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Background: In 2020, more than 2,260,000 women were diagnosed with breast cancer. Most patients are cured with surgery and adjuvant treatment, but despite that, approximately 700,000 women die of the disease every year. The historical focus on breast cancer progression has been on the malignant epithelial cell. However, cancer cells do not grow in isolation. In recent years, the importance of the tumour microenvironment in cancer progression has been highlighted. Perlecan and type IV collagen are basement membrane (BM) proteins in the normal mammary gland, and type I collagen is the main fibrillar collagen in the interstitial extracellular matrix (ECM). In cancer development, perlecan and type IV collagen have multifunctional roles and when degraded from the BM, bioactive substances and other fragments are released in the circulation. Significant ECM changes also occur that lead to an accumulation of fibrillar collagens. Given their abundance in the ECM; perlecan, type IV and type I collagen are of interest for breast cancer progression and may be of importance as new biomarkers to monitor disease, predict patient outcome and the treatment effect.

Aim: In this thesis, the protein and mRNA expression of perlecan, type IV and I collagen in breast cancer tissue is studied. The aim is to characterize the expression pattern of these proteins in breast cancer tissue and to see whether there is a correlation to known prognostic biomarkers and to the patient prognosis. Moreover, to evaluate circulating perlecan and type IV collagen as diagnostic and prognostic biomarkers in breast cancer patients.

Methods: In this thesis project, eight different patient cohorts were used. In freshly frozen normal breast and breast cancer tissue, perlecan protein expression was visualized using immunofluorescence. Type IV and I collagen protein expression were studied with immunohistochemistry in formalin-fixed, paraffin embedded primary breast cancer tissue, and type IV collagen in metastatic breast cancer tissue. For gene expression analysis, mRNA and clinicopathological data were extracted from the Cancer Genome Atlas and cBioportal database. Circulating plasma levels of perlecan were analysed in breast cancer patients and controls, circulating levels of CA15-3 and type IV collagen in patients with primary and metastatic breast cancer as well as controls. Perlecan and type IV collagen were measured with ELISA assays, and CA15-3 were using an electrochemiluminescence immunoassay.

Results: In breast cancer tissue, perlecan and type IV collagen protein expression in the epithelial BM was fragmented or completely lost, and perlecan and type IV collagen was expressed to varying extent in the tumour stroma. The mRNA analysis confirmed that type IV collagen mRNA was expressed in primary breast cancer tissue and highly expressed in metastatic tissue. Type I collagen was mostly highly expressed in the tumour stroma. Low type I collagen protein and mRNA expression correlated with biomarkers for aggressive breast cancer, but no effect on survival could be seen. Among patients receiving chemotherapy, low stromal type I collagen protein expression was associated with better survival compared to high expression, even after adjusting for other relevant factors. There was no correlation of perlecan or type IV collagen protein expression to clinically used prognostic biomarkers, but an oestrogen receptor dependent correlation between mRNA expression of perlecan and several matrix-degrading enzymes were found. Survival analysis showed that high stromal type IV collagen protein and mRNA expression in the primary tumour was significantly associated with a poorer survival, and high protein expression with a risk of developing distant metastasis. Metastatic breast cancer patients had higher levels of circulating type IV collagen compared to healthy controls and patients with primary breast cancer. High circulating type IV collagen levels correlated with poorer survival in metastatic breast cancer patients, and was superior to CA15-3 at detecting metastatic breast cancer.

Conclusions: The protein expression pattern of perlecan, type IV collagen and type I collagen become abnormal during breast cancer development. Stromal type IV collagen protein and mRNA in the primary tumour correlates to poorer prognosis, most likely due to a higher risk of developing metastatic disease. Circulating type IV collagen can function as a biomarker for detecting metastatic disease in patients with primary breast cancer and is prognostic in patients with metastatic breast cancer. Low stromal type I collagen is a marker for an aggressive breast cancer disease and can predict chemotherapy response.