GSTP1 does not modify MC1R effects on melanoma risk
2016-06-15

Glutathione S-transferases (GSTs) are a group of enzymes that act to detoxify reactive oxygen species resulting from oxidative stress processes and melanin production. GSTP1 is a polymorphic gene encoding variant proteins involved in metabolism. The role of the rs1695*A/G GSTP1 Ile105Val polymorphism in cutaneous malignant melanoma (CMM) susceptibility remains controversial. Two recent meta-analyses reported that the GSTP1 105Val allele was associated with an increased risk of CMM.1, 2 However, it was concluded by the authors of both publications that additional studies with larger cohorts were required. The melanocortin 1 receptor (MC1R) gene encodes a G-protein coupled receptor involved in the regulation of melanin production and the response to solar ultraviolet.3 MC1R RHC-variant alleles exercise major influence on hair colour, skin colour and sun sensitivity. The association between MC1R and susceptibility to CMM and skin cancer in general has been described previously.4, 5 A 2011 Spanish study reported that the GSTP1 polymorphism was associated with light coloured hair and increased CMM risk. Furthermore, the authors described a greater penetrance of CMM risk when the GSTP1 105Val allele was combined with MC1R RHC-variant alleles.6, 7 This study has further examined the association between rs1695*A/G GSTP1 Ile105Val polymorphism and CMM. In addition, we examined the previously described effect modification of the combination of GSTP1 Ile105Val with MC1R RHC-variant alleles in a cohort of Queensland control and CMM cases and CMM control and patient samples from Germany. This study included the largest population to date, a total of 27 574 participants. The Brisbane Nevus Morphology Study (BNMS) included a total of 738 participants, 342 at high risk of developing CMM were recruited from the Melanoma Unit and Dermatology Department of the Princess Alexandra Hospital (Brisbane, Australia) or from community private dermatology clinics. High risk was defined by previous CMM or CMM affecting a first-degree relative. An additional 396 moderate- and low-risk control volunteers were recruited by public advertisement or by a direct letter sent to previous participants of the Brisbane Twin Nevus Study (BTNS). A total of 761 participants, including 558 CMM and 203 non-CMM patients, were recruited from the Department of Dermatology, University Hospital Tuebingen. To match our German CMM cases, we sourced an additional 1988 ethnically appropriate controls from Augburg Germany as part of the KORA study. QIMR-Berghofer provided access to a large panel of 23 724 individuals unaffected by CMM with direct and imputed genotypes from several rounds of typing in the Brisbane Longitudinal Twin Study (BLTS), combined with several other studies that were made available to be used as a control group (Duffy et al., in submission). A further 363 European samples were accessed from the 1K genome project. Please refer to the supplementary section for further explanation of the genotyping and statistical methods. Six GSTP1 polymorphisms were examined to assess for a potential effect on CMM risk (Table 1). GSTP1 rs4891 was in strong linkage disequilibrium with rs1695 (r2=0.92, Table S1). Although previous studies have suggested the involvement of rs1695 in CMM risk (also apparent when stratified by study, Table S2), when the number of controls was augmented using the BLTS (n=23 724), German KORA and European samples from the 1K genome project (n=363), the total combined analysis did not show any statistically significant association between rs1695 or rs4891 and CMM risk (P=0.45). Moreover, the recently published CMM meta-analysis8 also does not support the role of GSTP1 polymorphism in increasing risk (rs1695 P=0.62; rs1138272 P=0.95; rs4891 P=0.72, M. Law personal communication). Consistent with the previously published literature,4 this analysis demonstrated a highly statistically significant association between MC1R variant genotypes designated using WT, r and R allele classification and risk of CMM (Table 2). These effects were not modified by GSTP1 rs1695 genotype (A/A, A/G, G/G) in the stratified analysis (overall P=0.79). This result was also confirmed when restricted to MC1R Arg151Cys genotype (WT/WT, WT/R, R/R), the MC1R allele of highest penetrance for CMM and red hair colour (Table S3). Moreover, examination of each MC1R allele individually did not show any interaction with GSTP1 rs1695 genotype and CMM risk (Table S4). The reported association of rs1695 and light hair colour 6 was not seen in our analysis. However, the rs1138272 GSTP1 Ala114Val SNP showed weak association with darker hair colour in the BTNS and German case samples. This was not replicated in the analysis of the larger BLTS, Genes and Blood Clotting study (GABC) or UK 1958 birth cohort studies. Neither of these SNPs was associated with skin colour. The role of GSTP1 polymorphism in CMM has been examined in several candidate studies with some positive reports of association.1, 2 In the most recent meta-analysis, an association of rs1695 with CMM risk was detected (P=.033 in a dominant model).1 However, this conclusion is not supported in larger GWAS analyses such as reported in Law et al.8 who studied 16 000 CMM cases and 26 000 controls. In this study, we similarly saw an association in our German case subsample that did not replicate in the larger German KORA controls or our full sample. Our conclusion is that GSTP1 polymorphisms do not have any large effect on risk of CMM. The effects of rare variants of GSTP1 on CMM remain to be clarified in larger studies. We also examined the hypothesis that GSTP1 may interact with other CMM risk loci as recently reported.6 This study is considerably larger than the group studied by Ibarrola-Villava et al. and should have good statistical power to replicate any true association. There was no modification effect found in the combination of GSTP1 Ile105Val with MC1R RHC-variant alleles. Finally, we also excluded the effects of GSTP1 on phenotypic risk factors for CMM, including hair and skin colour. GMD, AP, KJ, KJL and XLHY performed the research; GMD, DLD, KJ, HPS and RAS analysed the data; TKE, BW, BMS, NGM and CG contributed samples and genotypes to the study; and GMD, DLD, HPS and RAS designed the research study and wrote the manuscript. DLD, and RAS are Senior Research Fellows of the Australian NHMRC. This work was funded by project grant NHMRC APP1062935 and the Centre of Research Excellence for the Study of Naevi APP1099021. The authors have declared no conflicting interests. Data S1. Supplementary materials and methods Data S2. Supplementary references Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. 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