Biochemical and ultrastructural evidence of endoplasmic reticulum stress in LGMD2I

Abstract

Limb girdle muscular dystrophy type 2I (LGMD2I) is due to mutations in the fukutin-related protein gene (FKRP), encoding a putative glycosyltransferase involved in alpha-dystroglycan processing. To further characterize the molecular pathogenesis of LGMD2I, we conducted a histological, immunohistochemical, ultrastructural and molecular analysis of ten muscle biopsies from patients with molecularly diagnosed LGMD2I. Hypoglycosylation of alpha-dystroglycan was observed in all FKRP-mutated patients. Muscle histopathology was consistent with either severe muscular dystrophy or myopathy with a mild inflammatory response consisting of up-regulation of class I major histocompatibility complex in skeletal muscle fibers and small foci of mononuclear cells. At the ultrastructural level, muscle fibers showed focal thinning of basal lamina and swollen endoplasmic reticulum cisternae with membrane re-arrangement. The pathways of the unfolded protein response (UPR; glucose-regulated protein 78 and CHOP) were significantly activated in LGMD2I muscle tissue. Our data suggest that the UPR response is activated in LGMD2I muscle biopsies, and the observed histopathological and ultrastructural alterations may be related to sarcoplasmic structures involved in FKRP and alpha-dystroglycan metabolism and malfunctioning.

Limb girdle muscular dystrophy type 2I: No correlation between clinical severity, histopathology and glycosylated α-dystroglycan levels in patients homozygous for common FKRP mutation

“…The studies include few patients and have employed different methods for clinical and histopathological assessment. However, five homozygous patients studied by Boito et al [21] showed an apparent correlation between the clinical grading and pathology severity score, with an inverse correlation to age of onset. In contrast, four homozygous patients studied by Yamamoto et al [12] all showed a mild clinical course with no obvious correlation with age of onset and histopathological alterations.…”

“…However, Jimenez-Mallebrera et al [13] found some inconstancies between levels of α-DG glycosylation determined by IHC and the clinical course in patients harbouring mutations in fukutin and FKRP. Furthermore, Boito et al [21] found variable reduction of glycosylated α-DG in patients investigated by IHC and, interestingly, by immunoblot analysis they detected only trace amounts of glycosylated α-DG in three c.826C>A homozygotes of whom one was asymptomatic [4]. The result presented in this work demonstrates a huge variability in the levels of glycosylated α-DG among the homozygous patients, however, we observed no systematic correlation between the level of α-DG glycosylation and walking function or histopathological alterations.…”

“…The relationship between α-DG glycosylation and clinical severity have previously been studied in LGMD2I patients harbouring various FKRP mutant genotypes [1,7,10,13,21]. By employing IHC Brown et al [10] demonstrated a correlation between α-DG glycosylation levels and clinical phenotype.…”

Zebrafish Fukutin family proteins link the unfolded protein response with dystroglycanopathies

“…Up-regulation of BiP/GRP78 was reported in muscle biopsies from LGMD2I patients (67). To verify this result, we have studied muscle biopsies from patients with a number of molecularly characterized dystroglycanopathies.…”

Introduction to Human Glycosylation Disorders

“…It is thought to physically interact with POMGnT1 and regulate its activity. Its deficiency in tis sue seems to elicit the unfolded protein response (UPR) [91] . Further, POMGnT1 is co-precipitated with FKTN and mice with knockin mutations in FKTN have a decreased POMGnT1 activity [90] .…”