Mitochondrial diseases are clinically and genetically heterogeneous, often complicating diagnosis. Here, we describe four unrelated individuals with suspected mitochondrial disease who shared similar neuroimaging features, including bilateral symmetrical supra- and infratentorial white-matter abnormalities, together with variable movement disorders and intellectual impairment. Whole-genome sequencing identified the same homozygous MRPS22 variant (c.798_799delinsTA) in all four patients. MRPS22 encodes a component of the mitochondrial small ribosomal subunit (mtSSU). Functional studies in patient-derived fibroblasts showed impaired mitoribosome assembly and reduced de novo mitochondrial translation. Despite largely preserved steady-state levels of OXPHOS proteins, respiratory chain analysis identified a mild, isolated complex I deficiency. Proteomic profiling revealed reduced levels of mitochondrial ribosomal proteins and dysregulation of mitochondrial translation pathways. In line with the proteomic findings, RNA sequencing of fibroblasts from three patients revealed a distinct transcriptional signature compared with controls, with mitochondrial translation emerging as the most affected pathway. Mitochondrial-encoded transcripts were decreased, whereas nuclear-encoded mitochondrial genes were generally increased. Structural modelling suggested that the variant disrupts key interactions important for mitoribosome stability. While previously reported MRPS22 variants have been associated with severe, often prenatal-onset disease, the individuals described here exhibited a milder phenotype, thereby expanding the clinical spectrum of MRPS22-related disorders. Together, these findings support the pathogenicity of this variant and highlight the value of integrated genomic and functional analyses in diagnosing mitochondrial disease.
Keywords: MRPS22; adult-onset; mS22; mitochondrial ribosome; translation.
© The Author(s) 2026. Published by Oxford University Press.