Novel biallelic mutations in the PNPT1 gene encoding a mitochondrial‐RNA‐import protein PNPase cause delayed myelination

Abstract

Recent studies suggest that impaired transcription or mitochondrial translation of small RNAs can cause abnormal myelination. A polynucleotide phosphorylase (PNPase) encoded by PNPT1 facilitates the import of small RNAs into mitochondria. PNPT1 mutations have been reported in patients with neurodevelopmental diseases with mitochondrial dysfunction. We report here 2 siblings with PNPT1 mutations who presented delayed myelination as well as mitochondrial dysfunction. We identified compound heterozygous mutations (c.227G>A; p.Gly76Asp and c.574C>T; p.Arg192*) in PNPT1 by quartet whole-exome sequencing. Analyses of skin fibroblasts from the patient showed that PNPase expression was markedly decreased and that import of the small RNA RNaseP into mitochondria was impaired. Exogenous expression of wild-type PNPT1, but not mutants, rescued ATP production in patient skin fibroblasts, suggesting the pathogenicity of the identified mutations. Our cases expand the phenotypic spectrum of PNPT1 mutations that can cause delayed myelination.

Mitochondrial Trafficking and Processing of Telomerase RNA TERC

“…One of the key findings of this study is the mitochondrial import of TERC and the export of TERC-53 to the cytosol. Mitochondrial RNA import has been well established, with IMS localized protein PNPASE as a possible receptor (Mercer et al, 2011;Sato et al, 2018;Vedrenne et al, 2012;von Ameln et al, 2012;Wang et al, 2010;Zhang et al, 2014). Interestingly, PNPASE overexpression led to an accumulation of cytosolic TERC-53, but not the mitochondrial TERC-53.…”

“…Mitochondria import a wide array of non-coding RNAs, including tRNAs, rRNAs, microRNAs, and long non-coding RNAs (lncRNAs) (Alfonzo and Sö ll, 2009;Chang and Clayton, 1989;Mercer et al, 2011;Wang et al, 2010;Zhang et al, 2014). The import of most of these RNAs depends on PNPASE, a mitochondrial IMS (intermembrane space) protein, in mammalian cells (Sato et al, 2018;Vedrenne et al, 2012;von Ameln et al, 2012;Wang et al, 2010). The functions of most imported RNAs in mitochondria, however, remain unclear.…”

Crystal structure of dimeric human PNPase reveals why disease-linked mutants suffer from low RNA import and degradation activities

“…Any mutations that disrupt PNPase enzyme activity or its trimeric conformation inhibit its RNA degradation or import activities, leading to mitochondrial dysfunction and diseases. Investigations of the heterozygous variants Q254K/A510P ( 8 ) and G76D/R192* ( 10 ) will likely prove revealing, all of which are presumably defective in forming a functional trimeric PNPase.…”

“…Consequently, these mutants fail to import structured RNA into mitochondria ( 6 , 7 ). Disease-linked heterozygous variants have also been identified, including Q254K/A510P that causes severe multisystem disease ( 8 ), R136H/P140L that linked to Leigh syndrome ( 9 ) and G76D/R192* (* indicating nonsense mutation) that causes delayed myelination ( 10 ). It has been shown that the mutations of R136H/P140L produced RNA degradation-inactive PNPase enzyme ( 9 ), and Q254K/A510P resulted in a lower level of PNPase that cannot assemble into a trimeric complex ( 8 ).…”

Adapting CRISPR/Cas9 System for Targeting Mitochondrial Genome

“…One approach to deliver nuclear encoded RNA into mitochondria involves the utilization of a native RNA transport enzyme, polynucleotide phosphorylase (PNPase) encoded by PNPT1 gene, located in the mitochondrial inter-membranous space (26). There is increasing evidence of PNPase involved in the import of small RNAs, into mitochondria (27,28). Further, it has been shown that addition of twenty nucleotide stem-loop sequence of nuclear RNAse P, to the transcripts not normally heading to mitochondria has been shown to facilitate transportation of these RNAs into mitochondria in a PNPase-dependent manner (29).…”