A missense single-nucleotide polymorphism in a gene encoding a protein tyrosine phosphatase (PTPN22) is associated with rheumatoid arthritis

Abstract

Rheumatoid arthritis (RA) is the most common systemic autoimmune disease, affecting approximately 1% of the adult population worldwide, with an estimated heritability of 60%. To identify genes involved in RA susceptibility, we investigated the association between putative functional single-nucleotide polymorphisms (SNPs) and RA among white individuals by use of a case-control study design; a second sample was tested for replication. Here we report the association of RA susceptibility with the minor allele of a missense SNP in PTPN22 (discovery-study allelic P=6.6 x 10(-4); replication-study allelic P=5.6 x 10(-8)), which encodes a hematopoietic-specific protein tyrosine phosphatase also known as "Lyp." We show that the risk allele, which is present in approximately 17% of white individuals from the general population and in approximately 28% of white individuals with RA, disrupts the P1 proline-rich motif that is important for interaction with Csk, potentially altering these proteins' normal function as negative regulators of T-cell activation. The minor allele of this SNP recently was implicated in type 1 diabetes, suggesting that the variant phosphatase may increase overall reactivity of the immune system and may heighten an individual carrier's risk for autoimmune disease.

Figure 1

PTPN22 knockdown by RNAi increases antigen-receptor signaling in Jurkat T-cell line. Knockdown and NF-κB transcriptional response after T-cell receptor stimulation for cells transfected with two control siRNAs (Scramble) and two PTPN22 siRNAs. First, 4.5 μg siRNA (Dharmacon) and 1 μg pNF-κB-luc plasmid (Stratagene) were electroporated into 2×10⁶ Jurkat cells (ATCC, clone E6-1, homozygous for 620R) in Cell Line Nucleofector Solution V, according to manufacturer’s protocol #S18 (amaxa). After 24 h, 5.5×10⁴ cells were stimulated with anti-CD3 (2 μg/ml [BD Pharmingen, clone UCHT1]), anti-CD28 (2 μg/ml [BD Pharmingen, clone CD28.2]), and anti-mouse IgG1 (2 μg/ml [BD Pharmingen, clone A85-1]). Cells were lysed and assayed for luminescence 6 h after stimulation by use of Bright-Glo (Promega) and a 96-well MicroBeta plate reader (Wallac). PTPN22 siRNAs that were effective for knockdown were found by screening seven candidate siRNA sequences that were designed by use of Web-based tools from Ambion and Dharmacon. siRNA sequences used in this study were PTPN22.1 (5′-AAGGCAGACAAAACCTATCCT), PTPN22.2 (5′-GAGGATTCCAGCTACATCAAT), Scramble.1 (5′-AAGAACGGCATCAAGGTGAAC), and Scramble.2 (5′-AATTCTCCGAACGTGTCACGT). RNA was harvested according to the manufacturer’s protocol (RNeasy 96 [Qiagen]), and TaqMan real-time quantitative RT-PCR (SDS-7900 [Applied Biosystems]) was used to measure PTPN22 expression levels. PTPN22 mRNA levels in each sample were normalized to total RNA amounts (RiboGreen [Molecular Probes]) and then expressed relative to PTPN22 levels in cells that were untransfected with siRNA. The PTPN22 TaqMan sequences were 5′-GGCCCAAAGCAAGAAAATTACTAAA (forward), 5′-TGTCTGCCTTGTACTTGGTAGATTG (reverse), and 5′-TTCAGCTTCAGAAATT (MGB probe).

Figure 2

Western blot showing Csk coimmunoprecipitation by the two HA-tagged PTPN22 proteins (R620 and W620) expressed in 293T cells. W620 decreases the affinity of PTPN22 for Csk. Increasing the amount of PTPN22 W620 expressed does not increase the amount of Csk that coimmunoprecipitates. Quantitation of Csk band intensity indicates a 2.9-fold difference between R620 and W620. Similar results (2.6-fold) were seen in a second experiment. PTPN22 sequences were cloned by PCR and verified to encode a PTPN22 protein sequence corresponding to Swiss-Prot Q9Y2R2. The R620W variant was introduced (QuikChange [Stratagene]), and both PTPN22 variants with an N-terminal HA tag were cloned into the pCMV5 expression vector (Qbiogene). Csk was cloned by PCR, the sequence was verified, and it was introduced into the pcDNA-DEST40 expression vector (Invitrogen). The 293T (GenHunter) cells (3×10⁵ cells/well) were seeded in 12-well plates and were transfected 24 h later by use of 3.5 μl/well of Lipofectamine 2000 (Invitrogen). Cells were washed and resuspended in lysis buffer (50 mM Tris [pH 8.0], 2 mM EDTA, 1% NP-40, 50 mM NaF, 1 mM sodium orthovanadate, and 1× protease inhibitor cocktail [Sigma]) 48 h after transfection. HA-PTPN22 immunoprecipitations were performed as described elsewhere (Cloutier and Veillette 1999), with the following modifications. Before immunoprecipatation, detergent-insoluble material was removed by centrifugation (100,000×g) and lysates were pre-cleared by use of mouse IgG agarose beads (Sigma). The lysate (100 μg) was incubated with 15 μl of anti-HA conjugated beads (Sigma) for 2 h at 4°C. Beads were washed three times in wash buffer (50 mM Tris [pH 8.0], 100 mM NaCl, 2 mM EDTA, 1% NP-40, 50 mM NaF, and 1 mM sodium orthovanadate). Precipitated proteins and lysates were analyzed by western blot, by use of anti-HA (Covance) and anti-Csk (Upstate 06-566) antibodies, and were detected by use of HRP-conjugated secondary antibodies (Pierce 31430 and Biosource ALI3404) and chemiluminescence (Pierce).

Figure 3

RNA expression profile of PTPN22. Expression of the PTPN22 major splice variant (GenBank accession number NM_015967) was determined by kinetic RT-PCR analysis by use of total RNA from sorted hematopoeitic cells (AllCells) and selected tissues (Clontech), as described elsewhere (Rogge et al. 2000). Data for additional tissues are provided online (table A1 [online only]). To eliminate genomic DNA amplification, amplification primers (5′-GGTTGAGGAAGCTGGAGAAT and 5′-GGGAGAAGAACGATCTTGATGTA) were selected from different exons. Each RNA sample was also amplified with seven housekeeping genes, as described elsewhere (Rogge et al. 2000). The level of expression of these housekeeping genes was used to normalize the amount of message in all samples. The normalized expression levels of PTPN22 in all normal tissues (excluding tumor cell lines and purified hematopoietic cells) were averaged, and the results from each individual sample were expressed as a fold change relative to this average. A positive number indicates more PTPN22 message in the indicated sample relative to the average of all samples, whereas a negative number means there is less PTPN22 message relative to the overall average.