Vitamin D Receptor Gene Expression and Function in a South African Population: Ethnicity, Vitamin D and FokI

Abstract

Polymorphisms of the vitamin D receptor gene (VDR) have been associated inconsistently with various diseases, across populations of diverse origin. The T(f) allele of the functional SNP FokI, in exon 2 of VDR, results in a longer vitamin D receptor protein (VDR) isoform, proposed to be less active. Genetic association of VDR with disease is likely confounded by ethnicity and environmental factors such as plasma 25(OH)D3 status. We hypothesized that VDR expression, VDR level and transactivation of target genes, CAMP and CYP24A1, depend on vitamin D, ethnicity and FokI genotype. Healthy volunteers participated in the study (African, n = 40 and White, n = 20). Plasma 25(OH)D3 levels were quantified by LC-MS and monocytes cultured, with or without 1,25(OH)2D3. Gene expression and protein level was quantified using qRT-PCR and flow cytometry, respectively. Mean plasma 25(OH)D3 status was normal and not significantly different between ethnicities. Neither 25(OH)D3 status nor 1,25(OH)2D3 supplementation significantly influenced expression or level of VDR. Africans had significantly higher mean VDR protein levels (P<0.050), nonetheless transactivated less CAMP expression than Whites. Genotyping the FokI polymorphism by pyrosequencing together with HapMap data, showed a significantly higher (P<0.050) frequency of the CC genotype in Africans than in Whites. FokI genotype, however, did not influence VDR expression or VDR level, but influenced overall transactivation of CAMP and 1,25(OH)2D3-elicited CYP24A1 induction; the latter, interacting with ethnicity. In conclusion, differential VDR expression relates to ethnicity, rather than 25(OH)D3 status and FokI genotype. Instead, VDR transactivation of CAMP is influenced by FokI genotype and, together with ethnicity, influence 1,25(OH)2D3-elicited CYP24A1 expression. Thus, the expression and role of VDR to transactivate target genes is determined not only by genetics, but also by ethnicity and environment involving complex interactions which may confound disease association.

Association between vitamin D receptor polymorphisms and multiple sclerosis: systematic review and meta-analysis of case–control studies

“…One potential exception is the FokI variant that has a differential effect on the immune system. 51,52 Our results indicated that the ApaI polymorphism a functional effect on MS. Additionally, the TaqI polymorphism was implicated in MS risk. However, its expression was influenced by environmental factors such as age and publication year.…”

Vitamin D Status and Its Consequences for Health in South Africa

“…Of the eleven papers identified, seven were relevant experimental reports investigating VDR polymorphisms in South African populations (Table 4). Five related to disease [38,86,87,89,90], and two to ethnicity alone [85,88]. The first study on VDR single nucleotide polymorphisms (SNPs) in South African Whites, Blacks and Asian-Indians showed a significantly higher frequency of Taq I “TT” and ApaI “AA” genotypes in Blacks compared to Whites and Asian-Indians [85].…”

“…A recent study of Black African and Coloured adults in Cape Town found Fok I “AG” associated with lower serum 25(OH)D levels, as well as a lower 25(OH)D status in winter and following supplementation with vitamin D [38]. The impact of Fok I on VDR function was examined using monocytes from healthy South African Blacks and Whites [90]. Despite having higher levels of more active VDR, monocytes from Blacks showed reduced transactivation of the gene encoding the cathelicidin antimicrobial protein (CAMP), compared to Whites, unless supplemented in vitro with 1,25(OH) 2 D 3 .…”

Vitamin D Receptor (Vdr) Gene Polymorphism and Maternal Vitamin D Deficiency in Indian Women With Preterm Birth (Ptb)

“…Placental tissue obtained with PTB exhibited reduced mRNA expression of VDR gene compared to normal placental tissue [29]. Allele F in VDR produce shorter protein by three amino-acids which are more active form in transactivation of the VDR and producing 1, 25 (OH) D effect compared to f allele of VDR gene [30]. Results of the present study indicated that subjects with ff genotype have significantly more risk to develop PTB compared with the reference genotype FF.…”