The Human Protein Atlas project, launched in 2003, was initiated as a natural extension of the Human Genome Project, with the objective to explore the proteins encoded by the human genome. The primary focus was to analyze the distribution and relative abundance of all proteins in human normal cells and tissues, and to determine the subcellular localization of each protein. One main goal in this effort was to contribute to biomedical and clinical research, and because cancer is a major disease where diagnostics, classification and prognostic stratification is based on tissue morphology, a multitude of clinical cancer tissue samples were included in the comprehensive protein profiling. This has allowed researchers to utilize the protein profiling data for both biomarker discovery efforts and for validation of altered gene expression patterns at the protein level in both normal and cancer tissue.
The Human Protein Atlas project pursues a systematic high-throughput generation of affinity-purified polyclonal antibodies with the aim of generating a map of protein expression patterns on a proteome-wide scale in both human normal cells, tissues and organs, and in cancer tissues . Immunohistochemistry (IHC) is performed on tissue micro arrays (TMA), containing a multitude of different normal tissues and tumors, to enable a comprehensive mapping of protein expression patterns at cellular resolution in a tissue context. Altogether 144 different normal tissues are analyzed together with 216 different tumors representing the 20 most common forms of human cancer . Immunofluorescence (IF)-based profiling of protein expression in cell lines is performed to generate a map of subcellular localization patterns . All protein expression data, including the underlying images, are made publicly available at the Human Protein Atlas web portal (http://www.proteinatlas.org) . The current version of the Human Protein Atlas contains data for more than 14,000 unique proteins. This corresponds to more than 70% of all human protein encoding genes .
As the cell constitutes the smallest living entity, it is required to harbor specialized and distinct subcellular structures. Cells vary considerably in function and morphology and these differences form the basis for the concept of different cellular phenotypes. On a higher level, cell types with their distinct phenotypes are organized into tissues, commonly categorized as epithelial, muscle, vascular, nervous and connective tissue, and hematopoietic cells. Genetic changes leading to dysregulated signaling pathways with altered protein expression patterns cause a transformation from normal to the phenotypes and morphology that signifies cancer. Cancer is a heterogeneous disease associated with alterations in protein expression patterns leading to cell growth and 'anti-social behavior' of tumor cells. The deregulated expression patterns in tumor cells are caused by genetic and epigenetic alterations leading to distortion of multiple proteins and signaling pathways. Despite the complexity of cancer, microscopic evaluation of tissue morphology remains the gold standard for determining a cancer diagnosis in a clinical setting. Although morphology is crucial, adding a layer of information regarding protein expression on top of morphology appears to be beneficial for the stratification of different tumor types. Immunohistochemistry prevails as an invaluable method to provide such a tool for visualization of protein expression patterns in cells from a section of tumor tissue.