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Toward a consensus in the laboratory diagnostics of Fabry disease ‐ recommendations of a European expert group

2011; Springer Science+Business Media; Volume: 34; Issue: 2 Linguagem: Inglês

10.1007/s10545-010-9261-9

1573-2665

Andreas Gal, Derralynn Hughes, Bryan Winchester,

Glycogen Storage Diseases and Myoclonus

Guidelines on diagnostics and therapy of Fabry disease have already been compiled in a number of European countries and are being prepared in others. A synthesis of these national guidelines seems to be a sensible option for preparing a prospective European consensus document in the not too distant future. While clinical diagnostics is extensively discussed in the various guidelines, at present there is no consensus on laboratory diagnostic tests for Fabry disease either at national level or at the level of the European Union. There is a widespread variation concerning the diagnostic value of the various methods, such as enzyme activity testing, gene analysis, biopsies, Gb3 measurement etc. and on what should be used and how they compare, if indeed they do. This results in lack of agreement regarding the clinical pathology of the condition. While experts throughout Europe agree on many aspects of the laboratory diagnosis of Fabry disease (for a recent review see Winchester and Young 2010), there are some specific issues which need further discussion before a general recommendation can be made. In order to promote this process, three one-and-a-half day ad hoc meetings* of European experts** were organized in 2009 with the aim of defining a consensus on laboratory diagnostics of Fabry disease that should allow for laboratories performing this diagnostic service to work to the same standards. Invitations to attend the meetings were based on, in addition to geographic considerations, outstanding scientific accomplishments in the field and/or active involvement in the laboratory diagnostics of Fabry disease. *'Diagnostic use and value of Gb3 and lysoGb3 in Fabry disease' (April 22-23, 2009, Bad Nauheim, Germany); 'Diagnostic use and value of measuring alpha-galactosidase A activity in Fabry disease' (July 16-17, 2009, Berlin, Germany); 'Diagnostic use and value of mutations of the GLA gene in Fabry disease' (November 3-4, 2009, Barcelona, Spain). **J.M.F.G. Aerts (Amsterdam, The Netherlands), M. Beck (Mainz, Germany), O. Bodamer (Salzburg, Austria), A. Cooper (Manchester, United Kingdom), A. Gal (Hamburg, Germany), D. Germain (Paris, France), R. Giugliani (Zürich, Switzerland), D. Hughes (London, United Kingdom), L. Kuchar (Prague, Czech Republic), J. Ledvinova (Prague, Czech Republic), Z. Lukacs (Hamburg, Germany), C. Navarro (Vigo, Spain), E. Paschke (Graz, Austria), M. Piraud (Lyon, France), A. Rolfs (Rostock, Germany), C. Sa Miranda (Porto, Portugal), M. van Slegtenhorst (Rotterdam, The Netherlands), H. Treslova (Prague, Czech Republic), M.T. Vanier (Lyon, France), F.W. Verheijen (Rotterdam, The Netherlands), B. Winchester (London, United Kingdom). Primary objectives of the initiative were (i) establishment of 'gold standards' and practical algorithms for diagnosis of Fabry disease, (ii) promotion of a concept of certification/accreditation for diagnostic and treatment centres of excellence, and (iii) provision of a platform for future treatment guidelines. In particular, three specific issues were selected by the organizers: the diagnostic utility of measurement of Gb3 and lysoGb3, evaluation of α-galactosidase A activity, and analysing mutations of the GLA gene in patients with Fabry disease. The most important goal of these workshops was to provide a platform for the experts to exchange ideas and experiences and share experimental data with each other, even if preliminary or yet unpublished, on the use, practical value, and significance of different laboratory approaches in the diagnostics of Fabry disease. All data were critically discussed by the panel and used to prepare this recommendation for the medical community. We hope that this document will serve as a starting point for further discussion that, in the end, should result in a consensus, provide essential guidance to physicians, and ensure a better differential and timely diagnosis of patients with Fabry disease. In normal human tissues, there are two lysosomal glycosidases with α-galactosidase activity towards synthetic substrates. α-Galactosidase A (α-GAL; EC 3.2.1.22) which is deficient in Fabry disease, acts on terminal α-galactosyl residues in glycosphingolipids, whereas the so called α-galactosidase B is an α-N-acetylgalactosaminidase (α-NAGAL; EC 3.2.1.49) that acts on natural substrates with terminal α-N-acetylgalactosaminyl residues and is defective in Schindler disease. α-Galactosidase A does not catalyse the hydrolysis of the natural substrates of α-NAGAL whereas α-NAGAL may act on some natural substrates with terminal α-galactosyl residues (Clark and Garman 2009). As both enzymes act on synthetic α-galactoside substrates, α-N-acetylgalactosamine, a specific inhibitor of α-NAGAL is added to assays of α-galactosidase A activity for the diagnosis of Fabry disease. The genes (GLA and NAGA) encoding α-GAL and α-NAGAL are on chromosomes Xq22.1 and 22q13, respectively. Although the two genes show considerable homology, as they evolved from a common ancestral precursor (Clark and Garman 2009), α-galactosidase A and α-NAGAL have distinct physicochemical properties, and antibodies raised against one do not cross-react with the other. α-Galactosidase A is a typical lysosomal hydrolase with optimal activity towards natural and synthetic substrates at pH 3.8–4.6. It is a glycoprotein and is transported to the lysosomes via the mannose-6-phosphate pathway. It is synthesized as a precursor of 50 kDa and is processed to a mature lysosomal form of 46 kDa by partial proteolysis and modification of its carbohydrates. The enzyme is a homodimer with an active site in each of the monomers. Hydrolysis of its natural substrates in vivo requires saposin B whereas that in vitro requires the addition of a detergent, usually sodium taurocholate. Patients with a genetic deficiency of saposin B also accumulate the substrates for α-galactosidase A, as in Fabry disease. The three-dimensional structure of recombinant human α-galactosidase A has been determined by X-ray crystallography at a resolution of 3.25 Å, and used to understand the effects of mutations on the structure and function of the enzyme (Garman and Garboczi 2004, Garman 2007). After thorough clinical evaluation, determination of α-galactosidase A activity is the first step in the laboratory diagnosis of a patient suspected of having Fabry disease, unless there is a known familial GLA mutation, for which DNA analysis is straightforward (see later). The activity can be determined in various materials, such as plasma, leukocytes, fibroblasts, or dried blood spots (DBS); the assay is rapid, reliable, and cost-effective. Currently the assay of α-galactosidase A activity in leukocytes represents the diagnostic 'gold-standard'. A skin biopsy is rarely taken for initial diagnosis but studies on fibroblasts could be useful for molecular characterization of the enzyme deficiency. The most commonly used method of enzyme analysis is based upon cleavage of 4-methylumbelliferyl-α-D-galactoside, a synthetic fluorigenic substrate. Recently a substrate suitable to assay enzyme activity by electrospray ionization/tandem mass spectrometry (ESI/TMS) has also become available (Li et al. 2004, Zhang et al. 2010). The activity of α-galactosidase A should always be determined in the presence of α-N-acetylgalactosamine, which inhibits α-galactosidase B. In addition, another lysosomal enzyme, preferably β-galactosidase, should always be assayed to evaluate sample quality. As enzyme activity may diminish rapidly if the sample is inappropriately stored and/or transported, the diagnostic laboratory should be contacted in advance for information regarding appropriate sample storage and shipping conditions. About 2% of the population/Fabry patients carry the non-synonymous change c.937 G > T (p.Asp313Tyr). This variant has low α-galactosidase A activity in plasma compared to the wild-type and about 60% of the mean α-galactosidase A activity of the wild-type in cells. The p.Asp313Tyr variant is stable at lysosomal pH and is not disease-causing (pseudodeficiency). Males with the classic Fabry disease phenotype can be reliably diagnosed by detecting complete deficiency or only negligible ( T and c.639 + 919 G > A) apparently result in α-galactosidase A deficiency by causing complex changes in the pattern of splicing (Ishii et al. 2002, Filoni et al. 2008). About 50% of males with enzymatically proven Fabry disease carry a Class 1 mutation. Mutations that have been found in males with normal α-galactosidase A activities or with somewhat decreased enzyme activities, which are still, however, well above the pathologic range of values, are called Class 2 mutations and are almost certainly non-pathogenic. These include polymorphisms or sequence changes, e.g. c.937 G > T (p.D313Y) described as pseudodeficiency alleles. Routine mutation analysis of the GLA gene consists of sequencing of the coding region and exon-intron boundaries. The interpretation of gene alterations should be performed by an expert and genetic counselling should be offered. In more than 97% of males with pathologic α-galactosidase A activities, a sequence variant (Class 1 mutations or yet unclassified sequence changes) can be detected by routine mutation analysis. The identification of Class 1 mutations is considered a very useful and independent confirmation of the biochemical (and clinical) diagnosis. A small number of mutations, in particular exon-spanning duplications or inversions and deep intronic mutations, may escape detection by using the above method and can only be identified by more sophisticated procedures, such as the analysis of the mRNA or MLPA (multiplex ligation-dependent probe amplification). However, even if no highly probable disease-causing mutation is found in a male patient with an α-galactosidase A deficiency, the diagnosis of Fabry disease remains valid. For mutations not categorized as Class 1 or Class 2, a thorough clinical, biochemical and genetic analysis of the patient and his family may allow designation of the mutation as disease-causing or not. Analysis of evolutionary conservation of the relevant sequence and its absence in a large cohort of unaffected individuals may provide additional arguments for a pathogenic relevance of a change. An easily accessible and quality-controlled gene-specific mutation database would be an invaluable tool both for physicians treating patients, genetic counselors, and scientists involved in DNA diagnostics or research. Both Class 1 and Class 2 mutations also occur in females. Since enzyme activity measurements do not reliably detect heterozygotes, and many of the clinical features of Fabry disease are frequently observed in the general population, DNA diagnostics is much less efficient in identifying heterozygotes among women with clinical suspicion for Fabry disease than in confirming diagnosis in males with enzymatically proven Fabry disease. Carriers in families with Fabry disease can be identified by pedigree analysis and/or by showing that they have inherited the family-specific mutation. If bidirectional sequencing of the coding region and exon-intron boundaries do not reveal heterozygosity for a Class 1 mutation in a female DNA sample, MLPA may help to pick up the few cases of large DNA rearrangements in females, whereas the analysis of mRNA may be complicated by nonsense-mediated decay. In addition, a thorough clinical re-evaluation of the patient and additional biochemical tests are recommended. A careful analysis of the pedigree by a genetic counselor should also be offered, keeping in mind that nine out of ten patients with Fabry disease should have a positive family history for the trait. In females with typical signs and symptoms of Fabry disease but without positive family history, only the identification of a disease-relevant heterozygous GLA mutation allows definite diagnosis of carrier status. Conclusions (Fig. 1): (i) Thorough analysis of the patient's family and medical history together with a clinical examination are essential prior to laboratory testing. (ii) Diagnostic tests should be done in laboratories with appropriate quality control schemes, experience, and sample load. (iii) Analysis of α-galactosidase A activity is the standard diagnostic test of Fabry disease in males. Molecular analysis of the GLA gene is necessary to diagnose heterozygotes. Algorithm for the laboratory diagnosis of (a) male and (b) female patients with Fabry disease. The flow charts represent summaries of the discussion and conclusions in the text, with extra clarification in points 1-6. 1) Demonstration of a deficiency of α-galactosidase A in leukocytes and plasma from the same blood sample is supporting evidence for a diagnosis of Fabry disease. 2) Variants of Fabry disease with residual α-galactosidase A activity. 3) A patient with some symptoms of Fabry disease not due to deficiency of α-galactosidase A. 4) Class 1 mutation and mutations previously found in affected male or female Fabry patients. 5) Low α-galactosidase A activity can be suggestive of carrier status but not definitive. 6) To decide whether the proband has Fabry disease, all or some of the following investigations should be carried out: (a) re-evaluation of clinical presentation, (b) analysis of family pedigree, (c) measurement of urinary Gb3, and (d) in absence of a Class 1 mutation, any sequence change detected in the GLA gene must be expressed in vitro to investigate its effect on activity and/or structure of enzyme The European Consensus on Diagnostics in Fabry Disease was sponsored by an unrestricted education grant generously awarded by The Center for Extramural Clinical Research and Education of Shire Human Genetic Therapies, Inc. 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