A nonsense mutation in COQ9 causes autosomal-recessive neonatal-onset primary coenzyme Q10 deficiency: a potentially treatable form of mitochondrial disease

Abstract

Coenzyme Q(10) is a mobile lipophilic electron carrier located in the inner mitochondrial membrane. Defects of coenzyme Q(10) biosynthesis represent one of the few treatable mitochondrial diseases. We genotyped a patient with primary coenzyme Q(10) deficiency who presented with neonatal lactic acidosis and later developed multisytem disease including intractable seizures, global developmental delay, hypertrophic cardiomyopathy, and renal tubular dysfunction. Cultured skin fibroblasts from the patient had a coenzyme Q(10) biosynthetic rate of 11% of normal controls and accumulated an abnormal metabolite that we believe to be a biosynthetic intermediate. In view of the rarity of coenzyme Q(10) deficiency, we hypothesized that the disease-causing gene might lie in a region of ancestral homozygosity by descent. Data from an Illumina HumanHap550 array were analyzed with BeadStudio software. Sixteen regions of homozygosity >1.5 Mb were identified in the affected infant. Two of these regions included the loci of two of 16 candidate genes implicated in human coenzyme Q(10) biosynthesis. Sequence analysis demonstrated a homozygous stop mutation affecting a highly conserved residue of COQ9, leading to the truncation of 75 amino acids. Site-directed mutagenesis targeting the equivalent residue in the yeast Saccharomyces cerevisiae abolished respiratory growth.

Figure 1

Biosynthetic Pathway of Coenzyme Q₁₀

Figure 2

HPLC Analysis of Coenzyme Q₁₀ in Cultured Skin Fibroblasts Reverse phase EC-HPLC chromatograms obtained from lipid extracts of cultured skin fibroblasts from (A) healthy control fibroblasts and (B) the patient. Retention times are coenzyme Q₉ = 10.45 min; coenzyme Q₁₀ = 14.15 min. The patient's trace clearly shows an additional peak at 13.22 min (arrow) not resulting from coenzyme Q₉ or coenzyme Q₁₀, likely to be the result of accumulation of a metabolic intermediate of coenzyme Q₁₀ biosynthesis.

Figure 3

Sequence Analysis of COQ9 Sequencing electropherograms showing homozygous c.730C→T mutation in the patient's genomic DNA (top) and healthy control genomic DNA (bottom). The mutation predicts a change from arginine to a stop codon at amino acid 244 of the protein.

Figure 4

Evolutionary Conservation Data for COQ9

Figure 5

Molecular Model of COQ9 in Homodimeric Form One monomer is depicted in red, the other in blue. Figure created with Pymol.

Figure 6

Yeast Studies Left: Growth on respiratory (glycerol) medium. Right: Growth on fermentable (glucose) medium selective for the plasmids. (a) Δcoq9 mutant+ empty plasmid; (b) Δcoq9 mutant+ pYcoq9 (yeast COQ9); (c) Δcoq9 mutant+ pYcoq9Stop (yeast coq9 K191X); (d) Δcoq9 mutant+ pHcoq9 (human COQ9). COQ9 genes were placed under the control of a constitutive promoter on a multicopy plasmid.