**2. Molecular biology of renal cell carcinoma**

Most of the knowledge about molecular biology comes from the studies of a hereditary form of renal cell carcinoma. Studies of families with inherited RCC over the past twenty years lead to the identification of five inherited renal cancer syndromes and their related genes. Descrip‐ tion of all five syndromes is beyond the scope of this chapter; only Von Hippel-Lindau syndrome will be explored [2].

### **2.1. Von Hippel-Lindau tumor suppressor gene**

The von Hippel-Lindau (VHL) disease is a rare, autosomal dominantly inherited disease. Individuals with this syndrome are predisposed to development of multiple benign and malignant tumors. Most common are clear cell renal tumors, retinal and central nervous system hemangioblastomas, pheocromocytomas, pancreatic neuroendocrine tumors, endolymphatic sac tumors and pancreatic and kidney cists. VHL occurs in 1 in 36.000 and symptomatic disease develops in 70% of affected persons by the age of 60 years. Bilateral RCC develop in 25-45% of VHL patients. VHL results from mutation in the von Hippel-Lindau gene on chromosome 3p25-26. The VHL gene discovered in 1993 is a tumor suppressor gene; both copies of gene must be inactivated for tumor initiation. Different germline mutations predisposing to VHL include; large deletions, protein-truncating mutations and missense mutations that exchange the amino acids in the VHL protein. More than 1000 different mutations have been identified until now. According to the type of mutation, patients are classified in different groups, predisposed to different types of tumors. Group of patients bearing deletions or nonsense mutations, most often develop RCC [2,8].

The research on VHL gave light to the inside of molecular biology of sporadic kidney cancer. It is known, that loss of VHL function, including somatic mutations and epigenetic defects, is found in 70–90% of the sporadic clear cell RCC [8]. The pathophysiologic mechanism of such strong association is currently not very understood [8,9].

The VHL protein pVHL has several functions. The most studied is its role in the regulation of hypoxia inducible factor (HIF1α), member of transcription factors family. At normal cellular oxygen levels, pVHL binds to HIF1α and causes its degradation. In low oxygen or in the case when VHL gene is mutated pVHL does not bind to HIF1α. Consequently HIF1α dimerise with HIF1β and activate the transcription of genes involved in vessel development (vascular endothelial growth factor, platelet-derived growth factor B, erythropoietin) and genes involved in glucose uptake and metabolism. Up-regulation of targeted genes involved in neovascularization by HIF1α offers the explanation of high vascularity of RCC [2,8]. Beside this, pVHL has numerous other functions in the processes of regulation of extracellular matrix, senescence, phosphorylation enhancers and other. The importance of many physiologically relevant functions of pVHL is at present difficult to interpret [8].

Besides VHL, six other genes have been found to predispose to RCC (MET, FLCN, FH, SDH, TSC 1 and TSC 2). These genes interact trough common nutrient and energy sensing pathways. Understanding of the molecular mechanisms by which these genes interact in these pathways has enabled the development of targeted therapies [2].

### **2.2. VEGF-R pathway**

mRCC is considered an incurable disease [4]. In trying to overcome this, the mechanism of action and especially mechanisms of resistance to targeted therapies, need to be studied and

In this chapter evidence on sequential therapy after progression to the first line will be presented with the emphasis on changing mechanism of action. Additionally, mechanisms of resistance to targeted therapies and therapeutic options to overcome resistance will be

Most of the knowledge about molecular biology comes from the studies of a hereditary form of renal cell carcinoma. Studies of families with inherited RCC over the past twenty years lead to the identification of five inherited renal cancer syndromes and their related genes. Descrip‐ tion of all five syndromes is beyond the scope of this chapter; only Von Hippel-Lindau

The von Hippel-Lindau (VHL) disease is a rare, autosomal dominantly inherited disease. Individuals with this syndrome are predisposed to development of multiple benign and malignant tumors. Most common are clear cell renal tumors, retinal and central nervous system hemangioblastomas, pheocromocytomas, pancreatic neuroendocrine tumors, endolymphatic sac tumors and pancreatic and kidney cists. VHL occurs in 1 in 36.000 and symptomatic disease develops in 70% of affected persons by the age of 60 years. Bilateral RCC develop in 25-45% of VHL patients. VHL results from mutation in the von Hippel-Lindau gene on chromosome 3p25-26. The VHL gene discovered in 1993 is a tumor suppressor gene; both copies of gene must be inactivated for tumor initiation. Different germline mutations predisposing to VHL include; large deletions, protein-truncating mutations and missense mutations that exchange the amino acids in the VHL protein. More than 1000 different mutations have been identified until now. According to the type of mutation, patients are classified in different groups, predisposed to different types of tumors. Group of patients bearing deletions or nonsense

The research on VHL gave light to the inside of molecular biology of sporadic kidney cancer. It is known, that loss of VHL function, including somatic mutations and epigenetic defects, is found in 70–90% of the sporadic clear cell RCC [8]. The pathophysiologic mechanism of such

The VHL protein pVHL has several functions. The most studied is its role in the regulation of hypoxia inducible factor (HIF1α), member of transcription factors family. At normal cellular oxygen levels, pVHL binds to HIF1α and causes its degradation. In low oxygen or in the case when VHL gene is mutated pVHL does not bind to HIF1α. Consequently HIF1α dimerise with HIF1β and activate the transcription of genes involved in vessel development (vascular endothelial growth factor, platelet-derived growth factor B, erythropoietin) and genes

explained even more in detail [3-7].

syndrome will be explored [2].

**2. Molecular biology of renal cell carcinoma**

**2.1. Von Hippel-Lindau tumor suppressor gene**

mutations, most often develop RCC [2,8].

strong association is currently not very understood [8,9].

discussed.

188 Renal Tumor

Loss of both alleles of VHL gene leads to up-regulated transcription of growth factors such as VEGF, PDGF and TGF-α. These factors bind to their tyrosine kinase receptors. This leads to downstream signalling and ultimately to effects such as increased angiogenesis, increased cell proliferation and decreased apoptosis. As described previously pVHL mutations are inevita‐ bly connected to flawed HIF inactivation which results in production of VEGF. VEGF is the most prominent angiogenesis regulator. Its function is mediated through two tyrosine kinase receptors VEGF-R1 and VEGF-R2 in vascular endothelial cells. VEGF in the beginning binds to VEGF-R2, which promotes endothelial cell proliferation, migration and vascular permea‐ bility. In the next step VEGF binds to VEGF-R1 to assist the organization of new capillaries [9].

### **2.3. mTOR pathway**

mTOR is another regulator of HIF 1α, its signalling activity increases the cellular levels of HIF 1α, which worsens the already high levels of it because of absence of pVHL function. mTOR is a serine/threonine kinase that has a key function in apoptosis, cell growth and tumor proliferation. mTOR forms complexes with regulatory associated proteins named mTORC1 and mTORC2. mTORC 1 can be activated by growth factors including VEGFR, PDGFR, EGFR and IGFR and nutrients trough phosphatidylinositol-3 kinase/Akt (PI3K/Akt) pathway. Activated mTORC1 stimulate protein synthesis, entrance into G 1 phase, and proteins that regulate apoptosis [9].
