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Since the 17th century, the field of epidemiology has been responsible for the identification of external agents capable of causing cancer. In the last decades of the 20th century, geneticists isolated internal agents—genetic variations that cause inherited predisposition to specific tumour types. Also during that period and into the 21st century, scientists gained detailed knowledge about the molecules that cause cells to develop abnormal behaviours such as limitless reproduction, invasion of surrounding tissues, and spread (metastasis) to other regions of the body. As a result, there exists a great deal of information regarding the mechanisms by which various agents, external and internal, give rise to tumours. Whereas eliminating the ultimate causal agent is not always simple, knowing the immediate mechanism allows for interference with the abnormal, cancer-causing function, in turn facilitating the development of highly effective anticancer drugs.
The molecular basis of cancer
Discussion of the causes of cancers necessarily involves an examination of the molecular machinery in cells that guides the basic processes of proliferation (increase in cell number by cell division), differentiation (cell specialization into different tissue types), and apoptosis (programmed cell death). Those processes are guided by two innate programs in cells, the genetic code and the epigenetic code. In cancer each of those codes ultimately becomes altered regardless of whether the disease originated with an external or internal factor. Indeed, a fundamental characteristic of a tumour cell is that it begets a tumour cell. In other words, cancer, once manifest, becomes an inherited disease of the cell and is therefore self-perpetuating.
The hereditary nature of cancer at the cellular level explains why alterations have been found in both the genetic and the epigenetic codes in tumour cells. The number of alterations seen in the coded programs increases as tumours progress to more advanced stages. Their existence and accumulation also explain why principles of evolutionary theory provide insights of practical significance for cancer biology.
Genetic and epigenetic programs
One way to envision a cancer cell is to think of a cell that has rewired the normal control circuits for proliferation, differentiation, and death. The resulting alterations in the circuits’ functions, which are encoded by the genetic sequence and by the epigenetic configuration, enable the cell to escape programmed controls.
The genetic program, common to all cells in the body (whether noncancerous or cancerous), is found in the DNA sequence, which is packaged in chromosomes in the cell nucleus. Each person has a unique DNA sequence that is composed of approximately three billion base pairs (units of DNA) organized into roughly 25,000 genes. A gene can be thought of as a set of instructions that the cell follows to make a protein, each gene providing directions for a different protein. Some of the gene products that have been linked to cancer are organized in groups (pathways), which form networks that transmit information inside the cell and stimulate responses to changes in the cell’s environment.
The epigenetic code is responsible for providing cells with the memory of their particular specialization—for example, being part of the brain, the liver, or skin. The epigenetic code is embodied in chemical changes to DNA and in chemical and structural modifications of chromatin (the protein-DNA fibres in the nucleus that when condensed form the chromosomes). Modification of chromatin, such as when methyl groups attach to proteins in the chromatin structure, holds the fibre in a less-condensed (“open”) state and causes genes in the affected area to become or remain active. The resulting patterns of gene expression dictate and maintain cell differentiation.
The billions of cells that make up a tumour are descended from a single cell, in which disturbance of the genetic and epigenetic codes caused remodeling of the control circuits that governed that cell’s existence. A single damaging genetic or epigenetic event, however, is not enough to convert a healthy cell to a cancer cell. Rather, several insults must be inflicted upon the DNA or chromatin of a cell in order for it to become cancerous. The first of those, the damage that instigates transformation, is known as initiation. Ensuing damage that advances transformation is known as promotion. Initiation and promotion together are required for causing cancer. In many cases that is a slow process that takes years.
Hallmarks of cancer cells
No matter what tumour type, cancer cells display a number of characteristics that can be linked to specific molecular alterations and can be thought of as the “hallmarks of cancer.” In general, those features are associated with the aforementioned escape from coded cell programs. Among the hallmarks are: (1) increased proliferative activity, (2) evasion of growth suppression, (3) resistance to cell death, (4) acquired immortality, and (5) acquired ability to spread to and invade distant tissues and to stimulate angiogenesis (the formation of blood vessels).
The role of mutation
Proto-oncogenes, which encourage cell growth, and tumour suppressor genes, which inhibit it, are frequent targets of agents known to cause cancer, including chemicals, viruses, and radiation. Such agents exert their effects by inducing changes in those genes or by interfering with the function of the proteins that the genes encode. Mutations that convert proto-oncogenes to oncogenes tend to overstimulate cell growth, keeping the cell active when it should be at rest, whereas mutations in tumour suppressor genes eliminate necessary brakes on cell growth, also keeping the cell constantly active. (Proto-oncogenes are so-named because of their potential to mutate into cancer-causing genes.)
The normal cell is able to repair such genetic damage through its DNA repair mechanisms, such as the so-called mismatch repair genes, whose normal function is to identify and repair defective DNA segments that arise in the normal course of a cell’s life. However, if the cell’s repair mechanisms are faulty, mutations will accumulate, and genetic damage that has not been repaired will be reproduced and passed to all daughter cells whenever the cell divides. In this way malfunctioning DNA repair machinery contributes to the genesis of some cancers.
When a normal cell senses that its DNA has been damaged, it will stop dividing until the damage has been repaired. But when the damage is massive, the cell may abandon any attempt at repair and instead activate its apoptotic suicide program. Cells have a limited life span to begin with, and thus they are programmed to die some time after differentiation (the life span of cells varies according to type; some white blood cells, for instance, live for hours, whereas certain neurons live for decades). To execute the program of cell death, the integrity of the genes instrumental in triggering the program must be maintained. In cancer cells the program is rendered inoperative following mutation of a protein known as p53, which occurs in about half of all cancers. Cells can also acquire immortality by bypassing senescence, which normally marks the end of a cell’s functional existence. That is achieved by acquiring mutations that prevent the shortening of the ends of chromosomes, or telomeres. Telomeres can be thought of as clocks; their progressive shortening with each round of cell division brings the cell closer to death (see below Telomeres and the immortal cell).
Significant prolongation of a cell’s life, whether through defects in apoptosis or telomere shortening, increases the chances that it will accumulate mutations in its DNA that transform the cell. Once the cell has been transformed, the process of mutation does not end. Indeed, technologies capable of detecting abnormalities in the exome (the portions of the genome that code for proteins) have revealed on average some 100 mutations per tumour cell. The mutations that exert the greatest effect in causing tumour formation are referred to as driver mutations. Driver mutations presumably give selective advantage to tumour cells, whereas the remainder of the random mutations that occur in a cell’s genome are simply taken along during each replication cycle and hence are known as passenger mutations.
The additional mutations and changes in a tumour cell’s genetic and epigenetic program are not without consequence. In particular, they may facilitate invasion and metastasis, which enable cells originating within a tumour to migrate away, ultimately coming to rest in a distant organ, where they may give rise to a new tumour (see below Invasion and metastasis).