Human Molecular Genetics Advance Access originally published online on September 12, 2007
Human Molecular Genetics 2007 16(24):3027-3036; doi:10.1093/hmg/ddm261
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Functional characterization of NF-
B inhibitor-like protein 1 (NFBIL1), a candidate susceptibility gene for rheumatoid arthritis
1 School of Medicine and Biomedical Sciences, University of Sheffield, Sheffield S10 2RX, UK, 2 St. Vincent’s University Hospital, Dublin, Ireland and 3 The Conway Institute of Molecular Medicine, Dublin, Ireland
* To whom correspondence should be addressed. Tel: +44 1142712566; Fax: +44 1142711711; Email a.g.wilson{at}shef.ac.uk
Received June 13, 2007; Revised September 3, 2007; Accepted September 9, 2007
Several studies have implicated the NF-
B inhibitor-like protein 1 (NFkBIL1) gene located in the class III region of the major histocompatibility complex (MHC) as a possible susceptibility locus for rheumatoid arthritis (RA). Based on limited homology, it has been suggested to be a member of the inhibitor of NF-B (IB) family of proteins, but a role in mRNA processing has also been proposed. We have investigated the expression of NFkBIL1 in RA synovial tissue and characterized its function. Real-time PCR showed the two NFkBIL1 mRNA splice variants are expressed in a tissue-specific manner. Dual immunofluorescent staining of human RA synovium with polyclonal anti-NFkBIL1 antibodies and anti-CD68, anti-CD3 or anti-factor VIII showed that NFkBIL1 was expressed in the rheumatoid synovial lining and sub-lining layers and co-localized in CD68+ and CD3+, but not Factor VIII+ cells. Confocal microscopy of cultured synovial fibroblasts revealed expression in speckled nuclear and homogenous cytoplasmic distributions, suggesting shuttling between the cytoplasmic and nuclear compartments. Functional tests showed that NFkBIL1 isoforms were incapable of associating with NF-B and did not inhibit it, thus disproving the hypothesis that NFkBIL1 functions as an IB. Affinity purification of endogenous NFkBIL1 proteins and co-immunoprecipitation experiments showed that NFkBIL1 can associate with mRNA and with three protein partners, identified by mass spectrometry as leukophysin, translation elongation factor 1 
and CTP synthase I. These data support a potential role for NFkBL1 in the pathogenesis of RA and indicates that it may be involved in mRNA processing or the regulation of translation.
These authors contributed equally to this work.