Study on Leigh syndrome caused by SURF1 gene mutations and its mechanisms

Leigh syndrome (LS) is a prevalent mitochondrial encephalomyopathy in childhood, triggered by mutations in mitochondrial DNA (mtDNA) or nuclear DNA (nDNA). The protein encoded by the SURF1 gene localizes to the inner mitochondrial membrane and is involved in the biosynthesis of the cytochrome c oxidase (COX) complex. We enrolled 5 children harboring SURF1 gene variants whose clinical manifestations were highly consistent with LS. The clinical characteristics and potential pathogenic mechanisms of the disease were elucidated by systematic analysis of their clinical data. Among the 5 patients, 4 were female and 1 was male, with ages ranging from 13 months to 2 years and 7 months. Next-generation sequencing (NGS) results revealed 6 variant sites in the SURF1 gene among the 5 patients, of which 2 were known variants and 4 were unreported novel variants, namely c.314-317delTGCC (p.L105Qfs*7), c.588+1_588+3delGTA (splicing), c.655G>T (p.Glu219), and c.515+3G>C. Reverse transcription-quantitative polymerase chain reaction (RT-qPCR) was performed on the peripheral blood of 4 patients, and the results demonstrated that the messenger RNA (mRNA) expression level of the SURF1 gene was significantly lower than that in their parents. Using 10 healthy children as controls, we analyzed the ratios of mitochondria-related NADH-ubiquinone oxidoreductase core subunit 1 (ND1), Cytochrome c oxidase subunit I (COX1), Cytochrome c oxidase subunit II (COX2), NADH-ubiquinone oxidoreductase chain 4 (ND4), Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) was used as a nuclear reference gene. Mitochondrial DNA content was determined by measuring the ND1/GAPDH ratio using RT-qPCR, and further verified with COX1, COX2, and ND4. These ratios were all significantly decreased, indicating reduced mitochondrial DNA (mtDNA) copy number/mtDNA depletion. Iterative Threading ASSEmbly Refinement (I-TASSER)-based three-dimensional (3D) structural analysis indicated that all 6 variant sites induced alterations in the spatial structure of the SURF1 protein. The SURF1 protein is a hydrophilic protein, protein hydrophobicity and stability analyses showed that the 4 unreported novel variants could reduce the hydrophilicity, increase the hydrophobicity, and decrease the structural stability of the protein. The Saccharomyces cerevisiae Homolog of Yeast 1 (Shy1) domain serves as the key structural basis for SURF1 to exert its mitochondrial functions. We found that all 6 variant sites in the SURF1 gene were located within the Shy1 domain.