Association of Vitamin D Binding Protein Variants with Chronic Mucus Hypersecretion in Iceland

Background: Previous studies of vitamin D binding protein (VDBP, also known as group-specific component, Gc, encoded by the GC gene) have implicated two gene variants, GC*2 and GC*1F, as possible contributors with chronic obstructive pulmonary disease (COPD) protection and susceptibility, respectively. The objective of this study was to examine the association of VDBP to different subtypes of COPD. Study design: The association of the various GC genotypes to the COPD phenotype was examined in Icelandic COPD patients who were followed by pulmonary physicians at the University Hospital of Iceland. Methods: All patients were genotyped for the known alleles of the GC gene. The single nucleotide polymorphisms (SNPs) were identified by a restriction fragment length polymorphism procedure. Study power was estimated based on allele frequencies of the variants, and risk ratios were calculated from the prevalence of genotypes in the affected group divided by its prevalence in the control population. Statistical analyses were performed using the 2-tailed Fisher's Exact Test and χ² test, where appropriate. Patient group: One hundred and two COPD patients and 183 controls, together with 46 asthma patients and 48 patients with chronic mucous hypersecretion (CMH) were examined. Main outcome measure and results: The results demonstrate similar allele and genotype frequencies of GC in COPD patients overall and healthy controls. However, there was a higher prevalence of genotypes carrying a GC*1F allele and lower prevalence of genotypes with a GC*2 allele in the CMH patients than in controls. This difference was most notable in the homozygous form: 8.3% vs 1.1% for the GC±1F/±1F, and 0.0% vs 7.6% for the GC*2/*2 genotypes, respectively. When controlled for smoking, only the non-smoking CMH patients demonstrated a significantly altered frequency of the GC*1F/*1F genotype (p = 0.0001). The prevalence of the GC*2/*2 genotype was also significantly lower in patients with bronchial hypersecretion with airflow obstruction compared with the control group (2.9% vs 7.6%). Taken together, these results demonstrate that the GC*1F and GC*2 alleles are associated with sputum hypersecretion in individuals who are at increased risk of developing COPD.