Abstract
The rs7041 and rs4588 polymorphisms found in the GC gene, encoding vitamin D-binding protein (DBP), have distinct biochemical phenotypes. The aim of this study was to investigate vitamin D parameters with these polymorphisms, in individuals with possible vitamin D deficiency. The most common (49% of the cohort) genotype in rs7041 was GT, especially among individuals with high levels of free 25(OH)D calculated but with low levels of bioavailable 25(OH)D, and in rs4588 it was AC in particular among the individuals with low levels of bioavailable 25(OH)D. The most common phenotypes were Gc1s/2 (35.3%) and Gc1s/1s (31.4%), and Gc1f/1f was rare (5.9%). The variations in free and bioavailable 25(OH)D levels among healthy Turkish individuals may be attributed to the variations in total 25(OH)D as well as GC gene polymorphisms. The Turkish population shares a similarity for allele frequencies of rs7041 with the European population and similarity for allele frequencies of rs4588 with Gujarati Indians, and this may also be important in relation to certain ethnic populations showing associations between vitamin D and COVID-19.
Highlights
Vitamin D can be found in the serum as bound or unbound forms [1]
Several gene variants of proteins involved in the synthesis, transportation, and activation of vitamin D have been associated with changes in serum 25(OH)D including the group-specific complement (GC) gene, which encodes D-binding protein (DBP) [6,7,8]
Since there are two copies of the GC gene, this enables the generation of six possible DBP phenotypes (Gc1f-1s, Gc1f-1f, Gc1f-2, Gc1s-2, Gc1s-1s, Gc2-2) that can be identified from the genotypes
Summary
Vitamin D can be found in the serum as bound or unbound forms [1]. Several gene variants of proteins involved in the synthesis, transportation, and activation of vitamin D have been associated with changes in serum 25(OH)D including the group-specific complement (GC) gene, which encodes DBP [6,7,8]. Combinations of rs7041 and rs4588 genotypes can identify three phenotypic groups of DBP, namely, Gc1s, Gc1f, and Gc2, thought to vary in affinity for 25(OH)D [9]. These variants are Gc1f (1f): rs7041 (T) + rs4588 (C); Gc1s (1s): rs7041 (G) + rs4588 (C); and Gc2 (2): rs7041 (T) + rs4588 (A) [9]. Associations between various SNPs in this gene and the serum 25(OH)D concentrations have been investigated previously [7,8,10]; no study has examined this association in a Turkish population
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