Molecular analysis of the gene for the human vitamin-D-binding protein (group-specific component): allelic differences of the common genetic GC types

Abstract

DNA sequence analysis of the polymerase chain reaction products, including the coding region for amino acids 416 and 420, of the vitamin-D-binding protein (DBP, group-specific component, GC) shows allele-specific differences. The GC2 and GC1F phenotypes have an aspartic acid residue at amino acid position 416, whereas the GC1S phenotype has a glutamic acid at this position. In the GC2 phenotype, amino acid 420 is a lysine residue, and in the both common GC1 phenotypes, it is a threonine residue. The nucleotide exchanges involve a HaeIII (position 416) and a StyI (position 420) restriction site: the HaeIII restriction site is specific for the GC*1S allele and the StyI restriction site is specific for the GC*2 allele. We have tested 140 individual genomic DNA samples for the HaeIII site and 148 samples for the StyI site by restriction fragment length polymorphism (RFLP) analysis with a DBP-specific direct genomic DNA probe, and have compared these findings with the GC phenotype classification, by isoelectric focusing (IEF) of the corresponding plasma. The results of the HaeIII RFLP analysis and the IEF typing were in complete agreement. By using our DNA probe, we could disclose, in addition to the StyI site at amino acid position 420, two further StyI site downstream: one was specific for the GC*1S allele and another for the GC*1F allele. In 147 samples, there was agreement between the IEF GC typing and the analysis of the StyI restriction sites. In a single case, the observed result of the StyI-digest differed from the result expected after IEF classification: homozygous GC 1F-1F by IEF and heterozygous by StyI RFLP analysis. We discuss this finding as a recombination event or a possible silent allele in IEF typing. The GC polymorphism revealed by Southern blot analysis of StyI-digests provides an informative DNA marker system for chromosome 4q11-q13.

Amino acid variants of the vitamin D-binding protein and risk of diabetes in white Americans of European origin

“…The frequency of homozygotes for this haplotype was similar in all three groups: 3.0% (n ¼ 5) in controls, 3.3% (n ¼ 6) in type 1 patients, and 1.9% (n ¼ 4) in type 2 patients. Thus the frequencies of this variant were similar in all our groups and similar to the published data on white populations (18).…”

“…Thus it is possible that the effect of these variants of the DBP gene on the development of type 2 diabetes is peculiar to non-whites. It is interesting that the prevalence of homozygotes for the Asp-Thr haplotype, previously reported to be associated with quantitative prediabetic traits, is about seven to eight times greater in Pima Indians than in white populations (9,18).…”

Serum 25(OH)D and vitamin D status in relation to <i>VDR</i>, <i>GC</i> and <i>CYP2R1</i> variants in Chinese

“…Among several genetic variants of GC, rs7041 and rs4588, in exon 11, led to a Glu/Asp amino acid change at codon 416 and a Thy/Lys amino acid change at codon 420 [35], respectively. These two SNPs are the most commonly studied and repeatedly reported to be associated with lower levels of 25(OH)D. Our finding is in line with the results of several [11,17,19,23,31], but not all [12], recent studies that investigated the association between the genetic variants of GC and vitamin D. In a study of 3,210 Han Chinese from Beijing and Shanghai, Lu et al [11] found that the risk alleles of GC-rs2282679, rs4588 and rs7041 were significantly associated with lower levels of plasma 25(OH)D. The haplotype that contained all three risk alleles, and the C allele of rs1155563 was associated with lower levels of plasma 25(OH)D when compared with the haplotype that carried the four protective alleles.…”

Vitamin D metabolites in captivity? Should we measure free or total 25(OH)D to assess vitamin D status?

“…The three most common variants of DBP (also known as GC globulin) are GC1F, GC1S, and GC2. Each variant is characterized by a different combination of two single nucleotide polymorphisms (rs4588 and rs7041) resulting in two amino acid substitutions and differing glycosylation patterns (17). (Table 1, Figure 1).…”