Gene expression profiling to identify parity-induced changes in the human mammary gland

Background

The protective effect of an early first full-term pregnancy in relation to breast cancer risk is well established, but the molecular and cell-specific changes in the human mammary gland remain unclear. The search for these early predictive markers for breast cancer risk is of high priority because these markers could identify women at increased risk and could monitor preventive strategies.

Methods

To identify the molecular changes associated with pregnancy-induced differentiation, we analysed the global gene expression profiles of normal mammary tissues from both a parous and a nulliparous woman, using serial analysis of gene expression (SAGE) [1]. This approach allowed us to identify sets of genes, known and unknown, that are differentially expressed in parous versus age-matched nulliparous mammary gland tissues. Pair-wise comparisons of each tag were carried out in both SAGE libraries.

Results

The normal mammary gland of a multiparous woman is characterised by several known differentiation markers such as casein κ, casein β, keratin 14, CCAAT/enhancer binding protein β and δ, and adipsin. Candidate genes involved in cyto-architectural remodeling and growth inhibition with a potential role in pregnancy-induced protection against breast cancer were also observed.

Several of the genes that are highly expressed in the nulliparous mammary gland and that are lost after pregnancy encode for growth-promoting, cytoskeletal and extracellular matrix proteins. One of these genes, the small breast epithelial mucin, is almost completely downregulated upon first full-term pregnancy but is known to be expressed in more than 90% of invasive ductal carcinomas [2].

A subset of these differentially expressed genes was selected for real-time PCR validation experiments. Small-scale real-time PCR experiments revealed certain trends showing the upregulation of the genes BPAG1, VIM, DCN, RARRES2, EGR1 and ADH1B and the downregulation of SPARC and SBEM in the mammary gland after pregnancy, but nevertheless individual differences in expression levels were observed. These differences could be due to fluctuations in hormone levels at the time of surgical resection. However, these individual differences also may reflect the differential risk for breast cancer for each of the patients (nulliparous and parous) involved.

Conclusions

This study demonstrates that the SAGE technique is useful to detect alterations in gene expression, induced by an early first full-term pregnancy, in the human mammary gland. Our results show that pregnancy induces permanent gene expression modifications in the breast, which can be potentially valuable in the search for predictive markers for breast cancer. Ongoing additional research on these genes in a larger patient population will possibly lead to the development of early markers for breast cancer.