients experiencing end-stage renal failure. With age, progressive damage to vital organ systems develops and at some point, organs may start to fail in functioning. End-stage renal disease and life-threatening cardiovascular or cerebrovascular complications limit life-expectancy of untreated males and females with reductions of 20 and 10 years, respectively, as compared to the general population. While there is increasing evidence that long-term enzyme therapy can halt disease progression, the importance of adjunctive therapies should be emphasized and the possibility of developing an oral therapy drives research forward into active site specific chaperones.

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.

Structural aspects of therapeutic enzymes to treat metabolic disorders

Protein therapeutics represents a niche subset of pharmacological agents that is rapidly gaining importance in medicine. In addition to the exceptional specificity that is characteristic of protein therapeutics, several classes of proteins have also been effectively utilized for treatment of conditions that would otherwise lack effective pharmacotherapeutic options. A particularly striking class of protein therapeutics is exogenous enzymes administered for replacement therapy in patients afflicted with metabolic disorders. To date, at least 11 enzymes have either been approved for use, or are in clinical trials for the treatment of selected inherited metabolic disorders. With the recent advancement in structural biology, a significantly larger amount of structural information for several of these enzymes is now available. This article is an overview of the correlation between structural perturbations of these enzymes with the clinical presentation of the respective metabolic conditions, as well as a discussion of the relevant structural modification strategies engaged in improving these enzymes for replacement therapies.

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.

Chemical and pharmacological chaperones as new therapeutic agents

Proteins that are exported from the cell, or targeted to the cell surface or other organelles, are synthesised and assembled in the endoplasmic reticulum and then delivered to their destinations. Point mutations – the most common cause of human genetic diseases – can inhibit folding and assembly of the protein in the endoplasmic reticulum. The unstable or partially folded mutant protein does not undergo trafficking and is usually rapidly degraded. A potential therapy for protein misfolding is to correct defective protein folding and trafficking using pharmacological chaperones. Pharmacological chaperones are substrates or modulators that appear to function by directly binding to the partially folded biosynthetic intermediate to stabilise the protein and allow it to complete the folding process to yield a functional protein. Initial clinical studies with pharmacological chaperones have successfully reduced clinical symptoms of disease. Therefore, pharmacological chaperones show great promise as a new class of therapeutic agents that can be specifically tailored for a particular genetic disease.

Middelheim Fabry Study (MiFaS): a retrospective Belgian study on the prevalence of Fabry disease in young patients with cryptogenic stroke

OBJECTIVE

To assess the prevalence of Fabry disease in young patients with cryptogenic stroke.

PATIENTS AND METHODS

We retrospectively assessed the prevalence of Fabry disease in patients aged 16-60 years that were admitted to ZNA Middelheim Hospital from January 1, 2000 to December 31, 2004 for cryptogenic stroke. We screened for Fabry disease by measurement of alpha-galactosidase A and beta-glucuronidase activity on blood spot. In all patients with abnormal enzymatic activity and in all female patients with low normal values, genetic sequencing of the alpha-GAL-gene was performed.

RESULTS

In a population of 103 young patients with cryptogenic stroke that met the in- and exclusion criteria, we were unable to identify any patient with Fabry disease.

CONCLUSION

Based on the results of alpha-galactosidase A and beta-glucuronidase activity, genetic sequencing and the low prevalence of clinical signs and symptoms of Fabry disease in this population, we believe that the true prevalence of Fabry disease in patients with cryptogenic stroke may be less than currently accepted in literature.

Chemical chaperones and permissive temperatures alter localization of Gaucher disease associated glucocerebrosidase variants

Point mutations in the lysosomal hydrolase, glucocerebrosidase (GC), can cause Gaucher disease, a common lysosomal storage disease. Several clinically important GC mutations impede folding in the endoplasmic reticulum (ER) and target these enzymes for ER-associated degradation (ERAD). The removal of these misfolded proteins decreases the lysosomal concentration of GC, which results in glucosylceramide accumulation. The most common GC variant, N370S, and other clinically relevant variants, G202R and L444P, exhibit different cellular localization patterns in patient-derived fibroblasts. We show that these distributions can be altered by manipulation of the ER folding environment, either by chemical chaperones or by temperature shifts. N370S, L444P, and G202R GC are destabilized in the neutral pH environment of the ER, rendering them prone to ERAD. Fibroblasts harboring the G202R and L444P GC mutations grown at 30 degrees C localize the mutant proteins to the lysosome, and this increases total GC activity. Both of these temperature-sensitive mutants appear to be stable at 37 degrees C once they are trafficked to the low pH environment of the lysosome. Chemical chaperones correct the ER instability and significant ER retention of G202R GC. N370S is also destabilized under ER simulating conditions, a deficiency that is corrected by chemical chaperone binding. These data clearly show manipulating the ER environment with chemical chaperones increases the lysosomal concentration of partially active GC variants and suggest that small molecules could be used to treat Gaucher disease.

A synthetic chaperone corrects the trafficking defect and disease phenotype in a protein misfolding disorder

Mutations in proteins that induce misfolding and proteasomal degradation are common causes of inherited diseases. Fabry disease is a lysosomal storage disorder caused by a deficiency of alpha-galactosidase A activity in lysosomes resulting in an accumulation of glycosphingolipid globotriosylceramide (Gb3). Some classical Fabry hemizygotes and all cardiac variants have residual alpha-galactosidase A activity, but the mutant enzymes are unstable. Such mutant enzymes appear to be misfolded, recognized by the ER protein quality control, and degraded before sorting into lysosomes. Hence, correction of the trafficking defect of mutant but catalytically active enzyme into lysosomes would be beneficial for treatment of the disease. Here we show that a nontoxic competitive inhibitor (1-deoxygalactonojirimycin) of alpha-galactosidase A functions as a chemical chaperone by releasing ER-retained mutant enzyme from BiP. The treatment with subinhibitory doses resulted in efficient, long-term lysosomal trafficking of the ER-retained mutant alpha-galactosidase A. Successful clearance of lysosomal Gb3 storage and a near-normal lysosomal phenotype was achieved in human Fabry fibroblasts harboring different types of mutations. Small molecule chemical chaperones will be therapeutically useful for various lysosomal storage disorders as well as for other genetic metabolic disorders caused by mutant but nonetheless catalytically active enzymes.

Fabry disease: correlation between structural changes in alpha-galactosidase, and clinical and biochemical phenotypes

Fabry disease comprises classic and variant phenotypes. The former needs early enzyme replacement therapy, and galactose infusion is effective for some variant cases. Attempts of early diagnosis before manifestations appear will begin in the near future. However, it is difficult to predict the phenotype, to determine the therapeutic approach, only from genetic information. Thus we attempted structural analysis from a novel viewpoint. We built structural models of mutant alpha-galactosidases resulting from 161 missense mutations (147 classic and 14 variant), and evaluated the influence of each replacement on the structure by calculating the numbers of atoms affected. Among them, 11 mutants, biochemically characterized, were further investigated by color imaging of the influenced atoms. In the variant group, the number of atoms influenced by amino-acid replacement was small, especially in the main chain. In 85% of the cases, less than three atoms in the main chain are influenced. In this group, small structural changes, located apart from the active site, result in destabilization of the mutant enzymes, but galactose can stabilize them. Structural changes caused by classic Fabry mutations are generally large or are located in functionally important regions. In 82% of the cases, three atoms or more in the main chain are affected. The classic group comprises dysfunctional and unstable types, and galactose is not expected to stabilize the mutant enzymes. This study demonstrated the correlation of structural changes, and clinical and biochemical phenotypes. Structural investigation is useful for elucidating the bases of Fabry disease and clinical treatment.

Fabry disease: renal involvement limited to podocyte pathology and proteinuria in a septuagenarian cardiac variant. Pathologic and therapeutic implications

In men with classical Fabry disease (alpha-galactosidase A [alpha-Gal A] deficiency), kidney failure occurs as early as the second decade of life. In contrast, men with the mild "cardiac variant" have late-onset cardiac involvement and proteinuria but usually do not have renal failure. To investigate the nature of renal involvement in the cardiac variant of Fabry disease, the renal function and morphology were assessed in a 75-year-old affected man. He had mild congestive heart failure, a reduced left ventricular ejection fraction, and hypercholesterolemia but lacked the classical Fabry disease manifestations, including angiokeratoma, acroparesthesias, corneal and lenticular opacities, and hypohidrosis. At age 75 years, he had significant proteinuria, and mildly decreased renal function (serum creatinine, 1.8 mg/dL [159 micromol/L]), presumably secondary to hypertensive arteriosclerosis. He had about 4% residual alpha-Gal A activity in leukocytes, and mutation analysis identified the N215S missense mutation, the common lesion in cardiac variants. Histologic and ultrastructural studies of kidney tissue showed that lysosomal glycosphingolipid deposition was extensive in podocytes, rare in tubular epithelial cells, and absent in mesangial, interstitial, and vascular endothelial and smooth muscle cells. This cardiac variant serves as an "experiment of nature" showing that the residual alpha-Gal A activity precludes glycosphingolipid deposition in the renal endothelial and other cells that lead to early renal failure in classically affected men, whereas marked podocyte accumulation is associated with proteinuria and possibly late-onset renal dysfunction. These findings have important implications for the renal effectiveness of enzyme replacement therapy in classically affected patients and for the aggressive treatment of proteinuria in Fabry disease.

Preparation and properties of alpha-galactosidase chemically attached to activated chitin

alpha-Galactosidase (alpha-D-galactoside galactohydrolase, EC 3.2.1.22) from watermelon was covalently immobilized on chitin. The immobilized alpha-galactosidase exhibited an activity of 0.61 U per g of carrier and an activity yield of 67%. The properties of free and immobilized alpha-galactosidase were also searched and compared. The results showed that, optimum conditions for activity were not affected by immobilization. The optimum pH and temperature for free and immobilized enzyme found as pH 6.0 and 65 degress C, respectively. Compared with the free enzyme, the temperature and pH stabilities of the immobilized enzyme were similar. Both the enzymes were stable between pH 2-10 and below 50 degrees C. The Km values for free and immobilized enzyme were determined using p-nitrophenyl-alpha-D-galactopyranoside (PNPG) and raffinose as substrates. Operational stability of the immobilized enzyme was investigated by using both substrates. The operational half-life (t 1/2) was calculated as 34 h for PNPG and 28 h for raffinose. The immobilized alpha-galactosidase was also utilized in the hydrolysis of raffinose. The immobilization procedure on chitin was cheap and also easy to carry out, and the immobilized enzyme had good properties that the potential for practical application is considerable.

Renal pathological changes in Fabry disease

Fabry disease is a rare X-linked disorder, characterized by deficient activity of the lysosomal enzyme alpha-galactosidase A. This leads to systemic accumulation of the glycosphingolipid globotriaosylceramide (Gb3) in all body tissues and organs, including the kidney. Renal manifestations are less evident in female heterozygotes than in male hemizygotes, according to the Lyon hypothesis. Accumulation of Gb3 occurs mainly in the epithelial cells of Henle's loop and distal tubule, inducing early impairment in renal concentrating ability; involvement of the proximal tubule induces Fanconi syndrome. All types of glomerular cells are involved, especially podocytes, and glomerular proteinuria may occur at a young age. The evolution of renal Fabry disease is characterized by progressive deterioration of renal function to end-stage renal failure (ESRF). Ultrastructural study of kidney biopsies reveals typical bodies in the cytoplasm of all types of renal cells, characterized by concentric lamellation of clear and dark layers with a periodicity of 35-50 A. Management of progressive renal disease requires dietetic and therapeutic strategies, usually indicated in developing chronic renal failure, with dialysis and renal transplantation required for patients with ESRF. The recent development of enzyme replacement therapy, however, should make it possible to prevent or reverse the progressive renal dysfunction associated with Fabry disease.