Drivers of Hypoxia-induced Angiogenesis in Tumor Development

Background Cancer cell development requires a series of acquired capabilities to grow and spread: 1) self-sufficiency in growth signals, 2) insensitivity to growth inhibition signals, 3) evasion of apoptosis (programmed cell death), 4) limitless replicative potential, 5) sustained angiogenesis and 6) tissue invasion and metastasis (Hanahan and Weinberg, 2000). The acquisition of these capabilities is driven by mutations in key oncogenes and tumors suppressor genes, although the exact mechanisms are not yet fully understood. Especially angiogenesis is crucial to a cell's survival as it's continued multiplication depends on the oxygen and nutrients supplied in the vasculature(Hanahan and Weinberg, 2000). Angiogenesis can be initiated by lack of oxygen (hypoxia), and the cell's oxygen sensing pathway mediates a response. Under normal conditions and in the presence of oxygen, the VHL protein, pVHL mediates the binding of a ubiquitin ligase complex to a group of transcription factors called Hypoxia inducible Factors (HIFs) and directs the HIF-α subunits to proteosomal degradation. Thus in normal cells with enough oxygen, HIF-α -induced transcription of target genes is inhibited. During hypoxia, the HIF-α is not hydroxylated and is therefore not recognized by the VHL protein. The HIFs translocate to the nucleus and induce transcription of numerous genes, many encoding angiogenic factors that stimulate new vessel growth(Maher et al., 2011;Nordstrom-O'Brien et al., 2010). Cancer growth requires vast amounts of oxygen and most tumor cells are in a constant state of hypoxia.

If there is no functional pVHL in a cell it reacts as if it needs oxygen, as HIFs will stimulate angiogenesis irrespective of oxygen levels. Therefore patients with germline mutations in the VHL gene can serve as a model of hypoxia-induced angiogenesis. Patients with germline VHL mutations have von Hippel-Lindaus disease (vHL) and are prone to tumor development due to this mechanism, mainly renal cell carcinoma and central nervous system (CNS) hemangioblastomas(Maher et al., 2011). Even though hemangioblastomas are histologically benign tumors, they can have serious consequences. The natural development of hemangioblastomas is characterized by unpredictable periods of growth and stagnation. Often they develop associated cysts that affect adjacent nervous tissue and cause massive symptoms, as even small volume changes in the brain can cause severe neurological damage or even death(Ammerman et al., 2006;Glasker et al., 2010;Wanebo et al., 2003).

The mechanisms behind vHL-associated tumorgenesis are complex and not yet fully understood. A key event is loss of a functional VHL protein product as a result of inactivation of both alleles of the VHL gene in accordance with Knudson's two hit hypothesis(Vortmeyer et al., 2013). However, it is also clear that though inactivation of both copies of a person's VHL gene is necessary, it does not seem to be sufficient for hemangioblastoma development(Vortmeyer et al., 2013;Vortmeyer et al., 2006;Vortmeyer et al., 2004). Biallelic VHL inactivation may be present in the form of multiple tumor precursors throughout predisposed tissues, and most never develop into actual symptom-causing tumors(Vortmeyer et al., 2013;Vortmeyer et al., 2006;Vortmeyer et al., 2004).

The key question to a better understanding of how to slow or stop tumor development is identification of which specific additional factors initiate or promote tumor development and growth. Tumor development may be initiated in a single cell that evades normal control of cell division, but as the cell divides and multiplies, the daughter cells go through a sequence of multiple genetic events in many different genes that accumulate and provide the tumor with growth advantages(Hanahan and Weinberg, 2011). Such a sequence from benign adenoma to malignant carcinoma has previously been mapped for colorectal cancer development and has been of immense importance to our current understanding of cancer development(Fearon and Vogelstein, 1990). In the case of hemangioblastomas, further knowledge about any common genetic events in other genes than the VHL gene that occur in the early stages of hemangioblastoma progression will help determine which specific genes may be driving, i.e. promoting growth and/or cyst development.

One group recently identified loss of HNF1B on chromosome 17q to be a potential molecular driver of hemangioblastoma tumorigenesis using analysis of copy number variation in tumor DNA(Sun M et al., 2014). Other groups have found evidence that loss of ZAC1 on chromosome 6q plays a major role in both vHL-associated and sporadic CNS hemangioblastoma tumorigenesis(Lemeta et al., 2007;Zhou et al., 2010). However, more systematic approaches investigating hemangioblastomas' genetic alterations in a broader perspective could markedly increase our knowledge of the sequence of genetic events leading from early stage tumor precursors to fully grown tumors. This knowledge is of vast importance, both in relation to our general understanding of tumorigenesis, but also in relation to detection of early necessary genetics events that occur in all hemangioblastomas at early stages of tumor development and may be driving the process. Such necessary events in the tumor precursor cells may be used as biomarkers in tissue biopsies or tumor cells that make it into the blood stream to determine which patients are most at risk of aggressive tumor growth. Finally, changes in specific genes that are known to be key steps in turning a tumor precursors into clinical significant tumors would be obvious candidates to target in the development of anti-tumor drugs.