Objectives Altered signalling in B cells is a predominant feature of systemic lupus erythematosus (SLE). The genes BANK1 and BLK were recently described as associated with SLE. BANK1 codes for a B-cell-specific cytoplasmic protein involved in B-cell receptor signalling and BLK codes for an Src tyrosine kinase with important roles in B-cell development. To characterise the role of BANK1 and BLK in SLE, a genetic interaction analysis was performed hypothesising that genetic interactions could reveal functional pathways relevant to disease pathogenesis.
Methods The GPAT16 method was used to analyse the gene–gene interactions of BANK1 and BLK. Confocal microscopy was used to investigate co-localisation, and immunoprecipitation was used to verify the physical interaction of BANK1 and BLK.
Results Epistatic interactions between BANK1 and BLK polymorphisms associated with SLE were observed in a discovery set of 279 patients and 515 controls from northern Europe. A meta-analysis with 4399 European individuals confirmed the genetic interactions between BANK1 and BLK. As BANK1 was identified as a binding partner of the Src tyrosine kinase LYN, the possibility that BANK1 and BLK could also show a protein–protein interaction was tested. The co-immunoprecipitation and co-localisation of BLK and BANK1 were demonstrated. In a Daudi cell line and primary naive B cells endogenous binding was enhanced upon B-cell receptor stimulation using anti-IgM antibodies.
Conclusion This study shows a genetic interaction between BANK1 and BLK, and demonstrates that these molecules interact physically. The results have important consequences for the understanding of SLE and other autoimmune diseases and identify a potential new signalling pathway.
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BAPE is the coordinator of the Argentine collaborative group listed in the acknowledgements.
Funding This work was supported partly by grants from the European CVDIMMUNE project from the European Commission LSHM-CT-2006-037227, the Swedish Research Council for Medicine, the Swedish Association against Rheumatism, the Magnus Bergwalls Foundation, the Gustaf V:e 80th-year Jubilee, the Torsten and Ragnar Söderbergs Foundation and the Marcus Borsgtröms Foundation, the NIH-NCRR/COBRE grant P20 RR020143 to MEAR (PI JBH), the OCAST grant HR09-106 and the Instituto de Salud Carlos III partly financed through FEDER funds of the European Union to MEAR. This work was also partly supported by FISM, Regione Piemonte (CIPE and grant 2008) to SDA, the BMBF Kompetenznetz Rheuma C2.12, Germany to TW, grants SAF2006-00398, CTS-1180 and RETICS Program, RD08/0075 (RIER) from Instituto de Salud Carlos III (ISCIII) to JM. BAPE is the coordinator of the Argentine Collaborative group and his work was partly supported by the Federico Wihelm Agricola Foundation Research grant. National Institutes of Health RR020143 (JMG and JBH), RR015577 (JMG, JBH, JAJ), N01 AI050026-001 (JMG and JAJ), AR053483 (JMG and JAJ), AI063274 (PMG), AI031584 (JBH, JMG, JAJ), AR052125 (PMG), AR043247 (Kathy L Moser), Kirkland Scholar awards (JBH and JAJ), AR049084 (JBH), AR42460 (JBH), AR62277 (JBH), AI24717 (JBH), AR048940 (JBH, JAJ), AI083194 (JBH), R01 DE018209 (JBH), AI082714 (JBH), Alliance for Lupus Research (JBH), the US Department of Veterans Affairs (JBH), and the OHRS award for project number HR08-037 from the Oklahoma Center for the Advancement of Science and Technology (JMG).
Competing interests JW is an employee of MerckSerono Inc, and as such, the data or software he has developed belong to MerckSerono Inc.
Ethics approval All individuals provided informed consent as approved by the recruiting site institutional review boards at each of the affiliate institutions. All clinical investigation has been conducted according to the Declaration of Helsinki.
Patient consent Obtained.
Provenance and peer review Not commissioned; externally peer reviewed.