PMC10196124_01
Variants in the TRNT1 gene in the family revealed by sanger sequencing. Compound heterozygous variants in TRNT1 (NM_182916.2) of the patient: c.443C > T derived from the mother and c.692C > G derived from the father.
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F1
T derived from the mother and c.692C > G from the father (Figure 1). The pathogenicity of c.443C > T had been previously proved in a patient with SIFD. The variant c.692C > G was a novel variant and it was not present in the gnomAD and gnomAD_EAS databases.
Structures of the wild-type and mutated TRNT1 proteins. (A) Variant c.443C > T (p.Ala148Val) led to a substitution of hydrophobic valine (green) for small nonpolar alanine (orange) at position 148 (black arrow) showed by PSIPRED. (B) Variant c.692C > G (p.Ala231Gly) (red arrow) resulted in changes in secondary structures of the adjacent amino acids 226-227 (blue arrow) and 232-233 (green arrow) from a coil to a helix predicted by PSIPRED. (C) Variant c.443C > T (p.Ala148Val) and variant c.692C > G (p.Ala231Gly) caused changes in local spatial structure of TRNT1 proteins predicted by SWISS-MODEL.
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F2
T (p.Ala148Val) was a missense variant that led to a change in the polarity of amino acids (Figure 2A). The other missense variant c.692C > G (p.Ala231Gly) resulted in changes in the secondary structures of the adjacent amino acids 226-227 and 232-233 from a coil to a helix (Figure 2B). The changes in local spatial structure were predicted by SWISS-MODEL (https://swissmodel.expasy.org/) (Figure 2C).
Muscle biopsy of right quadriceps muscle. (A) In a light microscopy, haematoxylin and eosin (HE) stain showed that polygonal muscle fibers were closely packed to each other, with occasional necrosis muscle fibers (arrow). No abnormities were detected in modified Gomori trichrome (MGT) staining, succinate dehydrogenase (SDH) staining, and combined cytochrome oxidase (COX)/SDH staining. (B) Electron microscopy showed that the mitochondria around the muscle fibers were of different sizes and shapes. Some were empty or had discontinued ridges (red arrows). The control showed normal mitochondria (yellow arrows). (C) Western blot of TRNT1, cytochrome c oxidase subunit IV (COX IV), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in human skeletal muscle samples of 3 independent controls and the patient. The patient showed a decreased expression of TRNT1 and COX IV. The primary antibodies are anti-TRNT1 polyclonal antibody (Abcam, ab224536), anti-COX IV monoclonal antibody (Gene-Protein Link, P01L08), and anti-GAPDH monoclonal antibody (Cell Signaling Technology, #2118).
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F3
No typical "ragged-red" fibers (RRFs), "ragged-blue" fibers (RBFs), or cytochrome c oxidase (COX)-negative muscle fibers were found (Figure 3A). Some were empty or had discontinued ridges, supporting a diagnosis of mitochondrial myopathy (Figure 3B). A decreased expression of cytochrome c oxidase subunit IV (COX IV) protein was also detected (Figure 3C), indicating a damage of muscle mitochondrial respiratory chain complex IV (See the Supplementary Material for details of Western blot).