We studied the associations of HLA-DRB4*01:01, DRB1*07:01, and DQB1*03:03:2 genes and their amino acids located in functionally important positions of their protein chains with Vitiligo disorder. The co-occurrence of Vitiligo with other autoimmune disease reveals a common origin and genetic predisposition inducing autoimmunity, as the expression of HLA class II molecules is increased in the pre-lesional epidermis, the sites of melanocyte destruction and skin depigmentation in Vitiligo[16].The specific increase in the levels of HLA class II but not the class I molecules in the pre-lesions of Vitiligo patients suggests the prominent role of the HLA class II gene products in predisposition to Vitiligo skin disease. It is considered that a right level expression of the HLA antigen-presenting receptors and a strong interaction between HLA antigen-presenting receptors with T cell receptors is required for the proper immune response Overexpression of HLA class II molecules may modify the T-cell receptor assembly due to T-cell maturation in the thymus and consequently, influence the survival and development of mature CD4 + T cells. The converted CD4 + T cells have cytotoxic activity (CD4 CTL), and in terms of Vitiligo disorder, destruction of melanocytes[17].
Over 30% of the entire genetic effect for Vitiligo is suggested to be due to abnormal expression of different HLA genes[18], particularly, the alleles of DRB and DQB genes. In the present study, after stratifying these patients, based on family history, age of onset, and psychological trauma before the disease onset, the association of DRB4, DRB1, and DQB1 allelic genes with these distinct subtypes of Vitiligo were revealed.
Recent studies indicated differences in HLA frequencies between FVD (Familial Vitiligo Disease) and SVD (Sporadic Vitiligo Disease) patients which suggests a different genetic background and etiology for familial and non-familial Vitiligo [19]. When we divided the patients in FVD and SVD groups, our results showed a significant increase in the frequency of DRB1*07:01/X genotype in SVD patients (P = 0.030). This finding is in agreement with studies showing the association of DRB1*07:01/X genotype with susceptibility to SVD[20]. There is two hypotheses which could explain the different etiology for these two forms: 1. FVD could be the result of an immune attack against melanocytes presenting HLA Class II alleles; 2. SVD may be due to viral infections that could induce the expression of HLA II genes[21]. In addition, it has been shown that sporadic Vitiligo patients are at greater risk for certain autoimmune/auto inflammatory diseases [22]. Our finding is in line with the latter pathogenesis of Vitiligo.
We further stratified our patient groups by age to early-onset (under 30) and late-onset (above 30) divisions. Divided samples revealed that frequencies of HLA-DRB4*01:01 and DRB1*07:01 genes to be highly increased in early-onset patients as compared with healthy individuals (P = 0.024, P = 0.030 respectively). Our observations are in line with the study of Xiaojing et al., which reported an association of DRB1*07:01/X genotype with the earlier onset of the disease in the Uygar population [23].Our results also indicate a strong negative association of HLA-DRB4*01:01 with the late-onset Vitiligo (P = 0.0016, RR = 0.362), which confirms several studies involving Dutch patients [24]. The above data taken together, suggest susceptibility of HLA-DRB4*01:01 to early-onset of the disease (P = 0.024, RR = 1.362), with an autoimmune basis, and a more significant protective role for HLA-DRB4*01:01 to late-onset Vitiligo (P = 0.0016, RR = 0.360), with a non-immune etiology.
To extend the present study, we analyzed the correlation between DRB4*01:01, DRB1*07:01, and DQB1*03:03:2 and psychological trauma before Vitiligo infection in the Iranian population, an association analysis for Vitiligo that has not been addressed before. Our results showed that patients with a psychological trauma before infection who are carrying DRB4*01:01/X or DRB1*07:01/X genotypes have a decreased risk for Vitiligo development (P = 0.022, RR = 0.75) whereas, the frequency of DRB4*01:01/X or DRB1*07:01/X genotypes are increased in patients without psychological Trauma. These findings may suggest that the development of Vitiligo in patients with psychological trauma before infection may be caused by non-autoimmune event/s in contrast with Vitiligo patients without psychological trauma.
The relative risk or odd ratio, used in case-control studies provides the probability of risk alone. Thus, we calculated PcPPV to estimate the absolute risk and effectiveness of testing for selected genes [15]. For instance, based on the assumption that the prevalence of SVD in the Iranian patients is 1%, PcPPV of DRB*07:01/X genotype for SVD in our patient group is about 1.69.This indicates that patients carrying the DRB1*07:01/X genotype provide 1.69% absolute risk for developing SVD. In other words, in comparison with the prevalence of SVD (1%) in the general Iranian population, the risk of individuals carrying the DRB1*07:01/X genotype to develop SVD is increased by 0.69%.
Several studies reported that special amino acid residues in the HLA class II structure, especially in the pockets of antigen-binding grooves could determine the affinity of HLA molecules. These amino acid signatures are supposed to be critical for enhanced disease susceptibility. Both HLA-DRB4*01:01 and DRB1*07:01 molecules, shown in this study to be associated with Vitiligo, have the positively charged amino acid Arginine at position 71, situated within the pocket 4 (P4) of their antigen-binding groove. Arg71β is also shown to be associate with other autoimmune disorders [25]. Our findings proposed that elevated vulnerability of Vitiligo patients due to DRB4*01:01 and DRB1*07:01 alleles may be is correlated with the presence of amino acid Arginine at position 71 at pocket 4 on the antigen-binding site of the HLA-DRB1 receptor.
It is important to note that the pathogenesis of Vitiligo is complex and that the autoimmune nature of this disease is still under investigation. Nevertheless, despite the possible involvement of the non-autoimmune factors, including abnormal melanocytes or environmental factors that might complicate the understanding of the disease, due to accumulating data implicating different components of the immune system, the etiology for Vitiligo disorder is considered to be more of an autoimmune nature. On the other hand, several subtypes of Vitiligo do not show association with the HLA alleles as crucial elements of the autoimmune system, which might define a non-autoimmune etiology for the development of the disease. This contradiction is also observed in our Vitiligo patient group demonstrating a mixture of autoimmune and non-autoimmune genetic associations.
In conclusion, HLA-DRB1*07:01 allele is positively associated with sporadic form of Vitiligo and, HLA-DRB4*01:01, and DRB1*07:01 alleles are associate with the susceptibility to early-onset Vitiligo. Moreover, HLA-DRB4*01:01 allele provides protection for late-onset Vitiligo and our results also suggest for the first time that Vitiligo disorder in patients with psychological trauma before infection may has a non-autoimmune etiology.