Mutations in the UQCC1-interacting protein, UQCC2, cause human complex III deficiency associated with perturbed cytochrome b protein expression

PLoS Genet. 2013;9(12):e1004034. doi: 10.1371/journal.pgen.1004034.

Elena J Tucker ¹, Bas F J Wanschers ², Radek Szklarczyk ³, Hayley S Mountford ¹, Xiaonan W Wijeyeratne ⁴, Mariël A M van den Brand ⁵, Anne M Leenders ⁵, Richard J Rodenburg ⁵, Boris Reljić ⁴, Alison G Compton ¹, Ann E Frazier ¹, Damien L Bruno ⁶, John Christodoulou ⁷, Hitoshi Endo ⁸, Michael T Ryan ⁹, Leo G Nijtmans ⁵, Martijn A Huynen ³, David R Thorburn ¹⁰

Affiliations

1 Murdoch Children's Research Institute, Royal Children's Hospital, Parkville, Victoria, Australia ; Department of Paediatrics, University of Melbourne, Melbourne, Victoria, Australia.
2 Centre for Molecular and Biomolecular Informatics, Nijmegen Centre for Molecular Life Sciences, Radboud University Medical Centre, Nijmegen, The Netherlands ; Nijmegen Center for Mitochondrial Disorders, Radboud University Medical Centre, Nijmegen, The Netherlands.
3 Centre for Molecular and Biomolecular Informatics, Nijmegen Centre for Molecular Life Sciences, Radboud University Medical Centre, Nijmegen, The Netherlands.
4 Department of Biochemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, Australia.
5 Nijmegen Center for Mitochondrial Disorders, Radboud University Medical Centre, Nijmegen, The Netherlands.
6 Murdoch Children's Research Institute, Royal Children's Hospital, Parkville, Victoria, Australia ; Victorian Clinical Genetics Services, Royal Children's Hospital, Melbourne, Victoria, Australia.
7 Genetic Metabolic Disorders Research Unit, Children's Hospital at Westmead, Westmead, New South Wales, Australia ; Disciplines of Paediatrics & Child Health and Genetic Medicine, University of Sydney, Sydney, New South Wales, Australia.
8 Department of Biochemistry, Jichi Medical University, Tochigi, Japan.
9 Department of Biochemistry, La Trobe Institute for Molecular Science, La Trobe University, Melbourne, Victoria, Australia ; ARC Centre of Excellence for Coherent X-ray Science, La Trobe University, Melbourne, Australia.
10 Murdoch Children's Research Institute, Royal Children's Hospital, Parkville, Victoria, Australia ; Department of Paediatrics, University of Melbourne, Melbourne, Victoria, Australia ; Victorian Clinical Genetics Services, Royal Children's Hospital, Melbourne, Victoria, Australia.

PMID: 24385928 DOI: 10.1371/journal.pgen.1004034

Abstract

Mitochondrial Oxidative Phosphorylation (OXPHOS) is responsible for generating the majority of cellular ATP. Complex III (ubiquinol-cytochrome c oxidoreductase) is the third of five OXPHOS complexes. Complex III assembly relies on the coordinated expression of the mitochondrial and nuclear genomes, with 10 subunits encoded by nuclear DNA and one by mitochondrial DNA (mtDNA). Complex III deficiency is a debilitating and often fatal disorder that can arise from mutations in complex III subunit genes or one of three known complex III assembly factors. The molecular cause for complex III deficiency in about half of cases, however, is unknown and there are likely many complex III assembly factors yet to be identified. Here, we used Massively Parallel Sequencing to identify a homozygous splicing mutation in the gene encoding Ubiquinol-Cytochrome c Reductase Complex Assembly Factor 2 (UQCC2) in a consanguineous Lebanese patient displaying complex III deficiency, severe intrauterine growth retardation, neonatal lactic acidosis and renal tubular dysfunction. We prove causality of the mutation via lentiviral correction studies in patient fibroblasts. Sequence-profile based orthology prediction shows UQCC2 is an ortholog of the Saccharomyces cerevisiae complex III assembly factor, Cbp6p, although its sequence has diverged substantially. Co-purification studies show that UQCC2 interacts with UQCC1, the predicted ortholog of the Cbp6p binding partner, Cbp3p. Fibroblasts from the patient with UQCC2 mutations have deficiency of UQCC1, while UQCC1-depleted cells have reduced levels of UQCC2 and complex III. We show that UQCC1 binds the newly synthesized mtDNA-encoded cytochrome b subunit of complex III and that UQCC2 patient fibroblasts have specific defects in the synthesis or stability of cytochrome b. This work reveals a new cause for complex III deficiency that can assist future patient diagnosis, and provides insight into human complex III assembly by establishing that UQCC1 and UQCC2 are complex III assembly factors participating in cytochrome b biogenesis.

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