Mutations in NDUFAF3 (C3ORF60), encoding an NDUFAF4 (C6ORF66)-interacting complex I assembly protein, cause fatal neonatal mitochondrial disease.

Mitochondrial complex I deficiency is the most prevalent and least understood disorder of the oxidative phosphorylation system. The genetic cause of many cases of isolated complex I deficiency is unknown because of insufficient understanding of the complex I assembly process and the factors involved. We performed homozygosity mapping and gene sequencing to identify the genetic defect in five complex I-deficient patients from three different families. All patients harbored mutations in the NDUFAF3 (C3ORF60) gene, of which the pathogenic nature was assessed by NDUFAF3-GFP baculovirus complementation in fibroblasts. We found that NDUFAF3 is a genuine mitochondrial complex I assembly protein that interacts with complex I subunits. Furthermore, we show that NDUFAF3 tightly interacts with NDUFAF4 (C6ORF66), a protein previously implicated in complex I deficiency. Additional gene conservation analysis links NDUFAF3 to bacterial-membrane-insertion gene cluster SecF/SecD/YajC and to C8ORF38, also implicated in complex I deficiency. These data not only show that NDUFAF3 mutations cause complex I deficiency but also relate different complex I disease genes by the close cooperation of their encoded proteins during the assembly process.

NDUFA2 complex I mutation leads to Leigh disease.

Mitochondrial isolated complex I deficiency is the most frequently encountered OXPHOS defect. We report a patient with an isolated complex I deficiency expressed in skin fibroblasts as well as muscle tissue. Because the parents were consanguineous, we performed homozygosity mapping to identify homozygous regions containing candidate genes such as NDUFA2 on chromosome 5. Screening of this gene on genomic DNA revealed a mutation that interferes with correct splicing and results in the skipping of exon 2. Exon skipping was confirmed on the mRNA level. The mutation in this accessory subunit causes reduced activity and disturbed assembly of complex I. Furthermore, the mutation is associated with a mitochondrial depolarization. The expression and activity of complex I and the depolarization was (partially) rescued with a baculovirus system expressing the NDUFA2 gene.

Novel mutations in the NDUFS1 gene cause low residual activities in human complex I deficiencies.

Mitochondrial complex I deficiency is the most frequently encountered defect of the oxidative phosphorylation system. To identify the genetic cause of the complex I deficiency, we screened the gene encoding the NDUFS1 subunit. We report 3 patients with low residual complex I activity expressed in cultured fibroblasts, which displayed novel mutations in the NDUFS1 gene. One mutation introduces a premature stop codon, 3 mutations cause a substitution of amino acids and another mutation a deletion of one amino acid. The fibroblasts of the patients display a decreased amount and activity of complex I. In addition, a disturbed assembly pattern was observed. These results suggest that NDUFS1 is a prime candidate to screen for disease-causing mutations in patients with a very low residual complex I activity in cultured fibroblasts.

Baculovirus complementation restores a novel NDUFAF2 mutation causing complex I deficiency.

Mitochondrial complex I deficiency is the most common defect of the OXPHOS system. We report a patient from consanguineous parents with a complex I deficiency expressed in skin fibroblasts. Homozygosity mapping revealed several homozygous regions with candidate genes, including the gene encoding an assembly factor for complex I, NDUFAF2. Screening of this gene on genomic DNA revealed a homozygous stop-codon resulting in a truncation of the protein at position 38. The mutation causes a severely reduced activity and a disturbed assembly of complex I. A baculovirus containing the GFP-tagged wild-type NDUFAF2 gene was used to prove the functional consequences of the mutation. The expression and activity of complex I was almost completely rescued by complementation of the patient fibroblasts with the baculovirus. Therefore, the homozygous substitution in NDUFAF2 is the disease-causing mutation, which results in a complex I deficiency in the fibroblasts of the patient.

Molecular base of biochemical complex I deficiency.

The oxidative phosphorylation (OXPHOS) system, consisting of five enzyme complexes (I-V) together with 2 electron carriers, has an important role in the energy metabolism of the cell. With 45 subunits, complex I is the first and largest complex of the respiratory chain. It is under bigenomic control and a proper interaction between the mitochondrial and the nuclear genome is important for a good biogenesis and functioning of the complex. Isolated complex I deficiency is the most frequently diagnosed form of mitochondrial disorders caused by the disturbance of the OXPHOS system. It has a wide clinical variety and, at present, in many patients the underlying genetic cause of the complex I deficiency is still not known. In this review, the role of complex I in the oxidative phosphorylation and the localization and function of the different complex I subunits will be described. Furthermore, a brief overview of the assembly process and biochemical studies, performed when a patient is suspected of a mitochondrial disorder is given. Finally, the present knowledge for molecular base of complex I deficiency is described and the findings in a research cohort of patients with complex I deficiency are reported. Identifying new genes encoding proteins involved in complex I biogenesis is challenging and in the near future new powerful techniques will make high throughput screening possible. Progress in elucidating the genetic defect causing complex I deficiencies is important for a better genetic counseling, prenatal diagnostic possibilities and further development of new treatment strategies to cure the complex I deficiencies in the future.

NDUFA10 mutations cause complex I deficiency in a patient with Leigh disease.

Mitochondrial complex I deficiency is the most common defect of the oxidative phosphorylation system. We report a patient with Leigh syndrome who showed a complex I deficiency expressed in cultured fibroblasts and muscle tissue. To find the genetic cause of the complex I deficiency, we screened the mitochondrial DNA and the nuclear-encoded subunits of complex I. We identified compound-heterozygous mutations in the NDUFA10 gene, encoding an accessory subunit of complex I. The first mutation disrupted the start codon and the second mutation resulted in an amino acid substitution. The fibroblasts of the patient displayed decreased amount and activity, and a disturbed assembly of complex I. These results indicate that NDUFA10 is a novel candidate gene to screen for disease-causing mutations in patients with complex I deficiency.