Proliferation in molecular subtypes of breast cancer

Abstract

Breast cancer is a heterogeneous disease. Using immunohistochemistry (IHC) and in situ hybridization (ISH) breast cancers can be divided into molecular subtypes with different prognosis and response to treatment. Increased proliferation has been identified as one of the hallmarks of cancer and it is associated with poor prognosis in breast cancer. One of the goals of the Breast Cancer Subtypes project is to identify new prognostic factors and their relation to specific molecular subtypes of breast cancer. Breast cancer research has revealed new genes associated with proliferation. These can contribute to improved prognostication and potentially more targeted treatment of breast cancer patients. Two genes associated with proliferation in breast cancer are MRPS23 and ZNF703. This study thesis is based on four cohorts. Cohort 1, 3 and 4 comprise women from the county Trøndelag that were followed for breast cancer occurrence from 1961 to 2012. Cohort 1 comprises 25 727 women from Nord Trøndelag, of whom 1379 were diagnosed with breast cancer during follow up. Cohort 3 comprises 34 221 women from Nord Trøndelag of whom 731 were diagnosed with breast cancer, and cohort 4 comprises 23 350 women born at E.C Dahls foundation, of whom 885 were diagnosed with breast cancer. Tumour tissue is available for most of the breast cancer patients that were diagnosed at St. Olav’s hospital. In addition to tumour tissue, we have access to information about the patients and their tumours, and some information about the healthy background population. The breast cancer tumours included in cohorts 1, 3 and 4 have previously been reclassified into molecular subtypes using IHC and in situ hybridization (ISH). Cohort 2 comprises gene expression data from approximately 2000 breast cancer tumours from the METABRIC dataset. In the first study, we examined MRPS23 copy number using fluorescence in situ hybridization (FISH), and studied associations with molecular subtype, proliferation, and prognosis. In addition, we examined gene expression of MRPS23 and associations with molecular subtype and prognosis. We found that amplification of MRPS23 was associated with increased proliferation. We identified copy number increase in all subtypes except 5-negative phenotype. We found no clear association between MRPS23 copy number and prognosis. High gene expression of MRPS23 was associated with the luminal B subtype. There was no association between gene expression and prognosis. In the second study, we used FISH and IHC to study copy number and protein expression of ZNF703 in breast cancer, and associations with molecular subtype, proliferation and prognosis. We found that high copy number was associated with the luminal B subtypes. In addition, we found an association between high copy number and increased proliferation, higher histological grade and poorer prognosis. Furthermore, on looking at luminal A tumours separately, we found that tumours with high copy number had higher grade and poorer prognosis compared to tumours without copy number increase. We found a correlation between high copy number and high protein expression, but no correlation between protein expression and prognosis. In the third study we used Ki-67 and mitotic count as markers of proliferation to study incidence trends of high- and low-proliferative breast cancer tumours. We performed analyses on all breast cancer tumours combined, and separately for subgroup of molecular subtypes. Multiple imputation was used to estimate missing values of Ki-67 and mitotic count in cases with missing values. We compared incidence in women born in 1929 or later to women born before 1929. According to Ki-67, we found an increase in both high- and low-proliferative tumours among women born in 1929 or later. According to mitotic count, we only found an increase in low-proliferative tumours. We did separate analyses of HER2+ and triple negative tumours. In HER2+ tumours we found that there had been an increase of high-proliferative tumours according to Ki-67, while it according to mitotic count had been an increase of lowproliferative tumours among women born in 1929 or later. There was no change in incidence trends according to proliferation status for triple negative tumours.