Age- and gender-specific modulation of serum osteopontin and interferon-alpha by osteopontin genotype in systemic lupus erythematosus

Abstract

Osteopontin (OPN) is a multifunctional cytokine involved in long bone remodeling and immune system signaling. Additionally, OPN is critical for interferon-alpha (IFN-alpha) production in murine plasmacytoid dendritic cells. We have previously shown that IFN-alpha is a heritable risk factor for systemic lupus erythematosus (SLE). Genetic variants of OPN have been associated with SLE susceptibility, and one study suggests that this association is particular to men. In this study, the 3' UTR SLE-risk variant of OPN (rs9138C) was associated with higher serum OPN and IFN-alpha in men (P=0.0062 and P=0.0087, respectively). In women, the association between rs9138 C and higher serum OPN and IFN-alpha was restricted to younger subjects, and risk allele carriers showed a strong age-related genetic effect of rs9138 genotype on both serum OPN and IFN-alpha (P<0.0001). In African-American subjects, the 5' region single nucleotide polymorphisms, rs11730582 and rs28357094, were associated with anti-RNP antibodies (odds ratio (OR)=2.9, P=0.0038 and OR=3.9, P=0.021, respectively). Thus, we demonstrate two distinct genetic influences of OPN on serum protein traits in SLE patients, which correspond to previously reported SLE-risk variants. This study provides a biologic relevance for OPN variants at the protein level, and suggests an influence of this gene on the IFN-alpha pathway in SLE.

Figure 1

Haplotype structures observed in osteopontin (OPN) gene in each ancestral background studied. Darker gray shading in the box plot indicates higher r² values, and a gene diagram showing the location of each studied single nucleotide polymorphism (SNP) is shown above the box plot. Pairwise r² values in each square are shown as r² × 100, calculated using Haploview 4.0 software.

Figure 2

Serum interferon-α (IFN-α) in male and female systemic lupus erythematosus (SLE) patients stratified by osteopontin (OPN) rs9138 genotype. (a) Shows male SLE patients stratified by OPN genotype, P value calculated as the probability that a horizontal line was a better fit of the data than a semi-log line with a non-zero slope between genotype categories using the extra sum-of-squares F test. Each dot represents a unique patient; lines and error bars show the median and interquartile range. (b) Shows all female SLE patients stratified by OPN genotype, P value by Mann–Whitney U-test.

Figure 3

Serum interferon-α (IFN-α) activity in female patients stratified by age and osteopontin (OPN) rs9138 genotype. P value for a difference in serum IFN-α by OPN genotype in the ≤23-year-old group calculated as the probability that a horizontal line was a better fit of the data than a semi-log line with a non-zero slope between genotype categories using the extra sum-of-squares F test. P value for a difference in serum IFN-α in AC genotype ≤23 years old vs AC genotype in >23 years old calculated by Mann–Whitney U-test. Each dot represents a unique patient; lines and error bars show the median and interquartile range.

Figure 4

Serum osteopontin (OPN) in the overall systemic lupus erythematosus (SLE) cohort and in male SLE patients stratified by OPN rs9138 genotype. (a) Shows serum OPN in men, women and healthy unrelated controls, with P values by Mann–Whitney U-test. (b) Shows serum OPN in male SLE patients stratified by OPN genotype, P value calculated as the probability that a horizontal line was a better fit of the data than a semi-log line with a non-zero slope between genotype categories using the extra sum-of-squares F test. Each dot represents a unique patient; lines and error bars show the median and interquartile range.

Figure 5

Serum osteopontin (OPN) in female systemic lupus erythematosus (SLE) patients. (a) Shows female SLE patients stratified by OPN rs9138 genotype, all P values >0.05. (b) Shows female SLE patients stratified by age and OPN rs9138 genotype. P value for a difference in OPN levels by OPN genotype in the ≤23-year-old group calculated as the probability that a horizontal line was a better fit of the data than a semi-log line with a non-zero slope between genotype categories using the extra sum-of-squares F test. P value for a difference in OPN levels in AC genotype ≤23 years old vs AC genotype in >23 years old calculated by Mann-Whitney U-test.