Normal RNAi response in human fragile × fibroblasts
© Jensen et al; licensee BioMed Central Ltd. 2009
Received: 16 June 2009
Accepted: 9 September 2009
Published: 9 September 2009
Fragile × syndrome is caused by loss of expression of the FMRP protein involved in the control of a large number of mRNA targets. The Drosophila ortholog dFXR interacts with a protein complex that includes Argonaute2, an essential component of the RNA-induced silencing complex (RISC). Furthermore dFXR associates with Dicer, another essential processing enzyme of the RNAi pathway. Both microRNA and microRNA precursors can co-immunoprecipitate with dFXR. Consequently it has been suggested that the Fragile × syndrome may be due to a defect in an RNAi-related apparatus.
We have investigated the RNAi response in Fragile × patient cells lacking FMRP compared with normal controls. RNAi responses were successfully detected, but no statistically significant difference between the response in normal cells compared to patients cells was found - neither one nor two days after transfection.
Our data show that in human fibroblasts from Fragile × patients lacking FMRP the RNAi response is not significantly impaired.
Fragile × syndrome is caused by loss of expression of the FMRP protein involved in the control of a large number of mRNA targets . The Drosophila ortholog dFXR interacts with a protein complex that includes Argonaute2, an essential component of the RNA-induced silencing complex (RISC) . Furthermore dFXR associates with Dicer, another essential processing enzyme of the RNAi pathway . Both microRNA and microRNA precursors can co-immunoprecipitate with dFXR , and it was shown that FMRP can act as a miRNA acceptor protein for the ribonuclease Dicer and facilitate the assembly of miRNAs on specific target RNA sequences [4, 5].
Consequently, it has been suggested that the Fragile × syndrome may be due to a defect in an RNAi-related apparatus  although in Drosophila S2 cells dFXR is not required for an RNAi response . RNAi is a naturally occurring mechanism of gene regulation that induces sequence-specific knock-down of gene expression at the post-transcriptional level . Regulation of gene expression by RNAi utilizes endogenous cellular pathways in which double-stranded RNA molecules produced from endogenous or foreign DNA are processed into short double-stranded RNA molecules of 21-23 nucleotides. These small interfering RNA (siRNA) molecules are incorporated into RISC that facilitates degradation of the target.
Recently the RISC function was analysed in mouse embryo fibroblasts (MEFs) from FMRP wildtype or knockout (FMR1-/-) littermates . Here it was shown that cells lacking FMRP have normal RISC activity, since FMRP and RISC were associated with distinct pools of mRNAs.
Our data support the conclusions made by Didiot et al. that cells lacking FMRP have a normal RNAi response. Since FMRP together with the autosomal paralogs FXR1P and FXR2P constitutes a family of RNA bindings proteins, it is conceivable that functional redundance exists among these, contributing to the observed results . Further experiments are required to analyse the involvement of FMRP in the miRNA pathway in human cells.
We thank the technicians at the Kennedy Center for excellent assistance and Anders Lade Nielsen for discussions. This work was supported by The Velux Foundation, the Novo Nordisk Foundation, Fonden til Lægevidenskabens Fremme, the Danish Medical Research Council and grants from the Kennedy Center.
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