By Aarushi Bellani
In the UK, a woman is diagnosed with breast cancer every ten minutes, making it the most common type of cancer occurring in the female population. With the increased incidence of this disease, a lot of research has been done in order to find preventative diagnostic methods as well as lifestyle changes that can lower the incidence of a fatal form of this cancer.
The dependence of breast cancer occurrence on hormonal changes in female bodies has been proven with a multitude of epidemiological and experimental data. Endocrine conditions such as late menopause and early menarche which are controlled by the functioning of the ovaries, affect the risk of developing breast cancer significantly (Russo et al., 2005). As the process of pregnancy sees a drastic change in hormone levels, it has been hypothesized to either play a preventative role or increase the risk of developing breast cancer in women.
Multiple factors are taken into consideration when thinking about pregnancy as a risk factor, such as parity, duration and age at which the first and last birth occurs. Parity refers to the number of pregnancies that occur for more than twenty weeks (Bai et al., 2002). Studies have determined that women who are nulliparous (having no full-term pregnancies) are at a higher risk of developing mammary carcinomas than multiparous (having more than one full-term pregnancy) (Logan, 1953). A population case-control study conducted in Sweden confirmed this hypothesis and saw a ten percent reduction in breast cancer risk with each subsequent pregnancy after the first one (Lambe et al., 1996). The protective effect provided by full-term pregnancies was studied by Ginger and Rosen who proposed a ‘cell-fate’ model to explain the same. Using rat models, the study suggested that the hormonal changes induced by a full-term pregnancy lead to changes such as the accumulation of tumour suppressor p53 in the nucleus, in mammary epithelial cells, thus leading to changes in gene expression and cell fate (Ginger & Rosen, 2003). These changes then affected process such as cell growth and ability to differentiate, hence influencing tumour formation.
The age at which women have children, the time since birth and also the intervals between births is known to affect the prevalence of breast cancer. Giving birth before the age of 24 years has been found to decrease the risk of breast cancer development. Not only the age of the woman at her first birth, but also age at the last birth seems to determine cancer risk. A population study conducted in Norway, involving parous and non-parous women diagnosed with breast cancer, showed clearly the adverse effects of pregnancies after the age of 30. Another very interesting facet of this research was that in spite of multiple births, any pregnancies after 30 years of age appeared to increase the risk of developing mammary cancer (Albrektsen et al., 2005). Population studies backed up with in vitro and in vivo experiments have shown that there is an underlying hormonal basis for these emerging risk factors and so trying to induce an artificial environment, replicating that during pregnancy is an avenue for preventative research.
Human chorionic gonadotropin (hCG) is one of the most important hormones, playing a lot of different roles in pregnancy. Studies by Russo et al. have showed that hCG is very efficient at preventing tumour phenotypes in epithelial cells in young virgin rats (Russo et al., 1991). This has been replicated by Santucci-Pereira et al. in 2013, and they have devised a genetic mechanism by which these hormones provide protection against carcinogenesis. The promising effect of hormone therapy has led to it being considered as a prophylactic treatment in people that are at a higher risk of developing breast cancer (Santucci-Pereira et al., 2013).
Although there is increasing evidence for the protective effects of early and multiple pregnancies on breast cancer, there is still a requirement of more thorough research in order to derive the exact molecular mechanism of the involved hormones to devise an effective preventive care.
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