Muscular dystrophy with arrhythmia caused by loss-of-function mutations in BVES

Abstract

Objective: To study the genetic and phenotypic spectrum of patients harboring recessive mutations in BVES.

Methods: We performed whole-exome sequencing in a multicenter cohort of 1929 patients with a suspected hereditary myopathy, showing unexplained limb-girdle muscular weakness and/or elevated creatine kinase levels. Immunohistochemistry and mRNA experiments on patients' skeletal muscle tissue were performed to study the pathogenicity of identified loss-of-function (LOF) variants in BVES.

Results: We identified 4 individuals from 3 families harboring homozygous LOF variants in BVES, the gene that encodes for Popeye domain containing protein 1 (POPDC1). Patients showed skeletal muscle involvement and cardiac conduction abnormalities of varying nature and severity, but all exhibited at least subclinical signs of both skeletal muscle and cardiac disease. All identified mutations lead to a partial or complete loss of function of BVES through nonsense-mediated decay or through functional changes to the POPDC1 protein.

Conclusions: We report the identification of homozygous LOF mutations in BVES, causal in a young adult-onset myopathy with concomitant cardiac conduction disorders in the absence of structural heart disease. These findings underline the role of POPDC1, and by extension, other members of this protein family, in striated muscle physiology and disease. This disorder appears to have a low prevalence, although it is probably underdiagnosed because of its striking phenotypic variability and often subtle yet clinically relevant manifestations, particularly concerning the cardiac conduction abnormalities.

Figure 1. Segregation analysis of the 3 identified BVES mutations

Pedigrees of families A, B, and C are shown. Unaffected family members for whom DNA was available for segregation analysis of the BVES variants in the respective families, I-1, II-1, II-2, III-1, III-2, III-3, and III-4, were heterozygous for the respective variants.

Figure 2. BVES gene structure and mRNA analysis

(A) BVES mRNA transcript variant C (NM_001199563) contains 8 exons, of which 7 coding. Transcript length is 5,567 base pairs. Untranslated regions are filled in grey, translated in black. The location of the 3 identified variants is marked with an arrow, position of the amplified fragments for quantitative and quantitative PCR experiments with dashed lines. (B) Qualitative PCR: agarose gel of PCR products (amplicons A, B, and C) amplified from cDNA from control (C1) and patient 2 (P2). For every amplicon, the 2 lanes on the right contain mRNA without reverse transcriptase and H2O, respectively. (C and D) BVES quantitative mRNA analysis for amplicons A and C, respectively. BVES mRNA levels were first normalized internally to RPLP0 mRNA levels; subsequently, BVES mRNA levels of the patients were normalized (as a percentage) to the mean of the 2 controls. For both amplicons A and C, there is a significant difference in mRNA levels between both controls and patients 2 and 3, respectively, which is not the case for patient 4. Error bars: standard error of the mean (SEM). *p < 0.05, **p < 0.01, using the unpaired t test. bp = base pair; Ex = exon.

Figure 3. Muscle MRI findings in 3 patients harboring homozygous mutations in BVES

Axial T1-weighted images are shown for patients 1, 2, and 3, performed at ages 39, 35, and 56 years, respectively.

Figure 4. Reduction of POPDC1 and PODPC2 at the sarcolemma in muscle of patients harboring homozygous mutations in BVES

(A and B) Representative muscle sections of patients and controls immunostained for POPDC1 and POPDC2, respectively, and α-sarcoglycan (SGCA) as sarcolemmal marker. All images were acquired with identical settings and are displayed in the figure with identical intensity scaling for each channel. Scale bar = 50 μm. (C and D) Fluorescence intensities measured at the sarcolemma (n = 30 sarcolemmal segments at random positions on the section). AU = arbitrary unit.