Muscle involvement in limb-girdle muscular dystrophy with GMPPB deficiency (LGMD2T)

Abstract

Objective: In this study, muscle involvement assessed by MRI and levels of GMPPB and glycosylation of α-dystroglycan expression in muscle were examined in patients with limb-girdle muscular dystrophy (LGMD) type 2T.

Methods: Six new patients with genetically verified mutations in GMPPB were studied. T1-weighted magnetic resonance images were obtained in 4 participants. Muscle strength and potential involvement of extramuscular organs were examined. Glycosylation of α-dystroglycan in muscle was studied, and GMPPB and α-dystroglycan expression was analyzed by Western blotting. Prevalence of LGMD2T was calculated from the total LGMD population in Denmark. GMPPB was sequenced in all unclassified cases.

Results: Two patients carried 3 new mutations in GMPPB. The other 4 patients carried previously described pathogenic mutations in GMPPB. MRI showed that the paraspinal muscles were the most affected, followed by involvement of hamstrings. Our results showed a loss of glycosylation of α-dystroglycan as well as secondary loss of merosin expression on Western blotting. The prevalence of LGMD2T in the Danish cohort of patients with LGMD is 1.5%.

Conclusions: The new findings of this study are (1) the consistent finding of a preferential affection of paraspinal and hamstring muscles in LGMD2T, (2) 3 new mutations in GMPPB, (3) variable loss of glycosylation tested with IIH6 and VIA4 antibodies, and (4) a prevalence of LGMD2T of 1.5% in a well-characterized Danish LGMD cohort.

Figure 1. MRI and muscle biopsy findings

(A) T1-weighted, cross-sectional magnetic resonance images of muscles in cases 1–4. Images were acquired at C6 and L4 of the spine, at the middle of the thighs, and at the thickest part of the calves. (B) Expression of muscle proteins on Western blotting and immunofluorescence in patients with LGMD2T. Two patients express normal levels of GMPPB protein. Same analysis was not available for the rest of the participants. Expression of α-dystroglycan (α-DG) is reduced in all patients. (C) Merosin (clone 22B2) expression appeared near normal on immunofluorescence-stained muscle sections. IIH6C-specific glycosylation of α-DG is significantly reduced in patients, while VIA4-specific glycosylation appears normal. Bar is 50 μm. MHC = major histocompatibility complex.

Figure 2. Muscle involvement and muscle strength evaluation

(A) Levels of muscle involvement evaluated by T1-weighted MRI using the Mercuri scale. (B) Muscle strength evaluation using the Medical Research Council (MRC) scale. Values 0–5, including plus and minus for 4 and 5 (4+ equals 4.33 and 5− equals 4.66).