Genetic and physical interaction of the B-cell systemic lupus erythematosus-associated genes BANK1 and BLK

Abstract

Objectives Altered signaling 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 signaling and BLK codes for an Src tyrosine kinase with important roles in B-cell development. To characterize the role of BANK1 and BLK in SLE, we performed a genetic interaction analysis hypothesizing that genetic interactions could reveal functional pathways relevant to disease pathogenesis. Methods We Used the method GPAT16 to analyze the gene-gene interactions of BANK1 and BLK. Confocal microscopy was used to investigate co-localization, 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, we tested the possibility that BANK1 and BLK could also show a protein-protein interaction. We demonstrated co-immunoprecipitation and co-localization of BLK and BANK1. In a Daudi cell line and primary naïve B-cells the endogenous binding was enhanced upon B-cell receptor stimulation using anti-IgM antibodies. Conclusions Here, we show a genetic interaction between BANK1 and BLK, and demonstrate that these molecules interact physically. Our results have important consequences for the understanding of SLE and other autoimmune diseases and identify a potential new signaling pathway.

Autoimmunity risk alleles: hotspots in B cell regulatory signaling pathways

“…Lyn-knockout mice develop severe lupus-like disease consistent with lack of regulatory signaling. On the other hand, Blk-and Fyn-knockout mice do not develop autoimmunity (18,19). However, BLK is implicated in negative regulation of BCR signaling (18,19).…”

“…On the other hand, Blk-and Fyn-knockout mice do not develop autoimmunity (18,19). However, BLK is implicated in negative regulation of BCR signaling (18,19). Lyn mediates its inhibitory effects by phosphorylation of plasma membrane receptors such as CD22, CD72, and FcγRIIB, which act in turn by recruiting protein and inositol phosphatases (SHP-1 and SHIP-1, respectively) (20).…”

Genetic Interactions Between BANK1 and BLK in Chinese Patients with Systemic Lupus Erythematosus

“…We previously reported potential epistasis between BLK and TNFSF4 in both Chinese and white populations, suggesting that unbalanced functions of B cell and T cell signaling may be involved synergistically in the pathogenesis of SLE 2 . Another large-scale association study confirmed the genetic interactions between BANK1 and BLK in Europeans, indicating B cell activity and a B cell-specific pathway were crucial in lupus pathogenesis 3 . No further replications were conducted in Chinese subjects or other populations with independent sets of cases and controls.…”

“…The study was approved by the medical ethics committee of Peking University; all patients gave informed consent. The reported single-nucleotide polymorphisms (SNP) with top association signals of interactions were selected without optional discrimination 3 . Genotyping was undertaken as reported 2 .…”

B Cells Producing Pathogenic Autoantibodies

“…Among these signaling molecules, BANK1, a positive regulator of the BCR signaling pathway, has been identified by GWAS to be a susceptibility gene for SLE. The diseaseassociated variants of BANK1 are found to induce sustained BCR signaling and B cell hyperactivity, leading to the proliferation and expansion of autoreactive B cells [129,130]. Several negative regulators of BCR signaling have also been associated with autoimmunity, including PTPN22, Csk, Lyn, CD22, and FcγRIIB.…”