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Post-translational processing of GAA protein associated with GAA variants may explain the clinical variability observed amongst Pompe disease patients

Laboratory Genetics and Genomics
  • Primary Categories:
    • Metabolic Genetics
  • Secondary Categories:
    • Metabolic Genetics
Introduction:
Asymptomatic newborns who screen positive for Pompe disease may have one or more variants of uncertain significance (VUS) detected in GAA. The presence of a VUS often raises questions; both from patients’ families and from clinicians, particularly regarding confirmation of diagnosis, predicted age of onset of symptoms, and severity of symptoms.

 

Methods:
In order to better understand the consequences of a VUS, our laboratory has validated an in-vitro functional expression system using HEK293T cells for quantitative and qualitative analysis of GAA VUS (PMID: 36246652). For this study, six patients with GAA enzyme activity in the deficient range, each compound heterozygous for a VUS and a null variant, were characterized. Four patients had early disease manifestations with cardiomyopathy, consistent with infantile onset Pompe disease (IOPD) and two patients were asymptomatic newborns.

 

Results:
Transient expression of all six VUS demonstrated a quantitative reduction in GAA enzyme activity within the pathogenic range (≤ 14% of wildtype) in our model expression system. Qualitative characterization of the expressed protein from each VUS by western blot showed significant variability in the GAA protein banding patterns, emphasizing variability in protein processing. The unprocessed GAA protein band at 110 kDa was consistently observed for each VUS detected in cases of IOPD. For both VUS detected in the asymptomatic newborns, only the mature processed 76 kDa protein band was observed.

 

Conclusion:
Amongst individuals with similar levels of GAA residual activity in our in-vitro expression system, the observed clinical diversity could potentially be due to differences in protein processing within cells, with defective processing and trafficking leading to earlier glycogen accumulation associated with Pompe disease. Although this rationale needs in depth investigation, it may help explain some of the phenotypic variability amongst patients with IOPD and those with later onset Pompe disease. We expect that the combined approach of residual activity measurements and western blot analysis may help in identifying at risk asymptomatic LOPD patients identified via newborn screening.

 

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