ARTÍCULO DE REVISÍON

 

Genetics of breast cancer: Applications to the Mexican population

 

 

Elad Ziv, MD

Division of General Internal Medicine, Department of Medicine, Institute for Human Genetics, San Francisco, California; Helen Diller Family Comprehensive Cancer Center, San Francisco, California. Department of Epidemiology and Biostatistics, San Francisco, California, USA

Corresponding author

 

 

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ABSTRACT

Breast cancer research has yielded several important results including the strong susceptibility genes,BRCA1 and BRCA2 and more recently 19 genes and genetic loci that confer a more moderate risk.The pace of discovery is accelerating as genetic technology and computational methods improve. These discoveries will change the way that breast cancer risk is understood in Mexico over the next few decades.

Keywords: breast neoplasms; genetics; BRCA1; BRCA2; oncogenes; Mexico

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RESUMEN

La investigación en cáncer de mama ha dado varios resultados importantes incluyendo los genes fuertemente susceptibles, BRCA1 y BRCA2, y más recientemente 19 genes y loci genéticos que confieren un riesgo moderado. El ritmo de los descubrimientos se acelera conforme mejora la tecnología y métodos computacionales.Estosdescubrimientoscambiarán la forma en que la investigación del cáncer es comprendida en México en las próximas décadas.

Palabras clave: neoplasias de la mama; genética; BRCA1; BRCA2; oncogenes; México

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Evidence of a genetic effect for breast cancer

Family studies and twin studies can suggest a genetic contribution to the risk of breast cancer. Relatives of women with breast cancer have an increased risk of developing the disease,¹ suggesting a genetic effect on risk. Women with one first degree relative with breast cancer have approximately 2-fold higher risk of breast cancer compared to the general population. The risk among women with second degree relatives with breast cancer is also increased ~1.5-fold compared to the general population. Women with multiple affected relatives and women with relatives who are affected at a younger age tend to have higher risk.^2-4

Twin studies are a unique case of family studies in which monozygotic (MZ) twins are compared to dizygotic (DZ) twins. Both MZ and DZ twins share environmental influences but MZ twins share 100% of their genome while DZ twins only share 50% of their genome. Thus, if the risk among women with an affected MZ twin is higher than the risk of women with an affected DZ twin, a genetic effect is strongly suspected to underlie this. Monozygotic twins of women with breast cancer have a higher risk of developing the disease compared with dizygotic twins of women with breast cancer.^5,6 Lichtenstein et al. identified a 13% risk of breast cancer among MZ twins if the other twin had breast cancer, whereas the risk for DZ twins was 9%, a different that was statistically significant. Heritability, the proportion of a trait that is attributable to genetic factors, can be estimated from twin studies. Analyzing data from twin studies, Lichtenstein et al. estimated that heritability for breast cancer is approximately 27%.⁵ However, the estimate of heritability makes many assumptions and these have been criticized to lead to underestimates of the importance of genes.⁷ Other investigators, also using twin data, have argued that a higher proportion of breast cancers attributable to genetic factors.⁶ For example, Peto and Mack compared the risk of breast cancer among MZ twins, DZ twins and the risk of breast cancer in the contralateral breast of an affected woman. They found that the risk of breast cancer in the contralateral breast of an affected woman is the same as the risk in each of the breasts of her MZ twin. Based on this, they concluded that most breast cancers have some genetic effect.

 

Population genetics of Mexico

Mexicans are a complex population genetically.⁸ Historically, Mexico has included an Indigenous population, Spanish migrants and African migrants, originally brought as slaves. Using genetic markers, it is possible to estimate the genetic ancestry of a group or of an individual.⁹ Such estimates are most efficiently made with a set of markers that are selected to be specifically informative for continental ancestry,^10-13 but can also be ascertained from dense whole genome polymorphism data from SNP (single nucleotide polymorphism) arrays.¹⁴ Using these genetic markers, it is possible to see the ancestral components of Indigenous, European and African ancestors in modern day Mexicans and Mexican Americans. For example, a recent study of U.S. "Latinos" in the San Francisco Bay Area including Mexican Americans demonstrated that approximately 55% of the ancestry among Mexican Americans is European, approximately 40% is Indigenous American and approximately 5% is African.¹⁵ However, the proportions of ancestry may vary substantially. For example, in a study of breast cancer in Mexico, the investigators found a much higher proportion, ~60%, of Indigenous American ancestry.¹⁶ Furthermore, the proportions of ancestry may vary dramatically by geographic location and other factors. For example, individuals from Northern Mexico have much higher European ancestry and much lower Indigenous American ancestry compared to those from Southern Mexico.¹⁷

The relationship between breast cancer risk andgenetic ancestry has been investigated in Mexico and in Mexican Americans. Fejerman et al. found that among Mexican women with higher European ancestry there was a significantly higher risk of breast cancercompared to Mexican women with lower European ancestry and high Indigenous American ancestry.¹⁶ No significant association was found with African ancestry. The association between ancestry and risk was attenuated in multivariate regression models by knownnon-genetic risk factors such as reproductive historyand hormone use. Therefore, part of the association between genetic ancestry and breast cancer in Mexico is explained by the prevalence of different non-genetic risk factors among sub-populations within Mexico. However, some of the association between geneticancestry and breast cancer risk remained statistically significant after accounting for known non-genetic risk factors. Furthermore, this association was also seen in U.S. populations of Mexican and Central Americanorigin.¹⁸ Thus, there may be genetic variants that are more common among women with European ancestry that increase the risk of breast cancer.

 

Breast cancer genetics

High risk susceptibility genes

Two high risk genes, BRCA1 and BRCA2, have been identified by linkage analysis.^19,20 Women with mutations in these genes have a very high lifetime risk of breast cancer (~50-80%) and also a dramatically elevated risk of ovarian cancer (~20-40%). However, mutations in these genes are relatively rare and it is estimated that they account for only about 5% of breast cancer cases.²¹

Separately, other genes have been identified to increase breast cancer risk, including p53, PTEN, ATM and others.²² However, mutations in these genes are even less common than mutations in BRCA1 and BRCA2, so they likely account for a very small fractionof breast cancer cases.

High risk susceptibility genes in Mexico

Few studies exist of BRCA1 and BRCA2 mutations have been performed in Mexico. Ruiz-Flores et al. studied 51 Mexican women with breast cancer, including 36 women who developed breast cancer at age < 35.²³ They found 2 disease-causing protein truncating mutations, one in BRCA1 and one in BRCA2, which had not been reported before. Vildal-Millan et al. screened 40 women with breast cancer and either family history of breast and/or ovarian cancer and found two mutations in BRCA1.²⁴ Both studies found additional genetic variants that alter the protein sequence but do not completely abolish the activity and thus may or may not increase the risk of breast cancer. Clearly, additional studies of BRCA1 and BRCA2 in Mexican women would be helpful to further delineate the contribution of these genes.

Additional information can be obtained from studies in the U.S. of breast cancer patients who are classified as "Latina" or "Hispanic." In the Southwestern United States, the majority of women in suchstudies are usually of Mexican American origin and therefore the mutations identified in these women maybe helpful to understanding the risk contribution ofBRCA1 and BRCA2 in Mexico. Several studies have identified a series of BRCA1 mutations in HispanicAmerican women.^25-28 Some of these mutations have also been observed in other populations such as in Spain or in Jewish populations.^25-28 Other mutations have not been observed before and may be of Indigenous American origin.²⁹

Additional data on BRCA1 and BRCA2 mutations in Mexico, as well as data on other high risk susceptibility genes (P53, PTEN, ATM) will be important toobtain in order to understand the contribution of these genes to breast cancer risk in Mexican families withbreast cancer.

Moderate susceptibility alleles

Genome wide association studies (GWAS) have recently been used to identify new loci for breast cancer.^30-37 To date over 19 loci have been identified (Table I). While these loci have modest effects they influence the understanding of breast cancer risk in potentially important ways. First, these genes may help us to understand the underlying biology of breast cancer by implicating new pathways that have not been previously considered to be associated with breast cancer risk. Second, while each polymorphism may not be a strong risk factor for breast cancer, together tehse polymorphisms may help to develop more precise risk information. For example, Wacholder et al. evaluated the proportion of women reclassified into high and low risk categories based on using the Gail model with and without 10 genetic variants.³⁸ They found that 19% of women were classified into a high risk category (>0.575%/year) with just the regular Gail model, but 27.7% were in that category if SNPs were added. Although they concluded that these risk SNPs were not quite ready for use in the general population, as more SNPs associated with breast cancer are identified, risk prediction will continue to improve. Thus, breast cancer prevention may be improved with the results of GWAS.

The initial GWAS scans were focused on populations of mostly European ancestry.^32,39,40 Subsequently, East Asian populations were also included in GWAS scans and have yielded new results.³⁷ In addition, there is an ongoing effort to map loci in African Americans.^41,42

To date, very little work has been published on the Mexican population and the effect of these common variants and breast cancer. However, in U.S. Latinas, it appears that at least some of them also predict risk.⁴³ More data is needed on how these risk variants affect risk in Mexican women. In addition, there may be additional polymorphisms that affect risk that may be discovered in Mexican women.

 

New approaches to identifying and characterizing breast cancer susceptibility genes

With the ongoing development of genetic technology, it is becoming feasible now to sequence the entire genomes of individual patients.^44,45 As the feasibility of this technology improves, it will become possible to obtain more data from breast cancer cases that may ultimately help to identify new genes. In addition, sequencing of the tumor and understanding the relationship between the genetics of the tumor and the genetics of the patient in which it developed may also help to identify genes that increase the risk of cancer. Such approaches have recently yielded results in identifying PALB2 which is a risk factor in pancreatic cancer⁴⁶ and breast cancer.^47,48 Large scale tumor sequencing efforts such as the Cancer Genome Atlas project may help to identify genes involved in cancer susceptibility and progression.⁴⁹

Declaration of conflict of interests: The author declares not to have conflict of interests.

 

References

1. Pharoah PD, Day NE, Duffy S, Easton DF, Ponder BA. Family history and the risk of breast cancer: a systematic review and meta-analysis. Int J Cancer 1997;71(5):800-9.         [ Links ]

2. Claus EB, Risch N,Thompson WD. Genetic analysis of breast cancer in the cancer and steroid hormone study.Am J Hum Genet 1991;48(2):232-42.         [ Links ]

3. Claus EB, Risch NJ,Thompson WD. Using age of onset to distinguish between subforms of breast cancer.Ann Hum Genet 1990;54(Pt 2):169-77.         [ Links ]

4. Claus EB, Risch NJ,Thompson WD.Age at onset as an indicator of familial risk of breast cancer.Am J Epidemiol 1990;131(6):961-72.         [ Links ]

5. Lichtenstein P, Holm NV,Verkasalo PK, Iliadou A, Kaprio J, Koskenvuo M, et al. Environmental and heritable factors in the causation of cancer-- analyses of cohorts of twins from Sweden, Denmark, and Finland. N Engl J Med 2000;343(2):78-85.         [ Links ]

6. Peto J, Mack TM. High constant incidence in twins and other relatives of women with breast cancer. Nat Genet 2000;26(4):411-4.         [ Links ]

7. Risch N.The genetic epidemiology of cancer: interpreting family and twin studies and their implications for molecular genetic approaches. Cancer Epidemiol Biomarkers Prev 2001;10(7):733-41.         [ Links ]

8. Burchard EG, Borrell LN, Choudhry S, Naqvi M,Tsai HJ, Rodriguez-Santana JR, et al. Latino Populations:A Unique Opportunity for the Study of Race, Genetics, and Social Environment in Epidemiological Research. Am J Public Health 2005.         [ Links ]

9. Collins-Schramm HE, Chima B, Morii T,Wah K, Figueroa Y, Criswell LA, et al. Mexican American ancestry-informative markers: examination of population structure and marker characteristics in European Americans, Mexican Americans,Amerindians and Asians. Hum Genet 2004;114(3):263-71.         [ Links ]

10. Mao X, Bigham AW, Mei R, Gutierrez G,Weiss KM, Brutsaert TD, et al. A genomewide admixture mapping panel for Hispanic/Latino populations. Am J Hum Genet 2007;80(6):1171-8.         [ Links ]

11. Martinez-Marignac VL,Valladares A, Cameron E, Chan A, Perera A, Globus-Goldberg R, et al.Admixture in Mexico City: implications for admixture mapping of type 2 diabetes genetic risk factors. Hum Genet 2007;120(6):807-19.         [ Links ]

12. Bonilla C, Gutierrez G, Parra EJ, Kline C, Shriver MD.Admixture analysis of a rural population of the state of Guerrero, Mexico.Am J Phys Anthropol 2005;128(4):861-9.         [ Links ]

13. Halder I, Shriver MD. Measuring and using admixture to study the genetics of complex diseases. Hum Genomics 2003;1(1):52-62.         [ Links ]

14. Bryc K,Velez C, Karafet T, Moreno-Estrada A, Reynolds A,Auton A, et al. Colloquium paper: genome-wide patterns of population structure and admixture among Hispanic/Latino populations. Proc Natl Acad Sci USA 107 Suppl 2:8954-61.         [ Links ]

15. Ziv E, John EM, Choudhry S, Kho J, Lorizio W, Perez-Stable EJ, et al. Genetic ancestry and risk factors for breast cancer among Latinas in the San Francisco Bay Area. Cancer Epidemiol Biomarkers Prev 2006;15(10):1878-85.         [ Links ]

16. Fejerman L, Romieu I, John EM, Lazcano-Ponce E, Huntsman S, Beckman KB, et al. European ancestry is positively associated with breast cancer risk in Mexican women. Cancer Epidemiol Biomarkers Prev 19(4):1074-82.         [ Links ]

17. Gorodezky C,Alaez C,Vazquez-Garcia MN, de la Rosa G, Infante E, Balladares S, et al.The genetic structure of Mexican Mestizos of different locations: tracking back their origins through MHC genes, blood group systems, and microsatellites. Hum Immunol 2001;62(9):979-91.         [ Links ]

18. Fejerman L, John EM, Huntsman S, Beckman K, Choudhry S, Perez-Stable E, et al. Genetic ancestry and risk of breast cancer among U.S. Latinas. Cancer Res 2008 1;68(23):9723-8.         [ Links ]

19.Wooster R, Bignell G, Lancaster J, Swift S, Seal S, Mangion J, et al. Identification of the breast cancer susceptibility gene BRCA2. Nature 1995;378(6559):789-92.         [ Links ]

20. Miki Y, Swensen J, Shattuck-Eidens D, Futreal PA, Harshman K,Tavtigian S, et al.A strong candidate for the breast and ovarian cancer susceptibility gene BRCA1. Science 1994;266(5182):66-71.         [ Links ]

21. Pharoah PD,Antoniou A, Bobrow M, Zimmern RL, Easton DF, Ponder BA. Polygenic susceptibility to breast cancer and implications for prevention. Nat Genet 2002;31(1):33-6.         [ Links ]

22.Walsh T, King MC.Ten genes for inherited breast cancer. Cancer Cell 2007;11(2):103-5.         [ Links ]

23. Ruiz-Flores P, Sinilnikova OM, Badzioch M, Calderon-Garciduenas AL, Chopin S, Fabrice O, et al. BRCA1 and BRCA2 mutation analysis of early-onset and familial breast cancer cases in Mexico. Hum Mutat 2002;20(6):474-5.         [ Links ]

24.Vidal-Millan S,Taja-Chayeb L, Gutierrez-Hernandez O, Ramirez Ugalde MT, Robles-Vidal C, Bargallo-Rocha E, et al. Mutational analysis of BRCA1 and BRCA2 genes in Mexican breast cancer patients. Eur J Gynaecol Oncol 2009;30(5):527-30.         [ Links ]

25. Mullineaux LG, Castellano TM, Shaw J,Axell L,Wood ME, Diab S, et al. Identification of germline 185delAG BRCA1 mutations in non-Jewish Americans of Spanish ancestry from the San Luis Valley, Colorado. Cancer 2003;98(3):597-602.         [ Links ]

26. Nanda R, Schumm LP, Cummings S, Fackenthal JD, Sveen L,Ademuyiwa F, et al. Genetic testing in an ethnically diverse cohort of high-risk women: a comparative analysis of BRCA1 and BRCA2 mutations in American families of European and African ancestry. JAMA 2005;294(15):1925-33.         [ Links ]

27. John EM, Miron A, Gong G, Phipps AI, Felberg A, Li FP, et al. Prevalence of pathogenic BRCA1 mutation carriers in 5 US racial/ethnic groups. JAMA 2007;298(24):2869-76.         [ Links ]

28.Weitzel JN, Lagos V, Blazer KR, 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(7):1666-71.         [ Links ]

29.Weitzel JN, Lagos VI, Herzog JS, Judkins T, Hendrickson B, Ho JS, et al. Evidence for common ancestral origin of a recurring BRCA1 genomic rearrangement identified in high-risk Hispanic families. Cancer Epidemiol Biomarkers Prev 2007;16(8):1615-20.         [ Links ]

30.Turnbull C,Ahmed S, Morrison J, Pernet D, Renwick A, Maranian M, et al. Genome-wide association study identifies five new breast cancer susceptibility loci. Nat Genet 42(6):504-7.         [ Links ]

31. Easton DF, Pooley KA, Dunning AM, Pharoah PD,Thompson D, Ballinger DG, et al. Genome-wide association study identifies novel breast cancer susceptibility loci. Nature 2007;447(7148):1087-93.         [ Links ]

32. Hunter DJ, Kraft P, Jacobs KB, Cox DG,Yeager M, Hankinson SE, et al. A genome-wide association study identifies alleles in FGFR2 associated with risk of sporadic postmenopausal breast cancer. Nat Genet 2007 Jul;39(7):870-4.         [ Links ]

33. Gold B, Kirchhoff T, Stefanov S, Lautenberger J,Viale A, Garber J, et al. Genome-wide association study provides evidence for a breast cancer risk locus at 6q22.33. Proc Natl Acad Sci U S A 2008;105(11):4340-5.         [ Links ]

34. Stacey SN, Manolescu A, Sulem P,Thorlacius S, Gudjonsson SA, Jonsson GF, et al. Common variants on chromosome 5p12 confer susceptibility to estrogen receptor-positive breast cancer. Nat Genet 2008;40(6):703-6.         [ Links ]

35.Ahmed S,Thomas G, Ghoussaini M, Healey CS, Humphreys MK, Platte R, et al. Newly discovered breast cancer susceptibility loci on 3p24 and 17q23.2. Nat Genet 2009;41(5):585-90.         [ Links ]

36.Thomas G, Jacobs KB, Kraft P,Yeager M,Wacholder S, Cox DG, et al. A multistage genome-wide association study in breast cancer identifies two new risk alleles at 1p11.2 and 14q24.1 (RAD51L1). Nat Genet 2009;41(5):579-84.         [ Links ]

37. Zheng W, Long J, Gao YT, Li C, Zheng Y, Xiang YB, et al. Genome-wide association study identifies a new breast cancer susceptibility locus at 6q25.1. Nat Genet 2009;41(3):324-8.         [ Links ]

38.Wacholder S, Hartge P, Prentice R, Garcia-Closas M, Feigelson HS, Diver WR, et al. Performance of common genetic variants in breast-cancer risk models. N Engl J Med 362(11):986-93.         [ Links ]

39. Cox A, Dunning AM, Garcia-Closas M, Balasubramanian S, Reed MW, Pooley KA, et al.A common coding variant in CASP8 is associated with breast cancer risk. Nat Genet 2007;39(3):352-8.         [ Links ]

40. Stacey SN, Manolescu A, Sulem P, Rafnar T, Gudmundsson J, Gudjonsson SA, et al. Common variants on chromosomes 2q35 and 16q12 confer susceptibility to estrogen receptor-positive breast cancer. Nat Genet 2007;39(7):865-9.         [ Links ]

41. Fejerman L, Haiman CA, Reich D,Tandon A, Deo RC, John EM, et al. An Admixture Scan in 1,484 African American Women with Breast Cancer. Cancer Epidemiol Biomarkers Prev 2009.         [ Links ]

42. Udler MS, Meyer KB, Pooley KA, Karlins E, Struewing JP, Zhang J, et al. FGFR2 variants and breast cancer risk: fine-scale mapping using African American studies and analysis of chromatin conformation. Hum Mol Genet 2009;18(9):1692-703.         [ Links ]

43. Slattery ML, Baumgartner KB, Giuliano AR, Byers T, Herrick JS,Wolff RK. Replication of five GWAS-identified loci and breast cancer risk among Hispanic and non-Hispanic white women living in the Southwestern United States. Breast Cancer Res Treat 2011;129(2):531-539.         [ Links ]

44. Reis-Filho JS. Next-generation sequencing. Breast Cancer Res 2009;11 Suppl 3:S12.         [ Links ]

45. Mardis ER,Wilson RK. Cancer genome sequencing: a review. Hum Mol Genet 2009;18(R2):R163-8.         [ Links ]

46. Jones S, Hruban RH, Kamiyama M, Borges M, Zhang X, Parsons DW, et al. Exomic sequencing identifies PALB2 as a pancreatic cancer susceptibility gene. Science 2009;324(5924):217.         [ Links ]

47. Papi L, Putignano AL, Congregati C, Piaceri I, Zanna I, Sera F, et al. A PALB2 germline mutation associated with hereditary breast cancer in Italy. Fam Cancer 9(2):181-5.         [ Links ]

48. Heikkinen T, Karkkainen H,Aaltonen K, Milne RL, Heikkila P,Aittomaki K, et al.The breast cancer susceptibility mutation PALB2 1592delT is associated with an aggressive tumor phenotype. Clin Cancer Res 2009;15(9):3214-22.         [ Links ]

49. Hede K. Superhighway or blind alley? The cancer genome atlas releases first results. J Natl Cancer Inst 2008 Nov 19;100(22):1566-9.         [ Links ]

 

 

Corresponding author:
Elad Ziv. Helen Diller Familly Comprehensive Cancer Center.
14503^rdSt, Suite 272
San Francisco CA 94143. USA
E-mail: elad.ziv@ucsf.edu

 

 

Accepted on: September 27, 2011