Common variants of the vitamin D binding protein gene and adverse health outcomes.
Crit Rev Clin Lab Sci. 2013 Jan;50(1):1-22. doi: 10.3109/10408363.2012.750262.
Malik S, Fu L, Juras DJ, Karmali M, Wong BY, Gozdzik A, Cole DE.
Office of Biotechnology, Genomics and Population Health, Public Health Agency of Canada , Toronto, ON , Canada .
Table of contents
- DBP genotypes and chronic disease
- Neurodegenerative disorders
- Other chronic diseases
- Infectious diseases
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The vitamin D binding protein (DBP) is the major plasma carrier for vitamin D and its metabolites, but it is also an actin scavenger, and is the precursor to the immunomodulatory protein, Gc-MAF. Two missense variants of the DBP gene - rs7041 encoding Asp432Glu and rs4588 encoding Thr436Lys - change the amino acid sequence and alter the protein function.
They are common enough to generate population-wide constitutive differences in vitamin D status, based on assay of the serum metabolite, 25-hydroxyvitamin D (25OHD).
Whether these variants also influence the role of vitamin D in an immunologic milieu is not known. However, the issue is relevant, given the immunomodulatory effects of DBP and the role of protracted innate immune-related inflammation in response to tissue injury or repeated infection. Indeed, DBP and vitamin D may jointly or independently contribute to a variety of adverse health outcomes unrelated to classical notions of their function in bone and mineral metabolism.
This review summarizes the reports to date of associations between DBP variants, and various chronic and infectious diseases.
The available information leads us to conclude that DBP variants are a significant and common genetic factor in some common disorders, and therefore, are worthy of closer attention. In view of the heightened interest in vitamin D as a public health target, well-designed studies that look simultaneously at vitamin D and its carrier in relation to genotypes and adverse health outcome should be encouraged.
The PDF, attached at the bottom of this page, includes all text, extensive tables, and references
The following is just the text of the study which concerns diseases
Type I diabetes mellitus results from the autoimmune destruction of insulin producing pancreatic p-cells while type II diabetes is associated with insulin resistance and elevated levels of inflammatory mediators65,66. Vitamin D may play a role in the prevention and treatment of diabetes through its anti-inflammatory actions or by modifying calcium homeostasis to improve insulin secretion or enhance insulin sensitivity67. As such, polymorphisms in the DBP gene might provide insight into the role of vitamin D and diabetes.
Several studies have examined whether pre-diabetic phe-notypes are associated with DBP variants. In a Japanese study group, composed of 47 males and 35 females with normal glucose tolerance, carriers of Gc1S-2 and Gc1S-1S had significantly higher fasting plasma insulin levels than homo-zygotes for Gc1F (p5 0.01 and p5 0.03, respectively)68. These findings were supported in an ethnically distinct population made up of 144 Dogrib Indians of the Northwest Territories in Canada 69.
On the other hand, Baier et al. found no association with insulin or fasting plasma glucose concentrations in a Pima cohort, although they did report that exon 11 polymorphisms were associated with blood glucose responses to an oral glucose challenge in non-diabetics70. Specifically, individuals homozygous for Gc1F had the highest increment in glucose concentration at 30 and 60 min and the lowest glucose concentration at 180 min. Moreover, carriers homozygous for Gc2 had significantly lower mean incremental glucose concentrations at 60 and 120 min70.
Similarly, in a population-based case-control study composed of Hispanic and Anglo participants from the San Luis Valley Diabetes Study, DBP variants were not associated with insulin levels71. Also, and in contrast with Baier et al., DBP variants were not associated with postprandial glucose, but were associated with fasting glucose levels (F= 2.46; p= 0.033), the Gc1F genotype being more commonly observed in individuals with the highest levels of plasma glucose 71.
Other studies have examined the association between genetic variants of DBP and clinical diabetes. Results from a Japanese study, composed of 208 non-insulin-dependent diabetes mellitus patients and 209 control subjects with normal glucose tolerance, showed that Gc1F-1F homozygos-ity was less frequently and Gc1S/2 heterozygosity more frequently observed in patients compared to controls (p5 0.001 and p5 0.02, respectively)72. Sequence analysis identified D432E and T436K genotypes in a cohort of 912 Pima Indians (578 diabetic; 334 nondiabetic) from Gila River Indian Community in Arizona, USA70, but no association was reported between either variant and the prevalence of type II diabetes.
In a family-based study, the tetranucleotide repeat in intron 873 and D432E and T436K polymorphisms were studied in 152 Caucasian families of German origin with at least one child developing type I diabetes66. Transmission disequilibrium testing (TDT) showed neither allelic distortion of intron 8 or exon 11 alleles between cases and controls, nor of their
combined haplotypes66. Similarly, allele, genotype and hap-lotype frequencies of SNPs in exon 11 did not differ between non-diabetic controls (n = 163) and type I (n = 181) or type II (n = 215) diabetics in a Caucasian American population74. These findings were substantiated in French and Polish Caucasian case-control studies that additionally reported that phenotypes, defined by haplotype combinations, were equally distributed between Type II diabetics and non-diabetics75,76.
In a00re0ate, these studies fail to support a stron0, consistent role for DBP in the pathobiology of diabetes. It is likely that ethnicity and lifestyle factors contribute to the lack of concordance among these studies.
Osteoporosis is a common disorder of aging that disproportionately affects post-menopausal women. It is characterized by low bone mass and micro-architectural changes in bone which contribute to increased bone fragility and risk of fracture77. The primary clinical marker of osteoporosis, lower bone mineral density (BMD), is associated with vitamin D insufficiency78-80. However, the links between DBP and bone biology are not well understood. Whereas Gc-MAF is shown to increase phagocytosis and free radical (superoxide) production in macrophages 61,81, further work has implicated it as a potent activator of osteoclasts82. Indeed, osteopetrotic patients (characterized by excessively dense bone and high BMD) appear to lack the ability to generate Gc-MAF83. Osteopetrotic rats treated with Gc-MAF showed a reduction in BMD and associated skeletal defects 84.
In the ongoing 20-year Danish Osteoporosis Prevention Study comprised of 595 white postmenopausal women, Scandinavian investigators found a three-fold lower fracture risk associated with Gc2-2 compared with Gc1-1 genotype (p = 0.014, OR = 0.32, 95% CI = 0.13-0.80)77. A more comprehensive set of SNPs distributed across promoter, intronic and exonic regions of DBP was analyzed in 384 Japanese in a search for association with BMD85. Six SNPs (-39C>T, IVS1 + 827C>T, IVS1 + 1916C>T, IVS1-1154A>G, D432E and IVS11 + 1097G>C) were either significantly associated or suggestively associated with BMD, the IVS11 + 1097G>C SNP in intron 11 showing the strongest correlation (p 0.006). Furthermore, a common haplotype (T-C-C-G-T-C) derived from the six SNPs showed significant association with adjusted radial BMD (r = 0.15, p = 0.008). The functional consequence(s) of these SNPs/haplotypes are not known.
Another polymorphism in the form of an (TAAA)n-Alu repeat or repetitive element downstream of intron 8 was examined in a cohort of Caucasian men from northeast England for association with DBP serum levels, BMD and osteoporosis86. The (TAAA)8 (GC-I8*8) allele is the most prevalent one in this study group. Upon analyzing 26 men with vertebral fractures and 21 controls, the GC-I8*10/*8 genotype was found to be associated with decreased BMD lumbar spine and femoral neck BMD and increased vertebral fractures compared with GC-I8*10/*10 (p 5 0.05, OR = 56; 95% CI = 7-445)86. Interestingly, GC-I8*10/*8 was also associated with elevated serum DBP (p 0.049) which, in turn, may provide greater potential for Gc-MAF activation of osteoclasts in Gc1 carriers. Results of this study remained consistent in another male cohort made up of 56 individuals with idiopathic osteoporosis and 114 controls, but the odds ratio was much more modest87. Carriers of the GC-I8*10/*8 genotype and the GC-I8*9 allele were at increased risk of osteoporosis (OR = 2.88, CI = 1.31-6.32, p = 0.013 and OR 1.86, CI 1.07-3.24, p 0.038, respectively).
Moreover, combining all genotypes that include GC-I8*8 is associated with a two-fold risk of osteoporosis (OR 2.38,CI 1.24-4.58, p 0.014). However, GC-I8*10 allele carriers and GC-I8*10/*10 homozygotes showed increasingly substantial protection from osteoporosis (OR 0.40, CI 0.25-0.64, p 0.0005 and OR 0.13, CI 0.05-0.36, p 0.0005, respectively). Whether this effect is related in part to linkage disequilibrium with the nearby functional polymorphisms (D432E and T436K) has not been examined.
In a common aging disorder like osteoporosis, many genes are involved and interaction with non-genetic factors (e.g. calcium and vitamin D intake) is common. A recent prospective study examining more than 6100 elderly Caucasians found no association between DBP phenotype and osteoporosis; however, a 33% increased risk of fracture was detected in Gc1S carriers homozygous for polymorphisms in the 3' untranslated region (UTR) of the vitamin D receptor (VDR) in haploblock 5 compared with non-carriers (p = 0.005)88. Moreover, a hazard ratio of 1.47 (95% CI 1.06-2.05) was observed for homozygous Gc1S carriers (compared with non-carriers) with low dietary calcium intake (51.09 g/day).
Collectively, these studies emphasize the importance of gene-gene and gene-environment interactions in complex trait analysis and underscore the need to characterize the role of non-coding genetic variants associated with osteoporosis and related phenotypes.
Chronic obstructive pulmonary disease (COPD) is characterized by decreased pulmonary elastic recoil, expiratory obstruction and hyperinflation secondary to inflammation of peripheral airways89. The role of DBP as a modulator of inflammation suggests that it plays a role in the chronic pulmonary inflammation characteristic of COPD. Consistent with this hypothesis, an association study in a Caucasian population of 104 COPD patients and 413 controls indicated that Gc1F-1F and Gc1S carriers (at least one copy) were seven-fold and two-fold more likely, respectively, to have COPD than individuals with at least one Gc-2 allele90.
In another study of 75 Caucasian COPD patients, Gc-2 homozygosity was found to be underrepresented in COPD patients compared to 64 controls (OR 0.17, CI 0.03-0.83)89. This is in agreement with an earlier report examining 114 COPD cases in a match pair design study showing an excess of Gc2 homozygotes in the control group (p = 0.049)91.
Similarly, a recent study of 471 unrelated Caucasian subjects confirmed that the Gc2 variant was associated with a decreased risk of COPD (OR 0.79, CI 0.65-0.99; p 0.048), though it was associated with an increased risk of bronchiectasis in patients with a1-antitrypsin deficiency (OR 1.51, CI 1.02-2.22). The authors suggested this may be due to reduced capacity to produce Gc-MAF, thereby stimulating macrophage-induced pathogen clearing from the 92 airway .
A Japanese case-control study (103 cases, 88 controls) found that Gc1F homozygotes were more susceptible to chronic bronchitis and emphysema due to COPD than were healthy smokers (p = 0.01, OR = 2.3, 95% CI = 1.2-4.6)93.In a second Japanese cohort, Gc1F homozygotes were overrep-resented among COPD patients (36.5%) compared to controls (20.7%; OR = 2.2; 95% CI = 1.1-4.6). However, no association was found with diffuse panbronchitis, also characterized by chronic airflow limitation94. In a case-control study ofa Han Chinese cohort (100 cases, 100 controls), the COPD patient group was found to be enriched for Gc1F homozygous individuals while the frequency of Gc2 was significantly lower95.
Together, these findings are consistent with evidence suggesting that the presence of the 436 K minor variant (Gc2) precludes the conversion of DBP to Gc-MAF, potentially reducing macrophage-related inflammation96. Another recent report indicates that, 25(OH)D deficiency is more common in COPD patients from Belgium and correlates with disease severity97. In the same study, homozygous T436 subjects were found to have a 25% reduction in vitamin D serum levels (p5 0.0001) and were enriched amongst the COPD patient population (OR 2.11; CI 1.20-3.71). It is conceivable,
therefore, that anti-inflammatory actions exerted downstream of 25(OH)D52,97 are reduced in patients. The nature by which DBP influences COPD pathogenesis deserves further investigation, however unlikely it is to affect the rate or onset of deteriorating lung function98,99.
Asthma is a chronic inflammatory condition characterized by airway obstruction and elevated serum IgE levels100, most often found to be a consequence of enhanced acquired reactions to external allergens. It is associated with various reactive pro-inflammatory states including allergic rhinitis and eczema. Ecological studies indicate that vitamin D deficiency is associated with asthma and asthma related phenotypes101-105, but some studies contradict these findings reporting a positive association106,107, whereas others show no associa-tion108. Further evidence suggesting an inverse correlation between vitamin D and asthma is provided by genetic109,110 and epidemiologic (geospatial and solar radiation) studies111. However, in another study, Hughes et al. reported no association between ultraviolet radiation (UVR) or vitamin D serum levels; yet, cod liver oil supplementation earlier in life was associated with an increased risk of asthma112.
Overall evidence suggests that vitamin D protects against asthma, but the precise nature of this relationship is ambiguous and inconsistent findings are likely a function of epidemiological context. A molecular mechanism by which vitamin D may affect asthma is unknown and its relationship to atopy is unclear. However, emerging evidence suggests that 1,25(OH)2D3 inhibits Th2-related asthmatic inflammation either directly or via immunosuppressive T-regulatory cell induction113-115. However, these findings were not substantiated in a recent case-control study examining 467 asthmatic patients and 288 unrelated healthy controls in a Chinese Han population, which showed an enrichment of Gc2-2 in asthma cases (OR 1.35: CI 1.01-1.78; p 0.006)116. A TDT of more than 200 families from the German Asthma Family Study found no significant association with asthma, but rs222040 and rs7041 (D432E) were both weakly associated with total serum IgE (p5 0.02 and p5 0.03, respectively)117.
Risk of ischemic stroke is associated with elevated levels of vascular inflammation118,119 and lower 25(OH)D120, both of which are thereby linked to DBP. In six study populations from the United States, Europe and China (3550 cases, 6560 controls), a recent study evaluated 105 variants in 64 inflammation- and cardiovascular-related genes for association with ischemic stroke. Among the SNPs evaluated were DBP D432E and T436K, but neither was significantly associated119.
Endometriosis is a uterine disorder in which large overgrowths of endometrium accumulate inside or adjacent to the uterus121. The precise etiology remains unclear, but one theory holds that the peritoneal epithelia can differentiate into endometrial tissue as a result of chronic inflammation122. Furthermore, macrophage activation has been associated with disease progression123, thereby highlighting a potential role for DBP. In a recent cross-sectional study of premenopausal women undergoing laparoscopy, analysis of specific Gc allele products using nano-scale liquid chromatography-electro-spray ionization-mass spectrometry indicated that the Gc-2 allele product was enriched in serum samples in women with endometriosis (p 0.006)124. The authors speculate that the lack of activated macrophages' phagocytic function in those Gc2 carriers may increase the risk for implantation of endometriosis tissue in the peritoneal cavity. Further investigation of this hypothesis is warranted.
Inflammatory bowel disease (IBD) includes Crohn's disease and ulcerative colitis, both of which are characterized by chronic inflammation of the gastrointestinal tract125. Data collected as part of the Swiss Inflammatory Bowel Disease Cohort Study was used to test for associations between common DBP polymorphisms and risk of IBD125. Homozygotes for the K allele of T436K SNP were more common in non-IBD controls than in IBD patients (p 0.006). Significant associations were observed between SNP variants and disease risk in separate comparisons of healthy controls with ulcerative colitis (p = 0.022) and Crohn's disease patients (p = 0.016)125. The DBP-2 haplotype (consisting of D432-436 K) was found at higher frequencies in healthy controls than in IBD cases (OR = 2.53, CI = 1.48-4.34, p = 0.0005), especially when comparing healthy controls to the ulcerative colitis cases (OR = 4.39, CI = 1.87-10.31, p = 0.0003).
Several observational studies have demonstrated an inverse relationship between UVB irradiance (required for the cutaneous synthesis of vitamin D) or vitamin D status [serum 25(OH)D concentrations] and the incidence of mortality from endometrial1, lung127,128, breast127,129-133, Prostate1, ovarian138,139 and colorectal cancer140-149. The underlying mechanism by which vitamin D status affects cancer risk is unknown, however, 1,25(OH)2D3 has been shown to modulate cell proliferation and differentiation of both normal and malignant cells. Alternatively, inflammatory modulation effects of 1,25(OH)2D3 or Gc-MAF may play important roles in cancer pathogenesis.
The portion of the extensive tables in the PDF concerning Breast Cancer
Ecological case-control studies indicate an inverse correlation between vitamin D status and breast cancer risk, however, the relationship is less clear in longitudinal studies150,151. Much work has been done to elucidate a vitamin D-related mechanistic role in breast cancer. Gc-MAF exhibits anti-breast tumor activities in mice152 and shows immunothera-peutic properties in metastatic breast cancer patients153. Exposure to Gc-MAF significantly reduces vimentin expression in human breast cancer cells, suggesting a reversal of breast cancer progression154. Alternatively, the active metabolite of vitamin D, calcitriol, has been shown to inhibit cell proliferation and differentiation and promote apoptosis of breast tumor tissue155-158.
Genetic studies examining the relationship between functional DBP variants and breast cancer (in situ and invasive) have yielded inconsistent findings. In a nested case-control study derived from the Cancer Prevention Study II Nutrition Cohort (500 cases, 500 controls), D432E and T436K were not associated with postmenopausal breast cancer risk159. Interestingly, a German population-based case-control study comprised of 1,402 post-menopausal women with in situ or invasive breast cancer and 2608 population controls reported a protective effect of Gc2-2 carriers (OR 0.72, 95 % CI 0.54-0.96)160. Both these studies argue against a role for Gc-MAF or vitamin D in breast cancer pathogenesis. In contrast, subjects with the rs7041 TT genotype were at increased risk of breast cancer (OR 1.23; 95% CI 1.011.51) based on a study of 1560 invasive breast cancer patients and 1633 controls from the Ontario Women's Diet and Health Study161. Altogether, these findings underscore the complex pathogenesis of breast cancer and highlight the need to consider gene-environment interactions.
Studies have shown that vitamin D can inhibit proliferation and differentiation of human prostatic cells in vitro162 and 1,25(OH)2D3 has marked anti-tumor effects in animal models163. Furthermore, recent work has demonstrated that Gc-MAF directly inhibits proliferation and migration of prostate cancer cells as well as expression of a tumor metastasis-associated gene, urokinase plasminogen activator receptor164 consistent with an early study demonstrating the elevated risk of Gc2 genotype and carcinoma of the prostate165. However, in a comprehensive study of 749 cases and 781 controls from the PLCO Cancer Screening Trial, Ahn et al. found no association between risk of prostate cancer and 15 SNPs in GC, which included D432E and T436K166. An earlier study examining D432E and T436K in 181 prostate cancer patients (90 black and 91 white) and sex-, age- and race-matched controls also found no differences in allelic/genotypic frequencies between patients and controls; however, frequencies were strikingly different between blacks and whites 167.
In a well established colorectal cancer cohort (Colon Cancer Family Registry), Poynter et al. explored previous epidemiological evidence suggesting a reduced risk associated with vitamin D. No evidence for association between DBP genotype and risk of colorectal cancer was found, although associations between DBP SNPs and microsatellite-unstable colorectal cancer were reported168. A more recent study examining associations between polymorphisms in the GC and CASR genes similarly found no associations in any of the tested GC SNPs169.
Zhou et al.170 examined associations between variants of D432E (rs7041) and T436K (rs4588) and risk of four types of gastrointestinal cancers (hepatocellular, esophageal, gastric and colorectal) in a Han Chinese population. An elevated risk for colorectal cancer was observed in individuals homozygous for the 436 K allele, in comparison to individuals homozygous for the wildtype (OR = 3.41; CI = 1.85-6.57, p50.001). When all four types of cancer were combined in the analysis, the 436 K homozygotes had 1.15-fold increased risk over wildtype (CI 1.02-1.30, p 0.020) after adjustment for age, sex and smoking status. Gc2 allele carriers also had an increased risk for developing gastrointestinal cancer (OR 1.22; CI 104-1.39, p 0.015)
Associations between GC SNPs and basal cell carcinoma (BCC) were examined as part of the prospective cohort Rotterdam Study (n 7983). The study found no significant association between the GC genotypes and haplotypes and risk of developing at least one BCC. However, Gc1F carriers were more likely to develop a first BCC compared to non-carriers (HR = 1.40; 95% CI = 1.11-1.78). When stratified by age, individuals younger than 65 years of age and homozygous for Gc1S had about half the risk of developing a first BCC, compared to non-carriers (HR 0.53; CI = 0.31-0.91)171.
Multiple sclerosis (MS) is a form of disseminated encepha-lomyelitis that appears to be partly autoimmune in origin though its etiology is not known. The rates of MS exhibit a pattern of increasing prevalence with increasing latitude, indicating a protective role of ultraviolet radiation in the development of MS172. Moreover, several studies have shown an inverse correlation between serum levels of vitamin D and MS173,174, suggesting that polymorphisms influencing vitamin D status might contribute to the initiation or progression of disease.
Serum DBP was found to be upregulated along with other acute phase reactive proteins in a small sample (n 9) of pediatric MS patients175, however, the clinical relevance is not known. Early reports examining the distribution of DBP phenotypes among European adults did not find significant correlations with presence of disease, disease natural history or age of onset176,177. Similarly, in 107 Japanese MS patients and 109 controls, neither DBP phenotypes nor D432E/T436K genotypes (examined independently) were associated173,a 178,179 finding that was recently confirmed178,179. To control for population admixture/stratification, both SNPs were evaluated in a family-based design study of 187 Canadian families; however, the lack of association between genotype and disease was unchanged174. Ongoing interventional trials with vitamin D and association studies with larger cohorts may offer further opportunity to explore an interactive role of DBP SNPs with vitamin D in MS.
Amyotrophic lateral sclerosis (ALS) is another neuropathy of unknown etiology that is characterized by the progressive loss of motor neurons in the brain and spinal cord, leading to debilitating, usually fatal, muscle atrophy180. While the vast majority of ALS patients are sporadic, 10-15% of cases show familial clustering. Mutations in the cytosolic superoxide dismutase (SOD1) gene have been associated with ALS, but these account for less than 20% of all familial ALS (FALS)181. In Portuguese patients with FALS, proteomic analysis showed that the Gc-2 phenotype (436 K) was overrepresented in comparison to healthy controls182. The role of DBP in ALS is not known, but it is tempting to speculate that motor neuron damage resulting in the systemic release of actin and ensuing intravascular coagulation and local hypoxia-induced oxidative damage may be mitigated by the actin-scavenging properties of DBP. Whether the apparently reduced capacity of Gc-2 for actin-scavenging, compared with Gc130,34,35, might increase the likelihood of disease progression is not known.
A number of hypotheses have been put forth to explain the etiology of schizophrenia, but several lines of observational evidence suggest that vitamin D may be a risk factor183. The underlying molecular mechanism governing this association is not known, but prenatal vitamin D deficiency is associated with structural and functional deficits common to schizophrenic patients. Several early reports examined DBP pheno-types in relation to schizophrenia. An excess of Gc1-1 was observed in cases over controls in a study carried out in a German study (p5 0.01)184. This association was even more compelling for the hebephrenic subgroup (p5 0.01). On the other hand, analysis of a north east England cohort comparing 215 schizophrenic patients with healthy first-degree relatives or non-familial controls reported a significant reduction in Gc1S alleles (p 0.025) largely within the female subjects185. Findings from a Chinese male cohort of 423 schizophrenic patients also suggested a relative paucity of Gc1S (p5 0.001) and an excess of Gc2186. However, there are several other studies reporting lack of association between DBP and 187-189 schizophrenia 187-189, and firm conclusions would be premature. DBP gene variants and health outcomes 15
Parkinson disease is a progressive movement disorder, the severity of which has been found to be inversely associated with serum levels of 25(OH)D190. However, a study examining GC polymorphisms and severity of Parkinson disease did not identify any significant associations 191.
In liver disease, secretion of proteins belonging to the albumin family is usually reduced, and it is therefore not surprising that there are numerous studies reporting lower serum DBP concentrations in this group of disorders. This may also be associated with inability to sufficiently sequester cellular actin released by the dying hepatic cells themselves, thereby contributing to an increased risk of intravascular coagulation. Variants associated with DBP status might therefore be useful as biomarkers of worse outcomes in liver disease. Looking at 17 alcoholic liver cirrhosis patients and 100 healthy controls, French investigators observed an unusual electrophoretic form of non-ligand bound DBP that was characterized by the presence of an additional sialic acid moiety. This form of DBP was found only in Gc-1 carriers and was associated with worse clinical outcome35. To date, this novel phenotype has not been reported in other studies and, therefore, the larger relevance in non-hepatic disorders is unknown.
Sarcoidosis, a granulomatous inflammatory disease, typically affects young adults and is associated with altered immunoglobulin and increased 1,25(OH)2D3 production. DBP phenotypes were examined in a small case-control study of 88 patients, but found no relationship between disease, course of disease, or presentation192. Whether 1,25(OH)2D3 levels might be associated with the D432E or T436K genotypes, as others have observed, was not studied.
Graves' disease and Hashimoto thyroidosis are the two common forms of autoimmune thyroid disease. In such disorders, 1,25(OH)2D3 downregulates expression of thyrocyte-derived human leukocyte antigen class II mole-cules193 and inhibits secretion of proinflammatory cytokines and lymphocyte proliferation194. In 95 and 92 Western European pedigrees positive for Graves' disease and Hashimoto's thyroiditis, respectively, Pani et al. looked at TDT of the DBP gene195. Only the Alu repeat in intron 8 (I8*8) was found to be associated with Graves' disease (p5 0.03), and none were associated with Hashimoto's thyroiditis. Neither D432E nor T436K was associated with disease outcome. In contrast, association of GC SNPs with Graves' disease was found in a case-control study evaluating 332 cases and 185 healthy controls of Polish origin. In that study, cases were significantly enriched for the T436K heterozygotes (OR = 1.50; CI = 1.13-1.99; p 0.005)196.
Mycobacterium tuberculosis, the causative agent of tuberculosis, infects a third of the world's population and results in 3 million deaths per year197. Low serum levels of 25(OH)D have been correlated with extant tuberculosis198-204 and incident infection204. Oral supplementation of vitamin D given to tuberculosis contacts enhanced immunity towards the mycobacterium205. A significant advance in our understanding of innate immunity against tuberculosis came with a seminal report showing convergence of the vitamin D metabolic pathway with the Toll-like receptor (TLR2/1) signaling cascade resulting in the induction of cathelicidin, a potent antimicrobial peptide in the innate immunity path-way54,55. Subsequently, GC genotypes were examined in several ethnically diverse tuberculosis study populations206. In Gujarati Asians, the homozygous variant Gc2-2 phenotype was strongly associated with susceptibility to active tuberculosis, compared with Gc1-1 using logistic regression to adjust for age and sex (OR = 2.81, CI = 1.19-6.66, p = 0.009). Furthermore, this association was observed only under conditions of low 25(OH)D, suggesting an important gene-environment interaction in this condition.
Rheumatic fever is a systemic inflammatory illness characterized by excessive B cell activity leading to an overproduction of antibody to Group A streptococcus. DBP is found on the surface of B-cells, often in a molecular complex with actin and surface immunoglobulin, and may play a role in B cell activation by facilitating signal transduction activity207. Alternatively, inflammatory mediation of DBP may affect the course of disease. In a study of 39 cases and 90 controls of Arab ancestry, a positive association between the Gc-2 allele and rheumatic fever was observed (p 0.0024)208. The implications of this finding are not altogether clear, but suggest that the anti-inflammatory properties of Gc-2 may not be important in this disorder.
Untreated, human immunodeficiency virus (HIV) infection often progresses to acquired immunodeficiency syndrome (AIDS) with its subsequent complications of opportunistic infection and neoplasia. Macrophages are among the first cell types to be infected by HIV-1 and serve as reservoirs for the virus in affected persons209,210. Macrophage activation via Gc-MAF may therefore be an important factor in the acquisition of HIV-1 infection and progression to AIDS. Indeed, immunotherapy of HIV-infected individuals with GcMAF appeared to be significantly curative211. Several studies have examined the association between DBP variants and HIV/AIDS.
DBP phenotypes were examined in a cohort consisting of 86 HIV negative hospital workers, 351 homosexual men that were either HIV negative (as determined by the absence of HIV antibodies) or HIV positive (as determined by the presence of HIV antibodies with or without HIV antigen), and 96 AIDS cases212. It is thought that the presence of HIV antigen is a strong predictive marker of disease progression213-215. During the course of this study, a subset of 62 HIV negative homosexual men seroconverted. No significant difference in DBP distribution was observed between HIV negative heterosexuals, HIV negative homosexuals and patients with AIDS. Moreover, there was no difference in DBP phenotype distribution between homosexual men who remained HIV negative and those who sero-converted during follow-up, arguing against the involvement of DBP in susceptibility to HIV infection. Finally, DBP phenotypes were equally distributed among (i) HIV-positive homosexuals without the presence of HIV-antigen, (ii) HIV positivity with the presence of HIV antigen and (iii) AIDS - effectively excluding a role for DBP in either susceptibility or progression. Other investigators examining Spanish, German and Swedish populations also found no evidence of association between DBP variants and susceptibility to HIV/AIDS216-218.
However, an excess of Gc2 was observed in persistently seronegative sexual contacts (engaging in unprotected sex) of AIDS patients, although the sample size was small219.
In summary, there is little evidence to support a role for DBP in susceptibility to HIV/AIDS or progression to AIDS, based on these studies. The interplay between HIV and DBP, however, is complex211 and likely confounds any true associations between genetic variation and infection/disease outcome. Investigators may choose to focus on identifying new genetic variants in or near the GC gene that may provide further insight into a molecular mechanism bridging DBP and HIV/AIDS.