Genetic Deletions of GSTM 1 and GSTT 1 in Head and Neck Cancer : Review of the Literature from 2000 to 2012

Head and neck cancer incidence has risen significantly worldwide in the last few years (Toefil et al., 2007) and is now the fifth most common cancer worldwide (Jun et al., 2010). Head and neck cancer includes mainly the cancers of the oral cavity, pharynx and larynx. Cancer of oral cavity is common, followed by larynx and pharynx (Bhurgri et al., 2006). Its multifactorial etiology includes genetic susceptibility as well as environmental risk factors. Xenobiotics are detoxified by phase II enzymes, such as glutathione S-transferases (GSTs) which are involved in detoxification of polycyclic aromatic hydrocarbons (PAHs) and benzo(a)pyrene (Schneider et al., 2004). GSTs are family of dimeric protein enzymes known to play an important role in the Phase II detoxification of several carcinogens (Hayes and Pulford, 1995). GSTs catalyze the conjugation reactions between glutathione and carcinogen substrates and facilitate its excretion. GSTM1 and GSTT1 are known to exhibit deletion polymorphisms (Egan et al., 2004). Persons with homozygous deletions of either the GSTM1 or the GSTT1 gene have no enzymatic functional activity of the respective enzyme and are known as null gene (Egan et al., 2004). Evidence suggests that genetic polymorphisms of these genes might increase individual susceptibility to HNC. Number of published studies have focused on GSTM1 and GSTT1 genetic variation with respect to HNC and have yielded conflicting results (Toru et al., 2008). Whether GSTM1 or GSTT1 polymorphism is


Introduction
Head and neck cancer incidence has risen significantly worldwide in the last few years (Toefil et al., 2007) and is now the fifth most common cancer worldwide (Jun et al., 2010).Head and neck cancer includes mainly the cancers of the oral cavity, pharynx and larynx.Cancer of oral cavity is common, followed by larynx and pharynx (Bhurgri et al., 2006).Its multifactorial etiology includes genetic susceptibility as well as environmental risk factors.
Xenobiotics are detoxified by phase II enzymes, such as glutathione S-transferases (GSTs) which are involved in detoxification of polycyclic aromatic hydrocarbons (PAHs) and benzo(a)pyrene (Schneider et al., 2004).GSTs are family of dimeric protein enzymes known to play an important role in the Phase II detoxification of several carcinogens (Hayes and Pulford, 1995).GSTs catalyze the conjugation reactions between glutathione and carcinogen substrates and facilitate its excretion.GSTM1 and GSTT1 are known to exhibit deletion polymorphisms (Egan et al., 2004).Persons with homozygous deletions of either the GSTM1 or the GSTT1 gene have no enzymatic functional activity of the respective enzyme and are known as null gene (Egan et al., 2004).Evidence suggests that genetic polymorphisms of these genes might increase individual susceptibility to HNC.Number of published studies have focused on GSTM1 and GSTT1 genetic variation with respect to HNC and have yielded conflicting results (Toru et al., 2008).Whether GSTM1 or GSTT1 polymorphism is

Materials and Methods
We carried out a search in Medline for case control studies covering published online papers reported from Jan 2000 up to Oct 2012.Language was not a limitation and keywords used were GSTM1, GSTT1, HNC, head and neck cancer, carcinoma, null allele, deletions.We focused on null alleles of GSTM1 and GSTT1 genes in head and neck cancer.The search was limited to human study.Articles clearly describing case control study with association of these genes to head & neck cancer were selected.Citation lists of retrieved articles were checked to ensure sensitivity of the search strategy.We excluded studies that presented aggregate data for several cancers but not of HNC.A total of 47 publications for GSTM1 and 38 publications for GSTT1 were selected after exclusion from 473 publications that were searched.Positive controls were mentioned for GSTM1 and GSTT1 null genotypes.A total of 8063 patients and 9438 controls for GSTM1 and 6961 patients and 7954 controls for GSTT1

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were genotyped in these reports.
For each study we abstracted the publication date, country of research, ethnicity, statistical results, site of head and neck cancer, number of cases and controls.

Results
The literature search found 473 articles.After removal of duplicate entries and unrelated studies, 105 abstracts and 148 full-text articles were reviewed.Eventually, only 47 studies were identified that evaluated polymorphisms in GSTM1 and GSTT1 genes and HNC outcomes; the remaining abstracts and full-text articles did not pertain to HNC, polymorphisms or outcomes.All of these studies were case-control studies.For study size determination, we only included the subset of individuals who had genotyping done, not the entire study population.Study size varied widely for GSTM1 and GSTT1 genes (Table 1).The median of GSTM1 and GSTT1 cancer cases was 118 and 149, respectively and control was 144 and 180, respectively.
Table 2 gives an overview of GSTM1 and GSTT1 gene deletions with respect to year, ethnicity and site of cancer.Since 2000 to 2012, 36 studies of HNC, (including the subanatomic sites) ORs for GSTM1 null genotype vs the positive genotype were>1, suggesting that the null GSTM1 genotype may be associated with increased risk for HNC.Similarly for GSTT1 25 studies (ref) suggested ORs>1 for null genotype vs positive genotype (Table 3).Thus GSTT1 may also be considered as a risk factor of cancer initiation.
For GSTM1 null genotype in HNC 64% studies showed positive association with cancer and for larynx, pharynx and oral cavity the percentages were 62%, 86% and 84%, respectively.Similarly the percentages of positive association with HNC, larynx, pharynx and oral cavity for GSTT1 null genotype were 60%, 50%, 72% and 100%, respectively.
Study populations were predominantly Caucasian or Asian (reported or inferred based on the academic affiliation of authors or hospital location).Less than half of the studies (29) evaluated a mixed population of HNC sites, whereas 20 focused on specific subanatomic sites, particularly oral cavity, pharyngeal and laryngeal lesions (Table 4).Stratified according to ethnicity we found increased risk of HNC in Asians with GSTM1 and GSTT1 null genotype (Table 3) compared with Americans and Europeans.
Conflicting data were present for GSTM1 (Table 2) and GSTT1 null polymorphism with HNC (Table 5).However, current review showed that GSTM1 and GSTT1 null genotypes were found to be significantly associated

Discussion
Carcinogen detoxifying pathways are studied as a risk and prognostic factor of head and neck cancer.Two important genes of phase II detoxification process GSTM1 and GSTT1 have been found to be frequently deleted leading to null genotype of respective genes (Masood et al., 2011).GSTM1 and GSTT1 null genotype is associated with head and neck cancer risk.
In the current review, strikingly, the results of many studies showed contradictory associations between null genotypes and cancer risks.However we found significant association of GSTM1 and GSTT1 null genotype with head and neck cancers and sub sites of cancers in different ethnic groups also.Previous meta-analysis and pooled analysis have reported an association between the GSTM1 and GSTT1 null genotypes and head and neck tumors, but did not analyze ethnic specific or sub site specific differences and also many latest publications of 2010 were missing.Varela-Lema et al., (2008) evaluated ethnic specific and sub site specific differences in a pooled analysis and confirmed that there was no association of the GSTM1 genotype with oral and pharyngeal cancers in Caucasians although not statistically significant.But the data of Varela-Lema et al., (2008) was not up to date and included publications till 2007 and two genes were studied at a time.
Deletion of GSTM1 and GSTT1 contribute to the tumorigenesis and progression of head and neck cancer (Toru et al., 2008;Masood et al., 2010).For GSTT1, a gene that is highly conserved during evolution, major ethnic differences exist in frequency distribution (Cadoni et al., 2012).In Asia, highest percentages of individuals with the GSTT1 null genotype have been reported as evident from the current review.Additionally, as only few studies have been published in previous reviews, it is likely that the discrepancy may be by chance because studies with few papers may have insufficient statistical power to detect a slight effect or may yield a fluctuated risk estimate.
The overall assessment of the publications, several common concerns emerged about the published studies representing the challenges of a maturing field.Firstly, inadequate reporting of main aspects of the underlying population was a problem.Most studies had at least one to several of the following key categories incompletely reported: country of study and source of population, inclusion/exclusion criteria for participants, study design, population characteristics for general demographic variables and clinically important prognostic factors, demographic comparisons of patients included against those excluded from analysis and detailed descriptions of both genotyping quality control measures and statistical methods.Therefore these criteria should be considered in the future publications in order to deduce conclusive and confirmative results from review studies.
In conclusion, we reviewed the field of polymorphism variants of GSTM1 and GSTT1 null genotype and outcomes in HNC.Published studies have all used a

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standard candidate genetic polymorphism approach.We found that the genetic polymorphisms had consistent associations with HNC.Carcinogen detoxification pathways continue to be the most studied pathways for HNC outcomes.The vast majority of studies were exploratory in nature resulting in the need to validate or replicate results in larger, well-characterized populations of patients.Med Bras, 56, 299-303. Majumder M, Sikdar N, Paul RR, et al (2005) Glutathione S-transferase M1 gene polymorphism in Thai nasopharyngeal carcinoma.Asian Pac J Cancer Prev, 6, 270-5.To-Figueras J, Gene´ M, Go´mez-Catala´n J, et al (2002).
Microsomal epoxide hydrolase and glutathione S-transferase polymorphisms in relation to laryngeal carcinoma risk.