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s Nephron 2017;136:163–182 DOI: 10.1159/000475511 168 frame insertions/deletions, and multiple-site missense mutations may result in mutant forms amenable to the mechanism of action of migalastat. However, mutations that impair the synthesis of α-Gal A, severely affect protein structure (e.g., frameshift, truncation, large insertion/deletion), and/or catalytic activity are not expected to lead to amenable mutant forms. Splice site mutations have complex molecular consequences (e.g., cryptic splice-site usage, intron retention, exon skipping, and/or leaky wild-type splicing) resulting in severe effects on protein structure and/or catalytic activity that may or may not include some residual wild-type protein expression. Methods: The Good Laboratory Practice (GLP)-validated in vitro assay (GLP HEK/Migalastat Amenability Assay), is used to identify α-Gal A mutant forms amenable to migalastat. Fabry disease-associated missense, small in-frame insertion/deletion, and multiple-site missense mutations are each expressed in HEK-293 cells, and increases in α-Gal A activity in response to migalastat are measured. Mutant forms that show α-Gal A activity ≥1.20-fold over baseline with an absolute increase of ≥3.0% wild-type α-Gal A activity in the presence of 10 μmol/l migalastat are categorized as amenable. Mutations that do not qualify for testing in the GLP HEK assay, including large deletions, insertions, truncations, frameshift, and splice site mutations, are categorized as nonamenable. Results: The predictive value of the assay was assessed based on pharmacodynamic responses to migalastat in phase 2 and 3 clinical studies. Comparison of mutant α-Gal A responses to migalastat in the GLP HEK assay and those of white blood cell α-Gal A from male patients exhibited high sensitivity, specificity, and positive and negative predictive values (PPV and NPV, respectively; ≥0.875). GLP HEK assay results were predictive of decreases in kidney GL-3 in males and plasma lyso-Gb3 in both males and females (sensitivity: 1.0 and 0.93, respectively; specificity: 1.0 and 0.69; PPV: 1.0 and 0.84; NPV: 1.0 and 0.85). The clinical study amenable mutations (n = 51) were representative of all identified amenable mutations in terms of the GLP HEK assay absolute increase and α-Gal A activity fold over baseline in response to migalastat. To date (http://galafoldamenabilitytable.com), 675 α-Gal A mutant forms have been tested and 313 met the amenable mutation criteria, while 362 did not meet the criteria. In addition, 255 mutations did not qualify for testing and were categorized as nonamenable. Conclusion: The GLP HEK assay is used to identify migalastatamenable mutations. Male and female Fabry disease patients with amenable mutations are candidates for treatment with migalastat.