Molecular and functional characterization of breast cancer through a combination of MR imaging, transcriptomics and metabolomics

Abstract

Breast cancer is the most frequent cancer among women in Norway. The outcome of this disease is heterogeneous. Some patients have slowly growing tumors which stay confined within the mammary gland, whereas others have aggressive tumors that grow rapidly and metastasize to distant tissues. Based on gene expression profiles, breast cancer has been divided into at least five different subtypes with differences in clinical characteristics and prognosis. Improved understanding of the biology of these subtypes and how they should be treated is needed to benefit from this molecular subtyping in breast cancer diagnosis and treatment. The motivation for this work has been to map differences in breast cancer metabolism and vascularization using different MR imaging and spectroscopy methods. The work shows how different MR methods can be used to study the biology of animal breast cancer models representing the molecular subgroups luminal-like and basal-like breast cancer. The findings represent MR characteristics in breast cancer with good and poor prognosis, respectively. MR spectroscopy can be used to study metabolism in cells and tissues. Since cancer cells have a large need for energy and molecular building blocks in order to grow fast, they have metabolic properties that differ markedly from healthy cells. The thesis describes how MR-spectroscopy can detect differences in choline metabolism between luminal-like and basal-like breast cancer, and demonstrates how metabolic patterns found in the animal models are representative for findings in clinical samples. Since CCCs are proposed as biomarkers in diagnosis and evaluation of response to therapy in breast cancer, differences in choline metabolism between different molecular subtypes may be of clinical relevance. The thesis also describes how glucose metabolism can be studied using 13C-labeled glucose. Using a stable, MR-detectable isotope allows assessment of the metabolic fate of glucose. Abnormal glucose metabolism is a typical feature in cancer, and is the basis for PET imaging using FDG. There is also great interest for use of hyperpolarized MR spectroscopy for measurement of lactate production in cancer. The method that is established in this work allows ex vivo studies of metabolism and the relationship between gene expression and metabolic rates. In the luminal-like and basal-like animal models, it was found that the least aggressive model had the highest glycolytic rate, suggesting that tumor growth rate and aggressiveness not necessarily is directly linked to glycolytic rate. MR imaging can be used for anatomical imaging of the body, but also for studies of functional properties such as perfusion and cellular density. In tumors, the blood vessels are typically leaky, poorly organized and have suboptimal function. Without sufficient blood supply, tumor growth is limited. The tumor cells’ ability to stimulate growth of new vessels has impact on disease outcome. Vascularization and neoangiogenesis was studied in the luminal-like and basal-like models. Using dynamic contrast-enhanced MR imaging and immunohistochemistry, it was found that the basal-like xenograft model had higher contrast uptake, that it is better vascularised and has more active angiogenesis than the luminal-like xenograft. In a follow-up study, the effect of the antiangiogenic agent bevacizumab was studied. This drug inhibits the formation of new blood vessels and can therefore slow down tumor growth. Shortly after treatment, MR imaging demonstrated increased contrast agent uptake in the tumors. Immunohistochemistry showed a reduction in the number of microvessels and angiogenic activity. These findings were interpreted as normalization of blood vessels and improved vascular function. MR imaging of vascular normalization may have impact on the use of bevacizumab together with other cytotoxic drugs or radiotherapy. The thesis consists of four papers which all describe use of MR in animal xenograft models of luminal-like and basal-like breast cancer. In the two first papers, high resolution ex vivo MR spectroscopy has been combined with gene expression analysis for description of tumor-specific metabolic properties. In the last two papers, dynamic contrast-enhanced MR imaging and pharmacokinetic analysis of contrast agent distribution has been used to describe tumor vascularization and how this respond to antiangiogenic treatment. All four papers describe how different MR techniques can distinguish between animal models of varying aggressiveness. They also show how the MR parameters are associated with molecular or structural properties of the tumors. The thesis may therefore contribute to improved understanding of MR findings in breast cancer diagnosis and treatment. In addition, it forms a basis for further studies of these animal xenograft models of luminal-like and basal-like breast cancer, especially with regard to identification of new treatment regimens and MR imaging for therapy response monitoring