Novel COX11 Mutations Associated with Mitochondrial Disorder: Functional Characterization in Patient Fibroblasts and Saccharomyces cerevisiae.

Genetic defects in the nuclear encoded subunits and assembly factors of cytochrome c oxidase (mitochondrial complex IV) are very rare and are associated with a wide variety of phenotypes. Biallelic pathogenic variants in the COX11 protein were previously identified in two unrelated children with infantile-onset mitochondrial encephalopathies. Through comprehensive clinical, genetic and functional analyses, here we report on a new patient harboring novel heterozygous variants in COX11, presenting with Leigh-like features, and provide additional experimental evidence for a direct correlation between COX11 protein expression and sensitivity to oxidative stress. To sort out the contribution of the single mutations to the phenotype, we employed a multi-faceted approach using Saccharomyces cerevisiae as a genetically manipulable system, and in silico structure-based analysis of human COX11. Our results reveal differential effects of the two novel COX11 mutations on yeast growth, respiration, and cellular redox status, as well as their potential impact on human protein stability and function. Strikingly, the functional deficits observed in patient fibroblasts are recapitulated in yeast models, validating the conservation of COX11's role in mitochondrial integrity across evolutionarily distant organisms. This study not only expands the mutational landscape of COX11-associated mitochondrial disorders but also underscores the continued translational relevance of yeast models in dissecting complex molecular pathways.

Bi-allelic variants in MDH2: Expanding the clinical phenotype.

Bi-allelic alterations in the MDH2 gene have recently been reported in three unrelated toddlers with early-onset severe encephalopathy. Here, we describe a new case of a child carrying novel variants in MDH2. This child presented with early-onset encephalocardiopathy requiring heart transplant and showed cerebellar ataxia and drug-responsive epilepsy; his family history was significant for multiple cancers, a feature often associated with monoallelic variants in MDH2. Functional studies in cultured skin fibroblasts from the proband showed reduced protein levels and impaired enzyme activity, further corroborating the genetic results. The relatively mild neurological presentation and severe cardiac manifestations requiring heart transplant distinguish this case from previous reports. This patient thus expands the spectrum of clinical features associated with MDH2 variants.

Proteomics and lipidomic analysis reveal dysregulated pathways associated with loss of sacsin.

Introduction: Autosomal recessive spastic ataxia of Charlevoix-Saguenay (ARSACS) is a rare incurable neurodegenerative disease caused by mutations in the SACS gene, which codes for sacsin, a large protein involved in protein homeostasis, mitochondrial function, cytoskeletal dynamics, autophagy, cell adhesion and vesicle trafficking. However, the pathogenic mechanisms underlying sacsin dysfunction are still largely uncharacterized, and so attempts to develop therapies are still in the early stages. Methods: To achieve further understanding of how processes are altered by loss of sacsin, we used untargeted proteomics to compare protein profiles in ARSACS fibroblasts versus controls. Results: Our analyses confirmed the involvement of known biological pathways and also implicated calcium and lipid homeostasis in ARSACS skin fibroblasts, a finding further verified in SH-SY5Y SACS -/- cells. Validation through mass spectrometry-based analysis and comparative quantification of lipids by LC-MS in fibroblasts revealed increased levels of ceramides coupled with a reduction of diacylglycerols. Discussion: In addition to confirming aberrant Ca2+ homeostasis in ARSACS, this study described abnormal lipid levels associated with loss of sacsin.

Severe mitochondrial encephalomyopathy caused by de novo variants in OPA1 gene.

Background: Mitochondria adjust their shape in response to the different energetic and metabolic requirements of the cell, through extremely dynamic fusion and fission events. Several highly conserved dynamin-like GTPases are involved in these processes and, among those, the OPA1 protein is a key player in the fusion of inner mitochondrial membranes. Hundreds of monoallelic or biallelic pathogenic gene variants have been described in OPA1, all associated with a plethora of clinical phenotypes without a straightforward genotype-phenotype correlation. Methods: Here we report two patients harboring novel de novo variants in OPA1. DNA of two patients was analyzed using NGS technology and the pathogenicity has been evaluated through biochemical and morphological studies in patient's derived fibroblasts and in yeast model. Results: The two patients here reported manifest with neurological signs resembling Leigh syndrome, thus further expanding the clinical spectrum associated with variants in OPA1. In cultured skin fibroblasts we observed a reduced amount of mitochondrial DNA (mtDNA) and altered mitochondrial network characterized by more fragmented mitochondria. Modeling in yeast allowed to define the deleterious mechanism and the pathogenicity of the identified gene mutations. Conclusion: We have described two novel-single OPA1 mutations in two patients characterized by early-onset neurological signs, never documented, thus expanding the clinical spectrum of this complex syndrome. Moreover, both yeast model and patients derived fibroblasts showed mitochondrial defects, including decreased mtDNA maintenance, correlating with patients' clinical phenotypes.

A Multisystem Mitochondrial Disease Caused by a Novel MT-TL1 mtDNA Variant: A Case Report.

BACKGROUND: Mitochondrial tRNA (MTT) genes are hotspot for mitochondrial DNA mutation and are responsible of half mitochondrial disease. MTT mutations are associated with a broad spectrum of phenotype often with complex multisystem involvement and complex genotype-phenotype correlations. MT-TL1 mutations, among which the m.3243A>G mutation is the most frequent, are associated with myopathy, maternal inherited diabetes and deafness, MELAS, cardiomyopathy, and focal segmental glomerulosclerosis. CASE STUDY: Here we report the case of an Italian 49-years old female presenting with encephalomyopathy, chronic proteinuric kidney disease and a new heteroplasmic m.3274_3275delAC MT-TL1 gene mutation. CONCLUSIONS: Our case demonstrates a systemic mitochondrial disease caused by the heteroplasmic m.3274_3275delAC MT-TL1 gene mutation, not yet described in the literature. A mitochondrial disease should be suspected in case of complex multisystem phenotypes, including steroid-resistant nephrotic syndrome with multisystemic involvement.