Objective
Oculopharyngodistal myopathy (OPDM) is an adult‐onset neuromuscular disease characterized by progressive ptosis, dysarthria, ophthalmoplegia, and distal muscle weakness. Recent studies revealed that GGC repeat expansions in 5′‐UTR of LRP12, GIPC1, and NOTCH2NLC are associated with OPDM. Despite these advances, approximately 30% of OPDM patients remain genetically undiagnosed. Herein, we aim to investigate the genetic basis for undiagnosed OPDM patients in two unrelated Chinese Han families.
Methods
Parametric linkage analysis was performed. Long‐read sequencing followed by repeat‐primed polymerase chain reaction and amplicon length polymerase chain reaction were used to determine the genetic cause. Targeted methylation sequencing was implemented to detect epigenetic changes. The possible pathogenesis mechanism was investigated by quantitative polymerase chain reaction, immunoblotting, RNA fluorescence in situ hybridization, and immunofluorescence staining of muscle biopsy samples.
Results
The disease locus was mapped to 12q24.3. Subsequently, GGC repeat expansion in the promoter region of RILPL1 was identified in six OPDM patients from two families, findings consistent with a founder effect, designated as OPDM type 4. Targeted methylation sequencing revealed hypermethylation at the RILPL1 locus in unaffected individuals with ultralong expansion. Analysis of muscle samples showed no significant differences in RILPL1 mRNA or RILPL1 protein levels between patients and controls. Public CAGE‐seq data indicated that alternative transcription start sites exist upstream of the RefSeq‐annotated RILPL1 transcription start site. Strand‐specific RNA‐seq data revealed bidirectional transcription from the RILPL1 locus. Finally, fluorescence in situ hybridization/immunofluorescence staining showed that both sense and antisense transcripts formed RNA foci, and were co‐localized with hnRNPA2B1 and p62 in the intranuclear inclusions of OPDM type 4 patients.
Interpretation
Our findings implicate abnormal GGC repeat expansions in the promoter region of RILPL1 as a novel genetic cause for OPDM, and suggest a methylation mechanism and a potential RNA toxicity mechanism are involved in OPDM type 4 pathogenesis. ANN NEUROL 2022;92:512–526
Objective
Despite the increasing number of genes associated with Charcot–Marie‐Tooth (CMT) disease, many patients currently still lack appropriate genetic diagnosis for this disease. Autosomal dominant mutations in aminoacyl‐tRNA synthetases (ARSs) have been implicated in CMT. Here, we describe causal missense mutations in the gene encoding seryl‐tRNA synthetase 1 (SerRS) for 3 families affected with CMT.
Methods
Whole‐exome sequencing was performed in 16 patients and 14 unaffected members of 3 unrelated families. The functional impact of the genetic variants identified was investigated using bioinformatic prediction tools and confirmed using cellular and biochemical assays.
Results
Combined linkage analysis for the 3 families revealed significant linkage (Zmax LOD = 6.9) between the genomic co‐ordinates on chromosome 1: 108681600–110300504. Within the linkage region, heterozygous SerRS missense variants segregated with the clinical phenotype in the 3 families. The mutant SerRS proteins exhibited reduced aminoacylation activity and abnormal SerRS dimerization, which suggests the impairment of total protein synthesis and induction of eIF2α phosphorylation.
Interpretation
Our findings suggest the heterozygous SerRS variants identified represent a novel cause for autosomal dominant CMT. Mutant SerRS proteins are known to impact various molecular and cellular functions. Our findings provide significant advances on the current understanding of the molecular mechanisms associated with ARS‐related CMT. ANN NEUROL 2023;93:244–256
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