Mitochondrial Cytochrome C deficiency can show the first disease signs in the prenatal stage

Cytochrome C oxidase deficiency is a group of mitochondrial disorders characterized by defects in the COX biogenesis and/or activity. Genetically, they can be caused by maternally inherited defects in mitochondrial DNA (mtDNA) encoded core subunits proteins (MT-CO1, MT-CO2, and MT-CO3) or mendelian inherited pathogenetic variants in nuclear DNA (nDNA) encoded accessory subunits (COX4I1/I2, COX5A, COX5B, COX6A1/A2, COX6B1, COX6C, COX7A1, COX7B, COX7C, COX8A, and NDUFA4/COXFA4,) and assembly factors (COX14, COA3, TACO1, COX10, COX15, COX20, SCO1, SCO2, SURF1, COA5, COA6, COA7, COA8, PET100, PET117, FASTKD2, and LRPPRC) proteins [1]. The list is yet to be completed since an increasing number of proteins have been found to play a role in the assembly mechanism. Clinically, they are extremely variable for age at onset, signs/symptoms, affected organ/tissue (muscle, heart, brain, liver), and severity ranging from neonatal and fatal cardio-myo-encephalopathy to mild myopathy compatible with adult life. Ronchi et al. [2] reported for the first time a homozygous c.667 G > C variant (p. Asp223His) in the COX18 gene in a 19-month-old girl with cardio-myo-encephalopathy whose first sign was hypertrophic cardiomyopathy at 35th week of gestation. After birth, the disease course was characterized by failure to thrive, poor feeding, lactic acidosis, axonal polyneuropathy, psychomotor delay, and a confirmed hypertrophic ventricular cardiomyopathy together with conduction abnormalities (suggestive of Wolff-Parkinson-White syndrome) and aortic arc vascular annulus malformation.

The clinical relevance of the report is indeed in the prenatal onset of cardiac involvement, which has several important implications. Hypertrophic cardiomyopathy can be due to inborn errors of metabolism in 15–20% of cases and represents the main clinical feature in 30–40% of cases with mitochondrial disorders [3]. COX deficiency is one of the most frequent causes of neonatal onset of mitochondrial disorder and hypertrophic cardiomyopathy can be the first sign even at the prenatal gestational stage. An early detection and genetic diagnosis can prompt an appropriate surveillance and management plan at birth to prevent the rapid progression of the disease to a fatal outcome; it is fundamental for defining the natural history of the disease since the pre-symptomatic stage and identifying prognostic factors; it is important for stratifying patients based on the risk of organ failure and, consequently, guide the decision for a heart transplantation. The latter has been always a matter of debate in patients with mitochondrial disorders since they are often multiorgan and patients may develop later in life additional signs and symptoms of other affected organs. However, the heterogeneity of COX deficiency includes a subgroup of patients with tissue-specific and/or not progressive clinical syndrome that could benefit from heart transplants. Therefore, COX deficiency disease-causing genes must be included in targeted gene panels for early-onset hypertrophic cardiomyopathy, genetics testing should be considered at the earliest time point of the diagnostic workup and a knowledgeable clinical utilization and appropriate genetic counseling must be offered.

The observation of antenatal development of cardiac involvement also highlights the cardiomyocytes’ dependence on the bioenergetic OXPHOS metabolism and specifically, on COX activity. It has been already suggested that abolishment or severe reduction in both COX activity and synthesis is incompatible with life and only rare permissive or hypomorphic variants have been associated with diseases. In the specific case of COX18 variants, 36% of residual activity was found in the patient’s myoblasts, and a similar level was detected for steady-state protein level by western-blot and the fully assembled native protein by BN-PAGE confirming that a residual COX activity is needed for a live birth. This observation must be considered by medical geneticists and biologists while interpreting data from in silico analysis and in vitro functional studies for defining the pathogenicity of variants.

COX is the terminal enzyme of the electron transport chain that catalyses the oxidation of cytochrome c and the reduction of molecular oxygen to water consequently generating protons pumps from the mitochondrial matrix into the intermembrane space. COX biogenesis follows a linear mechanism in which an initial seed composed of COX4 and COX5A sequentially joins three protein modules pre-formed around the three central catalytic core proteins MT-CO1, MT-CO2, and MT-CO3. The assembly required several assembly factors such as chaperones, transcriptional and translational activators, metal-binding proteins, and enzymes. A study in HEK293T COX18 knock-out cell line constructed by TALENs demonstrated the role of COX18 in the assembly of MT-CO2, a protein composed of two transmembrane domains and a long hydrophilic C terminus protruding into the intermembrane space (IMS) containing the dinuclear copper center (CuA). Specifically, COX18 promotes the translocation of the MT-CO2 C-tail containing the apo-Cu_A site in the inner membrane while COX20 stabilizes the N-proximal transmembrane domain [4]. The release of COX18 coincides with the binding of the SCO1-SCO2-COA6 copper insertion module to complete the MT-CO2 biogenesis. Ronchi et al. study confirmed the role of COX18 in MT-CO2 biogenesis in the patient’s myoblasts carrying the c.667 G > C variant and silenced COX18 HEK293 cells. Interestingly, pathogenic variants in MT-CO2, SCO1, SCO2, COA6, and COX18, all involved in the process of the MT-CO2 module assembly, have been associated with human disease with isolated cardiomyopathy or cardio-myo-encephalopathy suggesting a high specificity of MT-CO2 assembly module for cardiac tissue.

Several clinical and biological questions remain still open. Clinical variability cannot be explained by genotype-phenotype correlation for defects in nDNA-encoded proteins nor by the level of heteroplasmy for defects in mtDNA-encoded subunits. Multi-omics genomics, transcriptomic, and proteomics studies in human disease models are warranted to identify key players in cell metabolism acting as disease modifiers. COX complex genes have peculiar characteristics, such as tissue-specific gene isoforms and post-translational modifications, but the mechanists’ details of the regulation are still unknown. Molecular genetics and biology studies in human disease models are fundamental to shed light on COX assembly pathways in health and disease and identify genetic and epigenetics targets for therapy.

In conclusion, a genetic test is a standard of care that must be offered to the family at the earliest time point of the disease course. The detection of antenatal signs of a potential metabolic and specifically, COX deficiency disorder can guide genetic diagnosis and management plan, thereby modifying the natural history of the patient.

Author contributions

CG is the only author of this commentary.

Funding

CG was supported by PNRR M4C2-Investimento 1.4 CN00000041 -mRNA National Center for Gene Therapy and Drugs based on RNA Technology - #NEXTGENERATIONEU (NGEU)—financed by European Union.

Competing interests

The author declares no competing interests.