Fig 2: GMPPB domain structure and conservation. GMPPB is a 360-aa polypeptide and consists of nine exons (shown with blocks on the scheme). It has two predicted PFAM domains: nucleotidyl transferase domain and bacterial transferase hexapeptide domain (shown with yellow blocks). The scheme shows CMS-associated mutations (above GMPPB scheme), and mutations associated with muscular dystrophy (underneath GMPPB scheme). Mutations described in this paper are shown in black, whereas mutations published previously are shown in grey for comparison. Protein alignment was performed in ClustalW2.

Fig 3: Effect of different variants on GMPPB expression and localization. (A) GMPPB constructs were transfected into HEK293 cells, protein lysates were prepared 48 h after transfection and analysed by western blot using anti-GMPPB antibody. (B) GMPPB constructs were transfected into C2C12 cells. Permeabilized cells were stained with anti-GMPPB antibody. Scale bar = 20 µm.

Fig 4: Mutation c.130-3C>G disrupts wild-type splicing pattern of GMPPB. (A) Schematic structure of GMPPB gene, the pET01 exon trap vector, and the exon trap vector with inserted GMPPB exons 2–4. (B) Gel electrophoresis of amplicons generated using vector-specific primers. The wild-type (WT) construct generated one transcript, whereas c.130-3C>G mutant construct generates two shorter transcripts. (C) Sequencing data and schematic diagrams showing aberrant splicing from the mutant construct. The nucleotide sequence around each splice site is shown.