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New insights on non-canonical DNA structures: G-Quadruplex folding of RPGR exon ORF15 might impair photoreceptor vesicular trafficking and ciliogenesis

Luigi DONATO

During last years it has becoming evident that non-Watson–Crick base pairing, resulting in the assembly of alternative DNA secondary structures, also occurs in the genome. Such noncanonical structures, called non-B form DNA, include the G-quadruplexes, stacked nucleic acid structures that form within G-rich DNA or RNA sequences. Dysregulation of DNA-G4 is associated with human disorders, including neurological dysfunction and accelerated ageing, even if its role in neurophysiology/neuropathology has not yet been fully elucidated. We present results coming from our recent experiments on X-linked retinitis pigmentosa (OMIM 26800), a group of hereditary disorders that can lead to blindness because of photoreceptor degenerations. The most frequently mutated Xlinked retinitis pigmentosa genes is RPGR (RP3), and codes for a protein with a series of six RCC1-like domains (RLDs), involved in ciliogenesis, microtubule organization and regulation of transport in primary cilia. RPGR presents a splicing variant, called exon ORF15, which constitutes a mutational hot spot in a huge number of patients. The most challenge peculiarity of exon ORF15 is its repetitive nature, particularly of guanine (G)-rich sequences, that makes it very difficult to screen. Thus, we investigated the possible molecular causes that determine such difficulties by a multiple in-silico approach, evaluating the possibility that, due to its nature, exon ORF15 could show a G-quadruplex structure. All exploited algorithms confirmed the possibility that several G-quadruplex could be folded in RPGR exon ORF15, providing new insights towards a better sequencing approach to RPGR screening and Xlinked retinitis pigmentosa diagnosis.