Abstract

Objective:

To evaluate whether the presence of polymorphisms of peroxisome proliferator-activated receptor gamma PPARγ (Pro 1 2Ala) and PPARGC1B (Ala203Pro) modifies the association between the inorganic arsenic (iAs) methylation capacity and breast cancer (BC).

Materials and methods:

Mexican women were interviewed, and blood and urine samples were collected from them (cases/controls= 197/220). The concentration of urinary arsenic species and the polymorphisms of interest were determined by high-performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) and polymerase chain reaction (PCR), respectively.

Results:

In women with a high %MMA (urinary monomethyl arsenic) and high primary methylation ratio (PM = MMA/iAs), the risk of BC was increased (odds ratio [OR]%_{MMA T3 vs.T1}= 3.60: 95% confidence interval [CI] 2.02-6.41, OR_{PMI T3 vs.T1}= 3.47: 95%CI 1.95-6.17), which was maintained after adjusting for polymorphisms. No significant interactions were observed between the polymorphisms and the arsenic variables on the risk of BC.

Conclusion:

Pro 12Ala and Ala203Pro polymorphisms did not modify the association between the iAs methylation capacity and BC.

Keywords:
breast cancer; arsenic; genetic polymorphisms; PPARγ; Mexico

Resumen

Objetivo:

Evaluar si la presencia de polimorfismos de PPARγ (Pro 1 2Ala) y PPARGC1B (Ala203Pro) modifica la asociación entre la capacidad de metilación del arsénico inorgánico (Asi) y el cáncer de mama (CM).

Material y métodos:

Se entrevistaron mujeres mexicanas y recolectaron muestras de sangre y orina de (casos/controles=197/220). La concentración de especies de arsénico urinario y los polimorfismos de interés se determinaron mediante cromatografía líquida de alta resolución acoplada a espectrometría de masas (HPLC-ICP-MS) y reacción en cadena de la polimerasa (PCR), respectivamente.

Resultados:

En mujeres con %MMA (monometilarsénico urinario) y razón de primera metilación altas (PM=MMA/Asi) se incrementó el riesgo de CM (RM%MMAT3vsT1=3.60: intervalo de confianza [IC]95%2.02-6.41, RMPMT3vs.T1=3.47:IC95%1.95-6.17), que se mantuvo, respectivamente, al ajustar por polimorfismos. No se observaron interacciones significativas entre los polimorfismos y las variables arsenicales sobre el riesgo de CM.

Conclusión:

Los polimorfismos Pro 12Ala y Ala203Pro no modificaron la asociación entre la capacidad de metilación del Asi y el CM.

Palabras clave:
cáncer de mama; arsénico; polimorfismos genéticos; PPARγ; México

The activation of peroxisome proliferator-activated receptor gamma (PPARγ) consistently induces apoptosis, inhibits cellular proliferation, and promotes cellular differentiation in both hormone-dependent and hormone-independent mammary tumors.¹1. Tachibana K, Yamasaki D, Ishimoto K, Doi T. Review article: The role of PPARs in cancer. Hindawi Publishing Corporation 2008;1-15.

2. Mueller E, Sarraf P, Tontonoz P, Evans RM, Martin KJ. Terminal differentiation of human breast cancer through PPAR?. Mol Cell 1998;1:465-470.

3. Wu M, Chu C, Chou Y, Chou W, Yang T, Hsu G et al. Joint effect of peroxisome proliferator-activated receptor gamma genetic polymorphisms and estrogen-related risk factor on breast cancer risk: results from a case-control study in Taiwan. Breast Cancer Res Treat 2011;127:777-784.

4. Grommes C, Landreth GE, Heneka MT. Antineoplastic effects of peroxisome proliferator-activated receptor gamma agonists. Lancet Oncol 2004;5:419-429.^-55. Dong J. Anticancer activities of PPAR gamma in breast cancer are context-dependent. Am J Pathol 2013;182:1972-1973.PPARγ is a ligand-dependent transcription factor that belongs to the superfamily of nuclear hormone receptors (estrogen, progesterone). Ligands modulate PPARγ functions, thereby releasing receptor corepressors and permitting binding of coactivators, which facilitates its transactivation.⁶6. Tontonoz P, Spiegelman B. Fat and beyond: the diverse biology of PPAR gamma. Annu Rev Biochem 2008;77:289-312. An important gene in said transactivation of PPARγ is PPARγ coactivator 1 beta (PPARGC1B), which can also bind to and activate the α and β estrogen receptors (ERs) in vitro in a ligand-independent manner, with greater affinity for ERβ.⁷7. Wirtenberger M, Tchatchou S, Hemminki K, Schmutahard J, Sutter C, Schumutzler R, et al. Associations of genetic variants in the estrogen receptor coactivators PPARGC1A, PPARGC1B y EP300. Carcinogenesis 2006;27:2201-2208.

Genetic variants of these genes, Pro12Ala of the PPARγ gene and Ala203Pro of the PPARGC1B gene, have been evaluated in relation to the risk of breast cancer (BC) in previous epidemiological studies. Evidence that the Pro12Ala polymorphism of the PPARγ gene is related to BC is contradictory. In two studies conducted, one in Taiwanese women and the other in the cohort of the Nurses' Health Study, no significant associations between the Pro12Ala variant and BC were found.³3. Wu M, Chu C, Chou Y, Chou W, Yang T, Hsu G et al. Joint effect of peroxisome proliferator-activated receptor gamma genetic polymorphisms and estrogen-related risk factor on breast cancer risk: results from a case-control study in Taiwan. Breast Cancer Res Treat 2011;127:777-784.^,88. Memisoglu A, Hankinson SE, Manson JE, Colditz JA, Hunter DJ. Lack of association of the codon 12 polymorphism of the peroxisome proliferator-activated receptor gamma genne with breast cancer and body mass. Pharmacogenetics 2002;12:597-603. In contrast, in a prospective study in the USA, a marginally significant increased risk of BC was detected in homozygous carriers of the G allele.⁹9. Wang Y, McCullough M, Stevens V, Rodriguez C, Jacobs E, Teras L, et al. Nested case-control study of energy regulation candidate gene single nucleotide polumorhisms and breast cancer. Anticancer Res 2007;27:589-593. For its part, the Ala203Pro polymorphism has scarcely been studied in relation to BC. Wirtenberger and colleagues reported an increased risk of familial BC for the carriers of the GC and CC genotypes of Ala203Pro in a German cohort.⁷7. Wirtenberger M, Tchatchou S, Hemminki K, Schmutahard J, Sutter C, Schumutzler R, et al. Associations of genetic variants in the estrogen receptor coactivators PPARGC1A, PPARGC1B y EP300. Carcinogenesis 2006;27:2201-2208. Li and colleagues, although not evaluating Ala203Pro, found an increased risk of ER-positive BC in Swedish and Finnish women who carried risk alleles of several polymorphisms of PPARGC1B.¹⁰10. Li Y, Li Y, Wedren S, Li G, Charn TH, Desai KV, et al. Genetic variation of ESR1 and its co-activator PPARGC1B is synergistic in augmenting the risk of estrogen receptor-positive breast cancer. Breast Cancer Res 2011;13:1-12. Our research group confirmed the absence of a significant association between the Pro12Ala variant of the PPARγ gene and BC and detected a marginally significant negative association in women who were heterozygous for the Ala203Pro polymorphism of the PPARGC1B gene.¹¹11. Martínez-Nava GA, Burguete-García AI, López-Carrillo L, Hérnandez-Ramírez RU, Madrid-Marina V, Cebrián ME. PPAR? y PPARG1CB polymorphisms modify the association between phthalate metabolites and breast cancer risk. Biomarkers 2013;18:493-501.

Recently, our research group has also identified that the adult female residents in northern Mexico, who are exposed to inorganic arsenic (iAs) principally through drinking water¹²12. Cebrián ME, Albores A, García-Vargas G, Del Razo LM. Chronic arsenic poisoning in humans. The case of Mexico. Arsenic in the environment part II: Human health and ecosystem effects. New York: John Wiley & Sons, 1994. and food,¹³13. Del Razo LM, García-Vargas GG, García-Salcedo J, Sanmiguel MF, Rivera M, Hernández MC, Cebrián ME. Arsenic levels in cooking food and assessment of adult dietary intake of arsenic in the Region Lagunera, Mexico. Food Chem Toxicol 2002; 40:1423-1431. with less capacity for urinary elimination, evaluated through the percentage of elimination of monomethyl arsenic (%MMA), presented an increased risk of BC (Odd Ratio [OR]%MMA_Q5vs.Q1= 2.63; 95% confidence interval [95%CI] 1.89-3.66; p for trend < 0.001).¹⁴14. López-Carrillo L, Hérnandez-Ramírez RU, Gandolfi AJ, Ornelas-Aguirre JM, Torres-Sánchez L, Cebrián ME. Arsenic methylation capacity is associated with breast cancer in northern Mexico. Toxicological and Applied Pharmacology 2014;280:53-59.^,1515. Torres-Sánchez LE, Rojas-Martínez R, Escamilla-Núñez C, de la Vara-Salazar E, Lazcano-Ponce E. Tendencias en la mortalidad por cáncer en México de 1980 a 2011. Salud Publica Mex 2014;56:473-491. While the mechanisms by which iAs could be a cofactor of BC have not been clearly elucidated, it has been observed that sodium arsenite mimics the effects of estradiol in the BC cell line MCF-7, inducing cellular proliferation, which, in turn, is a risk factor for this type of cancer.¹⁶16. Ruiz-Ramos R, López-Carrillo L, Ríos-Pérez AD, Vizcaya-Ruíz AD, Cebrián ME. Sodium arsenate induces ROS generation, DNA oxidative damage, HO-I and c-Myc proteins, NF-kB activation and cell proliferation in human breast cancer MCF-7 cells. Elsevier 2009;674:109-115. Additionally, during in vitro tests, iAs interferes with adipogenesis signaling by reducing the expression of PPARγ¹⁷17. Wauson E, Langan A, Vorce R. Sodium arsenite inhibits and reserves expresion of adipogenic and fat cell-specific genes during in vitro adipogenesis. Toxicol Sci 2002;65:211-219.^,1818. Yadav S, Anbalagan M, Shi Y, Wang F, Wang H. Arsenic inhibits the adipogenic differentiation of mesenchymal stem cells by down-regulating peroxisome proliferator-activated receptor gamma and CCAAT enhancer-binding proteins. Toxilogic in vitro 2012;27:211-219. and consequently affects the balance of cellular proliferation and differentiation in breast, colon, and prostate tumors.¹⁷17. Wauson E, Langan A, Vorce R. Sodium arsenite inhibits and reserves expresion of adipogenic and fat cell-specific genes during in vitro adipogenesis. Toxicol Sci 2002;65:211-219. Additionally, iAs can act as an endocrine disruptor and can thus be considered related to BC by means of the overexpression of aromatase, an enzyme that limits the rate of estrogen synthesis.¹⁹19. Xu Y, Tokar E, Waalkes M. Arsenic-induced cancer cell phenotype in human breast epithelia is strogen receptor-independent but involves aromatase activation. Arch Toxicol 2013;88:263-274. The expression of this enzyme can be inhibited by PPARγ activation via the overexpression of BRCA1 (tumor suppressor gene) and the inhibition of PGE2 (proinflammatory prostaglandin E2).²⁰20. Margalit O, Wang D, DuBois RN. PPAR? agonists target aromatase via both PGE2 and BRCA1. Cancer Pres Res 2012;5:1169-1172.

In accordance with the preceding findings, the objective of this study is to evaluate whether the previously observed association between the methylation capacity of iAs and BC is modified according to the presence of the Pro12Ala polymorphism of the PPARγ gene and the Ala203Pro polymorphism of the PPARGC1B gene in Mexican women, considering that BC is a major problem not only in Mexico but also in Latin America.²¹21. Amadou A, Torres-Mejía G, Hainaut P, Romieu I. Brest cancer in Latin America: global burden, patterns, and risk factors. Salud Publica Mex 2014;56:547-554.

Materials and methods

Study population

During the period from 2007 to 2011, a case-control study was conducted in five northern Mexico states (Chihuahua, Coahuila, Sonora, Durango, and Nuevo León) where the incidence of BC is higher than elsewhere in the country.¹⁴14. López-Carrillo L, Hérnandez-Ramírez RU, Gandolfi AJ, Ornelas-Aguirre JM, Torres-Sánchez L, Cebrián ME. Arsenic methylation capacity is associated with breast cancer in northern Mexico. Toxicological and Applied Pharmacology 2014;280:53-59. For the purposes of this report, the first 197 cases and 220 recruited controls were included.

The eligibility criteria of the cases were the following: women with histologically confirmed BC, a minimum age of 18 years, no history of any other type of cancer, and a minimum of one year of residence in the study area. The patients were identified in hospital units (n = 17) of the Mexican Institute of Social Security (Instituto Mexicano del Seguro Social - IMSS), Secretary of Health, University Hospitals, and the Institute of Security and Social Services for State Workers (Instituto de Seguridad y Servicios Sociales de los Trabajadores del Estado - ISSSTE) located in those states. The response rate was 94.8%.

The controls were healthy women without a history of cancer who were matched with the index case by age (±5 years) and place of residence. Controls were identified in the five aforementioned states, by means of the master sampling frame of the System of National Health Surveys of Mexico, composed of basic geostatistical areas (BGAs) with square-shaped (20-80) regions in urban areas and tracts of approximately 10 000 hectares with identifiable natural limits in rural areas. The BGAs were randomly selected and, within them, the squares, in which the domiciles were located systematically to identify eligible women. When an eligible woman was not found in the domicile or was not available to participate, a new one was located; however, if there was more than one eligible woman, one of them was selected randomly. The study was approved by the ethics committee of the National Institute of Public Health.

Interview

After signing the informed consent, participants were directly interviewed regarding their reproductive characteristics and their consumption of tobacco and alcohol, among other variables of interest in the original study. Anthropometric measurements were also obtained for calculating the body mass index [BMI = body weight (kg)/height (m²⁾].

Biological samples

The women donated a venous blood sample and a first morning urine sample (prior to any treatment for cancer). Urine samples were collected in sterile, latex-free, polypropylene containers and were stored at -20°C in the Department of Toxicology of the Center for Research and Advanced Studies (Centro de Investigaciones y Estudios Avanzados - CINVESTAV) of the National Polytechnic Institute until dispatched for analysis at the University of Arizona. The blood samples were stored at -70°C in the Genetic Epidemiology laboratory of the Center for Research on Infectious Diseases (Centro de Investigación Sobre Enfermedades Infecciosas -CISEI) of the National Institute of Public Health (Instituto Nacional de Salud Pública- INSP) until their analysis.

Determination of arsenic concentrations

The concentrations (µg/L) of the urinary species arsenite (As³⁺⁾, arsenate (As⁵⁺⁾, monomethylarsenate (MMA⁵⁺⁾, dimethylarsenate (DMA⁵⁺⁾, and arsenobetaine (AsB), were determined by high-performance liquid chromatography with inductively coupled plasma mass spectrometry (HPLC-ICP-MS) in the Analytical Section of the Center for Risk Identification at the University of Arizona, according to the methodology described by Gilbert-Diamond and colleagues.²²22. Gilbert-Diamond D, Cottingham KL, Gruber JF, Punshon T, Sayarath V, Gandolfi AJ, et al. Rice consumption contributes to arsenic exposure in US women. PNAS 2011;108: 20656-20660. The measurements that were below the limit of detection (LOD) were attributed with the corresponding LODs (AsB: 0.08; As³⁺: 0.15; As⁵⁺: 0.41; MMA⁵⁺: 0.12; DMA⁵⁺: 0.12) divided by two (LOD/2), in accordance with the suggestion of Barr and colleagues.²³23. Barr D, Landsittel D, Nishioka M, Thomas K, Curwin T, Raymer J, et al. A Survey of Laboratory and Statistical Issues Related to Farmworker Exposure Studies. Environ Health Perspect 2006;114:961-968. These proportions were similar between the cases and the controls (table I). The urinary creatinine concentration (mg/dL) was determined with a commercial kit with 1 mg/dL as the detection limit, according to the methodology described by the manufacturer.²⁴24. Randox. Manual RX Monza Técnicas Randox [internet monography]. Reino Unido, 2010 [consulted 2014 October] Available at: http://sistemainterno.com/web/wp-content/themes/aaaclientesflash/gaamsa2011/pdf/MANUAL_QC_2013.pdf.
http://sistemainterno.com/web/wp-content...

The iAs was obtained from the sum of the concentrations of the urinary species As³⁺ and As⁵⁺, while the total arsenic (AsT) was estimated based on the sum of the concentrations of iAs, MMA⁵⁺, DMA⁵⁺, and AsB. The proportions of the urinary iAs species (%iAs, %MMA, and %DMA) were obtained by dividing the concentration of each species by the difference between AsT and AsB (AsT-AsB). The following methylation ratios were calculated: primary methylation= MMA/iAs, secondary methylation= DMA/MMA, and total methylation= DMA/iAs.

Determination of the Pro12Ala polymorphism of the PPARγ gene and the Ala203Pro polymorphism of the PPARGC1B gene

DNA was extracted from the blood samples by means of a semi-automated method using an ABI PRISM 6100 Nucleic Acid PrepStation. The purity and integrity of the samples were evaluated using UV spectrophotometry and 0.8% agarose gel electrophoresis.¹¹11. Martínez-Nava GA, Burguete-García AI, López-Carrillo L, Hérnandez-Ramírez RU, Madrid-Marina V, Cebrián ME. PPAR? y PPARG1CB polymorphisms modify the association between phthalate metabolites and breast cancer risk. Biomarkers 2013;18:493-501.

The selected polymorphisms, rs1801282 (Pro12Ala) and rs7732671 (Ala203Pro), were analyzed by allelic discrimination by means of end-point PCR with TaqMan probes in an Applied Biosystems 7900 Real-Time PCR System. All of the assays were performed in duplicate and were determined and analyzed using <<Sequencing Detection System>> software (SDS 2.3, Applied Biosystems).

Statistical analysis

The age at menarche, parity, age at first pregnancy, and lactation throughout life presented normal distributions and were compared between the cases and controls with Student's t-test, while the BMI, the distributions of the arsenic species, the methylation ratios and proportions were normalized with a logarithmic transformation. The consumption of tobacco and alcohol could not be normalized, so the medians that were compared with the Mann-Whitney U test are presented. The genotype frequencies of interest were evaluated using the Hardy-Weinberg test.

By means of unconditional logistic regression models, an odds ratio (OR) was calculated with a confidence interval of 95% (95%CI) for each of the urinary arsenic species proportions and methylation ratios with the BC. These variables were categorized into tertiles according to the distribution in the controls. Two multiple models were considered: 1) adjusted for AsT-AsB, AsB, and normalized creatinine and the original variables of age (years), age at 1st pregnancy (years, categorized), lactation throughout life (months), BMI (kg/m²⁾, tobacco (packs per year), and alcohol (g per week); and 2) further adjusted by the polymorphisms of interest. Included as confounding factors were the covariables that modified the OR between the iAs metabolites and the BC.¹⁴14. López-Carrillo L, Hérnandez-Ramírez RU, Gandolfi AJ, Ornelas-Aguirre JM, Torres-Sánchez L, Cebrián ME. Arsenic methylation capacity is associated with breast cancer in northern Mexico. Toxicological and Applied Pharmacology 2014;280:53-59.

The multiple model, adjusted for the covariables mentioned, was stratified by genotypes grouped according to the dominant model of the genes of interest (GG+GC vs. CC and GC+CC vs. GG for PPARγ Pro12Ala and PPARGC1B Ala203Pro, respectively). It was not possible to evaluate the co-dominant and recessive models due to the reduced numbers of women homozygous for mutations in both genes. Additionally, to evaluate the interactions between the polymorphisms and the arsenic variables, the multiplicative term, genxAs, which consisted of the genotypes in the dominant model (AB+BB vs. AA) and the tertiles of each arsenic species, was added to the multiple models. A value ≤ 0.20 was established as the level of statistical significance. The data analysis was performed using the statistical package Stata 12.0 (StataCorp, College Station, TX, USA).

Results

Table I shows that by the design of the study, the mean age was similar between cases and controls (53.14 vs. 53.86 years). As for the reproductive characteristics, the cases had significantly lower ages at menarche, parity, and lactation throughout life and older age at first pregnancy compared with the controls. With respect to BMI, consumption of tobacco and of alcohol, no statistically significant differences between the cases and the controls were observed.

Significant differences in the concentrations of iAs, DMA, AsT, and AsT-AsB adjusted for creatinine were observed between the cases and the controls but not for MMA and AsB. Compared with the controls, the %MMA (11.00 vs 9.09) and the primary methylation (1.16 vs 0.88) were significantly higher in the cases. Contrary to this finding, the secondary methylation was significantly lower in the cases than in the controls (7.02 vs 8.57). No significant differences in %iAs, %DMA, the total methylation ratio, and the urinary creatinine levels were observed. The observed distributions of the genotype frequencies were determined to be in Hardy-Weinberg equilibrium for both polymorphisms.

Table II shows that women with high urinary %MMA showed increased risk of BC that remained after adjusting for the polymorphisms of interest (OR_T3vs.T1= 3.57, 95%CI 1.99-6.38; p for trend < 0.001), which was consistent with the OR observed for primary methylation (OR_T3vs.T1= 3.51, 95%CI 1.96-6.28; p for trend <0.001). In contrast, a decreased risk of BC was observed for those women who had high urinary %iAs (OR_T3vs.T1= 0.50, 95%CI 0.28-0.88; p for trend = 0.012) and higher values of secondary methylation (OR_T3vs.T1= 0.33, 95%CI 0.19-0.59; p for trend <0.001). All of the aforementioned results were similar when considering only the observations with detectable levels of the respective arsenic variables (data not shown).

Table III shows that no statistically significant interactions were obtained between the polymorphisms of interest as well as the arsenic variables and the risk of BC.

Discussion

According to our results, polymorphisms of the gene PPARγ Pro12Ala and of its coactivator PPARGC1B Ala203Pro do not modify the association between methylation capacity of iAs and BC, demonstrating that the association between the methylation capacity of iAs and BC (reported in a previous study)¹⁴14. López-Carrillo L, Hérnandez-Ramírez RU, Gandolfi AJ, Ornelas-Aguirre JM, Torres-Sánchez L, Cebrián ME. Arsenic methylation capacity is associated with breast cancer in northern Mexico. Toxicological and Applied Pharmacology 2014;280:53-59. was maintained after adjusting for these polymorphisms.

The association between polymorphism Pro12Ala of PPARγ and BC is inconclusive. The results of a recent meta-analysis suggest that the carriers of the GG+CG genotype may be less susceptible to developing this tumor (OR = 0.85, 95%CI 0.73-0.98).²⁵25. Mao Q, Guo H, Gao L, Wang H, Ma X. Peroxisome proliferator-activated receptor ?2 Pro12Ala (rs1801282) polymorphism and breast cancer susceptibility: a meta-analysis. Mol Med Rep 2013;8:1773-1778. This finding is consistent with previous observations in Danish women that carriers of the G allele exhibited a decreased risk of BC.²⁶26. Vogel U, Christensen J, Nexø BA, Wallin H, Friis S,Tjønneland A. Peroxisome profilerator-activated receptorg2 Pro12Ala, interaction with alcohol intake and NSAID use, in relation to risk of breast cancer in a prospective study of Danes. Carcinogenesis 2006;28:427-434. In contrast, an increased risk of BC was identified in North American women homozygous for the G allele (OR = 2.91, 95%CI 1.05-8.04).⁹9. Wang Y, McCullough M, Stevens V, Rodriguez C, Jacobs E, Teras L, et al. Nested case-control study of energy regulation candidate gene single nucleotide polumorhisms and breast cancer. Anticancer Res 2007;27:589-593. In our study, we observed a proportion (11.64%) of carriers of the G allele in the control group, close to the frequency reported in the Mexican population,²⁷27. Black MH, Fingerlin TE, Allayee H, Zhang W, Xiang AH, Trigo E, et al. Evidence of interaction between PPARG2 and HNF4A contributing to variation in insulin sensitivity in Mexican Americans. Diabetes 2008;57:1048-1056. and, as in other studies, we did not find an association between these polymorphisms and BC, even after adjusting for exposure to iAs (ORCG+GGvs.CC = 0.90, 95%CI 0.56-1.43) (data not included in the tables).

Regarding polymorphism Ala203Pro of the coactivator PPARGC1B, very few studies have evaluated its connection to the risk of BC, and the results have been inconsistent. In a population of German women with mutations negative for BRCA1 and BRCA2 and with familial BC, an increased risk of BC was observed in the carriers of the CC and CG genotypes.⁷7. Wirtenberger M, Tchatchou S, Hemminki K, Schmutahard J, Sutter C, Schumutzler R, et al. Associations of genetic variants in the estrogen receptor coactivators PPARGC1A, PPARGC1B y EP300. Carcinogenesis 2006;27:2201-2208. While these women are a highly selected study population, as the frequency of BC by mutations in BRCA1 and BRCA2 is only 5 to 10% at the population level, and are not necessarily comparable with the women studied,²⁸28. Weitzel J, Lagos V, Blazen K, Nelson R, Ricker C, Herzog J, et al. Prevalence of BRCA mutations and founder effect in high-risk hispanic families. Cancer Epidemiol Biomarkers Prev 2005;14:1666-1671. the results are opposite to those observed in this study, even after adjusting for the methylation capacity of iAs.

In the stratified models, according to the dominant model of the genotypes of interest, non-significant differences were observed between the ORs of the arsenic variables with BC and the heterozygous and homozygous mutations compared with the ancestral homozygotes. This finding may suggest that the risk of BC is modified in some genetic subgroups, especially at high levels of exposure to iAs.

Moreover, our results show that women with high %MMA and higher values of the primary methylation ratio presented an increased risk of BC that remained after adjusting for the polymorphisms of interest, in contrast with the negative association observed for those women who had higher values of secondary methylation. This finding confirms the evidence regarding the monomethylated metabolite of iAs (MMA) that has been consistently associated with increased risks of various types of cancer, in contrast to the dimethylated forms (expressed in higher values of secondary methylation), which have shown inverse relations.²⁹29. Hsueh M, Chiou HY, Huang YL, Wu WL, Huang CC, Yang MH, et al. Serum beta-carotene level, arsenic methylation capability, and incidence of skin cancer. Cancer Epidemiol Biomarkers Prev 1997;6:589-596

30. Yu RC, Hsu KH, Chen CJ, Froines JR. Arsenic methylation capacity and skin cancer. Cancer Epidemiol Biomarkers Prev 2000;9:1259-1262.

31. Chen YC, Guo YL, Su HJ, Hsueh YM, Smith TJ, Ryan LM, et al. Arsenic methylation and skin cancer risk in southwestern Taiwan. J Occup Environ Med 2003;45:241-248.

32. Chen YC, Su HJ, Guo YL, Hsueh YM, Smith TJ, Ryan LM, et al. Arsenic methylation and bladder cancer risk in Taiwan, Cancer Causes Control 2003;14:303-310.

33. Steinmaus C, Bates MN, Yuan Y, Kalman D, Atallah R, Rey OA, et al. Arsenic methylation and bladder cancer risk in case-control studies in Argentina and the United States. J Occup Environ Med 2003;48:478-488.

34. Pu YS, Yang SM, Huang YK, Chung CJ, Huang SK, Chiu AW, et al. Urinary arsenic profile affects the risk of urothelial carcinoma even at low arsenic exposure. Toxicol Appl Pharmacol 2007; 218:99-106.

35. Huang YK, Huang YL, Hsueh YM, Yang MH, Wu MM, Chen SY, et al. Arsenic exposure, urinary arsenic speciation, and the incidence of urothelil carcinoma: a twelve-year follow up study. Cancer Causes Control 2008;19:829-839.^-3636. Steinmaus C, Yuan Y, Kalman D, Rey OA, Skibola CF, Dauphine D, et al. Individual differences in arsenic metabolism and lung cancer in a case-control study in Cordoba, Argentina. Toxicol Appl Pharmacol 2010;247:138-145.

Our findings should be interpreted with caution, given the strengths and limitations of this study, which include the following: the blind measurement of biomarkers, which decreases the probability of error of differential measurement; the high response rates between cases and controls, which reduce the probability of selection bias; and the small sample size, which reduces the statistical power of the study.

Acknowledgements

This study was funded by the National Science and Technology Council (Consejo Nacional de Ciencia y Tecnología - Conacyt) and the Sectorial Fund for Research in Health and Safety (Fondo Sectorial de Investigación en Salud y Seguridad - FOSISS, 2005-C02-14373 and 2009-01-111384), Mexico.

References

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History