Protein Transduction Domain-Mediated Delivery of Recombinant Proteins and In Vitro Transcribed mRNAs for Protein Replacement Therapy of Human Severe Genetic Mitochondrial Disorders: The Case of Sco2 Deficiency.

Mitochondrial disorders represent a heterogeneous group of genetic disorders with variations in severity and clinical outcomes, mostly characterized by respiratory chain dysfunction and abnormal mitochondrial function. More specifically, mutations in the human SCO2 gene, encoding the mitochondrial inner membrane Sco2 cytochrome c oxidase (COX) assembly protein, have been implicated in the mitochondrial disorder fatal infantile cardioencephalomyopathy with COX deficiency. Since an effective treatment is still missing, a protein replacement therapy (PRT) was explored using protein transduction domain (PTD) technology. Therefore, the human recombinant full-length mitochondrial protein Sco2, fused to TAT peptide (a common PTD), was produced (fusion Sco2 protein) and successfully transduced into fibroblasts derived from a SCO2/COX-deficient patient. This PRT contributed to effective COX assembly and partial recovery of COX activity. In mice, radiolabeled fusion Sco2 protein was biodistributed in the peripheral tissues of mice and successfully delivered into their mitochondria. Complementary to that, an mRNA-based therapeutic approach has been more recently considered as an innovative treatment option. In particular, a patented, novel PTD-mediated IVT-mRNA delivery platform was developed and applied in recent research efforts. PTD-IVT-mRNA of full-length SCO2 was successfully transduced into the fibroblasts derived from a SCO2/COX-deficient patient, translated in host ribosomes into a nascent chain of human Sco2, imported into mitochondria, and processed to the mature protein. Consequently, the recovery of reduced COX activity was achieved, thus suggesting the potential of this mRNA-based technology for clinical translation as a PRT for metabolic/genetic disorders. In this review, such research efforts will be comprehensibly presented and discussed to elaborate their potential in clinical application and therapeutic usefulness.

Intracellular delivery of full length recombinant human mitochondrial L-Sco2 protein into the mitochondria of permanent cell lines and SCO2 deficient patient's primary cells.

Mutations in human SCO2 gene, encoding the mitochondrial inner membrane Sco2 protein, have been found to be responsible for fatal infantile cardioencephalomyopathy and cytochrome c oxidase (COX) deficiency. One potentially fruitful therapeutic approach for this mitochondrial disorder should be considered the production of human recombinant full length L-Sco2 protein and its deliberate transduction into the mitochondria. Recombinant L-Sco2 protein, fused with TAT, a Protein Transduction Domain (PTD), was produced in bacteria and purified from inclusion bodies (IBs). Following solubilisation with l-arginine, this fusion L-Sco2 protein was transduced in cultured mammalian cells of different origin (U-87 MG, T24, K-562, and patient's primary fibroblasts) and assessed for stability, transduction into mitochondria, processing and impact on recovery of COX activity. Our results indicate that: a) l-Arg solution was effective in solubilising recombinant fusion L-Sco2 protein, derived from IBs; b) fusion L-Sco2 protein was delivered successfully via a time- and concentration-dependent process into the mitochondria of human U-87 MG and T24 cells; c) fusion L-Sco2 protein was also transduced in human K-562 cells, transiently depleted of SCO2 transcripts and thus COX deficient; transduction of this fusion protein led to partial recovery of COX activity in such cells; d) [(35)S]Methionine-labelled fusion L-Sco2 protein, produced in a cell free transcription/translation system and incubated with intact isolated mitochondria derived from K-562 cells, was efficiently processed to yield the corresponding mature Sco2 protein, thus justifying the potential of the transduced fusion L-Sco2 protein to successfully activate COX holoenzyme; and finally, e) recombinant fusion L-Sco2 protein was successfully transduced into the mitochondria of primary fibroblasts derived from SCO2/COX deficient patient and facilitated recovery of COX activity. These findings provide the rationale of delivering recombinant proteins via PTD technology as a model for therapeutic approach of mitochondrial disorders.

Human recombinant mutated forms of the mitochondrial COX assembly Sco2 protein differ from wild-type in physical state and copper binding capacity.

The human Sco2 protein is a cytochrome c oxidase assembly protein that participates in mitochondrial copper pathway, acting downstream of Cox17 protein. In a previous work, we detected mutations in the human SCO2 gene in three unrelated infants with fatal cardioencephalomyopathy and COX deficiency. In this study, full-length processed recombinant wild-type and two mutated forms of hSco2p (w/t-rhSco2p, E140K-rhSco2p, and S225F-rhSco2p) were produced in bacteria as soluble recombinant peptides for the first time and evaluated for differences in their physical state and ability to bind copper. Our data indicate the following: (a) w/t-rhSco2p and S225F-rhSco2p were found to be in a monomeric form in contrast to E140K-rhSco2p that was in a major non-reducible dimer and a minor monomer form; (b) wild-type and mutated rhSco2p exhibited clear differences in their physical conformational state, as shown by circular dichroism and thermal denaturation analyses; (c) copper binding studies showed that E140K-rhSco2p bound markedly less copper while S225F-rhSco2p more than expected as compared to amount of the copper bound with w/t-rhSco2p. rhCox17p served as positive control experiment. These data indicate that S225F and E140K mutations found in the SCO2 gene derived from patients alter the physical conformational state of encoded hSco2p that may disturb the normal copper transport pathway in mitochondria. These findings are valuable for understanding the molecular basis of fatal cardioencephalomyopathy and COX deficiency and for designing appropriate pharmacological interventions.