Updated Meta-analysis on HER2 Polymorphisms and Risk of Breast Cancer: Evidence from 32 Studies

Breast cancer has a wide distinct range of clinical, pathological and molecular features that makes it a heterogeneous disease (Sorlie et al., 2001). It has been progressively increased with estimated 229,060 new diagnoses and 39, 510 deaths per year in the United States, and its incidence is currently increasing in the world (Linos et al., 2008; Ziegler et al., 2008; Siegel et al., 2012). Molecular genetic factors indicated the various structural and functional genetic alterations play an important role in the development and progression of breast cancer. HER2 (Human epidermal growth factor receptor 2, ERBB2/neu/EGFP2) gene is located on chromosome 17q12-21q, spans 38kb, a 1255 amino acid glycoprotein of 185 kDa, comprises 27 coding exons, and over expressed in 20%-30% of breast cancer (Tommasi et al., 2004). Sequence analysis identified a common genetic variant at codon 655 in the transmembrane coding region of HER2 gene, an Ile-to-Val single-nucleotide polymorphism was found, resulting in the substitution of isoleucine (Ile: ATC) with valine (Val: GTC) (Cooke et al., 2001; Uzan et al., 2009). Isoleucine to valine changes might alter the hydrophobicity of the protein which affects the conformational stability of the domains (Papewalis et


Introduction
Breast cancer has a wide distinct range of clinical, pathological and molecular features that makes it a heterogeneous disease (Sorlie et al., 2001). It has been progressively increased with estimated 229,060 new diagnoses and 39, 510 deaths per year in the United States, and its incidence is currently increasing in the world (Linos et al., 2008;Ziegler et al., 2008;Siegel et al., 2012). Molecular genetic factors indicated the various structural and functional genetic alterations play an important role in the development and progression of breast cancer.
HER2 (Human epidermal growth factor receptor 2, ERBB2/neu/EGFP2) gene is located on chromosome 17q12-21q, spans 38kb, a 1255 amino acid glycoprotein of 185 kDa, comprises 27 coding exons, and over expressed in 20%-30% of breast cancer (Tommasi et al., 2004). Sequence analysis identified a common genetic variant at codon 655 in the transmembrane coding region of HER2 gene, an Ile-to-Val single-nucleotide polymorphism was found, resulting in the substitution of isoleucine (Ile: ATC) with valine (Val: GTC) (Cooke et al., 2001;Uzan et al., 2009). Isoleucine to valine changes might alter the hydrophobicity of the protein which affects the conformational stability of the domains (Papewalis et But previous studies have produced inconsistent results. Therefore, we conducted a meta-analysis to estimate the possible influence of HER2 Ile655Val polymorphism on the risk of breast cancer.

Publication search
PubMed was searched using the search terms "HER2 (or ERBB2, or neu, or EGFP2)", "polymorphism" and "breast cancer" (last search update was on December, 2013). Case-control studies containing available genotype Ile655Val (rs 1136201) were chosen. Additional studies were identified by a manual search of the references of original studies.

Data extraction
Information was carefully extracted from all investigators independently by two of the authors (Chen and Yang) a consensus on inclusion criteria listed above. Disagreement was resolves by discussion between the two authors. Otherwise, other authors (Tang and Wei) were consulted to resolve. A final decision was made by the majority of the votes. The following data were sought for: first author's surname, publication year, country origin, ethnicity (categorized as African, Asian and Caucasian), genotyping total number of cases and controls.

Statistic analysis
Information was carefully extracted from each study, the following date of HER2 Ile655Val polymorphism was assessed for Hardy-Weinberg equilibrium in control group using the p text, and a P-value of<0.05 were not considered. The strength of association between HER2 Ile655Val polymorphism and canner was accessed by calculating crude ORs (odds rations) with 95%CIs (confidence intervals). The pooled ORs were performed for additive genetic model, dominant model and recessive model, respectively. Heterogeneity assumption was checked by a chi-square based Q-test. A significant Q-statistic (p<0.05) indicated heterogeneity among studies. The pooled OR estimate of each study was calculated by the fixed-effects mode (Mantel et al., 1959) if there was not significant heterogeneity. Otherwise, the random-effects model was used (DerSimonian et al., 1986).
The potential for publication bias was carried out by a Begg's test (funnel plot method) and Egger's linear regression test (p<0.05 considered representative of statistical significance) (Egger et al., 1997).
Meanwhile, to assess the influence of definition criteria on the pooled result, sensitivity analysis was performed in the meta-analysis. Sensitivity analysis was performed by methods to examine the impact of methodological quality, statistical models and types of study design on the results. First, leave-one-out to repeat analyses to test the influence of single studies on the summary effect; Second, reanalyze under different effects models; Third, reanalyze data categorized by study design.

Eligible studies
We identified 32 cases-control studies on the association between HER2 Ile655Val polymorphism and breast cancer, including 14, 926 cases and 15, 768 controls ( Table 1). The distribution of genotypes in the controls of all the studies was in agreement with Hardy-Weinberg equilibrium, except three studies (Millikan et al., 2003;Kallel et al., 2010;Ozturk et al., 2013).

Meta analysis
The results of the association between the HER2 Ile655Val polymorphism and breast cancer and the heterogeneity test were shown in Table 2 ( Figure. 1-3) Table 2.
Sensitivity analysis ensured all the data analysis was stability and dependability in Figure 4.

Discussion
It is biologically plausible that exposure to cancer is a result of the accumulation of genetic variation and a combination often environmental exposure. The genetic susceptibility to cancer may be attributed to the SNP of major genetic pathways. And genetic susceptibility to cancer has been a research focus on scientific community. HER2 gene variants in the etiology of breast cancer have drawn increasing attention. Some studies have attempted to discover a possible association between the HER2 Ile655Val polymorphism and the risk of breast cancer in population. It is possible that point HER2 Ile655Val polymorphism might be found in breast cancer.
In subgroup analysis, we found that HER2 Ile655Val polymorphism was significantly correlated with breast cancer in overall in additive and dominant models. The Val allele causes an increase susceptibility to breast cancer in worldwide populations, especially in Caucasian. However, we did not found Ile allele and Ile-carriers such association risk in African nor Asian. Maybe several factors contribute to these. Fist, the presence of Val in 655position stabilized the formation of an active dimer of the protein, thus predisposing to an auto-activity of the receptor (Fleishman et al., 2002). Second, clinical heterogeneity contribute to the discrepancy in different. Third, our dates did not evaluate enough African subgroup for lack of studies.
In conclusion, this meta-analysis suggests that allele contrast (Val vs Ile) of HER2 Ile655Val polymorphism is associated with the breast cancer risk, especially in Caucasian subgroups. Future well designed large studies might be necessary to validate this association in different populations incorporated with environmental factors in the susceptibility of singleness cancer.