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SCO2
synthesis of cytochrome C oxidase 2
SCO2 is located on the long (q) arm of chromosome 22, at band 22q13.33. Arm ratio per GRCh38 - banding schematic.
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Overview
The SCO2 gene, located on chromosome 22, encodes a mitochondrial protein required for the proper assembly of cytochrome c oxidase (also known as complex IV). This enzyme complex sits at the end of the electron transport chain within mitochondria, where it facilitates the final steps of cellular respiration that generate adenosine triphosphate (ATP), the cell's primary energy currency.
Pathogenic changes in SCO2 follow an autosomal recessive inheritance pattern, meaning that affected individuals typically inherit one altered copy from each parent. Carriers with a single pathogenic variant generally remain healthy, whilst those with variants in both gene copies may develop mitochondrial complex IV deficiency, a condition characterised by impaired energy production affecting multiple organ systems.
What the gene does
The SCO2 protein functions as a metallochaperone, specifically binding copper ions and transferring them to the CuB site of the COX2 subunit during cytochrome c oxidase assembly. This copper insertion step is essential for the catalytic activity of the fully assembled complex IV, which couples electron transfer from cytochrome c to oxygen with proton pumping across the inner mitochondrial membrane.
Without functional SCO2, the incorporation of copper into cytochrome c oxidase becomes inefficient, resulting in reduced or absent complex IV activity. Cells with defective complex IV cannot maintain normal ATP production through oxidative phosphorylation, forcing reliance on less efficient anaerobic metabolism. Tissues with high metabolic demands, particularly cardiac muscle and the central nervous system, are especially vulnerable to energy deficits caused by SCO2 dysfunction.
The protein resides within the mitochondrial inner membrane space, where it collaborates with other assembly factors to coordinate the multi-step process of building functional respiratory chain complexes. This coordinated assembly ensures that electron transport proceeds efficiently and safely, minimising the production of reactive oxygen species that can damage cellular components.
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Chromosome location
SCO2 is located on the long arm of chromosome 22 at position 13.33 (22q13.33), a region near the chromosome's terminus. The gene comprises seven exons that encode a 266-amino-acid protein. This chromosomal location places SCO2 within a gene-dense region containing multiple loci associated with diverse clinical phenotypes.
Protein structure
The SCO2 protein contains a thioredoxin domain spanning amino acids 85 to 259, which provides the structural framework for copper binding and transfer. This domain architecture is characteristic of redox-active proteins and supports the protein's role in coordinating metal ion chemistry within the mitochondrial environment. The thioredoxin fold enables SCO2 to maintain appropriate copper coordination during the handoff to cytochrome c oxidase subunits, ensuring proper metallocentre assembly in the target enzyme complex.
Key variants
Pathogenic variants in SCO2 typically result in loss of protein function, either through amino acid substitutions that disrupt copper binding or through changes that destabilise the protein structure. The majority of clinically significant variants are missense changes affecting residues critical for metal coordination or protein folding. Because SCO2-related conditions follow autosomal recessive inheritance, affected individuals generally carry pathogenic variants on both gene copies, whilst heterozygous carriers remain asymptomatic.
No pathogenic or likely-pathogenic ClinVar variants recorded yet for this gene.
Associated conditions
Biallelic pathogenic variants in SCO2 cause mitochondrial complex IV deficiency, a clinically heterogeneous disorder that can present in infancy or early childhood. The most common presentation includes hypertrophic cardiomyopathy, hypotonia, and encephalopathy, reflecting the vulnerability of cardiac and neural tissues to energy deficiency. Some affected individuals exhibit additional features such as metabolic acidosis, developmental delay, or distinctive facial characteristics. Disease severity varies depending on the specific variants present and the degree of residual complex IV activity they permit.
No disease links recorded for this gene in our reference set.
Inheritance pattern
Conditions caused by pathogenic SCO2 variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
SCO2 appears on multiple NHS Genomic Medicine Service gene panels with green (high-evidence) classifications, reflecting strong consensus regarding its clinical validity. The gene is included on panels for mitochondrial disorders, mitochondrial disorder with complex IV deficiency, likely inborn errors of metabolism, paediatric or syndromic cardiomyopathy, and intellectual disability, among others. This broad panel representation underscores the diverse clinical presentations associated with SCO2 deficiency and supports its inclusion in diagnostic genomic testing for individuals with unexplained multi-system disease involving energy metabolism.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Frequently asked questions
What does it mean to be a carrier of an SCO2 variant?
Carriers have one pathogenic SCO2 variant and one typical copy, which generally provides sufficient protein function for normal health. Carrier status becomes relevant for family planning, as two carrier parents have a 25% chance with each pregnancy of having a child with SCO2-related mitochondrial disease.
How is SCO2-related complex IV deficiency diagnosed?
Diagnosis typically combines clinical assessment, biochemical testing showing reduced complex IV enzyme activity in muscle or other tissues, and genetic testing confirming biallelic pathogenic SCO2 variants. Elevated lactate levels and abnormal imaging findings may support the diagnosis.
Can SCO2-related conditions be treated?
Management focuses on supportive care tailored to the specific organ systems affected, such as cardiac monitoring and intervention for cardiomyopathy, and developmental support for neurological impairment. No curative treatments currently exist, though experimental therapies targeting mitochondrial function are under investigation.