[Show abstract][Hide abstract] ABSTRACT: The c-Src tyrosine kinase, Csk, physically interacts with the intracellular phosphatase Lyp (encoded by PTPN22) and can modify the activation state of downstream Src kinases, such as Lyn, in lymphocytes. We identified an association of CSK with systemic lupus erythematosus (SLE) and refined its location to the intronic polymorphism rs34933034 (odds ratio (OR) = 1.32; P = 1.04 × 10(-9)). The risk allele at this SNP is associated with increased CSK expression and augments inhibitory phosphorylation of Lyn. In carriers of the risk allele, there is increased B-cell receptor (BCR)-mediated activation of mature B cells, as well as higher concentrations of plasma immunoglobulin M (IgM), relative to individuals with the non-risk haplotype. Moreover, the fraction of transitional B cells is doubled in the cord blood of carriers of the risk allele, due to an expansion of late transitional cells in a stage targeted by selection mechanisms. This suggests that the Lyp-Csk complex increases susceptibility to lupus at multiple maturation and activation points in B cells.
[Show abstract][Hide abstract] ABSTRACT: The gene B lymphocyte kinase (BLK) is associated with rheumatoid arthritis, systemic lupus erythematosus and several other autoimmune disorders. The disease risk haplotype is known to be associated with reduced expression of BLK mRNA transcript in human B cell lines; however, little is known about cis-regulation of BLK message or protein levels in native cell types. Here, we show that in primary human B lymphocytes, cis-regulatory effects of disease-associated single nucleotide polymorphisms in BLK are restricted to naïve and transitional B cells. Cis-regulatory effects are not observed in adult B cells in later stages of differentiation. Allelic expression bias was also identified in primary human T cells from adult peripheral and umbilical cord blood (UCB), thymus and tonsil, although mRNA levels were reduced compared with B cells. Allelic regulation of Blk expression at the protein level was confirmed in UCB B cell subsets by intracellular staining and flow cytometry. Blk protein expression in CD4(+) and CD8(+) T cells was documented by western blot analysis; however, differences in protein expression levels by BLK genotype were not observed in any T cell subset. Blk allele expression differences at the protein level are thus restricted to early B cells, indicating that the involvement of Blk in the risk for autoimmune disease likely acts during the very early stages of B cell development.
Human Molecular Genetics 06/2012; 21(17):3918-25. · 7.69 Impact Factor