Research paperSURF1 mutations in Chinese patients with Leigh syndrome: Novel mutations, mutation spectrum, and the functional consequences
Introduction
Mitochondrial oxidative phosphorylation (OXPHOS) defects, referred to as mitochondrial diseases as well, represent a group of inherited metabolic disorders, caused by mutations in genes of the OXPHOS complex subunits or the genes regulating OXPHOS assembly and function maintenance (Frazier et al., 2017). Leigh syndrome (LS) is a mitochondrial disease subtype characterized with the developmental delay, ataxia, and the progressive loss of mental abilities and movement, and visible symmetrical lesions due to the changes of the central nervous system and the inhibition of motor functions (Xu et al., 2017). LS affects at least 1 in 40,000 newborns, with extremely high frequencies in populations live in Faroe Islands and Saguenay Lac-Saint-Jean region of Quebec (Merante et al., 1993; Ostergaard et al., 2007). To date, over 75 LS causing genes in both nuclear DNA and mitochondrial DNA (mtDNA) have been identified (Lake et al., 2016). LS inheritance patterns show Mendelian or maternal inheritance pattern and de novo mutations (Craven et al., 2017; Frazier et al., 2017). Currently, many ongoing clinical trials are investigating novel LS treatments, but no effective therapy has been developed to date and the LS prognosis remains poor (Steele et al., 2017), particularly in those cases caused by the changes in the genes responsible for OXPHOS complexes assembly and stability.
Isolated cytochrome c oxidase (complex IV) deficiency was initially considered to be the leading cause of LS (van Riesen et al., 2006). Although it is currently generally accepted that the respiratory chain complex I deficiency is responsible for the development of the majority of LS cases, complex IV deficiency was found to be responsible for approximately 15% of LS cases worldwide (https://ghr.nlm.nih.gov/condition/leigh-syndrome#genes) (Lake et al., 2016). In China, we found that13% of LS patients were shown to have complex IV deficiency (Ma et al., 2013). Although novel pathogenic mutations related to complex IV activity continue to be reported (Calvo et al., 2012; Stroud et al., 2015; Lake et al., 2016), at least one third of the LS cases were shown to be caused by the mutations in the SURF1 gene (Darin et al., 2003; Lee et al., 2012), and this proportion was reported to vary between the populations (Shoubridge, 2001). Previously, we performed clinical molecular epidemiology studies in different mitochondrial disease centers, aiming to describe the genetic characteristics of Chinese LS patients (Yang et al., 2006; Fang et al., 2017), however, the SURF1 mutations in the LS patients have not been comprehensively studied in China to date.
SURF1 is a member of Surfeit locus protein localized at the mitochondrial inner membrane with multiple trans-membrane domains. Mutations in the SURF1 gene associated with the development of the LS were first reported 20 years ago, due to its important role in complex IV biogenesis (Tiranti et al., 1998; Zhu et al., 1998). Mechanistically, the loss of SURF1 functions results in a considerable decrease, but not complete loss of complex IV assembly, in cultured fibroblasts from patients and mice, and bacteria, indicating that SURF1 is important but not indispensable for complex IV assembly (Tiranti et al., 1999; Agostino et al., 2003; Bundschuh et al., 2008). Mutations in any region of SURF1 were shown to affect complex IV assembly, while the patients with frameshift mutations at C-terminus were shown to have a better prognosis (Tiranti et al., 1999; Lee et al., 2012), however, the associations between other SURF1 mutations and disease prognosis are currently unknown due to the lack of functional studies.
In this study, we performed a mutation screening in 178 Chinese patients. Newly-identified SURF1 mutations were confirmed using cell model-based complementation assay. Next, we comprehensively analyzed the mutation spectrum of the SURF1 gene in China, based on this and previous studies. Additionally, we investigated the effects of the SURF1 mutations on complex IV assembly and disease prognosis.
Section snippets
Editorial policies and ethical considerations
This study and all procedures used were approved by the Ethics Committee of Peking University First Hospital, China. Informed consent was obtained from all patients enrolled in this study.
Patients and sequencing
According to mitochondrial disease criteria scoring system (Morava et al., 2006), 178 patients with clinically suspected to have a mitochondrial disorder were recruited in the Department of Pediatrics, Peking University First Hospital, Beijing, China from February 2010 to June 2016 (Morava et al., 2006).
Mutations in the SURF1 gene in patients suspected to suffer from the mitochondrial OXPHOS defects
Fifteen SURF1 mutations were identified in 12 out of 178 patients, of which three, c.465_466delAA, c.532A > T, and c.826_827ins AGCATCTGCAGTACATCG, have not been reported previously (Table 1). Although c.772C > T mutation was identified in one patient with LS in our previous study (Fang et al., 2017), but not functionally validated, we considered it a novel mutation. Notably, two more patients carrying c.772C > T mutation were identified in this study, indicating that c.772C > T mutation is
Discussion
SURF1 mutations have been frequently associated with mitochondrial diseases, particularly the LS, and currently over 100 mutations in this gene have been identified. In China, the first analysis of the SURF1 mutations by using PCR/Sanger sequencing of 65 LS patients was performed by our group at the Peking University First Hospital in 2006, and c.G604C mutation was found to be predominant (Yang et al., 2006). Although c.G604C was considered likely benign or benign in Clinvar now, bioinformatics
Conclusion
We identified four novel SURF1 mutations and presented, to the best of our knowledge, the first analysis of the SURF1 mutation spectrum in Chinese LS patients. Our results suggest that the patients carrying SURF1 missense mutations may have better disease prognosis than those harboring the null mutations of this gene, while the C-terminus frameshift mutations may not be associated with the mild disease phenotype. Finally, the analysis of complex IV assembly using cell model or non-invasive
Conflict of interest
The authors declare no conflict of interest.
Acknowledgements
We thank Dr. Wei Li, Dr. Haihua Gu, and Dr. Zhenfen Chi for their kind gifts of B95-8 cells, 293T cells, and pX330 construct, respectively. This work was funded by the Chinese National Science Foundation (81471097) and Zhejiang Provincial Natural Science Foundation of China (LQ15H070005). We also thank Key Discipline of Zhejiang Province in Medical Technology (First Class, Category A).
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These authors contributed equally to this work.