Cancer occurs due to a malfunction in cell growth regulation. Gastric cancer is one the most prevalent types of cancer worldwide and is characterized by abnormal cell growth in the stomach. The XRCC2 gene is associated with DNA repair by homologous recombination and a faulty mechanism can lead to the development of several genetic diseases, such as cancer. Several types of cancer have been associated with polymorphisms in DNA repair genes, such as Arg188His of the XRCC2 gene. This study was aimed at examining the possible involvement of this polymorphism in the development of gastric cancer in patients in the municipality of Macapá-AP. PCR-RFLP analysis was carried out with the HphI enzyme (Haemophilus parahaemolyticus) to detect polymorphism in 30 gastric cancer patients and 55 control individuals. Our results revealed that 43.3% of patients had this polymorphism compared to 18.1% of the control group. Our findings demonstrated that the Arg188His polymorphism was statistically significant and therefore it was associated with gastric cancer in the study population, similar to other polymorphisms of repair genes already linked with this disease in the same population.

Cancer occurs due to a malfunction in cell growth regulation, initially causing a neoplasia or tumor [1]. In gastric cancer, abnormal cell growth may be present throughout the gastroenteric system, usually giving rise to lesions [2]. Studies have examined whether the large number of genetic polymorphisms found in the human genome can be associated with differences in protein expression capable of altering the cellular phenotype, increasing or not the risk of developing several disorders such as benign and malignant neoplasias [3].

During DNA replication, mechanisms of DNA repair act to prevent unintentional changes in the genome. One of them is performed by enzymes produced from genes such as XRCC2 [4,5] located in the long arm of chromosome 7, position 36.1, a member of family of the RAD51 gene [6]. The enzymes synthesized from this gene prevent DNA damage, decrease mutations, replication errors, and genomic instability.

Some polymorphisms occur due to variations in the DNA sequence of a single nucleotide (SNPs - single nucleotide polymorphisms) [7]. The SNP Arg188His in the XRCC2 gene is relatively rare and is located at the nucleotide 31479 in exon 3, where the wild-type G allele is replaced by the mutant A allele, resulting in the substitution of the amino acid arginine (Arg) for histidine (His) [8].

National Cancer Institute (INCA) [9] estimates that 12,920 new cases of gastric cancer in men and 7,600 in women will be diagnosed, indicating the importance of molecular studies on this disease. This study was aimed at examining the possible involvement of the Arg188His polymorphism in the development of gastric cancer in the municipality of Macapa-AP, Brazil.

Biological Samples
The study population was represented by 85 DNA samples, of which 55 comprised the control group from the Institute of Hematology and Hemotherapy of Amapa (HEMOAP) and 30 from gastric cancer patients from the Dr. Alberto Lima Hospital (HCAL).

After individuals signed the Informed Consent Term (ICF), blood samples were collected and sent to the Laboratory of Molecular Biology and Biotechnology (BIOMOL) of the Federal University of Amapá (UNIFAP).

Identification of the XRCC2 Arg188His polymorphism
The protocol developed by Gök et al. [10] was used to identify the Arg188His polymorphism of the XRCC2 gene. PCR was performed with the primer pairs F5'-TGTAGTCACCCATCTCTCTGC-3' and R5'-AGTTGCTGCCATGCCTTACA-3'. The initial denaturation was carried out for 3 min at 95°C, followed by 35 cycles at 94°C for 30s, 57°C for 30s, 72°C for 45s and a final extension of 1 cycle at 72°C for 7 minutes.

The PCR amplification product of 290 bp was visualized in 1.5% agarose gel. The amplified fragment was digested with the restriction enzyme HphI, under the following conditions: 10 ul of the product, 2ul of H2O, 2u of buffer, and 1ul of enzyme in a final volume of 15ul, at 37ºC for 16 hours. After the electrophoresis, the genotypes observed were wild homozygote (290 bp - Arg / Arg), heterozygote (148 bp + 142 bp - Arg / His) or the mutant homozygote (290 bp + 148 bp + 142 bp - His / His).

Statistical Analysis
Allele and genotype frequencies were analyzed with the Hardy-Weinberg equilibrium and the chi-square test (X2) using the software Bio Estat (Ayres, M. Pará, Brazil).

Of the 30 analyzed samples of gastric cancer patients, 43.3% (13) had the Arg188His and 56.6% had theArg188Arg genotypes, while in the 55 controls they accounted for 18.1% and 81.8% of samples, respectively (Table 1).

Regarding genotype frequency, 20% of the patients and 10.9% of the controls had the Arg188His genotype (2.6471 (0.7496-9.3478) p=0.2277). The frequency of the His188His genotype was 23.3% and 7.2% in patients and controls, respectively (4.6324 (1.2018-17.8560) p = 0.0446) (Table 2). Several studies have examined the association of Arg188His polymorphism and the development of several types of cancer. No association has been found with laryngeal cancer11 and variations in the coding region of the XRCC2 gene did not show any susceptibility to thyroid cancer [12,13].

Studies on the association with the development of ovarian cancer reported contrasting results, partly due to the small effect of the polymorphism on the disease and the insufficient amount of samples analyzed [14,15]. On the other hand, studies involving the Arg188His polymorphism and cancer of head and neck in a population of India in 2014 found a highly significant association in individuals with the heterozygous genotype (Arg/His), which was then considered a risk factor for the disease [16]. Studies in France also showed an association between polymorphism and the development of oral and pharyngeal cancer in carriers of the His allele [11]. In patientswith gastric cancer in Turkey, the Arg188His polymorphism was observed in 39% of samples [10].

Our results were statistically significant, although this polymorphism is not directly associated to gastric cancer but to breast cancer [17]. Gastric cancer is one of the leading causes of death in the state of Amapá, with patients developing one of the most aggressive forms of the disease. External factors associated with the onset of this disease in the state are still poorly known. However, the study population has a genetic predisposition for the development of gastric cancer, as the Arg188His polymorphism is associated with this disease. Other DNA repair gene polymorphisms have also been significantly correlated with gastric cancer in this population.

The sample size in the present study was small, but our findings regarding the association of Arg188His polymorphisms with gastric cancer in the study population may still be relevant.

Consent 
All authors declare that written informed consent was obtained from all the patient.

Ethical Approval 
All authors hereby declare that all experiments have been examined and approved by the appropriate ethics committee and have therefore been performed in accordance with the ethical standards laid down in the 1964 Declaration of Helsinki.

Competing Interests
Authors have declared that no competing interests exist.