The molecular and cellular basis of copper dysregulation and its relationship with human pathologies

Copper (Cu) is an essential micronutrient required for the activity of redox-active enzymes involved in critical metabolic reactions, signaling pathways, and biological functions. Transporters and chaperones control Cu ion levels and bioavailability to ensure proper subcellular and systemic Cu distribution. Intensive research has focused on understanding how mammalian cells maintain Cu homeostasis, and how molecular signals coordinate Cu acquisition and storage within organs. In humans, mutations of genes that regulate Cu homeostasis or facilitate interactions with Cu ions lead to numerous pathologic conditions. Malfunctions of the Cu⁺ -transporting ATPases ATP7A and ATP7B cause Menkes disease and Wilson disease, respectively. Additionally, defects in the mitochondrial and cellular distributions and homeostasis of Cu lead to severe neurodegenerative conditions, mitochondrial myopathies, and metabolic diseases. Cu has a dual nature in carcinogenesis as a promotor of tumor growth and an inducer of redox stress in cancer cells. Cu also plays role in cancer treatment as a component of drugs and a regulator of drug sensitivity and uptake. In this review, we provide an overview of the current knowledge of Cu metabolism and transport and its relation to various human pathologies.

Mitochondrial disorders of the OXPHOS system

Mitochondrial disorders are among the most frequent inborn errors of metabolism, their primary cause being the dysfunction of the oxidative phosphorylation system (OXPHOS). OXPHOS is composed of the electron transport chain (ETC), formed by four multimeric enzymes and two mobile electron carriers, plus an ATP synthase [also called complex V (cV)]. The ETC performs the redox reactions involved in cellular respiration while generating the proton motive force used by cV to synthesize ATP. OXPHOS biogenesis involves multiple steps, starting from the expression of genes encoded in physically separated genomes, namely the mitochondrial and nuclear DNA, to the coordinated assembly of components and cofactors building each individual complex and eventually the supercomplexes. The genetic cause underlying around half of the diagnosed mitochondrial disease cases is currently known. Many of these cases result from pathogenic variants in genes encoding structural subunits or additional factors directly involved in the assembly of the ETC complexes. Here, we review the historical and most recent findings concerning the clinical phenotypes and the molecular pathological mechanisms underlying this particular group of disorders.

A history of mitochondrial diseases

This articles reviews the development of mitochondrial medicine from the premolecular era (1962-1988), when mitochondrial diseases were defined on the basis of clinical examination, muscle biopsy, and biochemical criteria, through the molecular era, when the full complexity of these disorders became evident. In a chronological order, I have followed the introduction of new pathogenic concepts that have shaped a rational genetic classification of these clinically heterogeneous disorders. Thus, mitochondrial DNA (mtDNA)-related diseases can be divided into two main groups: those that impair mitochondrial protein synthesis in toto, and those that affect specific respiratory chain proteins. Mutations in nuclear DNA can affect components of respiratory chain complexes (direct hits) or assembly proteins (indirect hits), but they can also impair mtDNA integrity (multiple mtDNA mutations), replication (mtDNA depletion), or mtDNA translation. Besides these disorders that affect the respiratory chain directly, defects in other mitochondrial functions may also affect oxidative phosphorylation, including problems in mitochondrial protein import, alterations of the inner mitochondrial membrane lipid composition, and defects of mitochondrial dynamics. The enormous and still ongoing progress in our understanding of mitochondrial medicine was made possible by the intense collaboration of an international cadre of "mitochondriacs." Having published my first paper on a patient with mitochondrial myopathy 37 years ago (DiMauro et al., 1973), I feel qualified to write a history of the mitochondrial diseases, a fascinating, still evolving, and continuously puzzling area of medicine. In each section, I follow a chronological order of the salient discoveries and I show only the portraits of distinguished deceased mitochondriacs and those whose names became eponyms of mitochondrial diseases.

Structural analysis of tissues affected by cytochrome C oxidase deficiency due to mutations in the SCO2 gene

Structural and histochemical studies carried out in a series of seven cases (from five families) with isolated cytochrome c oxidase (COX) deficiency caused by mutations in the SCO2 gene (1, 2) disclosed changes concentrated in the nervous system, skeletal muscle and myocardium. In five patients homozygous for the E140K mutation, the phenotype was predominantly neuromuscular and the average life span ranged between 9 and 15 months. In two cases, the course was more rapid (death at 7 and 11 weeks of life) and featured marked cardiac hypertrophy (3- and 4-fold increase in heart weight). This predominantly cardiomyopathic phenotype was associated with compound heterozygosity (E140K with another nonsense mutation) in the SCO2 gene. Polioencephalopathy with neurodegeneration and neuronal drop out was present in all cases with evidence that retinal neurons might be seriously affected too. Involvement of spinal motoneurons together with cytochrome c oxidase deficiency in muscle represents a "double hit" for the skeletal muscle. The mitochondrial population was not found to be significantly increased or structurally altered, with the exception of two compound heterozygotes in which the cardiac mitochondria were increased in number and size. Our report extends knowledge of the pathology of COX deficiency caused by mutations in the SCO2 gene.