Mutations in the accessory subunitNDUFB10result in isolated complex I deficiency and illustrate the critical role of intermembrane space import for complex I holoenzyme assembly

Abstract

An infant presented with fatal infantile lactic acidosis and cardiomyopathy, and was found to have profoundly decreased activity of respiratory chain complex I in muscle, heart and liver. Exome sequencing revealed compound heterozygous mutations in NDUFB10, which encodes an accessory subunit located within the PD part of complex I. One mutation resulted in a premature stop codon and absent protein, while the second mutation replaced the highly conserved cysteine 107 with a serine residue. Protein expression of NDUFB10 was decreased in muscle and heart, and less so in the liver and fibroblasts, resulting in the perturbed assembly of the holoenzyme at the 830 kDa stage. NDUFB10 was identified together with three other complex I subunits as a substrate of the intermembrane space oxidoreductase CHCHD4 (also known as Mia40). We found that during its mitochondrial import and maturation NDUFB10 transiently interacts with CHCHD4 and acquires disulfide bonds. The mutation of cysteine residue 107 in NDUFB10 impaired oxidation and efficient mitochondrial accumulation of the protein and resulted in degradation of non-imported precursors. Our findings indicate that mutations in NDUFB10 are a novel cause of complex I deficiency associated with a late stage assembly defect and emphasize the role of intermembrane space proteins for the efficient assembly of complex I.

Chlamydomonas reinhardtii as a plant model system to study mitochondrial complex I dysfunction

“…To date, only one patient has been reported with mutations in NDUFB10. The patient, born to non‐symptomatic parents, exhibited fetal cardiomyopathy and fatal infantile lactic acidosis, and died at 27 hr after birth (Friederich et al, ). Exome sequencing identified compound heterozygous sequence variation in the NDUFB10 gene: (a) a paternally inherited nonsense mutation resulting in a premature stop codon, and (b) a maternally inherited missense mutation resulting in a cysteine‐to‐serine (C107S) substitution.…”

“…Both NUO5 (NDUFV2 in human) and NUOB10 (NDUFB10 in human) are highly conserved complex I subunits that have been identified as critical markers in human mitochondrial disorders (Benit et al, ; Cameron et al, ; Friederich et al, ; Nishioka et al, ; Zhang et al, ). NDUFV2 has been implicated in Alzheimer's disease, bipolar disorder, Parkinson's disease, and other pathologies.…”

“…Pulse‐chase experiments showed that cysteine thiols in NDUFB10 are no longer free after import into isolated human mitochondria and therefore presumed to be disulfide‐linked (Friederich et al, ). It is expected that two disulfide bonds are formed in NDUFB10 upon import (Friederich et al, ). Although the identity of the disulfide bond‐forming cysteines remains to be biochemically ascertained, single‐particle electron cryo‐microscopy of fungal, murine, bovine, and ovine complexes I model a disulfide bond between the cysteines within each motif of the NDUFB10 subunit (Agip et al, ; Letts, Fiedorczuk, Degliesposti, Skehel, & Sazanov, ; Parey et al, ; Zhu, Vinothkumar, & Hirst, ).…”

“…Although the identity of the disulfide bond‐forming cysteines remains to be biochemically ascertained, single‐particle electron cryo‐microscopy of fungal, murine, bovine, and ovine complexes I model a disulfide bond between the cysteines within each motif of the NDUFB10 subunit (Agip et al, ; Letts, Fiedorczuk, Degliesposti, Skehel, & Sazanov, ; Parey et al, ; Zhu, Vinothkumar, & Hirst, ). As NDUFB10 does not have a canonical mitochondrial targeting sequence that is cleaved upon import (Hirst et al, ) and NDUFB10 sulfhydryl oxidation was shown to be CHCHD4‐dependent, it was hypothesized to be imported to the IMS via the oxidative folding MIA mechanism (Friederich et al, ). In comparison with NDUFB10, the Chlamydomonas NUOB10 and other vascular plant orthologs contain only the C‐(X) 11 ‐C motif (Figure ).…”

“…This finding is in accordance with the observations in the C107S NDUFB10 patient tissues, where complex I deficiency was not consistent across all tissues. For example, the skin fibroblasts appeared to express more of the C107S NDUFB10 variant than other tissues, enabling normal range of complex I activity (Friederich et al, ). These results indicate that the C107S substitution did not abolish NDUFB10’s import into human mitochondria.…”