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Vincent A.,University of Toronto | Wright T.,University of Toronto | Billingsley G.,Program of Genetics and Genome Biology | Westall C.,University of Toronto | Heon E.,University of Toronto
Ophthalmic Genetics | Year: 2011

Purpose: To report the rare observation of CNGA3 mutation as a cause of oligocone trichromacy (OT) and present phenotypic characteristics. Methods: A 20 year old male patient underwent ophthalmological evaluation including detailed color vision assessment using Ishihara pseudoisochromatic plates, American Optical Hardy Rand Rittler plates (HRR) and Mollon-Reffin Minimalist test (MRM). Optical coherence tomography (OCT), fundus autofluorescence (FAF), visual field assessment and electrophysiological testing was also performed. The patient's DNA was sequenced for mutations in the coding sequence of CNGA3 and CNGB3 genes. Results: Best corrected visual acuity (BCVA) was 20/50 and 20/30 in the right and left eyes respectively. His color vision was normal to Ishihara, HRR and MRM tests. Fundus appearance, FAF, OCT and Goldmann visual fields (GVF) were all normal. Humphrey visual field analysis (HVF) demonstrated reduced sensitivity and paracentral scotomas (5-20°). The full-field electroretinogram (ERG) showed normal rod responses and severely reduced cone responses. The multifocal electroretinogram (mfERG) was non-recordable above noise. Compound heterozygous mutations in exon 8 of the CNGA3 coding sequence were identified; c.1070 A > G (Tyr357Cys; novel) and c.1694 C > T (Thr565Met). Allele-specific polymerase chain reaction confirmed that the mutations were located on separate alleles. No mutations were identified in CNGB3. Conclusion: This is the second reported case of CNGA3 associated OT. Mutations in CNGA3 have previously been associated with incomplete and complete achromatopsia. This report confirms that OT forms the mildest end of the spectrum of CNGA3 related diseases. © 2011 Informa Healthcare USA, Inc. Source

Pearson C.E.,Program of Genetics and Genome Biology | Pearson C.E.,University of Toronto
PLoS Genetics | Year: 2011

Diseases associated with unstable repetitive elements in the DNA, RNA, and amino acids have consistently revealed scientific surprises. Most diseases are caused by expansions of trinucleotide repeats, which ultimately lead to diseases like Huntington's disease, myotonic dystrophy, fragile X syndrome, and a series of spinocerebellar ataxias. These repeat mutations are dynamic, changing through generations and within an individual, and the repeats can be bi-directionally transcribed. Unsuspected modes of pathogenesis involve aberrant loss of protein expression; aberrant over-expression of non-mutant proteins; toxic-gain-of-protein function through expanded polyglutamine tracts that are encoded by expanded CAG tracts; and RNA-toxic-gain-of-function caused by transcripts harboring expanded CUG, CAG, or CGG tracts. A recent advance reveals that RNA transcripts with expanded CAG repeats can be translated in the complete absence of a starting ATG, and this Repeat Associated Non-ATG translation (RAN-translation) occurs across expanded CAG repeats in all reading frames (CAG, AGC, and GCA) to produce homopolymeric proteins of long polyglutamine, polyserine, and polyalanine tracts. Expanded CTG tracts expressing CUG transcripts also show RAN-translation occurring in all three frames (CUG, UGC, and GCU), to produce polyleucine, polycysteine, and polyalanine. These RAN-translation products can be toxic. Thus, one unstable (CAG)•(CTG) DNA can produce two expanded repeat transcripts and homopolymeric proteins with reading frames (the AUG-directed polyGln and six RAN-translation proteins), yielding a total of potentially nine toxic entities. The occurrence of RAN-translation in patient tissues expands our horizons of modes of disease pathogenesis. Moreover, since RAN-translation counters the canonical requirements of translation initiation, many new questions are now posed that must be addressed. This review covers RAN-translation and some of the pertinent questions. © 2011 Christopher E. Pearson. Source

Hick A.,Institute Of Genetique Et Of Biologie Moleculaire Et Cellulaire Igbmc | Hick A.,French Institute of Health and Medical Research | Hick A.,French National Center for Scientific Research | Hick A.,University of Strasbourg | And 51 more authors.
DMM Disease Models and Mechanisms | Year: 2013

Friedreich's ataxia (FRDA) is a recessive neurodegenerative disorder commonly associated with hypertrophic cardiomyopathy. FRDA is due to expanded GAA repeats within the first intron of the gene encoding frataxin, a conserved mitochondrial protein involved in iron-sulphur cluster biosynthesis. This mutation leads to partial gene silencing and substantial reduction of the frataxin level. To overcome limitations of current cellular models of FRDA, we derived induced pluripotent stem cells (iPSCs) from two FRDA patients and successfully differentiated them into neurons and cardiomyocytes, two affected cell types in FRDA. All FRDA iPSC lines displayed expanded GAA alleles prone to high instability and decreased levels of frataxin, but no biochemical phenotype was observed. Interestingly, both FRDA iPSC-derived neurons and cardiomyocytes exhibited signs of impaired mitochondrial function, with decreased mitochondrial membrane potential and progressive mitochondrial degeneration, respectively. Our data show for the first time that FRDA iPSCs and their neuronal and cardiac derivatives represent promising models for the study of mitochondrial damage and GAA expansion instability in FRDA. © 2013. Published by The Company of Biologists Ltd. Source

Zamiri B.,University of Toronto | Mirceta M.,Program of Genetics and Genome Biology | Mirceta M.,University of Toronto | Bomsztyk K.,University of Washington | And 3 more authors.
Nucleic Acids Research | Year: 2015

Unusual DNA/RNA structures of the C9orf72 repeat may participate in repeat expansions or pathogenesis of amyotrophic lateral sclerosis and frontotemporal dementia. Expanded repeats are CpG methylated with unknown consequences. Typically, quadruplex structures form by G-rich but not complementary C-rich strands. Using CD, UV and electrophoresis, we characterized the structures formed by (GGGGCC)8 and (GGCCCC)8 strands with and without 5-methylcytosine (5mCpG) or 5-hydroxymethylcytosine (5hmCpG) methylation. All strands formed heterogenous mixtures of structures, with features of quadruplexes (at pH 7.5, in K+, Na+ or Li+), but no feature typical of i-motifs. C-rich strands formed quadruplexes, likely stabilized by G•C•G•C-tetrads and C•C•C•C-tetrads. Unlike G•G•G•G-tetrads, some G•C•G•C-tetrad conformations do not require the N7-Guanine position, hence C9orf72 quadruplexes still formed when N7-deazaGuanine replace all Guanines. 5mCpG and 5hmCpG increased and decreased the thermal stability of these structures. hnRNPK, through band-shift analysis, bound C-rich but not G-rich strands, with a binding preference of unmethylated > 5hmCpG > 5mCpG, where methylated DNA-protein complexes were retained in the wells, distinct from unmethylated complexes. Our findings suggest that for C-rich sequences interspersed with G-residues, one must consider quadruplex formation and that methylation of quadruplexes may affect epigenetic processes. © The Author(s) 2015. Published by Oxford University Press on behalf of Nucleic Acids Research. Source

Reddy K.,Program of Genetics and Genome Biology | Reddy K.,University of Toronto | Zamiri B.,University of Toronto | Stanley S.Y.R.,University of Toronto | And 3 more authors.
Journal of Biological Chemistry | Year: 2013

Certain DNA and RNA sequences can form G-quadruplexes, which can affect promoter activity, genetic instability, RNA splicing, translation, and neurite mRNA localization. Amyotrophic lateral sclerosis and frontotemporal dementia were recently shown to be caused by expansion of a (GGGGCC)n· (GGCCCC)n repeat in the C9orf72 gene. Mutant r(GGGGCC) n-containing transcripts aggregate in nuclear foci possibly sequestering repeat-binding proteins, suggesting a toxic RNA pathogenesis. We demonstrate that the r(GGGGCC)n RNA but not the C-rich r(GGCCCC) n RNA forms extremely stable uni- and multimolecular parallel G-quadruplex structures (up to 95 °C). Multimolecular G-quadruplex formation is influenced by repeat number and RNA concentration. MBNL1, a splicing factor that is sequestered in myotonic dystrophy patients by binding to expanded r(CUG)n repeat hairpins, does not bind the C9orf72 repeats, but the splicing factor ASF/SF2 can bind the r(GGGGCC)n repeat. Because multimolecular G-quadruplexes are enhanced by repeat length, RNA-RNA interactions facilitated by G-quadruplex formation at expanded repeats might influence transcript aggregation and foci formation in amyotrophic lateral sclerosis-frontotemporal dementia cells. Tract length-dependent G-quadruplex formation by the C9orf72RNA should be considered when assessing the role of this repeat in C9orf72 gene activity, protein binding, transcript foci formation, and translation of the C9orf72 product, including the noncanonical repeat-associated non-ATG translation (RAN translation) into pathologic dipeptide repeats, as well as any oligonucleotide repeat-based therapy. © 2013 by The American Society for Biochemistry and Molecular Biology, Inc. Source

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