FokI polymorphism at the human vitamin D receptor gene locus in Europeans and Africans

To the Editor: Two potential translation initiation codons (ATG) were reported in the nucleotide cDNA sequence of the human vitamin D receptor (VDR) gene (1): a T/C polymorphism (ATGACG) was discovered at the first ATG codon (2), so individuals who have an ACG codon instead of the first ATG codon probably initiate from the second ATG codon, and have a VDR protein three amino acids shorter. Some other polymorphisms were previously described in the 3% part of the VDR gene, and their potential relationship to the pathophysiology of osteoporosis was studied (3). Using polymerase chain reaction (PCR) amplification and the subsequent restriction of the amplified fragment with the restriction enzyme FokI, a restriction fragment length polymorphism (RFLP) method was developed (4) to detect the T/C polymorphism at the first ATG codon. The presence of the FokI site (denoted allele f) indicates that the first ATG is present, and the absence of the FokI site (denoted allele F) indicates that the first ATG is absent. Genomic DNA was extracted from peripheral blood lymphocytes by a classical method using digestion by proteinase K and phenol-chloroform extraction. PCR amplification was performed using the VDR2a (5%-AGCTGGCCCTGGCACTGACTCTGCTCT-3%) and VDR2b (5%-ATGGAAACACCTTGCTTCTTCTCCCTC-3%) primers described by Gross et al. (4). PCR products were restricted with FokI (Gibco-BRL) at 37°C for 3 h, and then electrophoresed through a 3% agarose gel containing ethidium bromide. Individuals were scored as FF homozygotes, Ff heterozygotes and ff homozygotes according to the restriction pattern (Fig. 1). Table 1 shows that allele frequencies determined on 105 unrelated European females from the Paris area were 0.63 for the F allele and 0.37 for the f allele; the relative genotype frequencies were: FF 38.1; Ff 49.5; ff 12.4%, respectively. Allele frequencies determined on 98 unrelated African females originating from Central Africa were 0.77 for the F allele and 0.23 for the f allele; and the relative genotype frequencies were: FF 57.2; Ff 38.8, ff 4.0%, respectively. The two genotype distributions differed significantly (X=9.39, pB0.01). The structural difference caused by the two alleles may affect the function of the VDR protein; some evidence suggests that the longer f allele may be less active in a transfected cell system (5). In the first study concerning postmenopausal Mexican– American women (4), those of the ff genotype had higher rates of femoral bone loss than FF women. In white premenopausal women (6), femoral neck bone mineral density (BMD) of the ff genotype was 12.1% lower than that of the FF genotype, inferring that if this difference was maintained after menopause, women with the ff genotype have a substantially greater risk of osteoporosis. Some studies suggest that black people have a higher peak bone mass than white people (7); but it is uncertain whether these racial differences are attributed to environmental or genetic factors. The genotype frequencies of FokI VDR polymorphism reported above can be used to explain, in part, variance of BMD because they differed greatly between Caucasoid females and those of African