FRZB1 rs2242070 polymorphisms is associated with brick tea type skeletal fluorosis in Kazakhs, but not in Tibetans, China.

Skeletal fluorosis is a metabolic bone and joint disease caused by excessive accumulation of fluoride in the bones. Compared with Kazakhs, Tibetans are more likely to develop moderate and severe brick tea type skeletal fluorosis, although they have similar fluoride exposure. Single nucleotide polymorphisms (SNPs) in frizzled-related protein (FRZB) have been associated with osteoarthritis, but their association with the risk of skeletal fluorosis has not been reported. In this paper, we investigated the association of three SNPs (rs7775, rs2242070 and rs9288087) in FRZB1with brick tea type skeletal fluorosis risk in a cross-sectional case-control study conducted in Sinkiang and Qinghai, China. A total of 598 individuals, including 308 Tibetans and 290 Kazakhs, were enrolled in this study, in which cases and controls were 221 and 377, respectively. The skeletal fluorosis was diagnosed according to the Chinese diagnostic criteria of endemic skeletal fluorosis (WS192-2008). The fluoride content in tea water or urine was detected using the fluoride ion electrode. SNPs were assessed using the Sequenom MassARRAY system. Binary logistic regressions found evidence of association with rs2242070 AA genotype in only Kazakh participants [odds ratio (OR) 0.417, 95% CI 0.216-0.807, p = 0.009], but not in Tibetans. When stratified by age, this protective effect of AA genotype in rs2242070 was pronounced in Kazakh participants aged 46-65 (OR 0.321, 95% CI 0.135-0.764, p = 0.010). This protective association with AA genotype in rs2242070 in Kazakhs also appeared to be stronger with tea fluoride intake > 3.5 mg/day (OR 0.396, 95% CI 0.182-0.864, p = 0.020). Our data suggest there might be differential genetic influence on skeletal fluorosis risk in Kazakh and Tibetan participants and that this difference might be modified by tea fluoride intake.