Mutations in POLR3A and POLR3B encoding RNA Polymerase III subunits cause an autosomal-recessive hypomyelinating leukoencephalopathy.

Department of Human Genetics, Yokohama City University Graduate School of Medicine, 3-9 Fukuura, Kanazawa-ku, Yokohama 236-0004, Japan.
The American Journal of Human Genetics (Impact Factor: 10.93). 11/2011; 89(5):644-51. DOI: 10.1016/j.ajhg.2011.10.003
Source: PubMed


Congenital hypomyelinating disorders are a heterogeneous group of inherited leukoencephalopathies characterized by abnormal myelin formation. We have recently reported a hypomyelinating syndrome characterized by diffuse cerebral hypomyelination with cerebellar atrophy and hypoplasia of the corpus callosum (HCAHC). We performed whole-exome sequencing of three unrelated individuals with HCAHC and identified compound heterozygous mutations in POLR3B in two individuals. The mutations include a nonsense mutation, a splice-site mutation, and two missense mutations at evolutionally conserved amino acids. Using reverse transcription-PCR and sequencing, we demonstrated that the splice-site mutation caused deletion of exon 18 from POLR3B mRNA and that the transcript harboring the nonsense mutation underwent nonsense-mediated mRNA decay. We also identified compound heterozygous missense mutations in POLR3A in the remaining individual. POLR3A and POLR3B encode the largest and second largest subunits of RNA Polymerase III (Pol III), RPC1 and RPC2, respectively. RPC1 and RPC2 together form the active center of the polymerase and contribute to the catalytic activity of the polymerase. Pol III is involved in the transcription of small noncoding RNAs, such as 5S ribosomal RNA and all transfer RNAs (tRNA). We hypothesize that perturbation of Pol III target transcription, especially of tRNAs, could be a common pathological mechanism underlying POLR3A and POLR3B mutations.

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Available from: Hiroshi Doi
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    • "In eukaryotes, suppressor tRNAs in S. cerevisiae and S. pombe are among the most studied although some have been used in higher eukaryotes as well ((Koukuntla et al., 2013) and references therein). When a suppressor tRNA is used in combination with a selectable marker carrying a corresponding nonsense mutation as a premature stop codon, effects on the biogenesis of the suppressor tRNA can be monitored by changes in the functional recently, mutations in the two largest pol III subunits cause hypomyelinating leukodystrophy, and in the Bdp1 subunit of transcription initiation factor, TFIIIB, cause hearing loss (Bernard et al., 2011; Saitsu et al., 2011; Girotto et al., 2013). Thus it is tempting to speculate that the S. pombe TMS system may be useful toward examining the mechanistic basis of these mutations. "
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    ABSTRACT: Suppressor tRNAs bear anticodon mutations that allow them to decode premature stop codons in metabolic marker gene mRNAs, that can be used as in vivo reporters of functional tRNA biogenesis. Here, we review key components of a suppressor tRNA system specific to Schizosaccharomyces pombe and its adaptations for use to study specific steps in tRNA biogenesis. Eukaryotic tRNA biogenesis begins with transcription initiation by RNA polymerase (pol) III. The nascent pre-tRNAs must undergo folding, 5' and 3' processing to remove the leader and trailer, nuclear export, and splicing if applicable, while multiple complex chemical modifications occur throughout the process. We review evidence that precursor-tRNA processing begins with transcription termination at the oligo(T) terminator element, which forms a 3' oligo(U) tract on the nascent RNA, a sequence-specific binding site for the RNA chaperone, La protein. The processing pathway bifurcates depending on a poorly understood property of pol III termination that determines the 3' oligo(U) length and therefore the affinity for La. We thus review the pol III termination process and the factors involved including advances using gene-specific random mutagenesis by dNTP analogs that identify key residues important for transcription termination in certain pol III subunits. The review ends with a 'technical approaches' section that includes a parts lists of suppressor-tRNA alleles, strains and plasmids, and graphic examples of its diverse uses. Published by Elsevier B.V.
    Full-text · Article · Nov 2014 · Gene
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    • "Though the cerebellar abnormalities appeared more severe on MRI in patients with POLR3B mutations in this study, the patients with POLR3B mutations did not have more severe clinical examinations, including cerebellar signs, than ones with POLR3A mutations. Patients with both POLR3A and POLR3B mutations presented with intellectual disability and cerebellar signs, and some had additional clinical symptoms of hypodontia or hypogonadism, as described in the literature [2] [3] [4]. All three patients with POLR3B mutations in this study could walk without support at the age of 16– 31 years, although the two with POLR3A mutations were not independent walkers at 7 and 14 years. "
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    ABSTRACT: Background: Mutations of POLR3A and POLR3B have been reported to cause several allelic hypomyelinating disorders, including hypomyelination with hypogonadotropic hypogonadism and hypodontia (4H syndrome). Patients and methods: To clarify the difference in MRI between the two genotypes, we reviewed MRI in three patients with POLR3B mutations, and three with POLR3A mutations. Results: Though small cerebellar hemispheres and vermis are common MRI findings with both types of mutations, MRI in patients with POLR3B mutations revealed smaller cerebellar structures, especially vermis, than those in POLR3A mutations. MRI also showed milder hypomyelination in patients with POLR3B mutations than those with POLR3A mutations, which might explain milder clinical manifestations. Conclusions: MRI findings are distinct between patients with POLR3A and 3B mutations, and can provide important clues for the diagnosis, as these patients sometimes have no clinical symptoms suggesting 4H syndrome.
    Full-text · Article · May 2013 · Brain & development
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    • "Exome sequencing for extremely rare neurological conditions (for example in which patients are isolated cases in their families) has also been successful. Mutations in POLR3A and B encoding RNA polymerase subunits for example were shown to be the cause of autosomal-recessive hypomyelinating leukoencphalopathy in a study of 3 unrelated affected individuals [35]. Additionally, while not being able to find a causative mutation, exome sequencing of 4 unrelated individuals with monomelic amyotrophy showed that variants of 2 genes (KIAA1377 and C5ORF42) increased the risk of the disease significantly (OR = 61.69) [36]. "
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    ABSTRACT: Over the past year huge advances have been made in our ability to determine the genetic aetiology of many neurological diseases through the utilisation of next generation sequencing platforms. This technology is, on a daily basis, providing new breakthroughs in neurological disease. The aim of this article is to clearly describe the technological platforms, methods of data analysis, established breakthroughs, and potential future clinical and research applications of this innovative and exciting technique which has relevance to all those working within clinical neuroscience.
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