Tumor hypoxia and genetic alterations in sporadic cancers (2011)

Cancer is a disease of a cell that gains the ability to multiply in an uncontrolled way, to invade from the primary site to surrounding tissues, and to metastasize to distant sites. Throughout the past three decades, the field of cancer genetics has identified critical genes and the pathways1 whose dysfunction leads to major cancer phenotypes: self-sufficiency in growth signals, insensitivity to anti-growth signals, evading apoptosis, limitless replicative potential, sustained angiogenesis, tissue invasion and metastasis.2 Thus, cancer is a multi-genetic disease and exhibits a progressive process where the genetic or epigenetic alterations responsible for these phenotypes accumulate in time and space. To date, about 350 cancer genes have been identified.3 Results of recent systematic DNA sequencing of the cancer genome have shown the following characteristics. There are two types of mutations in cancer cells: ‘driver’ and ‘passenger’. Driver mutations contribute to tumor cell growth and survival under restricted conditions and are positively selected during the course of cancer development. The rest of the mutations are ‘passenger’ mutations, which have not contributed to cancer development or been positively or negatively selected. Most of the mutations found in the cancer genome are passengers. The frequency of mutation in each driver is low, suggesting that the number of ‘drivers’ in common adult sporadic cancers could be greater than the 5–7 which have been estimated by age-incidence statistics. There is heterogeneity in the number of gene mutations among cancers; some genomes have >100 000 point mutations whereas others have fewer than 1000.3 This suggests that more than 99% of mutations in the cancer genome do not contribute to the carcinogenic process. So how do cancer cells gain such excessive numbers of mutations? There are three types of cancer genes: oncogenes, tumor suppressor genes and stability genes.1 Oncogenes encode proteins that promote cell multiplication and survival. Their expression or functions are activated by point gene mutation, fusion to another gene by chromosomal translocation and/or gene amplification. About 90% of cancer genes are dominant-acting oncogenes.3 Tumor suppressor genes encode proteins that inhibit cell multiplication and promote cell death. Inactivation of tumor suppressor genes is achieved by point mutation, gene deletion or insertion, or by epigenetic silencing. Activation of oncogenes or inactivation of tumor suppressor genes confers cell growth and gives the cancer cell a survival advantage. On the other hand, stability genes encode proteins whose loss or over-expression increases genetic alterations all over the genome. Stability genes include DNA repair genes, DNA damage sensor genes and cell cycle checkpoint genes. Malfunction of stability genes could be the driving force of the carcinogenic process.4–6 Alternatively they may not be necessary for carcinogenesis, but may merely promote this process.7 This topic is one of issues that will be discussed in this review. Most solid tumor tissues, even when they are microscopically small, contain acute and chronic hypoxic and/or anoxic areas where oxygen pressure is lower than is physiologically normal.8,9 As an adaptive response to the lack of oxygen, cancer cells may change their genome to increase their survival. In 1996, Glazer's group first presented evidence that the tumor microenvironment, especially hypoxia, induces high levels of gene mutations in cancer cells. This study was based on their hypothesis that ‘the microenvironment may give conditions that either increase DNA damage or compromise the DNA repair process’.10 Since then, this hypothesis has been tested by many research groups.11 The results of these studies generated a new concept that the microenvironment (hypoxia) induces genetic instability.12 This hypothesis accepts the idea of ‘genetic instability as a hallmark of cancer’; however, the extension of the hypothesis does not necessarily require the idea that cancer, especially sporadic cancer, gains gene mutations in putative stability genes that may drive the carcinogenic process.4–6 If hypoxia (the microenvironment) plays a role for the carcinogenic process, it should contribute to the progression stage of tumor development when a part of the tumor tissue starts to receive less oxygen. In fact, many clinical and experimental observations indicate that hypoxia is associated with the aggressiveness of tumor cells, leading to poor prognosis and metastasis in a variety of human cancers. Within tumor tissues, oxygen concentrations fluctuate both spatially and temporally. Hypoxic tumor cells may be re-exposed by a higher concentration of oxygen (re-oxygenation), which can alter the cancer genome and contribute to tumor progression. In this review, mechanisms by which hypoxia and re-oxygenation induce genetic alterations in sporadic cancer will be considered. Toward this goal, literature relating to tumor hypoxia, cellular pathways affected by hypoxia, types of genetic alterations and DNA repair systems affected by hypoxia and re-oxygenation has been compiled. The impact of hypoxia on human cancer in medicine was first recognized by radiologists. In the 1930s, the presence of hypoxia in solid tumor tissues was first hypothesized based on the observation that low levels of oxygen (hypoxia) protect a cell from the lethal effects of ionizing radiation and that some solid tumors are resistant to radiation.13 In 1955, Thomlinson and Gray reported histological observations of tumor cords with and without central necrosis in human lung tumors, suggesting the presence of an oxygen gradient within a tumor cord. They found that: (i) all of the tumor cords surrounded by the stroma and >200 µm in radius contained central necrosis; (ii) none of the tumor cords <160 µm in radius contained central necrosis; and (iii) no intact tumor cells were found at a distant of 180 µm from the stroma. Based on these results and the calculated distance of oxygen diffusion (150 µm), they proposed the presence of radio-resistant hypoxic cells at the edge of the necrotic area.14 Until the late 1980s when polarographic electrodes were used to directly measure levels of oxygen in human cancer tissues, the presence of tumor hypoxia was speculative.15,16 During the 1990s, several key findings were made using various methods for directly detecting tumor hypoxia in human tumor tissues.9,15 These findings are as follows: (i) hypoxic and anoxic areas exist in most solid tumors (areas with <2.5 mmHg of oxygen pressure); (ii) there is no predictable association between tumor hypoxia and other clinical factors, including size, stage, grade and site; (iii) tumor hypoxia may be an adverse prognostic factor;9,17 and (iv) tumor hypoxia not only induces radiation-resistance, but it may also induce resistance to chemotherapeutic agents.9,18 Using DNA-binding chemical Hoechst 33 432, cell sorting and radiation, Chaplin et al. first demonstrated that two types of hypoxia exist in solid tumor tissues.19 Because of the abnormal structure, distribution and function of microvessels formed by unregulated angiogenesis within tumor tissues, an inadequate blood flow occurs in geometrically different parts of tumor tissues. This type of hypoxia, called acute hypoxia, lasts from minutes to hours and is followed by re-oxygenation.16,19 Another type of hypoxia is caused by the reduction of oxygen diffusion due to an increase in the distance of the tumor cells from the tumor or host vasculature. This type of hypoxia is called diffusion-limited hypoxia or chronic hypoxia. It may last days, followed by re-oxygenation or cell death.16 It has been suggested that a different biology may exist between acute and chronic hypoxia and this might influence the interpretation of clinical and experimental data, and the design of treatments for hypoxic tumors.20 While struggling to overcome the radiation-resistance of hypoxic tumors, many aspects of the cellular response to hypoxia have been recognized and studied. These hypoxic responses are related to angiogenesis, glycolysis, metastasis, stress response, erythropoiesis and genomic stability.20,21 Hypoxia-inducible factors (HIFs) play a central role in these responses to hypoxia. In 1995, Wang et al. identified one of the HIFs, HIF1, a complex between HIF1α and HIFβ subunits, which is stabilized in response to hypoxia and regulates of to the hypoxia response in the of genes, such as and of at genes, and is by of There are three subunits, and and they a with a common HIFβ the and function as factors for genes under hypoxia. HIF1α and but not the HIFβ subunits, are by the in conditions tumor suppressor to and of is a part of the complex and directly to and a the The between the and is of a within by the of or Because the of oxygen and the lack of oxygen or in a cell leads to the of Another and which of on subunits, the of and their such as which results in of a complex and leads to of In sustained hypoxia is by the following (i) of a with a which to and between and and the of in hypoxic (ii) levels are by leading to of even oxygen levels are and (iii) HIF1α is from the in an major for a cell to to hypoxia is by using the that pathways the of oxygen and as can be also directly or regulates the expression of other genes in stability of the cellular genome. There are two other cellular pathways in response to hypoxia. 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