Characterization of Canine Mammary Carcinoma using Dog-Specific cDNA arrays

Abstract

Breast cancer is the most common neoplasm which is frequently diagnosed in women. One in eight women carries a risk of developing breast tumor in her life time. Similarly, mammary tumors in non-spayed female dogs are even more frequent, whereby; one in three female dogs carries a risk of developing mammary tumors in its life time. Ovarian hormones are very important in this context as they are involved in normal development as well as neoplastic transformations of the mammary gland. Among them progesterone is an important risk factor for canine mammary tumor development as progestins are known to induce mammary gland hyperplasia and mammary gland-derived growth hormone (GH) excess in dogs. Downstream mechanism of progesterone action in the mammary gland is far from being fully resolved. It is generally believed that progesterone acts on its target tissue in a paracrine manner by producing local growth factors. The main aim of this thesis was to improve the understanding of molecular events involved in canine mammary tumorigenesis with special emphasis on role of progesterone. In order to develop laboratory tool which are necessary for characterizing canine mammary tumors, a new canine specific cDNA microarray which could quantify expression of 20000 genes was developed. Microarray was validated by quantifying the basal gene expression in canine mammary tumor cell lines. This study also identified various altered biological pathways in these cell lines which may contribute to our understanding of tumor biology. In mouse mammary gland, Wnt4, a ligand of Wnt signaling is reported to be a downstream growth factor induced by synthetic progestins. We quantified, activity of Wnt signaling in canine mammary cell lines, progestin-induced canine mammary hyperplasia and spontaneous canine mammary tumors. From this study we conclude that, observed activity canonical Wnt signaling in progesterone induced canine mammary hyperplasia and spontaneous mammary tumors is due to overexpression of ligands of Wnt signaling. Further, spontaneous canine mammary tumors as well as progesterone-induced mammary hyperplasia were characterized using canine microarray. This study identified altered expression of genes in canine mammary tumors which may serve as potential tumors markers and/or therapeutic targets. Gene expression profile of progestin-induced canine mammary hyperplasia identified altered expression of many genes including growth factors which are potentially indueced by progestin. Taken together, prolonged exposure to progesterone during canine luteal phase, through locally produced growth factors such as Wnt and growth hormones, may stimulate mammary stem cells. This may lead to mammary hyperplasia as an early event of oncogenic transformation. Ligands of Wnt singling and growth hormone are consistently established to be overexpressed in progesterone treated mammary tissues. The potential relation between Wnt pathway activity and growth hormone is currently being investigated in a new project.