A mutation in C2orf64 causes impaired cytochrome c oxidase assembly and mitochondrial cardiomyopathy

Abstract

The assembly of mitochondrial respiratory chain complex IV (cytochrome c oxidase) involves the coordinated action of several assembly chaperones. In Saccharomyces cerevisiae, at least 30 different assembly chaperones have been identified. To date, pathogenic mutations leading to a mitochondrial disorder have been identified in only seven of the corresponding human genes. One of the genes for which the relevance to human pathology is unknown is C2orf64, an ortholog of the S. cerevisiae gene PET191. This gene has previously been shown to be a complex IV assembly factor in yeast, although its exact role is still unknown. Previous research in a large cohort of complex IV deficient patients did not support an etiological role of C2orf64 in complex IV deficiency. In this report, a homozygous mutation in C2orf64 is described in two siblings affected by fatal neonatal cardiomyopathy. Pathogenicity of the mutation is supported by the results of a complementation experiment, showing that complex IV activity can be fully restored by retroviral transduction of wild-type C2orf64 in patient-derived fibroblasts. Detailed analysis of complex IV assembly intermediates in patient fibroblasts by 2D-BN PAGE revealed the accumulation of a small assembly intermediate containing subunit COX1 but not the COX2, COX4, or COX5b subunits, indicating that C2orf64 is involved in an early step of the complex IV assembly process. The results of this study demonstrate that C2orf64 is essential for human complex IV assembly and that C2orf64 mutational analysis should be considered for complex IV deficient patients, in particular those with hypertrophic cardiomyopathy.

Figure 1

Family Pedigree and Molecular Genetic Analysis of the C2orf64 cDNA (A) Pedigree of the family of the two patients described in this report. (B) Electropherograms showing the wild-type sequence of C2orf64 (top panel) and the nucleotide changes in the complex IV deficient patients P1 (VI-1 in A) and P2 (VI-2) and the healthy siblings S3 (VI-3) and S4 (VI-4). The arrow indicates the mutated nucleotide c.157G>C. P1 and P2 are homozygous for the c.157G>C mutation, whereas S3 is a heterozygous carrier and S4 carries the wild-type sequence. Please note that the reverse sequence is shown.

Figure 2

Reduced Complex IV Activity and Amount in Fibroblasts from the Patients with a C2orf64 Mutation (A) Fibroblasts from patients P1 and P2 show a severely reduced in-gel activity of complex IV compared to the unaffected siblings S3 and S4 (top panel). The lower three panels show the results of immunoblots after nondenaturing BN-PAGE, revealing a severely reduced complex IV amount in patients P1 and P2. Complex IV was stained with anti-COX4 antibodies. Equal loading of the gel was tested by staining for complex III (by using anti-CORE2) and complex II (by using anti-70 kDa subunit). Holocomplex IV is indicated by the arrowhead; the asterisk indicates a nonspecific band. (B) Immunoblot after SDS-PAGE of fibroblast extracts of patients P1 and P2 and healthy siblings S3 and S4 showing reduced expression levels of complex IV subunits COX1, COX2, COX4, and COX5a in fibroblasts of both patients. The complex II 70 kDa subunit (CII 70 kDa) was used as a loading control. (C) Two dimensional BN-PAGE analysis of fibroblasts from patients P1 and P2 and the healthy siblings S3 and S4 was performed in accordance with standard procedures. Blots were stained by using antibodies against different complex IV subunits as indicated. Holocomplex IV is indicated (H). Patients P1 and P2 show accumulation of a small subcomplex (indicated by S1) that contains COX1 but not COX2, COX4, and COX5a. This indicates that mutation of C2orf64 results in accumulation of a COX1 containing early complex IV assembly intermediate. Note that blots of different fibroblasts and blots stained with different antibodies can only be compared qualitatively because exposure times are not the same.

Figure 3

Complementation with Wild-Type C2orf64 Restores Complex IV Fibroblasts from patient P2 and healthy siblings S3 and S4 were infected with retrovirus containing no insert (pDS) or C2orf64 (C2). The C2orf64 complementation was done by cloning full length human C2orf64 cDNA clone IOH26651 (GeneID: 493753; Imagenes, Berlin, Germany) into the Gateway retroviral destination vector pDS_FBneo (# MBA-295, LGC, Middlesex, UK) with the Gateway LR Clonase II enzyme mix (Invitrogen). Recombinant viruses were produced by using the amphotropic packaging cell line PA317 (#CRL-9078, LGC, Middlesex, UK) according to the manufacturer's protocol (Invitrogen, Breda, The Netherlands). P2, S3, and S4 fibroblasts were incubated for 24 hr with a 1:1 mixture of growth medium and virus-containing medium in the presence of 4 μg/ml polybrene, followed by 14 days selection with 500 μg/ml geneticin (G418, PAA, Pasching, Austria). G418-resistant cells were used for biochemical analyses within 5 passages after transduction. P2 fibroblasts grew very slowly and all cells failed to survive the selection procedure upon retroviral transduction of the empty vector. Mitochondrion-enriched fractions from all other transduced cell lines were tested for in-gel activity of complex IV after BN-PAGE. In addition, amounts of OXPHOS complexes were visualized with anti-NDUFA9 (complex I; CI), anti-70 kDa subunit of complex II (CII), anti-CORE2 for complex III (CIII), anti-COX1 for complex IV (CIV) and anti-complex Vα for complex V (CV). The results show a clear complex IV deficiency, consistent with the results in Figure 2A, which is rescued by complementation with wild-type C2orf64 (indicated by C2). Lanes indicated with wt are nontransduced fibroblasts, lanes with pDS indicate fibroblasts transduced with empty virus. Note that the complex III panel shows three bands, the lowest of which represents holo-complex III, whereas the two more slowly migrating bands most likely represent supercomplexes containing complex III. The middle band is absent in patient P2 and returns after complementation with wild-type C2orf64. This middle band therefore most likely represents a supercomplex containing complex III and complex IV.

Figure 4

The p.Ala53Pro Mutation in the Protein Sequence of C2orf64 (A) Alignment of the C2orf64 protein with its eukaryotic orthologs. Predicted α helices are indicated above the alignment. (B) Helical-wheel projection of the second Cx₉C motif. Hydrophobic and hydrophilic residues are marked in red and blue, respectively. Cysteine residues (yellow) on the hydrophobic face of the helix could form disulfide bridges with the first cysteine motif.