FokI polymorphism in the vitamin D receptor gene (VDR) and its association with lumbar spine pathologies in the Italian population: a case-control study

Abstract

Alterations in vitamin D homeostasis, mainly involving its nuclear receptor (VDR), could have a role in the pathophysiology of the spine. The association between VDR polymorphisms and spine disorders has been analyzed in different ethnic groups, focusing on the functional FokI polymorphism. However, so far, inconsistent findings were reported. The aims of this study were to evaluate, in the Italian white population, the VDR FokI polymorphism frequencies distribution in subjects with clearly defined lumbar spinal pathologies compared to asymptomatic controls and to analyze the interplay of genetic and conventional risk factors. Using a case-control design, 267 patients with spinal disorders and 220 asymptomatic controls were enrolled, evaluating their exposition to putative risk factors. Patients' clinical assessment was performed by Magnetic Resonance Imaging. FokI polymorphism (rs2228570) was detected by PCR-RFLP. Genotypes were designated by a lowercase letter (f allele, T nucleotide) for the presence of the restriction site and by a capital letter (F allele, C nucleotide) for its absence. Family history, higher age and BMI, exposure to vibration, physical job demand, smoking habit and lower practice of leisure physical activity were associated with spinal disorders. The FF genotype and F allele represented approximately 2-fold risk factors to develop discopathies and/or osteochondrosis concomitant with disc herniation, while f allele was protective. In conclusion, the link we observed between VDR FokI variants and specific lumbar spine pathologies suggests that spinal tissue degeneration is influenced by the genetic background. Future studies should evaluate the signaling pathways involving alterations in VDR and influencing the development and/or progression of spine disorders.

Figure 1. Patient’s clinical assessment and classification in subgroups.

a) Subgroup 1, patients with disc herniation only. b) Subgroup 2, patients with discopathies and/or osteochondrosis associated with disc herniation. c) Subgroup 3, patients with discopathies and/or osteochondrosis, without disc herniation, d–f) Subgroup 4, patients with stenosis (d), spondylolisthesis (e) or both (f). White arrows indicate the characteristic pathological features of each subgroup.

Figure 2. Structure of the genomic region of the VDR and location of the FokI polymorphism (a).

The VDR is located on chromosome 12 and contains nine exons: number 1, designated 1f–1c and indicated with white bars, contains six untranslated subunits in the promotor region, while the other eight exons, numbers 2–9, indicated with black bars in the coding region, encode proteins. FokI polymorphism is located in the exon 2, it is a C>T point mutation, the FokI enzyme cleaves when the restriction site ATG, a start codon, is present. A representative gel for the determination of FokI genotypes in three patients is showed (b). In the first lane there is a molecular weight DNA ladder (M) for size estimation of the DNA fragments. The letter “A” in the second, fourth and sixth lanes indicates the 265 bp PCR amplicons of the three patients. After digestion of the PCR product with FokI restriction enzyme an undigested 265 bp fragment (third lane, homozygous genotype FF, CC nucleotides), partially digested 265, 196 and 69 bp fragments (fifth lane, heterozygous genotype Ff, CT nucleotides), or totally digested 196 and 69 bp fragments (seventh lane, homozygous genotype ff, TT nucleotides) are present for the first, second and third patient, respectively.