Respiratory complex I deficiency caused by a novel multi-exonic PUS1 deletion.

Myopathy, lactic acidosis, and sideroblastic anemia type 1 (MLASA1) is an extremely rare mitochondrial disorder caused by biallelic pathogenic variants in PUS1, which encodes a mitochondrial tRNA pseudouridine synthase essential for mitochondrial protein synthesis. We describe two affected siblings presenting with progressive myopathy, lactic acidosis, sideroblastic anemia, short stature, developmental delay, and mild cognitive impairment. Depth-based copy number variation analysis of whole-exome sequencing data revealed a novel homozygous multi-exonic deletion in PUS1. The deletion breakpoints were defined by Sanger sequencing as a 9964 bp deletion spanning part of intron 3 through exons 4-6 and extending into the 3' untranslated region, resulting in complete loss of the C-terminal coding region. Skeletal muscle histology demonstrated ragged red fibers, whereas immunohistochemistry showed a selective and near-complete loss of NDUFB8, indicating impaired assembly of respiratory chain complex I. A systematic review of previously reported MLASA1 cases revealed marked clinical heterogeneity, including frequent developmental delay, dysmorphic features, and multi-organ involvement. These findings expand the genotypic and phenotypic landscape of MLASA1 and highlight the diagnostic value of copy number variation analysis in unresolved mitochondrial disorders. The impairment of complex I underscores the particular vulnerability of translation-dependent respiratory chain components in PUS1-related diseases.