The zebrafish orthologue of the human hepatocerebral disease geneMPV17plays pleiotropic roles in mitochondria

Abstract

Mitochondrial 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.

Genome and Transcriptome Sequencing of casper and roy Zebrafish Mutants Provides Novel Genetic Clues for Iridophore Loss

“…The roy strain has a spontaneous mutation that leads to complete iridophore defects and a transparent trunk [5,8,9]. As both casper and roy mutants have the same phenotype of iridophore loss, they were treated as the same transparent group.…”

“…A mpv17-knockdown zebrafish exhibited a moderate loss of iridophores along the body axis [8]. A recent study [9] further demonstrated that the mpv17 −/− zebrafish knockouts exhibited iridophore loss. It was therefore postulated that the mpv17 gene in zebrafish could play a key role in pyrimidine synthesis for iridophore development and maintenance [9].…”

“…A recent study [9] further demonstrated that the mpv17 −/− zebrafish knockouts exhibited iridophore loss. It was therefore postulated that the mpv17 gene in zebrafish could play a key role in pyrimidine synthesis for iridophore development and maintenance [9]. Its detailed genome lesions have not been sequenced either.…”

Opa1overexpression protects from early onsetMpv17-/--related mouse kidney disease

“…To search for possible modifier genes responsible of the phenotype anticipation, we first analysed two genes previously proposed to modify the severity of the clinical phenotype in some mouse models of mitochondrial dysfunction, i.e. Prkdc, encoding for the catalytic subunit of the DNA-dependent protein kinase (DNA-PK) (Papeta et al, JCI 2010), and Nnt, encoding the nicotinamide nucleotide transhydrogenase (McManus et al, 2019). However, the Prkdc mutation described in Balb/c and other inbred strains was not present in our C57Bl/6 colonies, and the Nnt mutation, present in the of pure C57Bl/6J strain, did not segregate with the disease (not shown).…”

“…Other data showed that reduced MPV17 expression was associated with impaired dTMP synthesis without affecting de novo or salvage synthesis of dTMP, suggesting that MPV17 may be involved in maintaining dTMP levels in mitochondria through a still uncharacterized pathway, which may be involved in transporting dTMP or one of its precursors from the cytosol to mitochondria (Alonzo et al, 2018). In zebrafish, Mpv17 has been related to pyrimidine nucleotide metabolism via impairment of dihydroorotate dehydrogenase (Martorano et al, 2019). All these observations have been reported in single publications and do need further confirmation.…”