The value of muscle biopsies in Pompe disease: identifying lipofuscin inclusions in juvenile- and adult-onset patients

Erin J Feeney,Stephanie Austin,Hanna Mandel,Yin-Hsiu Chien,Sean Prater,Evelyn Ralston,Nina Raben,Priya S Kishnani,Wuh-Liang Hwu,Benedikt Schoser

doi:10.1186/2051-5960-2-2

Abstract

BackgroundPompe disease, an inherited deficiency of lysosomal acid alpha-glucosidase (GAA), is a metabolic myopathy with heterogeneous clinical presentations. Late-onset Pompe disease (LOPD) is a debilitating progressive muscle disorder that can occur anytime from early childhood to late adulthood. Enzyme replacement therapy (ERT) with recombinant human GAA is currently available for Pompe patients. Although ERT shows some benefits, the reversal of skeletal muscle pathology - lysosomal glycogen accumulation and autophagic buildup - remains a challenge. In this study, we examined the clinical status and muscle pathology of 22 LOPD patients and one atypical infantile patient on ERT to understand the reasons for muscle resistance to ERT.ResultsThe patients were divided into three groups for analysis, based on the age of onset and diagnosis: adult-onset patients, juvenile-onset patients, and those identified through newborn screening (NBS). The areas of autophagic buildup found in patients’ biopsies of all three groups, contained large autofluorescent inclusions which we show are made of lipofuscin, an indigestible intralysosomal material typically associated with ageing. These inclusions, analysed by staining, spectral analysis, time-resolved Fluorescence Lifetime Imaging (FLIM), and Second Harmonic Generation (SHG) imaging, were the major pathology remaining in many fibers after ERT. The best outcome of ERT both clinically and morphologically was observed in the NBS patients.ConclusionsThe muscle biopsy, in spite of its shortcomings, allowed us to recognize an underreported, ERT-resistant pathology in LOPD; numerous lysosomes and autolysosomes loaded with lipofuscin appear to be a hallmark of LOPD skeletal muscle. Lipofuscin accumulation - a result of inefficient lysosomal degradation - may in turn exacerbate both lysosomal and autophagic abnormalities.

Highlights

Pompe disease, an inherited deficiency of lysosomal acid alpha-glucosidase (GAA), is a metabolic myopathy with heterogeneous clinical presentations
With the exception of one patient with atypical infantile-onset disease, the remaining 22 individuals were classified as Late-onset Pompe disease (LOPD) patients; these patients differed dramatically in clinical manifestations and age at diagnosis, ranging from infancy in patients identified through newborn screening (NBS) to adolescence and late adulthood
We have previously shown in the Acid alpha-glucosidase (GAA)-KO mouse model that lysosomal glycogen storage leads to dysfunctional autophagy, accumulation of autophagic substrates, and impaired fusion between autophagic and lysosomal vesicles, thereby initiating the process of autophagic buildup which spreads throughout the fiber [21,45,61]

Summary

Introduction

An inherited deficiency of lysosomal acid alpha-glucosidase (GAA), is a metabolic myopathy with heterogeneous clinical presentations. Enzyme replacement therapy (ERT) with recombinant human GAA is currently available for Pompe patients. ERT shows some benefits, the reversal of skeletal muscle pathology - lysosomal glycogen accumulation and autophagic buildup - remains a challenge. Pompe disease (glycogen storage disease type II; OMIM 232300) is an inherited neuromuscular disorder caused by a deficiency of acid alpha-glucosidase (GAA; OMIM 606800), the sole enzyme responsible for the breakdown of glycogen in the lysosomal compartment [1,2]. In late-onset patients, the therapy shows some benefits (e.g., improvements in walking distance and respiratory function) [12,13], but skeletal muscle weakness often persists [14]. The accumulation of autophagic debris - likely resulting from impaired fusion of lysosomes and autophagosomes (the vesicles which bring substrates and worn-out organelles to lysosomes) is a well-recognized phenomenon in Pompe disease as well as in other lysosomal storage disorders [22]

Methods

Results

Discussion

Conclusion