title: Analysis of acyclic nucleoside modifications in siRNAs finds sensitivity at position 1 that is restored by 5'-terminal phosphorylation both in vitro and in vivo.
abstract: Small interfering RNAs (siRNAs) are short, double-stranded RNAs that use the endogenous RNAi pathway to mediate gene silencing. Phosphorylation facilitates loading of a siRNA into the Ago2 complex and subsequent cleavage of the target mRNA. In this study, 2', 3' seco nucleoside modifications, which contain an acylic ribose ring and are commonly called unlocked nucleic acids (UNAs), were evaluated at all positions along the guide strand of a siRNA targeting apolipoprotein B (ApoB). UNA modifications at positions 1, 2 and 3 were detrimental to siRNA activity. UNAs at positions 1 and 2 prevented phosphorylation by Clp1 kinase, abrogated binding to Ago2, and impaired Ago2-mediated cleavage of the mRNA target. The addition of a 5'-terminal phosphate to siRNA containing a position 1 UNA restored ApoB mRNA silencing, Ago2 binding, and Ago2 mediated cleavage activity. Position 1 UNA modified siRNA containing a 5'-terminal phosphate exhibited a partial restoration of siRNA silencing activity in vivo. These data reveal the complexity of interpreting the effects of chemical modification on siRNA activity, and exemplify the importance of using multiple biochemical, cell-based and in vivo assays to rationally design chemically modified siRNA destined for therapeutic use.
authors.affiliation: Sirna Therapeutics, a wholly owned subsidiary of Merck and Co., 1700 Owens St, 4th Floor, San Francisco, CA 94158, USA.
authors.forename: Denise M
authors.initials: DM
authors.lastname: Kenski
authors.forename: Abby J
authors.initials: AJ
authors.lastname: Cooper
authors.forename: Jenny J
authors.initials: JJ
authors.lastname: Li
authors.forename: Aarron T
authors.initials: AT
authors.lastname: Willingham
authors.forename: Henry J
authors.initials: HJ
authors.lastname: Haringsma
authors.forename: Tracy A
authors.initials: TA
authors.lastname: Young
authors.forename: Nelly A
authors.initials: NA
authors.lastname: Kuklin
authors.forename: Jeffrey J
authors.initials: JJ
authors.lastname: Jones
authors.forename: Mark T
authors.initials: MT
authors.lastname: Cancilla
authors.forename: Daniel R
authors.initials: DR
authors.lastname: McMasters
authors.forename: Melina
authors.initials: M
authors.lastname: Mathur
authors.forename: Alan B
authors.initials: AB
authors.lastname: Sachs
authors.forename: W Michael
authors.initials: WM
authors.lastname: Flanagan
mesh_terms: D000818:Animals; D001055:Apolipoproteins B / Q000235:genetics; D002460:Cell Line; D015852:Eukaryotic Initiation Factor-2 / Q000378:metabolism; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D009687:Nuclear Proteins / Q000378:metabolism; D009705:Nucleosides / Q000737:chemistry*; D010766:Phosphorylation; D010770:Phosphotransferases / Q000378:metabolism; D034622:RNA Interference; D034741:RNA, Small Interfering / Q000737:chemistry* / Q000378:metabolism; D013816:Thermodynamics; D014157:Transcription Factors / Q000378:metabolism; D058727:RNA, Small Untranslated
chemical_list: D001055:Apolipoproteins B; D015852:Eukaryotic Initiation Factor-2; D009687:Nuclear Proteins; D009705:Nucleosides; D034741:RNA, Small Interfering; D014157:Transcription Factors; C520030:CLP1 protein, human; D010770:Phosphotransferases; D058727:RNA, Small Untranslated