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28 August 2008
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Breastfeeding protects against triple-negative breast cancer
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MedWire News: Women who breastfeed for at least 6 months have a diminshed risk for developing triple-negative breast cancer, US study results show.
The research team, from the Fred Hutchinson Cancer Research Center in Seattle, Washington, also found that certain other reproductive factors appeared to be differentially associated with certain breast cancer subtypes.
Gene expression studies have identified and validated the existence of four "intrinsic" breast cancer subtypes: luminal A, luminal B, human epidermal growth factor receptor (HER)-2-overexpressing, and basal-like.
Luminal tumors express hormone receptors while nonluminal subtypes lack these markers. The triple negative phenotype - which expresses neither estrogen receptor, progesterone receptor, nor HER-2 - is often used a surrogate marker for the basal subtype.
The five-year survival rate for triple negative breast cancer is 15% lower than for other types of the disease, in part because the disease responds poorly to most breast cancer treatments. Notably, nearly 50% of Black women younger than 55 years who are diagnosed with breast cancer have the triple negative type, compared with 22% of White women with the disease.
In the current study, lead researcher Amanda Phipps and colleagues sought to determine what puts women at risk for the triple negative type of breast cancer.
The authors pooled two population-based, case-control studies of breast cancer in women aged 55 to 79 years for analysis. These included 1476 controls and 1023 cases of luminal breast cancer, 39 cases of HER-2-overexpressing breast cancer, and 78 cases of triple-negative breast cancer.
Clinical and lifestyle factors were obtained and analyzed using the polytomous logistic regression method.
As reported in the journal Cancer, breastfeeding for at least 6 months was found to reduce the risk for triple-negative disease (odds ratio [OR]=0.5) and to a lesser extent luminal breast cancer (OR=0.80) compared with breastfeeding for less than 6 months.
The researchers say it is unclear why breastfeeding influenced hormonal cancer risks. One possible explanation is that while women are breastfeeding, they are not menstruating and thus their hormones are not cycling. Alternatively, breastfeeding may alter the structure of breast cells in a way that makes them less prone to develop into cancer cells, Phipps and team said.
Another finding of the study was that early age at menarche was associated with risk for HER-2-overexpressing disease only (OR=2.7, relative to population average menarche) whereas both late age at menopause and the use of estrogen plus progestin hormone therapy were found to be associated with risk of luminal disease only (OR=1.6, 1.7)
Phipps and collegues comment: "Certain reproductive factors may have a greater impact on the risk of certain molecular subtypes of disease compared with others.
"Future studies that further define the etiology of breast cancer subtypes will add to the biologic understanding of this disease."
Cancer 2008; Advance online publication
http://www3.interscience.wiley.com/journal/121387534/abstract
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