Characterising a homozygous two‐exon deletion in UQCRH : comparing human and mouse phenotypes

Abstract

Mitochondrial disorders are clinically and genetically diverse, with isolated complex III (CIII) deficiency being relatively rare. Here, we describe two affected cousins, presenting with recurrent episodes of severe lactic acidosis, hyperammonaemia, hypoglycaemia and encephalopathy. Genetic investigations in both cases identified a homozygous deletion of exons 2 and 3 of UQCRH, which encodes a structural complex III (CIII) subunit. We generated a mouse model with the equivalent homozygous Uqcrh deletion (Uqcrh À/À ), which also presented with lactic acidosis and hyperammonaemia, but had a more severe, non-episodic phenotype, resulting in failure to thrive and early death. The biochemical phenotypes observed in patient and Uqcrh À/À mouse tissues were remarkably similar, displaying impaired CIII activity, decreased molecular weight of fully assembled holoenzyme and an increase of an unexpected large supercomplex (S XL ), comprising mostly of one complex I (CI) dimer and one CIII dimer. This phenotypic similarity along with lentiviral rescue experiments in patient fibroblasts verifies the pathogenicity of the shared genetic defect, demonstrating that the Uqcrh À/À mouse is a valuable model for future studies of human CIII deficiency.

Knockout of the Complex III subunit Uqcrh causes bioenergetic impairment and cardiac contractile dysfunction

“…Uqcrh -KO mice were born with a lower Mendelian ratio and showed failure to thrive, which was particularly evident after weaning, suggesting that the metabolic disturbance exacerbates with the switch in diet from breast milk to regular chow. We measured body mass at 6, 7, 8, and 9 weeks and confirmed a previously described growth arrest (Vidali et al 2021 ) resulting in significantly lower body mass of approximately 75% (or 4–5 g less) at 6 weeks and approximately 65% (or 7–9 g less) at 9 weeks of age compared to wildtype littermate (C57BL/6 N) control mice (Table 1 ). The growth arrest was accompanied by significantly elevated blood glucose levels in Uqcrh -KO with > 25 mmol/L at 6 weeks of age, which further increased to > 29 mmol/L at 9 weeks of age compared to 3.4–9.7 mmol/L measured throughout in wildtype littermate controls (Table 1 ).…”

“…Uqcrh -KO mice were generated by a two-exon deletion and show clinical signs with striking similarities, albeit a more severe phenotype compared with patients diagnosed with the corresponding deletion (Vidali et al 2021 ). As a result of the mutation, both humans and mice show marked impairment of CIII activity.…”

“…Here, we set out to test the consequences of global ablation of another CIII-related protein on cardiac contractile function, the ubiquinol cytochrome c reductase hinge protein (UQCRH), a regulator of electron transfer between cytochrome c and c of CIII (Kim and King 1983 ; Mukai et al 1985 ; Kim et al 1987 ; Ohta et al 1987 ; Park et al 2017 ). A two-exon deletion in UQCRH has recently been identified in two pediatric patients who developed a clinical condition characterized by recurrent episodes of severe ketoacidosis, excess blood ammonia, hypoglycemia, and signs of encephalopathy concomitant with impaired function of CIII (Vidali et al 2021 ). Although UQCRH is broadly expressed across tissues, its function seems to be particularly important for organs with high-energy metabolism (Modena et al 2003 ).…”

“…However, an association between UQCRH deletion and cardiac function has not yet been investigated. Here, we took advantage of a recently generated and characterized mouse model with corresponding two-exon deletion ( Uqcrh -KO) (Vidali et al 2021 ) to explore the relationship between mitochondrial metabolism and cardiac contractile function in greater detail. The Uqcrh -KO mouse model largely replicates the human condition and most notably shows an impaired activity of CIII, hyperglycemia, and premature death at the age of approximately 12 weeks.…”

Integration of Human Protein Sequence and Protein-Protein Interaction Data by Graph Autoencoder to Identify Novel Protein-Abnormal Phenotype Associations

“…Our predictions about 2 DAMRC-associated genes were validated by recent papers. An article published in November 2021 reported that UQCRH (Rank:8)-deficient mouse model shows impaired CIII activity [ 42 ]. ATP5MC3 (Rank:24) encodes a structural complex V (CV) subunit, and heterozygous ATP5MC3 variants were reported to reduce mitochondrial complex V activity in a paper published in October 2021 [ 43 ].…”