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Alina Radziwon, Ian M MacDonald; Identification of the first CHM promoter mutation in a patient with choroideremia. Invest. Ophthalmol. Vis. Sci. 2016;57(12):3143.
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© ARVO (1962-2015); The Authors (2016-present)
Choroideremia (CHM) is an X-linked inherited chorioretinal disease known to be caused by mutations in the CHM gene responsible for the production of Rab escort protein 1. In some cases, exomic sequencing fails to identify a causative mutation. In this study we carried out genetic analysis of the upstream untranscribed region in two related CHM patients with no mutations found within the CHM gene, to evaluate the possibility of a regulatory mutation
Two males were identified in whom clinical features were consistent with a clinical diagnosis of CHM. Sequencing of the coding region and adjacent splice sites did not identify a mutation, but a novel variant (c.-98C>A) was found upstream of exon 1. The variant was not listed in either the latest release of dbSNP or the 1000 Genomes Project. We generated two luciferase reporter plasmids containing a 1kb DNA fragment from upstream of the CHM transcription start site, one with wild type sequence the other bearing the c.-98C>A mutation. The constructs and a promoterless control plasmid were transfected into HEK293T cells and assayed for the ability to drive transcription as measured by luciferase activity. Bioinformatic analysis of sequences adjacent to the variant was used to elucidate its significance. Future mobility shift assay will be used to biochemically confirm the disruption of transcription factor binding to DNA in the mutant sequence
In vitro functional assay using luciferase reporter constructs driven by sequence upstream of CHM demonstrated that the mutation c.-98C>A diminished the transcriptional activity by 98%. While the promoter of CHM has not been previously identified, these results show that it must span the location of this mutation. In silico analysis of sequence near the mutation shows it alters an invariant CCCA motif that is part of the binding sequence for Staf/ZNF143, a widespread but little studied transcription factor with less than ten known protein coding gene promoters among its targets
Disruption of a transcription factor binding site in the previously unidentified promoter of CHM markedly reduces transcription and causes choroideremia. While likely rare, mutations within the CHM promoter could be considered as an explanation for CHM cases in which no mutation is found by exomic sequence analysis, and promoter screening should be included in the evaluation of patients with unexplained choroideremia
This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.
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