Abstract
ABSTRACTMitochondrial DNA depletion syndromes (MDS) are a group of rare autosomal recessive disorders with early onset and no cure available. MDS are caused by mutations in nuclear genes involved in mitochondrial DNA (mtDNA) maintenance, and characterized by both a strong reduction in mtDNA content and severe mitochondrial defects in affected tissues. Mutations in MPV17, a nuclear gene encoding a mitochondrial inner membrane protein, have been associated with hepatocerebral forms of MDS. The zebrafish mpv17 null mutant lacks the guanine-based reflective skin cells named iridophores and represents a promising model to clarify the role of Mpv17. In this study, we characterized the mitochondrial phenotype of mpv17−/− larvae and found early and severe ultrastructural alterations in liver mitochondria, as well as significant impairment of the respiratory chain, leading to activation of the mitochondrial quality control. Our results provide evidence for zebrafish Mpv17 being essential for maintaining mitochondrial structure and functionality, while its effects on mtDNA copy number seem to be subordinate. Considering that a role in nucleotide availability had already been postulated for MPV17, that embryos blocked in pyrimidine synthesis do phenocopy mpv17−/− knockouts (KOs) and that mpv17−/− KOs have impaired Dihydroorotate dehydrogenase activity, we provided mpv17 mutants with the pyrimidine precursor orotic acid (OA). Treatment with OA, an easily available food supplement, significantly increased both iridophore number and mtDNA content in mpv17−/− mutants, thus linking the loss of Mpv17 to pyrimidine de novo synthesis and opening a new simple therapeutic approach for MPV17-related MDS.
Highlights
In the past few years, the scientific scenario identified mitochondria as key players in a growing spectrum of human diseases
By performing reverse transcription and quantitative polymerase chain reaction (RT-Quantitative PCR (qPCR)) analysis, we observed a significant overexpression of mpv17-like2, whereas the mpv17-like expression level remained unchanged in both wild-type and homozygous knockout (KO) larvae at 6 days post-fertilization (Fig. 1A)
The pyrimidine de novo synthesis pathway is strictly linked to the iridophore phenotype in mpv17 null mutants By performing a bioinformatics search on CORD platform to identify MPV17 co-regulated genes, we found that the trifunctional Carbamoyl-phosphate synthetase 2, aspartate transcarbamylase and dihydroorotase (CAD) enzyme involved in pyrimidine production showed 66.48% of concordance in 176 experiments (Pearson coefficient 0.504 and P=1.1×10−28), among the highest scores for MPV17 co-regulated genes (Fig. S3)
Summary
In the past few years, the scientific scenario identified mitochondria as key players in a growing spectrum of human diseases. Beside their main functions in oxidative phosphorylation and bioenergetics, mitochondria participate in a plethora of. Mitochondrial DNA (mtDNA) encodes only 13 subunits of the respiratory chain (RC), while the remaining ∼1500 mitochondrial proteins, encoded by nuclear DNA (nDNA), are imported into the organelle (Calvo et al, 2016). MtDNA depletion has been discovered to cause a continuously increasing group of severe human diseases called mtDNA depletion syndromes (MDS), first described in 1991 (Moraes et al, 1991)
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