Cdx-2 polymorphism in the vitamin D receptor gene (VDR) marks VDR expression in monocyte/macrophages through VDR...

Caudal-type homeobox protein 2 (CDX-2) is an intestine-specific transcription factor (TF), with a polymorphic binding site (Cdx-2, rs11568820, A/G) in the vitamin D receptor gene (VDR). The molecular mechanism underlying Cdx-2 association with conditions like osteoporosis, which depends on intestinal VDR expression and calcium absorption, is believed to be due to higher affinity of CDX-2 for the ancestral A allele compared to the G allele. However, it is unclear why the polymorphism is associated with diseases like tuberculosis, which is dependent on VDR expression in immune cells that do not express CDX-2. This study aimed to explain Cdx-2 variant association with immune-related conditions. We hypothesised that the effect of Cdx-2 polymorphism on VDR expression in monocytes/macrophages, devoid of the CDX-2 TF, is indirect and dependent on circulating 25(OH)D₃ and VDR methylation. Primary monocyte/macrophages from healthy donors (n = 100) were activated though TLR2/1 elicitation. VDR mRNA and 25(OH)D₃ were quantified by RT-qPCR and LC-MS/MS, respectively. Genotyping and methylation analysis were done by pyrosequencing. AA vs. AG/GG showed reduced levels of 25(OH)D₃ (P < 0.010), higher VDR promoter methylation (P < 0.050) and lower VDR mRNA induction (P < 0.050). Analysis of covariance confirmed that the effect of Cdx-2 variants depends primarily on VDR methylation. Thus, VDR methylation may confound association studies linking VDR polymorphisms to disease.