The genetic concept of vitiligo

Beyond genome-wide association studies: genetic heterogeneity and individual predisposition to cancer

Genome-wide association studies (GWAS) using population-based designs have identified many genetic loci associated with risk of a range of complex diseases including cancer; however, each locus exerts a very small effect and most heritability remains unexplained. Family-based pedigree studies have also suggested tentative loci linked to increased cancer risk, often characterized by pedigree-specificity. However, comparison between the results of population- and family-based studies shows little concordance. Explanations for this unidentified genetic 'dark matter' of cancer include phenotype ascertainment issues, limited power, gene-gene and gene-environment interactions, population heterogeneity, parent-of-origin-specific effects, and rare and unexplored variants. Many of these reasons converge towards the concept of genetic heterogeneity that might implicate hundreds of genetic variants in regulating cancer risk. Dissecting the dark matter is a challenging task. Further insights can be gained from both population association and pedigree studies.

Infections and autoimmunity--friends or foes?

Autoimmunity can be triggered by many environmental factors, among which infectious agents are pivotal. Here, we summarize current knowledge of the relationship between infection and autoimmunity. An autoimmune disease can be induced or triggered by infectious agents, which can also determine its clinical manifestations. Most infectious agents, such as viruses, bacteria and parasites, can induce autoimmunity via different mechanisms. In many cases, it is not a single infection but rather the 'burden of infections' from childhood that is responsible for the induction of autoimmunity. The development of an autoimmune disease after infection tends to occur in genetically susceptible individuals. By contrast, some infections can protect individuals from specific autoimmune diseases (i.e. the hygiene hypothesis).

Behçet's disease and vitiligo in two brothers: coincidence or association?

Behçet's disease (BD) and vitiligo are disorders with unknown aetiology. We report on two brothers with ocular Behcet's disease who had advanced unilateral ophthalmic symptoms associated with vitiligo. The two brothers had recurrent oral and genital ulceration, uveitis and white patches on their skin. The most probable hypothesis for the aetiology of BD is that of an autoimmune reaction in genetically predisposed individuals, with vasculitis as the main pathological hallmark. Despite many years of research, the specific causes of vitiligo remain obscure, and the most advanced aetiological hypothesis remains that of autoimmunity. To our knowledge, this is the first reported case within the literature of BD associated with vitiligo. The existence of the two different disorders is noteworthy as they were observed in two brothers during the same period in their lives, with very similar clinical observations.

Increased micronucleus frequency in phytohaemagglutinin-stimulated blood cells of patients with vitiligo

BACKGROUND

Vitiligo is a relatively common, acquired pigmentary disorder characterized by areas of depigmented skin resulting from loss of melanocytes in the epidermis. Although several hypotheses have been proposed for the aetiology and pathogenesis of vitiligo, the cause of vitiligo remains unclear.

OBJECTIVE

To evaluate spontaneous micronucleus (MN) frequency using the cytokinesis block MN assay to determine damages at the DNA or chromosome level in phytohaemagglutinin (PHA)-stimulated blood cells of patients with vitiligo and healthy control subjects.

METHODS

Peripheral blood samples were obtained and cultured from 21 patients with vitiligo (mean age: 21.48 +/- 9.78 years) and 21 age- and sex-matched healthy control subjects (mean age: 21.52 +/- 9.80 years). MN values were scored in binucleated cells obtained from whole-blood cultures of patients and control subjects.

RESULTS

MN frequencies (mean +/- SD) in PHA-stimulated blood cells of patients with vitiligo and control subjects were 0.94 +/- 0.58 and 0.58 +/- 0.32, respectively. Compared with control subjects, MN frequencies of patients with vitiligo were found significantly higher than those of the control subjects (P = 0.012).

CONCLUSION

Our results indicate unexpectedly some chromosomal/DNA damage in whole-blood cultures of patients with vitiligo. We do not know, however, if these chromosome/DNA instabilities observed in the cells of vitiligo patients resulted from the cause or from the consequences of the disorder.

Vitiligo pathogenesis: autoimmune disease, genetic defect, excessive reactive oxygen species, calcium imbalance, or what else?

The pathobiology of vitiligo has been hotly disputed for as long as one remembers, and has been a magnet for endless speculation. Evidently, the different schools of thought--ranging, e.g. from the concept that vitiligo essentially is a free-radical disorder to that of vitiligo being a primary autoimmune disease--imply very different consequences for the best therapeutic strategies that one should adopt. As a more effective therapy for this common, often disfiguring pigmentary disorder is direly needed, we must strive harder to settle the pathogenesis debate definitively--on the basis of sound experimental evidence, rather than by a war of dogmatic theories. Recognizing, however, that it is theories which tend to guide our experimental designs and choice of study parameters, the various pathogenesis theories on the market deserve to be critically, yet unemotionally re-evaluated. This Controversies feature invites you to do so, and to ask yourself: is there something important or worthwhile exploring in other pathogenesis scenarios than those already favoured by you that may help you improve your own study design, next time you have a fresh look at vitiligo? Vitiligo provides a superb model for the study of many fundamental problems in skin biology and pathology. Therefore, even if it later turns out that, as far as your own vitiligo pathogenesis concept is concerned, you have barked-up the wrong tree most of the time, chances are that you shall anyway have generated priceless new insights into skin function along the way.

Keratinocyte cultures from involved skin in vitiligo patients show an impaired in vitro behaviour

Vitiligo depigmentation is considered a consequence of either melanocyte disappearance or loss of functioning melanocytes in the involved areas. However, it has been reported that keratinocytes in involved vitiligo skin are damaged too. Based on this evidence, we evaluated the in vitro behaviour, in life span cultures, of involved and uninvolved vitiligo keratinocytes and their expression of proliferation, differentiation and senescence markers. An additional purpose was to investigate whether vitiligo keratinocytes from depigmented skin are able to sustain survival and growth of normal melanocytes (when added in co-culture experiments), as normal human keratinocytes manage to do. Our results demonstrate that almost all involved vitiligo keratinocytes have a shorter life span in vitro than the uninvolved cells and all of them do not maintain melanocytes in culture in a physiological ratio. Modification of proliferation and senescence marker expression also occurs. Indeed, we detected low initial expression levels of the senescence marker p16 in involved vitiligo keratinocytes, despite their shorter in vitro life span, and increased expression of proliferating cell nuclear antigen and p53. This preliminary analysis of a small number of in vitro cultured vitiligo keratinocytes suggests an impaired senescence process in lesional vitiligo keratinocytes and attempts to regulate it.

Two therapeutic challenges: periocular and genital vitiligo in children successfully treated with pimecrolimus cream

Vitiligo is characterized by the selective destruction of melanocytes resulting in patches of skin depigmentation. Vitiligo is a therapeutic challenge. Eyebrows, eyelids and genital vitiligo are a therapeutic dilemma, especially in children. The possible side effects of topical corticosteroids and the difficulties for choosing any other adequate treatment option are a major concern. We present two children, one with vitiligo of the eyelids and the other with genital vitiligo, both treated with pimecrolimus 1% cream with almost full repigmentation of the lesions, showing pimecrolimus could be an adequate option for the treatment of vitiligo for these special vitiligo areas in children.

Vitiligo puzzle: the pieces fall in place

Over the years, the role of biochemical, immunological, genetic, and other biological aspects in the pathogenesis of vitiligo has been studied. So far, no convincing model describing the interplay of these contributing factors has been formulated. Based on existing research, we propose that vitiligo has a multi-factorial etiology, characterized by multiple steps, but always involving an increase of external or internal phenol/catechol concentration, serving as a preferred surrogate substrate of tyrosinase, competing with its physiological substrate tyrosine. The conversion of these substrates into reactive quinones is reinforced by a disturbed redox balance (increasing hydrogen peroxide). Such reactive quinones can be covalently bound to the catalytic centre of tyrosinase (haptenation). This could give rise to a new antigen, carried by Langerhans cells to the regional lymph node, stimulating the proliferation of cytotoxic T cells. However, the activation of such cytotoxic cells is only a first step in skin melanocyte killing, which also depends on a shift in the balance between immune defence and tolerance, e.g. resulting from a decrease in properly functioning T-regulatory cells. With this new model, based on a synthesis of several of the existing theories, in mind, the external and internal factors involved in the etiopathogenesis of vitiligo are reviewed, against the background of reported clinical data, experimental studies and existing and potential new therapies. A similar complex mechanism may also lead to some other autoimmune diseases.

The association between endothelin-1 gene polymorphisms and susceptibility to vitiligo in a Korean population

BACKGROUND

Vitiligo is a skin disorder affected by genetic, environmental, local and endocrine factors. Endothelin-1, which is expressed by keratinocytes, has paracrine effects on melanocytes, influencing their homeostasis, proliferation and pigmentation. It is thought to play a role in the skin response to 311-nm, narrow-band ultraviolet irradiation.

OBJECTIVE

To investigate the association of endothelin-1 gene (EDN1) polymorphisms with vitiligo in a Korean population.

METHODS

To evaluate the expression of endothelin-1 in cultured human keratinocytes after irradiation with narrow-band ultraviolet B (NBUVB), we performed RT-PCR and ELISA. In addition, we genotyped 312 vitiligo patients and 313 matched-healthy controls, and compared the genotype, allele and haplotype frequencies of EDN1 polymorphisms (intron 4 G/A, rs2071942 and exon 5 G/T, rs5370) between the two groups, using PCR-restriction fragment length polymorphism. The effects of sex, onset age, the presence of autoimmune diseases, family history and clinical type were analysed statistically.

RESULTS

NBUVB induced the expression of endothelin-1 in cultured human keratinocytes. The genotype distributions and allele frequencies of EDN1 polymorphisms did not differ significantly between vitiligo patients and healthy controls. Moreover, the results were not related to sex, onset age, the presence of autoimmune diseases or family history. Interestingly, the haplotype frequencies of EDN1 polymorphisms differed significantly between vitiligo patients and healthy controls. When analysed according to clinical type, the haplotype frequencies in the focal and segmental clinical types differed significantly from healthy controls.

CONCLUSION

This study suggests that EDN1 is related to the development of vitiligo in the Korean population.

Pigmentary Disorders in China

Because variations in the degrees of pigmentation occur in various regions of the body, there are different pigmentary diseases in various ethnic groups. We usually divide the disorders of melanin pigmentation into two types: hypermelanosis and hypomelanosis. These disorders may be due to genetic and environmental factors. Pigmentary disorders are more visible on the Chinese skin and are of great cosmetic concern to patients. In this article we introduce characteristic pigmentary disorders in China.

Association of glutathione S-transferase gene polymorphisms (GSTM1 and GSTT1) of vitiligo in Korean population

Vitiligo is an acquired pigmentary disorder of the skin involving melanocyte dysfunction. It has been reported that melanocyte impairment could be related to increased oxidative stress. The glutathione S-transferases (GSTs) are group of polymorphic enzymes that are important in protection against oxidative stress. To find the relationship between GSTM1 and GSTT1 polymorphisms with vitiligo susceptibility, GSTM1 and GSTT1 (homozygous deletion vs. non-deleted) polymorphisms between vitiligo patients (n=310) and healthy controls (n=549) were analyzed. We observed significant association in null alleles of the GSTM1 (P<0.001, OR=2.048, 95% CI=1.529-2.743). GSTM1 null type was also statistically different between two vitiligo subtypes and controls (Focal P<0.001, OR=2.224, 95% CI=1.499-3.298; Generalized P=0.001, OR=1.974, 95% CI=1.342-2.904). However, no significant association in GSTT1 (P=0.869, OR=1.024, 95% CI=0.775-1.353) was observed with vitiligo. In combined analysis of GSTM1 and GSTT1, both null type and GSTM1/GSTT1 (null/present) group showed significant differences between controls and vitiligo patients. These results suggest that GSTM1 null type might be associated with vitiligo susceptibility in Korean population.

Know thy neighbor: a survey of diseases and complex syndromes that map to chromosomal regions encoding TRP channels

On the basis of their ever-expanding roles, not only in sensory signaling but also in a plethora of other, often Ca(2+)-mediated actions in cell and whole body homeostasis, it is suggested that mutations in TRP channel genes not only cause disease states but also contribute in more subtle ways to simple and complex diseases. A survey is therefore presented of diseases and syndromes that map to one or multiple chromosomal loci containing TRP channel genes. A visual map of the chromosomal locations of TRP channel genes in man and mouse is also presented.

Lack of association with TNF-alpha-308 promoter polymorphism in patients with vitiligo

Vitiligo is an acquired depigmentary disorder of the skin, characterized by incomplete penetrance, multiple susceptibility loci and genetic heterogeneity. An immunologic hypothesis is currently advanced as a possible pathogenesis of vitiligo. The cytokines have an important role in pathogenesis of autoimmunity in which tumor necrosis factor-alpha (TNF-alpha), a paracrine inhibitor of melanocytes, is especially important. Several single-nucleotide polymorphisms (SNP) have been identified in the human TNF gene promoter. The polymorphism at position -308 (TNF-308), which involves substituting G for A and designing the AA genotype, leads to a higher rate of TNF gene transcription than the wild-type GG genotype in in vitro expression studies. It has also been linked to increased susceptibility to several chronic metabolic, degenerative, inflammatory and autoimmune diseases. Therefore, we investigated the TNF-alpha-308 SNP in patients with vitiligo. We examined 61 patients with vitiligo. Healthy age-, ethnically- and sex-matched individuals (n = 123) served as controls. Polymerase chain reaction amplification was used for analysis of the polymorphism at position -308 in promoter of TNF-alpha gene. We found that the distribution of TNF-alpha genotypes in vitiligo patients did not differ from that in control subjects (P > 0.05). Moreover, there was no association between TNF-alpha genotypes and types of vitiligo. In conclusion, we suggest that TNF-alpha-308 SNP is not a genetic risk factor for vitiligo susceptibility.