Vitamin D receptor polymorphisms in primary biliary cirrhosis: A functional connection?

Abstract

Vitamin D receptor polymorphisms are associated with increased susceptibility to primary biliary cirrhosis in Japanese and Italian populationsJournal of HepatologyVol. 50Issue 6PreviewVitamin D receptor (VDR) agonists have recently been identified as potent immunomodulators capable of inhibiting Th1-mediated immune response, leading us to consider the hypothesis that functional VDR polymorphisms might contribute to enhanced risk for developing primary biliary cirrhosis (PBC), a Th1-mediated autoimmune disease. In the current study, we aimed at elucidating the genetic association of VDR polymorphisms with susceptibility to PBC in Japanese and Italian populations. Full-Text PDF The bioactive form of vitamin D3, 1,25-dihydroxyvitamin D3 [1,25(OH)2D3], is a secosteroid hormone with a central function in calcium and bone metabolism, able to regulate cell proliferation and differentiation, and endowed with exquisite immunoregulatory properties [[1]Adorini L. Penna G. Control of autoimmune diseases by the vitamin D endocrine system.Nat Clin Pract Rheumatol. 2008; 4: 404-412Crossref PubMed Scopus (416) Google Scholar]. Vitamin D3 can be obtained through the diet, but it is mainly biosynthesized from 7-dehydrocholesterol in skin exposed to ultraviolet light. Vitamin D3 is then transported to the liver where it is hydroxylated to produce 25(OH)D, a reliable indicator of vitamin D status, and is further hydroxylated in the kidney to form the active hormone, 1,25(OH)2D3 [[2]Holick M.F. Vitamin D deficiency.N Engl J Med. 2007; 357: 266-281Crossref PubMed Scopus (10536) Google Scholar]. The biological effects of 1,25(OH)2D3 are mediated by the vitamin D receptor (VDR), a nuclear hormone receptor expressed in most cell types, functioning as a ligand-activated transcription factor that binds to vitamin D-responsive elements in the promoter region of target genes, and ultimately influences their rate of RNA polymerase II-mediated transcription [[3]Norman A.W. From vitamin D to hormone D: fundamentals of the vitamin D endocrine system essential for good health.Am J Clin Nutr. 2008; 88: 491S-499SCrossref PubMed Scopus (30) Google Scholar]. The vitamin D endocrine system is involved in a variety of biological processes that modulate immune responses [[4]Mora J.R. Iwata M. von Andrian U.H. Vitamin effects on the immune system: vitamins A and D take centre stage.Nat Rev Immunol. 2008; 8: 685-698Crossref PubMed Scopus (1091) Google Scholar], and has an important role in the control of autoimmune diseases [[1]Adorini L. Penna G. Control of autoimmune diseases by the vitamin D endocrine system.Nat Clin Pract Rheumatol. 2008; 4: 404-412Crossref PubMed Scopus (416) Google Scholar]. Vitamin D status is a crucial environmental factor that affects the prevalence of autoimmune diseases, as shown by the higher incidence of several autoimmune diseases in northern latitudes where lower amounts of vitamin D3 are synthesized from sunlight exposure [2Holick M.F. Vitamin D deficiency.N Engl J Med. 2007; 357: 266-281Crossref PubMed Scopus (10536) Google Scholar, 5Cantorna M.T. Mahon B.D. Mounting evidence for vitamin D as an environmental factor affecting autoimmune disease prevalence.Exp Biol Med (Maywood). 2004; 229: 1136-1142Crossref PubMed Scopus (451) Google Scholar]. Epidemiological analysis reveals strong ecological and case-control evidence that the vitamin D system reduces the risk of several autoimmune diseases, including multiple sclerosis, type 1 diabetes, inflammatory bowel disease, rheumatoid arthritis, and systemic lupus erithematosus [2Holick M.F. Vitamin D deficiency.N Engl J Med. 2007; 357: 266-281Crossref PubMed Scopus (10536) Google Scholar, 6Grant W.B. Epidemiology of disease risks in relation to vitamin D insufficiency.Prog Biophys Mol Biol. 2006; 92: 65-79Crossref PubMed Scopus (142) Google Scholar]. 1,25(OH)2D3 is produced by macrophages, dendritic cells (DCs), T and B cells, and contributes physiologically, via the VDR expressed in these cell types, to the autocrine and paracrine regulation of both innate and adaptive immune responses [[4]Mora J.R. Iwata M. von Andrian U.H. Vitamin effects on the immune system: vitamins A and D take centre stage.Nat Rev Immunol. 2008; 8: 685-698Crossref PubMed Scopus (1091) Google Scholar]. In addition to exerting direct modulatory effects on T and B cell function, VDR agonists influence the phenotype and function of DCs, promoting tolerogenic properties that favor the induction of regulatory, rather than effector, T cells [[7]Adorini L. Penna G. Induction of tolerogenic dendritic cells by vitamin D receptor agonists.Handb Exp Pharmacol. 2009; : 251-273Crossref PubMed Scopus (84) Google Scholar]. These intriguing actions of VDR agonists have been demonstrated in several experimental models of autoimmune diseases and appear to represent important components of immune system homeostasis. In addition to psoriasis, a Th1 and Th17 cell-mediated autoimmune disease of the skin, VDR agonists could be exploited to treat a variety of autoimmune diseases and other immune-mediated pathologies that are characterized by chronic inflammatory responses [[1]Adorini L. Penna G. Control of autoimmune diseases by the vitamin D endocrine system.Nat Clin Pract Rheumatol. 2008; 4: 404-412Crossref PubMed Scopus (416) Google Scholar]. Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by a progressive portal lymphocytic inflammatory response with destruction of intrahepatic bile duct epithelial cells leading to cholestasis, liver fibrosis and eventually cirrhosis [[8]Kaplan M.M. Gershwin M.E. Primary biliary cirrhosis.N Engl J Med. 2005; 353: 1261-1273Crossref PubMed Scopus (1012) Google Scholar]. The serological hallmark of the disease is the presence of circulating anti-mitochondrial antibodies, reflecting the induction of autoreactive T and B cells to the inciting antigens, mainly E2 subunits of mitochondrial pyruvate dehydrogenase [[9]Lleo A. Invernizzi P. Mackay I.R. Prince H. Zhong R.Q. Gershwin M.E. Etiopathogenesis of primary biliary cirrhosis.World J Gastroenterol. 2008; 14: 3328-3337Crossref PubMed Scopus (79) Google Scholar]. Although the etiology of PBC is still unclear, both genetic and environmental components are known to contribute to susceptibility and progression of the disease [[10]Gershwin M.E. Mackay I.R. The causes of primary biliary cirrhosis: convenient and inconvenient truths.Hepatology. 2008; 47: 737-745Crossref PubMed Scopus (241) Google Scholar]. Familiarity among PBC patients is frequently observed, and the prevalence of PBC clustering in a representative family with affected patients is much higher than in the general population [[11]Invernizzi P. Selmi C. Mackay I.R. Podda M. Gershwin M.E. From bases to basis: linking genetics to causation in primary biliary cirrhosis.Clin Gastroenterol Hepatol. 2005; 3: 401-410Abstract Full Text Full Text PDF PubMed Scopus (82) Google Scholar]. In addition, the concordance rate for PBC is remarkably higher in monozygotic compared to dizygotic twins, scoring as the highest among autoimmune diseases [[12]Selmi C. Mayo M.J. Bach N. Ishibashi H. Invernizzi P. Gish R.G. et al.Primary biliary cirrhosis in monozygotic and dizygotic twins: genetics, epigenetics, and environment.Gastroenterology. 2004; 127: 485-492Abstract Full Text Full Text PDF PubMed Scopus (402) Google Scholar]. MHC class II polymorphisms were found to be associated with both increased risk and protection from PBC in recent large-size studies [13Donaldson P.T. Baragiotta A. Heneghan M.A. Floreani A. Venturi C. Underhill J.A. et al.HLA class II alleles, genotypes, haplotypes, and amino acids in primary biliary cirrhosis: a large-scale study.Hepatology. 2006; 44: 667-674Crossref PubMed Scopus (111) Google Scholar, 14Invernizzi P. Selmi C. Poli F. Frison S. Floreani A. Alvaro D. et al.Human leukocyte antigen polymorphisms in Italian primary biliary cirrhosis: a multicenter study of 664 patients and 1992 healthy controls.Hepatology. 2008; 48: 1906-1912Crossref PubMed Scopus (110) Google Scholar]. Conversely, analysis of non-MHC gene polymorphisms has provided inconsistent results, possibly due to small sample size and ethnic differences in the frequency of minor alleles. As a notable exception, analysis of VDR polymorphisms has consistently demonstrated an association with susceptibility to PBC in different populations [15Halmos B. Szalay F. Cserniczky T. Nemesanszky E. Lakatos P. Barlage S. et al.Association of primary biliary cirrhosis with vitamin D receptor BsmI genotype polymorphism in a Hungarian population.Dig Dis Sci. 2000; 45: 1091-1095Crossref PubMed Scopus (51) Google Scholar, 16Vogel A. Strassburg C.P. Manns M.P. Genetic association of vitamin D receptor polymorphisms with primary biliary cirrhosis and autoimmune hepatitis.Hepatology. 2002; 35: 126-131Crossref PubMed Scopus (218) Google Scholar, 17Lakatos L.P. Bajnok E. Hegedus D. Toth T. Lakatos P. Szalay F. Vitamin D receptor, oestrogen receptor-alpha gene and interleukin-1 receptor antagonist gene polymorphisms in Hungarian patients with primary biliary cirrhosis.Eur J Gastroenterol Hepatol. 2002; 14: 733-740Crossref PubMed Scopus (51) Google Scholar, 18Fan L. Tu X. Zhu Y. Zhou L. Pfeiffer T. Feltens R. et al.Genetic association of vitamin D receptor polymorphisms with autoimmune hepatitis and primary biliary cirrhosis in the Chinese.J Gastroenterol Hepatol. 2005; 20: 249-255Crossref PubMed Scopus (111) Google Scholar]. DNA sequence variations occurring frequently in the population are defined as “polymorphisms” and can have subtle but real biological effects. Several polymorphisms have been identified in the VDR gene, but their influence on VDR protein function and signalling is largely unknown. The genomic organization of the human VDR locus at chromosome 12q13.1 shows that the VDR gene itself is quite large (over 100 kb) with 11 exons, and has an extensive promoter region capable of generating multiple tissue-specific transcripts [[19]Uitterlinden A.G. Fang Y. Van Meurs J.B. Pols H.A. Van Leeuwen J.P. Genetics and biology of vitamin D receptor polymorphisms.Gene. 2004; 338: 143-156Crossref PubMed Scopus (1159) Google Scholar]. VDR polymorphisms have been analyzed so far mostly in Caucasians, usually by studying three adjacent restriction fragment length polymorphisms (RFLP) for BsmI, ApaI, and TaqI, respectively, at the 3′ end of the VDR gene [[19]Uitterlinden A.G. Fang Y. Van Meurs J.B. Pols H.A. Van Leeuwen J.P. Genetics and biology of vitamin D receptor polymorphisms.Gene. 2004; 338: 143-156Crossref PubMed Scopus (1159) Google Scholar]. Because these polymorphisms are probably non-functional, linkage disequilibrium with one or more truly functional polymorphisms elsewhere in the VDR gene, or in nearby gene/s, is assumed to explain the associations observed [[20]Valdivielso J.M. Fernandez E. Vitamin D receptor polymorphisms and diseases.Clin Chim Acta. 2006; 371: 1-12Crossref PubMed Scopus (406) Google Scholar]. VDR gene polymorphisms have been found to be associated with osteoporosis, a common complication observed in about 25% of PBC patients although not specific to the disease [[21]Collier J. Bone disorders in chronic liver disease.Hepatology. 2007; 46: 1271-1278Crossref PubMed Scopus (179) Google Scholar], in which VDR genotypes have been reported to predict lower bone mineral density [[22]Springer J.E. Cole D.E. Rubin L.A. Cauch-Dudek K. Harewood L. Evrovski J. et al.Vitamin D-receptor genotypes as independent genetic predictors of decreased bone mineral density in primary biliary cirrhosis.Gastroenterology. 2000; 118: 145-151Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar]. However, VDR genotypes may play only a minor role in the development of osteoporosis in PBC, where duration and severity of cholestasis are associated with degree of bone loss and may exceed the potential effect of gene polymorphisms [[23]Pares A. Guanabens N. Rodes J. Gene polymorphisms as predictors of decreased bone mineral density and osteoporosis in primary biliary cirrhosis.Eur J Gastroenterol Hepatol. 2005; 17: 311-315Crossref PubMed Scopus (36) Google Scholar]. Conflicting results have also been reported for the association of VDR gene polymorphisms with PBC. Studies performed in the Hungarian population have supported a positive association between susceptibility to PBC and BsmI polymorphism at the VDR locus [15Halmos B. Szalay F. Cserniczky T. Nemesanszky E. Lakatos P. Barlage S. et al.Association of primary biliary cirrhosis with vitamin D receptor BsmI genotype polymorphism in a Hungarian population.Dig Dis Sci. 2000; 45: 1091-1095Crossref PubMed Scopus (51) Google Scholar, 17Lakatos L.P. Bajnok E. Hegedus D. Toth T. Lakatos P. Szalay F. Vitamin D receptor, oestrogen receptor-alpha gene and interleukin-1 receptor antagonist gene polymorphisms in Hungarian patients with primary biliary cirrhosis.Eur J Gastroenterol Hepatol. 2002; 14: 733-740Crossref PubMed Scopus (51) Google Scholar]. Conversely, the German [[16]Vogel A. Strassburg C.P. Manns M.P. Genetic association of vitamin D receptor polymorphisms with primary biliary cirrhosis and autoimmune hepatitis.Hepatology. 2002; 35: 126-131Crossref PubMed Scopus (218) Google Scholar] and Chinese [[18]Fan L. Tu X. Zhu Y. Zhou L. Pfeiffer T. Feltens R. et al.Genetic association of vitamin D receptor polymorphisms with autoimmune hepatitis and primary biliary cirrhosis in the Chinese.J Gastroenterol Hepatol. 2005; 20: 249-255Crossref PubMed Scopus (111) Google Scholar] studies have revealed a protective association of BsmI VDR polymorphism with PBC. These discrepant results could be due to different causes, including ethnic diversity, relatively small sample size, linkage disequilibrium, population stratification and environmental confounding factors. In this issue, Tanaka et al. [[24]Tanaka A. Nezu S. Uegaki S. Kikuchi K. Shibuya A. Miyakawa H. et al.Vitamin D receptor polymorphisms are associated with increased susceptibility to primary biliary cirrhosis in Japanese and Italian populations.J Hepatol. 2009; 50: 1202-1209Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar] report the genetic association of BsmI, ApaI, and TaqI VDR gene polymorphisms with PBC in large and well characterized patient series from Japan and Italy. Results in this paper indicate that RFLPs for BsmI and ApaI are significantly associated with the susceptibility of PBC, and that the ‘B’ allele is more frequently distributed in PBC patients [[24]Tanaka A. Nezu S. Uegaki S. Kikuchi K. Shibuya A. Miyakawa H. et al.Vitamin D receptor polymorphisms are associated with increased susceptibility to primary biliary cirrhosis in Japanese and Italian populations.J Hepatol. 2009; 50: 1202-1209Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar]. In contrast to previous studies, Tanaka et al. have analyzed a large number of PBC cases and controls, 334 and 335, respectively, performing three statistical tests for each genotype. Importantly, this study has investigated VDR allele frequencies in Japanese and Italian subjects, completely diverse populations but both ethnically homogeneous, thereby preventing population stratification. The significant association of BsmI polymorphism in both Japanese and Italian populations indeed suggests a general contribution of VDR gene polymorphism to PBC susceptibility [[24]Tanaka A. Nezu S. Uegaki S. Kikuchi K. Shibuya A. Miyakawa H. et al.Vitamin D receptor polymorphisms are associated with increased susceptibility to primary biliary cirrhosis in Japanese and Italian populations.J Hepatol. 2009; 50: 1202-1209Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar]. The interpretation of polymorphic variations in the VDR gene is blurred by the fact that until now only a few polymorphisms in this large gene have been studied and most are anonymous RFLPs, with an unknown functional effect. Nevertheless, results from the current study demonstrate the possibility that VDR polymorphism represents a genetic marker for the risk of PBC. In particular, the odds ratio (OR) of genotype ‘BB’ at the BsmI polymorphism in Japanese subjects was 13.7, supporting the rationale that genotype ‘BB’ might be used as a diagnostic tool and/or a predictive marker for PBC [[24]Tanaka A. Nezu S. Uegaki S. Kikuchi K. Shibuya A. Miyakawa H. et al.Vitamin D receptor polymorphisms are associated with increased susceptibility to primary biliary cirrhosis in Japanese and Italian populations.J Hepatol. 2009; 50: 1202-1209Abstract Full Text Full Text PDF PubMed Scopus (80) Google Scholar]. In the absence of a clear link between VDR polymorphisms and function of the vitamin D endocrine system, it is possible only to speculate about the underlying mechanisms. Vitamin D deficiency does not appear to be widespread among PBC patients [[25]Phillips J.R. Angulo P. Petterson T. Lindor K.D. Fat-soluble vitamin levels in patients with primary biliary cirrhosis.Am J Gastroenterol. 2001; 96: 2745-2750Crossref PubMed Google Scholar], who experience a lower frequency of 25(OH)D deficiency compared to other autoimmune diseases [[2]Holick M.F. Vitamin D deficiency.N Engl J Med. 2007; 357: 266-281Crossref PubMed Scopus (10536) Google Scholar]. 25-hydroxylation of vitamin D3, which takes place in the liver, is normal in PBC patients [[26]Danielsson A. Lorentzon R. Larsson S.E. Intestinal absorption and 25-hydroxylation of vitamin D in patients with primary biliary cirrhosis.Scand J Gastroenterol. 1982; 17: 349-355Crossref PubMed Scopus (28) Google Scholar]. Cholestatic liver injury is counteracted by a variety of adaptive hepatoprotective mechanisms, including modulation of bile acid transport, synthesis and detoxification, which are mediated by a complex network of nuclear receptors involving the farnesoid X receptor, pregnane X receptor, constitutive androstane receptor, and VDR [[27]Zollner G. Trauner M. Nuclear receptors as therapeutic targets in cholestatic liver diseases.Br J Pharmacol. 2009; 156: 7-27Crossref PubMed Scopus (114) Google Scholar]. VDR also functions as a receptor for the secondary bile acid lithocholic acid [[28]Makishima M. Lu T.T. Xie W. Whitfield G.K. Domoto H. Evans R.M. et al.Vitamin D receptor as an intestinal bile acid sensor.Science. 2002; 296: 1313-1316Crossref PubMed Scopus (948) Google Scholar], which is hepatotoxic, providing a possible association between PBC and VDR polymorphisms. Mice treated with 1,25(OH)2D3 after bile duct ligation (BDL) did not show decreased bile acid levels in plasma and liver, but the treatment suppressed mRNA expression of proinflammatory cytokines in the liver and strongly decreased plasma levels of proinflammatory cytokines [[29]Ogura M. Nishida S. Ishizawa M. Sakurai K. Shimizu M. Matsuo S. et al.Vitamin D3 modulates the expression of bile acid regulatory genes and represses inflammation in bile duct-ligated mice.J Pharmacol Exp Ther. 2009; 328: 564-570Crossref PubMed Scopus (69) Google Scholar], suggesting that 1,25(OH)2D3 targets the inflammatory rather than the cholestatic aspect of the disease. If the VDR polymorphisms described by Tanaka et al. reflect a deranged capacity of the vitamin D system to promote tolerogenic DCs, enhance regulatory T cells and/or inhibit pathogenic Th1 and Th17 cells in PBC patients, these would represent plausible explanations for the association of the observed VDR polymorphisms with PBC. Future research should document additional polymorphisms across the VDR gene to verify these hypotheses, trying to understand the functional consequences of the receptor variations. Until then, the role of VDR polymorphisms in PBC, as in any other autoimmune disease, will remain a topic for debate. Unfortunately, the complex organization of the VDR gene will make the identification of these functional polymorphisms a somewhat daunting task.