Netrin-1 in Mammary Gland Development & Cancer Lindsay Hinck, MCD Biology Lindsay Hinck's laboratory studies the regulatory role of the protein Netrin-1 in mammary gland development. Loss or misexpression of Netrin-1 has been implicated in tumor progression in the nervous system and the human netrin-1 gene maps to chromosomal region 17p12, a frequent site of loss of heterozygosity (LOH) in breast tumors. Hinck's data indicates that Netrin-1 regulates cell-cell interactions in the murine mammary gland and preliminary results suggest that loss of its function leads to an increased incidence of hyperplasias and tumors in mice.

Figure 1: Loss of either netrin-1 leads to abnormal end buds A. Longitudinal section through wild type end bud demonstrates the close juxtaposition between the cap cell and body cell layers. B-D, Longitudinal sections through netrin-1 -/- end buds. C. Exaggerated space between the cap cell layer and body cells. D. Loose cells detected in the subcapsular space. D. Break in basal lamina at the leading edge of the cap cell layer (box) and loose cells outside of the end bud (arrowheads). Arrows point to the cap cell layer. L indicates the lumen of the end bud. Scale bar: 5

In order to understand dysfunctional biology, it is essential to identify and analyze mechanisms that characterize healthy tissue. For example, cancer reverses many normal developmental processes, causing a single cell to dedifferentiate, proliferate and migrate. Thus, study of mammary gland development can shed light on the genesis and spread of breast cancer.

Lindsay Hinck's laboratory investigates the developmental role of a protein called Netrin-1. This protein was originally identified in the nervous system where it acts during development as a guidance cue for the construction of elaborate networks of neuronal connections. Netrin-1 functions as either an attractant that guides neurons to a target, or as a repellent that directs them away from a non-target zone. Half of the projects in the Hinck lab focus on the role of Netrin-1 and its receptors in the nervous system, while the remaining projects focus on the protein's role in the development of the mammary gland.

The mammary gland is an example of an organ that undergoes an elaborate and highly regulated morphogenesis. During development, the mammary epithelium grows to establish the mammary tree through a process of ductal elongation and branching. The ducts of the mammary gland comprise an outer tube of myoepithelial cells juxtaposing an inner tube of lumenal epithelial cells that surround a central space or lumen. Studies of Netrin-1 in the Hinck laboratory focus on the end buds. These are highly specialized growth structures of the gland whose function is to establish the ductal architecture.

Hinck and her colleagues have found that there are severe structural abnormalities in the end buds in the absence of Netrin-1. In these abnormal end buds, the cap cells are significantly pulled away from pre-lumenal cells, creating an exaggerated subcapsular space, with dissociated cells often found in this subcapsular space. In some of the abnormal end buds, disruptions have been observed in the basal lamina in front of the cap cell layer, as well as in the loose cells in the stroma ahead of the cap cell layer.

Many studies have shown that one of the first changes observed in the development of solid malignant tumors, such as breast tumors, is loss of normal tissue morphology, particularly the loss of cell-cell and cell-extracellular matrix interactions. The mammary gland phenotype of the netrin-1 knock-out mice displays these types of morphological changes. Hinck's laboratory is now investigating the molecular mechanisms that underlie these morphological changes and determining whether loss of Netrin-1 plays a role in the etiology of human breast cancer