Analysis of mitochondrial SCO proteins in mammals: Relevance to human mitochondrial disease

In mammals, there are two related SCO proteins, SCO1 and SCO2, that are required for the proper assembly of cytochrome c oxidase (COX), one of the complexes of the mitochondrial respiratory chain, but their exact function is unknown. Since both proteins contain a copper-binding domain, they have been suggested to function as copper chaperones for COX, specifically for the insertion of Cu into the CuA site in subunit II. Mutations in either gene result in fatal infantile encephalomyopathies in humans, with COX deficiency in affected tissues. SCO1 patients also suffer from hepatopathy, whereas SCO2 patients have cardiac involvement. The origin of the distinct clinical phenotypes is unknown.Here, we show that there is tissue specific expression of the two proteins. SCO1 is expressed predominantly in the vasculature, an unexpected and novel feature for a mitochondrial protein. In addition, we observed high levels of SCO1 expression in liver associated with very low levels of SCO2, a possible explanation for the hepatopathy in SCO1 patients. In skeletal muscle we saw the reverse, namely, large amounts of SCO2 and small amounts of SCO1. This differential localization of the SCO proteins can explain in part the distinct clinical presentations of the patients, and should help in further studies of the non-overlapping functions of the two SCO proteins.We have created two mouse models for Scot: one is a knock-in (KI) mouse homozygous for a missense mutation common in almost all patients, which causes the conversion of Glu-140 to Lys (E14OK) the other is a compound heterozygote with the above-mentioned missense mutation on one allele and a knockout (KO) of SCO2 on the other allele. Homozygous KO mice are embryonic lethal during early development, around day 7.5dpc. Both the homozygous KI and the compound heterozygous KI/KO mice were viable with a normal lifespan, but biochemical and histochemical analysis of the respiratory chain enzymes showed COX deficiency in brain, heart, liver, and muscle. The COX deficiency was accompanied by reduction in fully-assembled COX and the accumulation of assembly intermediates. Furthermore, the mice had muscle weakness when exposed to an endurance test on a treadmill and a hanging wire test. However, we did not find any cardiac malfunction, as is observed in humans with similar SCO2 mutations. In addition we found a minor reduction in copper content in mitochondria isolated from tissues, but not in the tissues overall. These mouse models should be a useful tool for further functional studies and for testing potential treatments for the human disease.