Introduction
Leigh syndrome (LS, MIM 256000) is the most prevalent childhood-onset mitochondrial disease, with neurodegenerative disorders as its most distinctive feature (Rahman et al., 1996). LS has a prevalence of approximately 1:40,000 and is highly genetically heterogeneous, primarily with impaired mitochondrial energy production (Darin et al., 2001). LS exhibits several different modes of inheritance (X-linked, autosomal or maternal). To data, more than 75 causing genes including both mitochondrial and nuclear genes have been identified (Schubert Baldo & Vilarinho, 2020). Although defects in each of the five oxidative phosphorylation system (OXPHOS) complexes have been observed in LS patients (Lake et al., 2016), nearly one-third of LS cases are related to complex I deficiency. Mitochondrial complex I is essential for aerobic respiratory and is located in the first of the mitochondrial oxidative respiratory chain. It oxidizes NADH, which reduces ubiquinone and transports protons through the inner mitochondrial membrane, resulting in proton driving force (Hirst, 2013). The complete assembly of complex I requires at least 45 subunits and 15 assembly factors, each of which can lead to mitochondrial complex I disruption and ultimately cause diverse human diseases including neurodegenerative diseases, aging, mitochondrial disorder and etc (Formosa et al., 2018; Mimaki et al., 2012; Schapira, 1993; Tinker et al., 2021).
TMEM126B (MIM 615533) is a 7968-bp gene containing five exons that encodes a component of mitochondrial complex I intermediate assembly (MCIA) complex, which is required for assembly of complex I but is not part of the mature complex (Heide et al., 2012). Mutations inTMEM126B would cause an isolated mitochondrial complex I deficiency and result in various clinical phenotypes such as exercise intolerance, muscle weakness, hyperlactic acidemia, hypertrophic cardiomyopathy and renal tubular acidosis (Alston et al., 2016; Sánchez-Caballero et al., 2016; Theunissen et al., 2017). To date, ten patients with a total of four mutations in TMEM126B have been reported worldwide, but all patients reported had a normal in neurological presentation (Alston et al., 2016; Sánchez-Caballero et al., 2016; Theunissen et al., 2017). Notably, the genetic spectrum of the TMEM126B mutations in China remains unclear.
In this study, using next-generation sequencing in a Chinese patient manifested with LS, we identified two novel heterozygous mutations of TMEM126B (c.82-2A>G and c.290dupT). Bioinformatics analysis and functional assays revealed that c.82-2A>G mutation caused complete exon 2 skipping and c.290dupT induced partial and complete exon 3 skipping. Patient-derived lymphoblastoid cells carrying biallelic mutations exhibited complex I content and assembly defect and mitochondrial dysfunction. Our findings uncovered the functional effect and the molecular mechanism of the pathogenic TMEM126B variants c.82-2A>G and c.290dupT, which not only expand the gene mutation spectrum of LS, but also expand the clinical spectrum caused by TMEM126B mutations.