The developmental origins of mammographic density and breast cancer risk

Abstract

Mammographic density is one of the most significant risk factors for breast cancer. Breast tissue with high mammographic density is characterised by increased abundance of fibroglandular tissue and reduced abundance of adipose tissue compared to breasts of low density. Mammographic density is a consequence of cellular and molecular events that occur during adolescent breast development. Epidemiological studies show that increased body mass index in adolescence is associated with low mammographic density as an adult and reduced lifetime risk of breast cancer. This suggests that adiposity during pubertal development could be a significant, and modifiable factor that affects adult breast health. However, the causal relationships are yet to be investigated. The studies described in this thesis investigated whether increased pubertal adiposity is causal in mammary gland density and cancer development in adulthood in a mouse model. Alms1 bbb/bbb mice exhibit hyperphagia, resulting in increased adiposity and body weight relative to wildtype (+/+) and heterozygous (bbb/+) littermates when fed a normal mouse chow diet. To investigate the impact of increased adiposity on mammary gland development during puberty, mammary glands were dissected from controls (bbb/+ or +/+) and bbb/bbb female mice during puberty (6 weeks; n=10/gp). At puberty, the adipocyte size in the mammary adipose tissue was significantly increased in bbb/bbb mice compared to controls (p<0.001). Further, bbb/bbb mice exhibited increased number of terminal end buds (p=0.003), increased number of proliferating epithelial cells (p=0.002), and increased abundance of macrophages around terminal end buds (p=0.008) and in the mammary adipose tissue (p<0.001), compared to controls. To investigate the impact of increased pubertal adiposity on mammary gland development and density in adulthood, bbb/bbb mice were calorie-matched with controls from 7 weeks of age, such that weight of adult bbb/bbb mice was comparable to that of wildtype. Mammary glands were dissected from calorie-restricted bbb/bbb mice (and controls) at adulthood (12 weeks; n=10/gp). At adulthood, the adipocyte size in the mammary adipose tissue was significantly increased in matched bbb/bbb mice compared to controls (p<0.01). Interestingly, matched bbb/bbb mice exhibited significantly reduced percent fibroglandular density (p<0.001), reduced stroma/epithelium ratio (p=0.003), and reduced collagen deposition around ducts (p<0.001), compared to controls. Further, mammary glands of matched bbb/bbb mice demonstrated increased expression of mRNA encoding adiponectin (p<0.01), TGFB1 (p=0.044), CSF1 (p=0.033), IGF1 (p=0.007), and STAT3 (p=0.033), compared to controls. To determine the impact of increased pubertal adiposity on mammary cancer development, the MMTV-PyMT tumour model was crossed with Alms1 bbb/+ mice to generate female PyMTcontrol and PyMT-bbb/bbb mice. At 18 weeks, PyMT-bbb/bbb mice exhibited significant increase in tumour free survival (LogRank p=0.002), greater tumour latency (p<0.01) and reduced number of tumours (p=0.006), compared to PyMT-control mice. Interestingly, matched PyMT-bbb/bbb mice exhibited significantly increased expression of mRNA encoding TGFB1, compared to PyMT-control mice (p=0.037). This research in mice provides the first evidence that increased pubertal adiposity might be causative in affecting mammographic density and breast cancer risk in adulthood. Together with epidemiological studies, this research provides the foundation for a new paradigm for the origins of mammographic density and breast cancer risk during pubertal breast development.