Molecular Genetic of Atopic dermatitis: An Update

Atopic dermatitis (AD) is a chronic multifactorial inflammatory skin disease. The pathogenesis of AD remains unclear, but the disease results from dysfunctions of skin barrier and immune response, where both genetic and environmental factors play a key role. Recent studies demonstrate the substantial evidences that show a strong genetic association with AD. As for example, AD patients have a positive family history and have a concordance rate in twins. Moreover, several candidate genes have now been suspected that play a central role in the genetic background of AD. In last decade advanced procedures similar to genome-wide association (GWA) and single nucleotide polymorphism (SNP) have been applied on different population and now it has been clarified that AD is significantly associated with genes of innate/adaptive immune systems, human leukocyte antigens (HLA), cytokines, chemokines, drug-metabolizing genes or various other genes. In this review, we will highlight the recent advancements in the molecular genetics of AD, especially on possible functional relevance of genetic variants discovered to date.


Introduction
Atopic dermatitis (AD) is a very frequent multifactorial chronic inflammatory skin disorder and is characterized by xerosis, pruritus and erythematous lesions with increased transepidermal water loss. (1) Despite the power of molecular approaches and persistent investigative efforts, AD remains an enigmatic disorder and the agent (or agents) triggering this skin disorder remains to be completely identified. AD is thought to be associated with the dysfunctioning of skin barrier and Th2 cell adaptive immune responses to common environmental allergens. It is a disease with complex genetic and environmental susceptibility factors. Although it is likely that many genetic loci are involved, the association of filaggrin (FLG) null mutations with AD has provided a major step forward in our understanding of disease pathogenesis. (2) FLG is expressed in keratinocytes and is thought to have a role in skin barrier function, cutaneous pH, and hydration. (3) Several clinical phenotypes of AD have now been discovered, but they exhibit great variations in disease severity among affected individuals. (4) Moreover, some cases have the atopic march, while others are susceptible to skin infections such as staphylococcus aureus, eczema herpeticum and malassezia. (4) Approximately 80% of AD patients have raised serum of immunoglobulin E (IgE) and/or immediate skin test reactivity to allergens. (5)(6)(7) Advances in genomic medicine have improved our understanding of the human genome's contribution to health and disease. Genome wide association studies (GWAS) are proved to be a powerful method for identifying disease susceptibility genes for common human diseases and has begun to reveal underlying cellular pathways and point to new therapeutic approaches. (8) This review focuses on the recent advancement of the molecular genetics of atopic dermatitis, especially on the genes located within and around the susceptible loci and their possible roles in pathogenesis of atopic dermatitis. Before beginning of discussion on the molecular genetics, we will highlight the factors that influence on the skin barrier abnormalities.

Factors influence on skin barrier dysfunctions
It has now been well established that FLG derived natural moisturizing factors (NMFs) are important in maintenance of epidermal water and low acidity thus preserving the barrier function of outermost stratum corneum. (9) FLG is involved in the development of keratinocytes to maintain epidermal integrity and it is an important marker of keratinocytes differentiation.
During keratinocytes differentiation, profilaggrin is dephosphorylated and degraded into 10-12 FLG molecules, which condense in the cytoskeleton of keratin to form an intensive protein-lipid matrix. Consequently, these FLG monomers are degraded into NMFs, which are important to maintain skin water, a low pH or maintaining the barrier function of the stratum corneum ( Fig. 1).
Recently, it has been reported that intragenic copy number variation (20-24 copies in one person) within a FLG gene contributes to the risk of AD with a dosedependent effect. (10) Profilaggrin may prevent the extracellular secretion of lamellar bodies, as contain lipids, and may block or decrease the degradation of FLG, which results in reduce in the production of NMFs. Reduced production of acidic metabolites of FLG results an increase of skin surface pH or activating neutral pH dependent kallikreins with downstream influences on skin barrier function. (11,12) An increased pH may also decrease the β-glucocerebrosidase activity and sharing lipid-processing defect may delay epidermal barrier recovery. (11)(12)(13) Also, the elevated pH takes place a decrease in acidic sphingomyelinase catalytic activity and causes degradation of β-glucocerebrosidase and acidic sphingomyelinase. (11)(12)(13) These enzymes playing main role in ceramide synthesis.
Ceramides are essential lipid moieties that are included in preserving epidermal permeability barriers. Thereby, a change in epidermal surface pH may regulate homeostasis of the epidermal permeability barrier, as well as stratum corneum integrity and cohesion. The following are the factors influence on FLG expression in AD. (A) Lossof-function mutations of FLG result in decreased or totally absent levels of FLG. (14)(15)(16) The prevalence of FLG mutation in Northern European subjects about 10% whereas in Chinese, Japanese, and Korean subjects range from 3% to 6%, (16) however, in certain Northern European populations with AD, the prevalence of FLG mutations is reach to 25%: 50%. (2) Consistently focusing in the research of AD for more than 20 year on genetic factors, recent strong role of a genetically predetermined skin barrier disturbance came to a large extent from the discovery the linkage between the development of AD and loss-offunction mutations in the gene encoding FLG. (14)(15)(16) (B) Expression of epidermal FLG is down regulated by cytokines IL-4, IL-13, IL-17A, IL-22, IL-25, IL-31, TNF-α in AD patients. (17)(18)(19)(20)(21)(22)(23) (C) Effect of environmental factors which lead to FLG proteolysis such as low ambient humidity, skin irritants, pruritus / excoriations / mechanical damage, age and water. (24,25) Later will be reviewed the genetic variation of FLG.
Study on single nucleotide polymorphism of the claudin-1 gene (CLDN1) in AD patients demonstrated that tight junctions contribute to the barrier dysfunction and immune dysregulation in AD patients and this may be mediated in part by reductions in tight junction protein claudin-1. (26) In addition, it is also reported that activation of toll like receptor (TLR)-2 plays a role on tight junction barrier via the protein kinase in the tight junction biogenesis. (27) Moreover, IL-4 and IL-13 also promote keratinocyte CLDN-1 expression. (27) Thus TLR-2 activation enhanced expression of CLDN-1 in keratinocytes of AD patients. In addition, TLR-2 lacking mice showed delayed or incomplete barrier recovery. (28) These genetic modifications of TLR-2 might lead to barrier dysfunctions and predispose in AD patients.

Evidences of the genetics role in atopic dermatitis
Recent advancements in clinical experience and molecular reseach on atopic dermatitis have strongly been influenced by genetically alterations. (29) All identified major candidate genes for AD pathogenesis has been listed in Figure 2. Studies on twin have provided the role of a genetic background with a concordance rate of 72-86 % in identical twins and 21-23 % in dizygotic twins, indicating high heritability of AD. (30,31) In addition multiple studies on affected individuals and their families have a positive family history of AD. However the heritability of AD was determined at 72 % by study on Norwegian twin. (31) Recently, a total of 19 susceptibility loci have been estimated at a genome-wide level of significance. Contribution of such these genetic factors may affect diverse phenotypes of AD among individuals. Hence the disease seems to be caused by genetic factor that lead to an immune system deregulation. However deficiencies in innate and adaptive immunity based on, endogenous factor as genetic predisposition result in skin barrier dysfunction and exogenous factor as hyper reactivity to environmental stimuli and susceptibility to skin infections which influence the course and severity of AD. In fact the pathogenesis of AD has been attributed to a complex and multifactorial interactions of the environment and host susceptibility genes, altered skin barrier function, the immune system, and pruritus. (6)

Molecular genetic tools of atopic dermatitis
In progress of biotechnology and molecular biology, more candidate genes have now been determined to be associated with AD. According to the available knowledge, the genes that are related to the structural abnormalities of the epidermis and immune dysregulation play a pivotal role in the etiology of AD. (32) Human genetic variants are either common or rare, common variants, known as polymorphisms and are defined as variants with a minor allele frequency of at least 1% within the population. (30,31) Single nucleotide polymorphisms (SNP) are the most common uses for determining of genetic variations within individuals. (33) Recently GWAS comprehensively surveys associations between SNP in common diseases. (34) Genome-wide linkage analysis had been used in different populations to date, and multiple candidate regions on multiple chromosomes had been associated with AD. (35)(36)(37)(38)(39) The linkage regions vary in different populations, and there is no extensive overlap among studies. Some candidate regions are close to functional genes linked to the various phenotypes of AD. Systemic, well-powered, genome-wide surveys using GWAS and immunochip analyses have estimated the relationship between SNP and susceptibility to AD. (31,39,40) It is well established that common loss of function variants of FLG are a major predisposing factor for AD. As for example, the FLG mutations are involved in AD and are featured in some review articles. (31,(42)(43)(44)(45) The AD associated major candidate genes and their chromosomal location or genetic variations have been summarized in Table 1.
The following genes play a central role in the pathogenesis of AD and now have been offered a several novel potential therapeutic opportunities for AD.
In addition three novel identified FLG stop-gain mutations, Q570X, R3409X and S3707X, were investigated only once. S2392X and S2377X in FLG2 were indicated 1 and 16 times, respectively. However, none of these variants could be detected in other members of African-American PEER children cohort. In TCHHL1, the variant Q294X was noted twice. (43-45, 55, 56) These findings are in agreement with those of Winge et al., who also failed to detect common FLG loss-of-function mutations in people of African ancestry with AD. (55) Their report is from the largest whole-exome sequencing study of African Americans with AD suggesting that S2377X (FLG2) and Q294X (TCHHL1) variants may not be clinically important with respect to incident AD and it seems unlikely that FLG stop-gain mutations have a prominent role with respect to incident AD in African Americans children. (55,56) Whereas, the recent demonstration that FLG2 mutations increase the persistence of AD in African Americans (57) further suggests a pathogenic role for FLG2 mutations (rs12568784 and rs16833974) and supports the sequencing of FLG2in African Americans with IV with or without AD. (44) Polcari data was demonstrated a prevalence of filaggrin mutations including R501X, 2282del4, E2554X, R2447X, 1249insG, R826X, 2767insT, and E2422X in the African American population that exceeds previously published data, although the overall prevalence is still lower than in other populations. (58) In addition other study was added that the FLG P478S polymorphism alone and combined with other factors influences free fatty acids levels and increases the susceptibility to AD among Chinese population. (59)

2-SPINK5 and LEKTI gene
The Serine Protease Inhibitor Kazal-Type 5 (SPINK5) gene located on chromosome 5q32 within genomic region has been linked to AD and encodes a 15-domain protease inhibitor Lymphoepithelial Kazal-Type-Related Inhibitor (LEKTI) which is expressed in epithelial and mucosal surfaces and in the thymus. In fact Serine protease enzymes play an important role in skin barrier homeostasis, including SC desquamation, lipid barrier construction and cornified cell envelope. While, SPINK5 participates to the regulation of proteolysis in keratinocyte differentiation and the generation of normal epithelium, and LEKTI had been found to be involved in maintaining the permeability of normal skin. SPINK5 polymorphisms are associated with the incidence and severity of AD among Chainese, (60,61) athough no association was confirmed with AD in German. (62) Otherwise, a recent study was suggested that the E420K LEKTI variant is a risk factor for AD through an increase in the TSLP expression. (63) Subsequently, recent studies found a significant association between (1258G > A) SPINK5 polymorphism and AD in Japanese, (64,65) and in German. (66) In individuals affected by AD, a significant maternal over-transmission of the risk allele to their children was demonstrated Walley et al. (67) Folster-Holset al., studied 8 SNPs in different regions of the SPINK5 gene, including 4 non-synonymous SNPs leading to an amino acid change (Asp106Asn (G316A), Asn368Ser (1103A > G) and Asp386Asn (1156G > A), and, Gly463Gly (A1389G), Val553Val (C1659T), Leu756Leu (C2358T) and Gly804Gly C2412T) and Glu 825Asp (C2475T)). (62) None of the SNPs were associated with an increased risk of Ad. Kato et al. examined associations between 8 SNPs (IV12-26C > T, IVS12-10A > G, IVS14 + 19 G > A, IVS 13-50 G > A, Asn 368Ser (1103A > G), Asp 386Asn (1156 G > A), His 396His (1188T > C), Glu 420Lys (1258G > A)) of the SPINK5 gene and AD in a Japanese population and found a positive association of 7 SPINK5 SNPs (except for 1156G > A) with AD. (64,65) In addition a study of Six SNPs rs17718511, rs17860502, KN0001820, rs60978485, rs17718737, and rs1422985 in the SPINK5 gene provides evidence for a significant interaction between the SPINK5 gene that may contribute to AD susceptibility among Korean. (68)

4-Innate Immune system genes
As components of innate immunity, antimicrobial peptides (AMPs) produced by keratinocytes play a crucial role in the clearance of microbial pathogens and in preserving epidermal barrier effectiveness. Consequently deficiencies in these AMPs play roles in the pathogenesis of AD. Mainly AMPs include proteins from the β-defensin family (hBD-1, 2, and 3), cathelicidins, psoriasin, and ribonuclease (RNase). (71) Mutations in PRRs s and AMPs are play roles in the initiation and exacerbation of AD. However patternrecognition receptors (PRRs) help protect organisms from microbial pathogens such as Staphylococcus aureus (S. aureus) and Malassezia furfur (M. furfur). Generally, in AD patients, AMPs expression is not reduced, but significantly varies according to the type of AMPs. (5) Recent progress has revealed that innate immune responses are initiated by pattern-recognition receptors (PRR) a family of proteins that enhance certain cytokine gene transcription in response to various pathogenic ligands and control acquired immune responses such as Th1 responses. (72,73) Mutations in PRRs such as toll-like receptors (TLRs) and nucleotide-binding oligomerization domain like receptors (NLRs), as well as mutations in AMPs are associated with susceptibility to skin infections and play key roles in the initiation and aggravation of AD. (74,75) The following are the genes associated innate immune system and have a role in AD pathogenesis.

(b) -CARD15 (or NOD2) gene
CARD15 is located at chromosome 16q21 and about 39.45 kb in length. No significant associations between AD and any polymorphisms (2104C > T, 2722 G > C, 802T > C, 534 G > C, rs1077861 (intron 10A > T), 2863 G > A, 4278A > G, -60A > G) or haplotype of CARD15 were observed. (77) Associations of three SNPs (2104C > T, 2722 G > C and 3020iC) with AD in children have been reported. Children with the C allele of 2722 G > C SNP had a 1.85-fold risk of developing AD in German. (78)

(c) -Monocyte differentiation antigen (or CD14) gene
CD14 gene is located at chromosome 5q22-q32 and about 1.95 kb in length. In a small study, children with the CT genotype of the CD14−159C > T SNP had a significantly lower prevalence of AD at three years of age compared with those with the genotypes CC and TT combined, (79) although the CD14-159C > T SNP was not associated with an increased risk of AD in German children. (80) No significant difference was found in the genotype frequencies of -159C > T, -1145 G > A, 1359 G > T and -550C > T SNPs between AD patients and controls. (81)

(d) -MBL2 gene
The mannose-binding lectin (MBL2) gene is located at 10q11.2-q21 chromosome and about 6.32 kb in length. Recently, three variants at codons 52, 54, and 57 of exon 1 of the MBL2 gene have been identified. The MBL2 Gly54 Asp SNP was not associated with an increased risk of AD in a Japanese population. (82)

(f) -DEFB1 gene
The human β-defensin 1 (DEFB1) gene is located at 8p23.2266T/C and 1241T/GSNPs of DEFB1 gene had been demonstrated to be associated with AD among Korean. (94) While 692 A>G and 1654 A>GSNPs of DEFB1 gene had been demonstrated to be associated with AD among Egyptian, (107) and among Mexican populations. (95,96) As controversial findings have been achieved, a study had been not confirmed that the role of -20 G/A (rs11362), -44 C/G (rs1800972), and -52 G/A (rs1799946) at 5'-UTR of DEFB1 gene in the development of AD among Brazilian population. (97)

5-Adaptive immune system genes:
Alterations in the adaptive immune system are also play key role with AD. Where AD is going through a biphases, the first phase is prevailed by T helper type 2 (Th2) cytokines that later turns to the second phase a more chronic Th1-dominated eczematous phase. The effective elevation of IgE in atopic disease by B cells depends on support by Th2 cells, which mainly produce interleukin-4 (IL-4), IL-5, IL-9 and IL-13. Adaptive immune genes are including cytokine genes and chemokine.

5(a) -Cytokines and related genes:
Cytokines have been classified into two subgroups according to their function: Th1 cytokines, mainly interleukin (IL) 2, IL12, interferon (IFN)γ, and tumor necrosis factor (TNF)α, which activate the cellular machinery of the immune system; and Th2 (IL4, IL5, IL6, IL10 and IL13) cytokines, which activate the humeral machinery. Some cytokines, such as IL-1α, IL-2, and TGF-βwere found to be decreased in AD, (98,99) while other such as IFN-γ, IL-12 and GM-CSF were elevated in chronic AD. Several data have been shown the association between cytokine polymorphism and AD. Some reports show positive association of certain cytokine polymorphisms with AD, while others are controversial. (100)

5(a-1) -IL-4 gene
Interleukin (IL)-4 is a glycoprotein encoded by the IL-4 gene which has been mapped to chromosome 5q31-33 and about 9.01 kb in length. Promoter polymorphisms of IL-4-590C/T were significantly associated with atopic AD in Egyptian population, (101) Czech population, (102) and Japanese population. (67) On contrast, IL-4-590C/T SNP of IL-4 was not associated with AD in Chinese, (103) Egyptian (101) and Czech population. (102) IL-4 -1098G/T polymorphism was significantly associated with atopic AD in Czech population, (102) while no association in Macedonians. (104) No association was found between AD and -33C/T,IL-4 polymorphism in Chinese, (103) Czech (102) and Macedonians population. (104) In Caucasians the T allele of IL-4 -589C>T SNP was significantly associated with the development of AD at 24 months of age. (105)

5(a-2) -IL-4Rα gene
Interleukin 4 receptor alpha gene (IL-4Rα) is located at chromosome 16p11.2-12.1 and about 50.86 kb in length. Several IL4R polymorphisms (-3112C > T, -1803T > C, -327C > A, -326A > C and -186G > A) have been found to associated with AD in Japanese. (106) Seven polymorphisms (223C > G > T > A, 1199C > A, 1291C > T, 1307T > C, 1727G > A, 2356C > T) and a silent 1242T > G have been demonstrated to have functional significance. Caucasian children with the rare homozygous 1727G > A polymorphism had a higher prevalence of flexural eczema in the first 6 months compared with the heterozygote and the wild type homozygote genotypes combined in British. (107) It has been demonstrated that the 1727G > A SNP (Gln551Arg) was significantly associated with AD in another Japanese population. (108) In contrast no association between (1199C > A, 1242T > G, 1507C > T and 1727G > A) of IL4R polymorphism and AD in a Chinese population. (103) Inaddition in Egyptian population the IL-4Rα I50 V and Q576R, polymorphisms were significantly associated with the development of AD. (101) Another polymorphism 3223C/T IL4Rα was associated with AD in Japanese. (106)

5(a-3) -STAT6 gene
The Signal transducer and activator of transcription (STAT)6 gene have been mapped to chromosome 12q13.3-q14.1, and about 16.79 in length. There was no association between AD risk and the 2964 G > A SNP of the STAT6 gene while the 1315-GT repeat allele het erozygosity of the dinucleotide repeat in exon 1 (13-, 14-, 15-and 16-GT repeat alleles) was significantly associated with allergic disease including AD in Japanese. (109) However, the short tandem repeat in exon 1 was not associated with AD risk in Chinese. (103) In Egyptian STAT6 2964 G/A and 2892 C/T polymorphisms were significantly associated with the development of AD. (101)

5(a-5) -IL-6 gene
IL-6 is located at chromosome 7p21 and about 6.12 kb in length. No association was found between the -174C > G SNP of the IL6 gene and AD. (106) As well as the -174C > G and -922A > G SNPs were not found. (106) Whereas -174C/G and nt565A/G were found that significantly associated with atopic AD in a Czech population. (102)

5(a-6) -TNF-α gene
Tumor necrosis factor (TNF)-α gene is located at chromosome 6p21.3. No significant association was found between -308 G > A SNP of the TNFα gene and AD in English. (110) Neither -1031T > C, -863C > A, -857C > T, -308G >A nor -238G > A SNPs of the TNFα gene was associated with AD in a Chinese population. (103) Whereas, no association was not found between AG cdn25 and AD in population of Macedonians. (104)

5(a-8)-IL-1α gene
Interleukin (IL)-1α gene is located at chromosome 2q14 and about 11.48 kb in length. The -899T > C SNP of the IL1A gene was not associated with AD in Macedonians. (104)

5(a-9) -IFNγ gene
Interferon gamma (IFNγ) gene is located at chromosome 12q14and about 16.25 kb in length. Study on a Chinese population was demonstrated no association between short tandem repeats at the first intron of IFNγ gene and AD. (103) In addition, no association was found between 874 A>T IFNγ gene and AD in population of Macedonians. (104)

5(a-10) -IL-β gene
IL-1β is located at chromosome 2q14 and about 7.16 kb in length. No association was found between either the -511C > T, 3953T > C, 3953T > C, -1418T > C or the 315T > C SNPs of theIL1B gene and AD in American, German, English and Macedonians. (104,106,110)

5(a-11) -IL1RN gene
Interleukin 1 receptor antagonist (IL1RN) gene is located at chromosome 2q14.2 and about 34.70 kb in length. The polymorphism in intron 2 of the IL1RN gene is caused by a variable copy number of an 86-bp sequence. The 4-repeat (IL1RN *1) and 2-repeat (IL1RN *2) alleles are most common, while the other alleles occur at a combined frequency of less than 5%. No association was found between the variable number of tandem repeat polymorphisms in intron 2 of the IL1RN gene and AD. (106)

5(a-13) -IL-5 gene
IL-5 is located at chromosome and about 2.08 kb in length. The -703C > T SNP of IL5was not significantly associated with AD in Japanese. (112)

5(a-14) -IL-12 β gene
IL-12 β is located at chromosome 5q31.1-q33.1 and about 15.69 kb in length. The AA genotype of IL12B 1188A > C SNP was associated with decreased risk of AD in a Japanese population [113]. The 4237 G > A, 4496A > G and 4510G > A SNPs of theIL12B gene were not associated to the development of AD. (103) In addition 30 untranslated region of the IL-12B gene was associated with the AD phenotype. (113)

5(a-16) -IL-13 gene
IL-13 is located at chromosome 5q31 and about 4.85 kb in direct. No association between the -1111C > T SNP of IL13and AD in Chinese population (103) and Japanese population. (115) While significant association between the -1024C > T SNP of the IL13 gene and AD was confirmed in German. (116) In the Japanese population there was no significant association between two SNPs of 704A > C and 1103C > T whereas the Arg allele of Arg144Gln SNP was significantly associated with an increased risk of AD in Japanese. (115) As well as the Arg 130 Gln polymorphism was confirmed in Canadian, (117) German, (116) and Japanese. (115) The A allele of the Arg144Gln SNP was associated with AD in German. (118) In Caucasians, haplotypes consisting of IL13 Arg144Gln with AD were associated with AD. None of the three SNPs (-1111C > T, 1293C > T, and Arg144Gln) were associated with AD during the first year. (106)

5(a-18) -TGF-β1 gene
Transforming growth factor β1 (TGF-β1) is located at chromosome 19q13.2 and about 52.34 kb in length. The C allele of the TGF-β1 915 G > C SNP was associated with an increased risk of AD in British, whereas there was no significant difference in the frequencies of the 869T > C genotypes [120]. No association between AD and the -590C > T SNP was not found. (106)

5(a-19) -GM-CSF gene
Granulocyte macrophage colony-stimulating factor (GM-CSF) is located at chromosome 5q31.1 and about 2.38 kb in length. The A allele of the -677A > C SNP in the promoter region of the GM-CSF gene was associated with an increased risk of AD in British. (110) Although -1916T > C SNP was significantly associated with an increased risk of AD, there was a strong linkage disequilibrium existed between the -677A > C and -1916T > C SNPs. (110) The 3606T > C and 3928C > T SNPs of the GM-CSF gene was not associated with susceptibility to AD in Japanese. (121) While a strong linkage disequilibrium between 3606T > C and 3928C > T SNPs of the GM-CSF with AD in Canadian. (117)

5(b-3)-CCL17 (or TARC) gene
The Chemokine(C-Cmotif) ligand 17 (CCL17) or thymus and activation-regulated chemokine (TARC) levels expression of AD patients were associated with disease activity. (130) In spite of some CCL17 polymorphisms are candidates as a genetic factor in AD, no correlation between AD and the -431C > T SNP of the CCL17 gene was found in a Japanese population. (131)

5(b-4)-CCR3 gene
Chemokine (C-Cmotif) receptor3 (CCR3) gene is located at chromosome 3p21.3 and about 143.70 kb in length. As mentioned before, CCR3 is a receptor for different chemokines which are play important role in AD pathogenesis. Hence, the biological activities of CCR3 suggest that polymorphisms of CCR3 may be an increased risk for AD. There was no significant difference in genotype frequencies of 51T > C SNP of the CCR3 gene between AD patients and controls among Japanese. (131)

5(b-5)-CCR4 gene
Chemokine (C-Cmotif) receptor-4 (CCR4) gene is located at chromosome 3p24 and about 3.36 kb in length. One study was not demonstrated a significant association between the 1014C > T CCR4 polymorphism and AD patients. (132)

5(b-6)-CMA1 gene
Mast cell chymase 1 (CMA1) gene is located at chromosome 14q11.2 and about 2.91 kb in length. A family-based association study in Caucasians revealed a significant association of this polymorphism with total IgE levels in patients with self-reported AD. (133) A significant association between the CMA1-1903A > G polymorphism and AD was observed by Weidinger et al. (134) It may be speculated whether this DNA variant alters the expression of chymase. It has also been shown that CMA1 is increased in chronic atopic eczema skin lesions (135) and a potential role of chymase in the promotion of skin barrier defects and cutaneous neovascularization has been suggested. (136)

6-Drug-metabolizing genes:
The metabolism of xenobiotic involves oxidation, reduction, and hydrolysis (phase I) and conjugation (phase II) reactions.

6(a-1)-GST genes
Certain genes within the glutathione Stransferase (GST) M, GSTT and GSTP subfamilies (GSTM1, GSTT1and GSTP1) are polymorphic in humans and the levels of individual enzymes expressed can be influenced by induction and genetic polymorphism. The GSTM1, GSTT1and GSTP1genes are located on chromosomes 1p13.3, 22q11.23 and 11q13, respectively. The 1404A > G (Ile105Val) and 2294C > T (Ala114 Val) SNPs of the GSTP1 gene were associated with a significantly increased risk of AD in Russian. (137) 6(a-2)-NAT-2 gene N-acetyl transferase (NAT)2 gene is located at chromosome 8p23.1-p21.3and about 9.97 kb in length. N-acetylation is an important genetic polymorphic pathway in the biotransformation of one or more single-based mutations in the NAT2 gene known to cause low expression levels of functional NAT2 enzyme. 481C > T (synonymous mutation) and 590G > A SNPs were not correlated with susceptible to AD in Russian. (138) Moreover, 481C > T, 590 G >A or 857G > A, were not associated with an increased risk of AD in Caucasian. (139)

7-Other genes associated with atopic dermatitis 7(a-1)-CTLA-4 (or CD152) gene
Cytotoxic T-lymphocyte-associated antigen-4 (CTLA-4) gene is located at chromosome 2q33 and about 5.55 kb in length. CTLA -4 is associated to an increased the risk of autoimmune diseases. While, the 49A > G SNP of CTLA-4 gene was not associated with AD in Chinese. (140)

7(a-2)-KLK ( or SCCE) gene
Kallikrein (KLK) or stratum corneum chymotrypsin enzyme (KLK) gene is located at chromosome 19q13.33 and about 7.57 kb in length. A significant genetic association was found between the rare AACCAACC variant of the KLK7 gene and AD in British. (141)

7(a-3)-RUNX1 gene
Runt-related transcription factor1 binding site between solute carriers (RUNX1) gene is located at chromosome 21q22.3. There was no significant allelic association between the RUNX1 polymorphism (rs734232) and AD in a small Japanese adult population. (142)

Conclusions
Immune system plays a key role in the pathogensis of atopic dermatitis, therefore it is important to design an appropriate epidemiological investigation of immune system polymorphism for atopic dermatitis patients. Continued advances in molecular genetics and in high-throughput of genotyping methods will facilitate the analysis of multiple polymorphisms within genes and the analysis of multiple genes pathways. The effects of polymorphisms are best represented by their haplotypes. Data from multiple polymorphisms within a gene can be combined to create haplotypes and the set of multiple alleles on a single chromosome. Polymorphisms, even those not significantly associated with AD, should be considered as potentially important public health issues. In addition, it is important to keep in mind that a susceptibility factor in one population may not be a factor in another. There are differences in the prevalence rates of immune system polymorphisms among populations. In a population where the frequency of an "at-risk" genotype in a given polymorphism is too low, the "at-risk" allele or "at-risk" genotype can be rare to assess its associated risk.
processing enzymes and profound alterations of barrier function and stratum