Haploinsufficiency of COQ4 causes coenzyme Q10 deficiency

Abstract

Background: COQ4 encodes a protein that organises the multienzyme complex for the synthesis of coenzyme Q(10) (CoQ(10)). A 3.9 Mb deletion of chromosome 9q34.13 was identified in a 3-year-old boy with mental retardation, encephalomyopathy and dysmorphic features. Because the deletion encompassed COQ4, the patient was screened for CoQ(10) deficiency.

Methods: A complete molecular and biochemical characterisation of the patient's fibroblasts and of a yeast model were performed.

Results: The study found reduced COQ4 expression (48% of controls), CoQ(10) content and biosynthetic rate (44% and 43% of controls), and activities of respiratory chain complex II+III. Cells displayed a growth defect that was corrected by the addition of CoQ(10) to the culture medium. Knockdown of COQ4 in HeLa cells also resulted in a reduction of CoQ(10.) Diploid yeast haploinsufficient for COQ4 displayed similar CoQ deficiency. Haploinsufficency of other genes involved in CoQ(10) biosynthesis does not cause CoQ deficiency, underscoring the critical role of COQ4. Oral CoQ(10) supplementation resulted in a significant improvement of neuromuscular symptoms, which reappeared after supplementation was temporarily discontinued.

Conclusion: Mutations of COQ4 should be searched for in patients with CoQ(10) deficiency and encephalomyopathy; patients with genomic rearrangements involving COQ4 should be screened for CoQ(10) deficiency, as they could benefit from supplementation.

Figure 1

(A) Array comparative genomic hybridisation (Agilent 44 K chip) of PT 1 showing the 3.9 Mb deletion: profile of chromosome 9 showing a series of spots having an about −1 log₂ ratio at 9q34.11-q34.13. (B) Enlargement of the deleted region ranging from 129 531 to 133 523 Mb (assembly hg18). Oligos at 129 481 and 133 575 Mb resulted in normal log₂ ratio. The quality of the experiment has been considered excellent on the bases of QC metric parameters (DNA analytics). The arrow indicates the position of the COQ4 gene within the deleted region. (C) COQ4 copy number in genomic DNA of cultured skin fibroblasts of the patient determined by real-time quantitative PCR.

Figure 2

(A) COQ4 mRNA expression and (B) respiratory chain enzyme activities, in PT1 cultured skin fibroblasts. (C) Coenzyme Q₁₀ (CoQ₁₀) content in PT1 cells and in cells of PT2 and PT3 who harbour heterozygous mutations in COQ1-PDSS2. (D) CoQ₁₀ biosynthetic rate in PT1 cells measured by incorporation of ¹⁴C labelled p-HB. (E) Growth profile of PT1 skin fibroblasts. Incubation with 10 μM CoQ₁₀ restores normal growth in patient cells. (F) Effect of CoQ₁₀ supplementation on complex II+III activity in patient cells. (G) COQ4 mRNA levels and (H) CoQ₁₀ levels in cells stably expressing an anti-COQ4 shRNA. (I) CoQ₆ content in diploid yeast strains harbouring a heterozygous deletion of COQ4. Transformation with a plasmid-expressing COQ4 restores normal CoQ₆ content in these cells. (J) Complex II+III activity in diploid yeast strains harbouring heterozygous deletions of COQ4, COQ2 and COQ6. Activities of other RC enzymes were normal (note that yeast does not have complex I). *Significant versus controls; †Significant versus untreated cells.