Why test for alpha-galactosidase A (α-Gal A)?

Deficient activity of the lysosomal enzyme alpha-galactosidase A (α-Gal A) leads to an accumulation of the glycosphingolipids globotriaosylceramide (Gb3) and globotriaosylsphingosine (lyso-Gb3) within almost all cell types and various organs in patients with Fabry disease.1-3 Some variants in the GLA gene encoding α-Gal A lead to these alterations in enzyme activity.4 Enzyme activity levels of <1% for α-Gal A are suggestive of classical Fabry disease.4,5 Patients with residual enzyme activity of levels of α-Gal A >3‒30% typically have late-onset Fabry disease.6

What is the α-Gal A enzyme activity assay?
Enzymatic activity of α-Gal A can be assessed in plasma, leukocytes, cultured fibroblasts or dried blood spots.7,8 Determination of α-Gal A activity using leukocyte and dried blood spot assays are performed using fluorometric methods; however, tandem mass spectrometry is also available for the evaluation of dried blood spots.9-13 In one study, the enzymatic activity of α-Gal A, evaluated using dried blood spots in male patients with Fabry disease, ranged from 0‒1.7 μmol/h/L, compared with 4.2‒17.3 μmol/h/L in unaffected individuals without Fabry disease.14 Therefore, the α-Gal A enzyme assay, in addition to genetic testing, can be used to confirm a diagnosis of Fabry disease in males.7,15-17 Female patients who are heterozygous for variants in the GLA gene had enzymatic activity levels of α-Gal A ranging from 0‒12.6 μmol/h/L in dried blood spots, which may be comparable with individuals without Fabry disease.14 Consequently, the α-Gal A enzyme assay is considered less reliable for diagnosing Fabry disease in females, because many patients exhibit enzyme activity levels within the normal range.15,16 Genetic testing is therefore used to confirm a diagnosis of Fabry disease in female patients.7,16,17

How sensitive is the α-Gal A enzyme activity assay in diagnosing Fabry disease?
The relative sensitivity of testing for α-Gal A enzyme activity in dried blood spots, analysing plasma lyso-Gb3, and GLA gene sequencing for diagnosing suspected Fabry disease in both males and females was assessed during a 2-year clinical diagnostics programme. In this study, enzymatic activity of α-Gal A was reported as pmol/punch/h, where activity ≤11 pmol/punch/h in males was considered low. In males, the sensitivity of the enzyme assay for α-Gal A for confirming a suspected diagnosis of Fabry disease was 100%; however, in females, the reported sensitivity was 49%. The findings from this study further indicate that testing for α-Gal A enzyme activity in males can be considered the first step in the diagnostic work-up. Whereas, a diagnosis of Fabry disease in females can instead be more efficiently made using concurrent analysis of plasma lyso-Gb3 and GLA gene sequencing.18

C-ANPROM/INT/FAB/0016; Date of preparation: March 2021