Objective: Recent case-control studies show associations of the minor T allele (of the C1858T single-nucleotide polymorphism corresponding to the R620W amino acid substitution) of PTPN22 with multiple autoimmune diseases, including systemic lupus erythematosus (SLE). We performed family-based association studies of this polymorphism in 4 independent cohorts containing SLE patients and their parents and/or other family members.
Methods: A total of 2,689 individuals from 902 independent Caucasian families with SLE were genotyped using polymerase chain reaction pyrosequencing (cohorts 1 and 2) and the Sequenom MassArray system (cohorts 3 and 4). The transmission disequilibrium test (TDT) and the pedigree disequilibrium test (PDT) were conducted to assess the evidence of association.
Results: The 1858 C > T allele frequencies of the parents showed no deviation from Hardy-Weinberg equilibrium within each cohort. No evidence of preferential transmission of the T allele from heterozygous parents to their affected offspring was observed in each of the 4 cohorts or in the combined sample. Consistent with the TDT result, the PDT analysis revealed no significant association between the T allele and SLE. In 54 of the 661 SLE patients (cohorts 1 and 3) with documented autoimmune thyroid disease, the T allele frequency was higher than in individuals with SLE alone (16.7% versus 8.5%; P = 0.008, odds ratio 2.16 [95% confidence interval 1.25-3.72]).
Conclusion: The R620W polymorphism of the PTPN22 gene is not a major risk allele for SLE susceptibility in our sample of Caucasian individuals from northern America, the UK, or Finland, but it appears to be a risk factor for the concurrent autoimmune diseases of autoimmune thyroid disease and SLE.