SPP1 Promoter Polymorphisms: Identification of the First Modifier Gene for Pseudoxanthoma Elasticum

Abstract

Abstract Background: Progressive calcification and fragmentation of elastic fibers are characteristic hallmarks of pseudoxanthoma elasticum (PXE), which is caused by mutations in ABCC6 encoding multidrug resistance–associated protein 6 (MRP6). Because of the great clinical variability of PXE, secondary genetic risk factors are suspected to exist. We investigated whether SPP1 (secreted phosphoprotein 1; previously OPN, osteopontin) promoter polymorphisms are associated with PXE. Methods: We screened an ∼2-kb region spanning the theoretical promoter of the SPP1 gene for sequence variations by denaturing HPLC and direct sequencing in 93 PXE patients. Sequence variations with a prevalence >5% were genotyped in 93 age- and sex-matched healthy controls. Statistical and haplotype association analyses were performed using Fisher exact test, PHASE v2.1.1, and Haploview 3.2. Results: Mutational screening revealed 9 different sequence variations. Three SPP1 promoter polymorphisms (c.−1748A>G, c.−155_156insG, and c.244_245insTG) were significantly more frequent in PXE patients than in 93 age- and sex-matched healthy controls (Pcorrected < 0.05 each). The odds ratios (95% CI) for PXE among carriers of the 3 alleles were, respectively, 2.16 (1.34–3.48), 2.41 (1.51–3.82), and 1.97 (1.23–3.15). Haplotype analysis of 6 SPP1 promoter polymorphisms revealed 1 haplotype to be significantly reduced among PXE patients (Pcorrected = 0.035, odds ratio 1.80, 95% CI 1.19–2.71). Conclusions: Polymorphisms in the SPP1 promoter are secondary genetic risk factors contributing to PXE susceptibility.