Abstract
Background:
Generalized vitiligo is an autoimmune disease of skin pigmentation that is associated with increased prevalence of other autoimmune diseases, particularly autoimmune thyroid disease (AITD; principally Hashimoto's disease and Graves' disease), both in vitiligo patients and their close relatives, suggesting a heritable predisposition involving, in part, shared susceptibility genes.
Summary:
This review summarizes current knowledge of vitiligo epidemiology and genetics, highlighting recent findings from genome-wide approaches to disease gene identification, emphasizing susceptibility loci shared with other autoimmune diseases, particularly AITD, as well as some important differences.
Conclusions:
Inherited susceptibility to generalized vitiligo involves a number of specific genes, many of which are shared with other autoimmune diseases that are epidemiologically associated with vitiligo, including AITD, confirming a longstanding hypothesis about the genetic basis of these disorders. These genes provide potential therapeutic targets for novel approaches to treatment as well as for approaches to presymptomatic diagnosis and disease prevention in individuals with inherited susceptibility to this group of autoimmune diseases.
Introduction
Generalized vitiligo is characterized clinically by acquired patches of white skin and overlying hair, usually multifocal and often bilateral (7,8) (Fig. 1), resulting from progressive autoimmune loss of melanocytes from the involved areas. Development of generalized vitiligo results from a complex interaction of predisposing genetic factors and unknown environmental triggers that initiate the process of melanocyte destruction, resulting in the characteristic depigmented lesions (9,10).
A patient with generalized vitiligo. Note obvious patches of white skin in typical distribution involving the periorbital region and hands.
It is well known that there is a tendency for multiple autoimmune diseases to occur concomitantly in some patients. Perhaps the first description was in 1855, when Addison reported a patient with idiopathic adrenal insufficiency, vitiligo, and pernicious anemia (11). In 1926 Schmidt reported co-occurrence of multiple autoimmune diseases in what came to be called Schmidt syndrome (12). In 1980 Neufeld and Blizzard suggested classification of the so-called autoimmune polyglandular syndromes based on clinical grounds (13), of which Schmidt syndrome was denoted type II. Over the past few years it has become recognized that APS II is more complex, and in particular that generalized vitiligo is part of a genetically determined, autoimmune-autoinflammatory disease diathesis that also includes AITD, rheumatoid arthritis, adult-onset type 1 diabetes mellitus, psoriasis, pernicious anemia, Addison's disease, and systemic lupus erythematosus, all of which occur at elevated frequencies both among vitiligo patients themselves, and also among their first-degree relatives, even those who are not themselves affected with vitiligo (14 –16). This has led to the hypothesis that general susceptibility to this constellation of diseases is a complex trait involving various shared susceptibility genes, while other genes and exposure to environmental triggers determine specific disease occurrence in individual patients (17).
Approaches to identification of genes involved in vitiligo pathogenesis have taken a number of forms, initially focusing on biological candidates and differential expression analyses. In recent years, technological advances enabled by the human genome project, and methodological advances applied to analyses of polygenic, multifactorial diseases, have permitted more global approaches, including a recent genome-wide association study. As the result, there has been considerable progress in identifying susceptibility genes for generalized vitiligo, some of which are shared with other autoimmune diseases and some of which are specific to vitiligo. These genes may thus provide novel therapeutic and even prophylactic targets for new interventional approaches to treat and prevent both generalized vitiligo and other autoimmune diseases in the APS type II constellation.
Here, we review the epidemiology of generalized vitiligo and associated autoimmune diseases, progress toward identifying vitiligo susceptibility genes and their occurrence in other, epidemiologically associated autoimmune diseases (especially AITD), and discuss biological pathways that appear to be specific to vitiligo but which may provide biological insight into other autoimmune diseases.
Epidemiology of Generalized Vitiligo and Associated Autoimmune Diseases
Generalized vitiligo is the most common depigmenting disorder, occurring with a frequency of ∼0.5% in various populations around the world (18 –20) and an average age of onset of ∼24 years (14). Women have been overrepresented in virtually all large patient surveys, whereas men and women are affected equally among probands' affected first-degree relatives (14,15). Large-scale epidemiological surveys have shown that most cases of generalized vitiligo occur sporadically, though about 15%–20% of patients report one or more affected first-degree relatives (14). Very rarely, large multigeneration families segregate vitiligo in an autosomal dominant pattern with incomplete penetrance (21). More typically, however, family clustering of generalized vitiligo cases exhibits a non-Mendelian pattern (14 –16,22 –29), with overall vitiligo prevalence among probands' first-degree relatives ∼7.0% in Caucasians, 6.1% in Indian-Pakistanis, 4.8% in U.S. Hispanic-Latinos (14), and 2.6% in Han Chinese (16). Concordance of generalized vitiligo in monozygotic twin-pairs is ∼23% (14), >60 times the general population risk. Together, these findings indicate that susceptibility to generalized vitiligo is a complex, polygenic multifactorial trait.
Melanocyte loss in generalized vitiligo occurs primarily on an autoimmune basis (7,30 –32), although the triggers of the autoimmune response remain unknown. Many patients have circulating antibodies to various melanocyte components, most frequently tyrosinase (33), the key enzyme of melanin biosynthesis (34), as well as circulating skin-homing melanocyte-specific cytotoxic T-lymphocytes (35). Sparse infiltrates of activated and cytotoxic T cells are seen at the margins of active lesions (36 –38). Moreover, generalized vitiligo is a component of the APECED (APS type I) and Schmidt (APS type II) multiple autoimmune disease syndromes, and in small studies has been associated with AITD (39,40), pernicious anemia (41,42), Addison's disease (43), and perhaps alopecia areata (44,45). Much larger surveys of vitiligo patients (14,15,46 –48) have generally found elevated frequencies of AITD, pernicious anemia, rheumatoid arthritis, psoriasis, type 1 diabetes, Addison's disease, and systemic lupus erythematosus; about 15%–25% of patients with generalized vitiligo have at least one additional concomitant autoimmune disorder. Moreover, these same autoimmune diseases also occur with increased prevalence in vitiligo patients' first-degree relatives, regardless of whether or not those relatives have vitiligo themselves (14,15). Together, these findings indicate that vitiligo patients and their close relatives have a genetically determined susceptibility to this specific group of autoimmune diseases, most likely mediated by shared susceptibility genes that predispose to these diseases, with other genes and environmental triggers determining disease specificity.
Vitiligo Susceptibility Genes
Four very different approaches have been used to identify genes that mediate susceptibility to vitiligo: gene expression analyses, candidate gene association studies, genome-wide linkage studies, and genome-wide association studies.
Gene expression studies
Gene expression studies, either of candidate genes or via global analysis using microarrays, can identify genes that are differentially expressed, in cells from vitiligo patients versus controls, or in involved skin versus uninvolved skin. However, gene expression differences cannot distinguish between genes with primary effects versus the many more genes whose expression may be dysregulated on a secondary basis or whose expression merely varies on the outbred genetic background of humans, unrelated to vitiligo.
VIT1 (subsequently renamed FBXO11) was originally so-named on the basis of its apparent aberrant expression in intralesional vitiligo melanocytes (49). Similarly, MYG1 is a widely expressed gene shown on the basis of differential hybridization to have elevated expression in melanocytes from vitiligo patients (50). Variation in MYG1 has been found to affect levels of gene expression and to be marginally associated with active vitiligo (51), although this study did not apply appropriate correction for extensive multiple testing. A recent more global analysis of 16,000 transcripts in melanocytes cultured from vitiligo patients versus controls identified a list of 859 differentially expressed genes (52). However, neither FBOX11 nor MYG1, nor any of the top-ranked genes from the more global expression analysis have been identified as potential vitiligo susceptibility genes by either genome-wide linkage studies or a recent genome-wide association study of generalized vitiligo (53), suggesting that none of these genes may be causally involved in vitiligo pathogenesis.
Candidate gene association studies
Candidate gene association studies are best suited to detecting genetic signals that represent relatively common causal variants with modest effect sizes, and hence the typical occurrence of these diseases in singleton patients who have limited or no family history of the disease. However, candidate gene association studies are highly subject to false-positive results, due to population stratification, inadequate statistical power and fluctuation, and inadequate correction for multiple testing, both within and across studies.
Apparent association of vitiligo with many different biological candidate genes has been reported in a large number of studies (Table 1). However, many of these studies reported only marginally significant nominal associations, with inadequate correction for multiple testing, and most have not been replicated in independent studies. Most of these associations thus most likely represent false-positives due to population stratification, low statistical power and fluctuation, and failure to adequately correct for multiple testing. As the result, only two biological candidate genes have been supported by positive results in multiple studies, HLA and PTPN22, and findings for a third, CTLA4, have been difficult to interpret.
MHC, major histocompatibility complex.
The earliest genetic studies of vitiligo were case–control allelic association studies of genes in the major histocompatibility complex (MHC), carried out by genotyping various MHC markers in patients with various different vitiligo phenotypes versus in controls, from many different populations (e.g., 54–59). In general, these early studies found no consistent association between the occurrence of vitiligo and specific HLA alleles. However, re-analysis of these studies as a group shows that several found association between vitiligo and HLA-DR4 alleles (60), and meta-analysis found association of vitiligo with HLA-A2 (61). More recent studies that used modern analytical and statistical methods found association between generalized vitiligo and HLA-DRB4*0101 and HLA-DQB1*0303 in Dutch patients (62), with HLA-DRB1*03, DRB1*04, and HLA-DRB1*07 alleles in Turkish patients (63), and with alleles of microsatellites located in the MHC in Colombian patients (64). In Caucasian multiplex generalized vitiligo families, the MHC class II haplotype HLA DRB1A *04-(DQA1*0302)-DQB1*0301 is associated with both increased risk of vitiligo and with relatively early disease onset (60), and in Han Chinese generalized vitiligo is associated with the MHC haplotype HLA-A25-Cw*0602-DQA1*0302 (65). Association has also been reported between generalized vitiligo and genes of the LMP/TAP gene region of the MHC (66), although this may merely reflect long-range linkage disequilibrium with the MCH class II gene region.
Three independent candidate gene studies have shown association of the PTPN22 R620W polymorphism with generalized vitiligo in Caucasians (67 –69), thus strongly supporting true association with what is believed to be the causal variant for PTPN22-related autoimmune susceptibility (70). An additional study, carried out in Indians from the Gujarat region, failed to observe association of vitiligo with the R620W variant (71), an unsurprising result given the rarity of the R620W variant in non-Caucasian populations and the small size (and thus very limited statistical power) of that study.
Interpretation of findings for CTLA4 has been more problematic. Several studies have observed apparent association of CTLA4 and generalized vitiligo (72 –74), but principally limited to the subset of patients who have other concomitant autoimmune diseases, although even in this group association has been inconsistent (74,75). A meta-analysis (74) indicated that, overall, association of CTLA4 with vitiligo is weak, and probably is secondary, driven by primary genetic association of CTLA4 with other autoimmune diseases that are epidemiologically associated with vitiligo, particularly AITD.
Genome-wide linkage studies
Genetic linkage studies are best suited to detecting genetic signals that represent relatively rare causal variants with large effect sizes; hence, segregation of the corresponding phenotype in the multiplex families needed for linkage analysis. However, it must be borne in mind that most vitiligo patients are singleton cases, with few or no affected relatives, and thus the susceptibility genes and variants detected by linkage in multiplex families may not be typical of the majority of cases. Moreover, in many instances it has proved difficult to identify causal genes that underlie candidate genetic linkage signals.
The first genome-wide linkage study of vitiligo was of a single large family with generalized vitiligo and other autoimmune diseases, inherited as an apparent autosomal dominant trait with incomplete penetrance. Vitiligo in this family was mapped to a 7.4 Mb interval in chromosome segment 1p31.3–p32.2 (76), and detailed studies of genes in this region of chromosome subsequently identified a promoter variant in FOXD3, which encodes an embryonic transcription factor that regulates melanoblast differentiation and development (77). However, this family with autosomal dominant vitiligo appears to be unique, and other generalized vitiligo patients do not have mutations of FOXD3 or show linkage to this region of chromosome 1p.
Genome-wide linkage analyses of more typical small multiplex generalized vitiligo families have yielded a number of other linkage signals (Table 1), for only a few of which have corresponding genes been identified. In Caucasians, besides chromosome 1p, significant vitiligo linkage signals were detected on chromosomes 7p13–q21, 8p12, and 17p, and suggestive signals on chromosomes 9q22, 11p15, 13q33, 19p13, and 22q11, the chromosome 7 and 17p linkages deriving principally from families with other autoimmune diseases, mainly AITD (78,79). In Chinese, a major vitiligo linkage signal was detected on chromosome 4q13–q21, and weaker signals at 1p36, 6p21–p22, 6q24–q25, 14q12–q13, and 22q12 (80,81). Except perhaps for the signals on proximal chromosome 22q, none of linkages observed in Caucasians align with those observed in Chinese, suggesting that, if these linkages are valid, different genes may be involved in the pathogenesis of vitiligo in different populations around the world.
The 17p vitiligo linkage signal, detected principally in multiplex vitiligo families with other associated autoimmune diseases, principally AITD (79), coincided with the location of SLEV1, a linkage signal originally detected in multiplex lupus families that included at least one case of vitiligo (82) and subsequently confirmed in multiplex lupus families with various other autoimmune diseases (83). Together, these findings suggested that this locus mediates susceptibility to multiple autoimmune diseases, including at least generalized vitiligo and lupus. Association analysis across the 17p linkage region, first in the same multiplex vitiligo families used for linkage and subsequently in a second set of multiplex vitiligo families, identified NLRP1 (previously, NALP1) as the vitiligo-autoimmunity gene in this region (84). NLRP1 encodes a key regulator of the innate immune system that, on interaction with unknown bacterial or viral triggers, stimulates formation of an inflammasome complex that activates the interleukin-1β inflammatory pathway and perhaps also modulates apoptosis. Subsequent studies have confirmed association of variation in NLRP1 with generalized vitiligo (85), as well as with Addison's disease (86,87) and type 1 diabetes (86).
A similar approach led to identification of the 22q12 locus detected in Chinese as Xbp1 (88), encoding a DNA-binding protein, the downstream targets of which are X boxes in many genes, including several HLA loci. Association of Xbp1 with vitiligo has not yet been confirmed in other populations, although variation in Xbp1 has also been associated with inflammatory bowel disease (89).
Two of the other generalized vitiligo linkage signals, on chromosomes 7p13–q21 and 9q22, have also been subjected to detailed analysis (90). However, specific vitiligo susceptibility genes in these regions have not yet been identified.
Genome-wide association studies
Genome-wide association studies, like candidate gene association studies, are best suited to detecting genetic signals that represent relatively common causal variants with modest effect sizes. However, unlike candidate gene association studies, genome-wide association studies are sufficiently powered to minimize statistical fluctuation and permit adequate control for multiple testing within the study, and provide full-genome datasets that enable both detection of and adjustment for population stratification. As the result, genome-wide association studies have provided robust, highly replicable genetic findings for many different complex human diseases.
Two different genome-wide association studies have been reported for generalized vitiligo, both in Caucasians. The first, of a founder population in an isolated Romanian village with a high prevalence of generalized vitiligo, AITD, and other autoimmune diseases (91), detected association at chromosome 6qter near IDDM8, a type 1 diabetes–rheumatoid arthritis locus. This study tentatively identified the causal gene as SMOC1 (92). The second genome-wide association study of generalized vitiligo studied over 1500 unrelated Caucasian cases in the initial genome-wide screening phase (53). This analysis detected and replicated association with at least 12 different loci (53,93). Most of these appear to represent genes with known functions in the immune response, including HLA class I (specifically, HLA-A*02), HLA class II, PTPN22, RERE, FOXP1, LPP, IL2RA, GZMB, UBASH3A, C1QTNF6, and probably CCR6. Many of these genes have been associated with susceptibility to various other autoimmune diseases. Like the Romanian study, this larger genome-wide association study also detected association at a locus at 6qter near IDDM8, though apparently at CCR6, located 1.44 Mb proximal to SMOC2; the relationship between these two association signals remains to be clarified. Interestingly, although association with NLRP1 was not detected in the initial genome-wide screen, which included mostly sporadic cases, association with NLRP1 was confirmed in a subset analysis consisting of multiplex vitiligo families, consistent with a large odds ratio for variation in NLRP1 that enabled its original mapping by genetic linkage analysis.
In addition to immune-related genes, the vitiligo genome-wide association study detected association to only a single non-immune-related gene, TYR, which encodes tyrosinase, the key enzyme of melanin biosynthesis and an important autoantigen in generalized vitiligo. Generalized vitiligo is thus analogous to type 1 diabetes and AITD, in which inherited variation in INS and TG, respectively, each encoding an important intracellular component of the target cell type, and each constituting a major autoantigen in the corresponding disease, each predisposes to disease susceptibility. For generalized vitiligo, the causal TYR vitiligo susceptibility variant appears to be the major allele of the common R402Q polymorphism. The minor TYR402R allele encodes a thermosensitive tyrosinase polypeptide that is present in greatly reduced amounts and is incorrectly glycosylated, and which thus is less available for presentation to the immune system. In fact, tyrosinase peptide is presented on the melanocyte surface by HLA-A*0201, which corresponds to the high-risk allele of HLA-A, and there was significant genetic interaction between the most associated single-nucleotide polymorphisms in TYR and HLA-A (53). Importantly, the TYR402Q allele, while conferring apparent susceptibility to generalized vitiligo, is also associated with genetic protection from malignant melanoma (94,95). A unifying hypothesis is that the TYR R402Q variant may mediate alternative susceptibility to vitiligo versus melanoma via modulating immune surveillance of malignant melanomas.
Genetic Relationships Between Generalized Vitiligo and AITD
There is strong epidemiological association between generalized vitiligo and AITD, which occurs (88% hypothyroidism and 12% hyperthyroidism) in 17%–25% of vitiligo probands and 6%–15% of their first-degree relatives (14,15), while the prevalence of vitiligo among patients with AITD is ∼7% (96,97). This epidemiological association strongly suggests that generalized vitiligo and AITD share common genetic susceptibility factors.
Genome-wide linkage and candidate gene association studies have identified a number of loci potentially involved in AITD (98 –100). The strongest association is to HLA-DR3 (101), in which a 74R variant allele appears to be causal for disease predisposition (102 –104). In contrast, generalized vitiligo is associated with HLA-DR4 on an MHC class II DRB1*04–DQB1*0301 haplotype (60).
A number of non-MHC candidate genes have been associated with AITD, although only six have been confirmed in multiple studies (105). These include four immunomodulatory genes, CTLA4 (106), PTPN22 (107), CD40 (108), and FCRL3 (109), and two thyroid-specific genes, thyroglobulin (TG) (110), and TSHR (111), though undoubtedly additional unidentified genes contribute to AITD susceptibility. Of these, genetic association of PTPN22 with generalized vitiligo is well established, whereas association of CTLA4 with generalized vitiligo appears to be indirect, reflecting primary association with other concomitant autoimmune diseases that occur in some vitiligo patients, principally AITD. At present, there is no evidence for association of either CD40 or FCRL3 with generalized vitiligo, and association signals were not observed in the vicinity of these genes in the recent vitiligo genome-wide association studies (53,91). Likewise, there was association with TSHR or TG, even in the subset of vitiligo patients with concomitant AITD, suggesting that these loci may be relevant principally to patients with isolated AITD, and that broader autoimmunity susceptibility genes may be of greater importance in patients with concomitant generalized vitiligo and AITD.
Conclusions
Recent studies have identified a number of confirmed genes that underlie susceptibility to generalized vitiligo. Whereas one of these, TYR, encodes a melanocyte-specific protein that likely plays a key role in target cell specificity of the autoimmune response, most vitiligo susceptibility genes encode proteins involved in immune functions. Indeed, many of these have also been identified as susceptibility genes for other autoimmune diseases with which generalized vitiligo is epidemiologically associated. The overall situation in AITD appears to be similar, with both general autoimmunity genes and thyroid-specific genes identified as conferring disease susceptibility. Generalized vitiligo and AITD are closely associated epidemiologically, and it is likely that, as additional susceptibility genes are identified for both of these autoimmune diseases, many will be shared in common, leading to a convergence of shared autoimmune susceptibility genes as well as sets of specific genes and ultimately environmental triggers that determine disease specificity and onset.
Footnotes
Acknowledgments
This work was supported by grants AR45584 and AR056292 from the National Institutes of Health.
Disclosure Statement
The author declares that no competing financial interests exist.
Portions of this review were presented at the Spring 2010 Meeting of the American Thyroid Association, “Thyroid Disorders in the Era of Personalized Medicine,” Minneapolis, MN, May 13–16, 2010.