Correlation of GAA Genotype and Acid-α-Glucosidase Enzyme Activity in Hungarian Patients with Pompe Disease

Abstract

Pompe disease is caused by the accumulation of glycogen in the lysosomes due to a deficiency of the lysosomal acid-α-glucosidase (GAA) enzyme. Depending on residual enzyme activity, the disease manifests two distinct phenotypes. In this study, we assess an enzymatic and genetic analysis of Hungarian patients with Pompe disease. Twenty-four patients diagnosed with Pompe disease were included. Enzyme activity of acid-α-glucosidase was measured by mass spectrometry. Sanger sequencing and an MLPA of the GAA gene were performed in all patients. Twenty (83.33%) patients were classified as having late-onset Pompe disease and four (16.66%) had infantile-onset Pompe disease. Fifteen different pathogenic GAA variants were detected. The most common finding was the c.-32-13 T > G splice site alteration. Comparing the α-glucosidase enzyme activity of homozygous cases to the compound heterozygous cases of the c.-32-13 T > G disease-causing variant, the mean GAA activity in homozygous cases was significantly higher. The lowest enzyme activity was found in cases where the c.-32-13 T > G variant was not present. The localization of the identified sequence variations in regions encoding the crucial protein domains of GAA correlates with severe effects on enzyme activity. A better understanding of the impact of pathogenic gene variations may help earlier initiation of enzyme replacement therapy (ERT) if subtle symptoms occur. Further information on the effect of GAA gene variation on the efficacy of treatment and the extent of immune response to ERT would be of importance for optimal disease management and designing effective treatment plans.

Keywords: GAA enzyme activity; GAA genotype; Pompe disease.

Figure 1

Association between GAA enzyme activity and patient’s genotypes in LOPD and IOPD: (A) The average α-glucosidase enzyme activity in different GAA genotypes in our cohort (the average for each genotype was calculated from the average values of the individual patient’s enzyme activity). (B) Correlation between the α-glucosidase enzyme activity and age of onset in different GAA genotypes in LOPD cases (genotypes showing high association with age of onset are indicated by a dashed line—the trends observed in these genotypes ([c.-32-13 T > G/c.525delT] and [c.-32-13 T > G/c.2269 C > T]) using the Pearson correlation did not show any significant differences). (C) Correlation between the α-glucosidase enzyme activity and age of onset in different GAA genotype in IOPD cases. (Abbreviations: LOPD—late-onset Pompe disease; IOPD—infantile-onset Pompe disease; Non-Cl—non-classical, Cl—classical, N—number of affected patients).

Figure 2

The localization of the found GAA mutations in protein structure and their effect on α-glucosidase enzyme activity: (A) Schematic representation of localization of the GAA mutations in protein structure. (B) The average α-glucosidase enzyme activity linked to different protein domain involvement in our LOPD/IOPD cohort (data analyses includes all 3 available enzyme activity values from all patients). (Abbreviations: SP—signal peptide; PP—propeptide, LOPD—late-onset Pompe disease; IOPD—infantile-onset Pompe disease; Cl—classical; non-Cl—non classical; TTP domain—trefoil type-P domain; NTB domain—N-terminal β-sheet domain; CCB domain—catalytic CH31 (α/β)8 barrel domain; PB domain—proximal β-sheet domain; N=—number of affected patients). (* p < 0.05). The lines represent the median values. To calculate significance, the groups LOPD 2, 3, 4 and 5 were compared to LOPD group 1 [c.-32-13 C > T/c.-32-13 C > T]. In the case of group LOPD 6, because the variant only affected a single patient, significance could not be calculated.