Carcinogenesis

Contents

• Introduction
• Mutation
• Mutation

Introduction

Carcinogenesis is the process by which a normal, well behaved, obedient, considerate cell that works for the good of the organism as a whole is transformed into a rebellious, ill-disciplined, selfish menace to life. Much of the work in this field focusses on malignant tumours, although many of the principles are applicable to benign tumouors.

In order to become malignant, a cell must acquire a multitude of properties, most of which it will not possess during its normal life.

• Indepedent, unrestricted proliferative activity
• Evasion of detection and deletion by T lymphocytes (which can identify incipient neoplastic cells)
• Movement (most cells of the body do not need to move to do their job)
• Detachment from adjacent cells (movement capability is useless if the cell is anchored)
• Degradation of basement membranes and extracellular matrix (movement can only occur if a path can be forged)
• Survival in an alien environment (many cells are specialised for their host tissue environment)
• Angiogenesis (as the tumour grows it needs a blood supply and must induce one by secreting growth factors)
• Invasion of blood vessels and lymphatics
• Survival in the lymphovascular systems (which is a strange environment for most cells)
• Attachment to endothelial cells to allow exit from the lymphovascular system
• Infiltration of completely alien tissue and survival therein (successful metastasis)

Most cells lack most of these properties. However, with the possible exceptions of independent growth and evasion of T cells (which can be achieved by altering the expression of MHC molecules and other cytokines), all of the abilities are found and are essential in at least one normal cell type and therefore are encoded within the genome (for example, angiogenesis is part of healing and growth factors such as VEGF and FGF exist which stimulate the growth of new blood vessels; degrading basement membrane and the extracellular matrix is also required in healing and enzymes such as metallomatrix proteinases are available to accomplish this). The challenge facing a cell which has aspirations to malignancy is to activate and deactivate as necessary all the genes that are involved in conferring these skills.

Mutation

In a certain sense neoplasia is a genetic disease in that it is caused by damage to DNA. Damage to DNA which alters its sequence and/or function is known as a mutation. A mutation may be either harmful (by causing inherited genetic disease or neoplasia; a mutation can also be fatal to the cell), be beneficial (evolution) or have neither harmful nor beneficial consequences.

Mutations may arise either through the action of carcinogens, or through mistakes that occur during the replication of DNA. The latter event can be influenced by oncogenes. Carcinogens are agents that can injure DNA and are discussed below.

Several different types of mutations exist.

• Point
• Methylation
• Translocation
• Deletions

The designations 'gain of function' and 'loss of function' are sometimes used to describe the consequences of a mutation on the affected protein.

Point Mutation

A point mutation affects a single base in a DNA molecule. The correct base is mutated into one of the three alternatives. This can have several consequences.

The mutation can result in a different amino acid being incorporated into the protein. The different amino acid alters the function of the protein.

The mutation can convert the codon into a stop codon, resulting in an abnormally short protein that may well be useless.

A mutation in the promoter / inhibitor regions of a gene may cause them to be overactive, underactive, or unable to bind the usual molecules that regulate them.

A point insertion is a type of mutation in which one or a few extra bases are inserted into the DNA sequence. If the number of bases is a multiple of three this results in extra amino acids being incorporated into the protein. If the number is not a multiple of three the mutation disrupts the codon sequence that follows (frameshift mutation) resulting in complete misreading of the remaining part of the gene.

Abnormal Methylation

Methylation of parts of genes reduces or completely blocks their expression and is used to regulate genetic activity in a cell. This can be important in ensuring that cells only manufacture the proteins they need to do their job; while many genes code proteins that are essential for all cells, others confer specialist functions. If a methylated gene is inappropriately demethylated it is overexpressed. Conversely, aberrant methylation may inhibit a gene that is involved in protecting the cell against neoplastic transformation.

Translocations

Translocations are larger scale mutations and involve part of a chromosome splitting off and fusing with another chromosome. They are written in the short form t(a;b) or the full form t(a;b)(c;d), where a and b are the two chromosomes and c and d are the specific regions of the chromosomes involved. Translocations result either in fused (chimeric proteins) that behave abnormally or place the protein coding sequence of one gene under the control of the regulator sequence of a different gene such that the levels of the protein are over or underexpressed.

The Philadelphia chromosome that is found in chronic myeloid leukaemia is an example of an abnormal fusion protein. The translocation is between region 34 of the long arm of chromosome 9 and region 11 of the short arm of chromosome 22 (t9;22)(34;11) and creates the bcr-abl fusion protein (bcr is on chromosome 22). The fusion protein is a tyrosine kinase that has automatous, persistent activity and stimulates the cell to divide, as well as inhibiting DNA repair mechanisms.

The bcl-2 translocation which is encountered in follicular lymphoma is an example of a translocation that affects gene expression. The translocation shifts the coding region of the bcl-2 gene from chromosome 18q21 to the control of the heavy chain gene promoter (14q32), leading to overexpression of bcl-2. Follicular lymphoma is derived from germinal centre B cells and these cells normally do not express bcl-2.

Translocations only occur during cell division.

Deletions

Deletions are another form of larger scale mutation. As with translocations they happen only during cell division and involve deletion of all or part of a gene. An inversion is a related process in which the sequence of a segment of DNA is reversed.

Carcinogens

A carcinogen is a substance or other agent that is able to damage DNA and induce a mutation. A few different classes of carcinogen exist.

• Chemical
• Radiation
• Viral

Chemical carcinogens operate either by generating free radicals, which then injure DNA, or by binding to the DNA directly and causing the mutations themselves, rather than through an intermediary free radial. Ionising radiation also acts through free radicals.

The structures of the two purine bases are similar to each other, as are the structures of the two pyrimidine bases and mutations often take the form of interchanges between adenine and guanine, or between cytosine and thymine. This is sufficient to alter the meaning of a codon. Cytosine and thymine are particularly vulnerable to radiation-induced damage.

Cigarette smoke is the most common and best known source of chemical carcinogens. The carcinogenic components include polycyclic hydrocarbons, such as benzopyrene.

Some chemotherapeutic drugs have mutagenic properties and destroy malignant cells by inducing DNA damage. The high proliferative rate of many malignant tumours means that they are more susceptible to the harm than normal cells. Nevertheless, chemotherapy can be carcinogenic and patients are at an increased risk of developing a second malignant tumour.

Certain chemicals that are employed in industry are carcinogenic. Examples include asbestos (mesothelioma and lung carcinoma) and the aniline dyes employed in the rubber industry (transitional cell carcinoma of the bladder).

Cells are especially vulnerable to chemical and radiation carcinogens when they are dividing. A dividing cell has uncoiled and exposed its DNA and is in the act of manipulating it. An uncorrected mutation at this point will be transmitted to one or both daughter cells. Some carcinogens can only operate when the cell is dividing.

Virally mediated carcinogenesis has a different mechanism of action. Viruses are basically packets of DNA or RNA that reprogram the nucleus of a cell to make lots of copies of the virus. In a few instances this reprogramming can put the cell at risk of subsequent neoplasia.

Squamous cell carcinoma of the cervix is very strongly associated with infection by certain subtypes (16, 18, 31, 33, 35, 39, 45, 51, 52, 56, 58, 59 ) of the human papilloma virus (HPV). The viral DNA includes genes for two proteins, E6 and E7, that interfere with the normal regulation of the cell cycle. The viral E6 protein increases the degradation of p53 by the infected cell. The E7 protein binds to the retinoblastoma protein and liberates the human transcription factor E2F: the normal inhibitory of the retinoblastoma protein on cell division is neutralised.

Other virally related cancers are Burkitt lymphoma and B cell lymphomas in immunocompromised patients (both Epstein-Barr virus), some cases of hepatocellular carcinoma (hepatitis B and hepatitis C viruses) and human adult T cell leukaemia/lymphoma (HTLV-1 virus).

The bacterial species helicobacter pylori may also have a role to play in the carcinogenesis of gastric adenocarcinoma and gastric marginal zone lymphoma. The association with the lymphoma reflects the ability of the bacteria to stimulate a chronic inflammatory response in the stomach that over time becomes monoclonal.

Promoters and Initiators

The concepts of promoters and initiators of carcinogenesis were derived from studies in mice, although they are believed to be extrapolatable to human neoplasms. An initiator causes a permanent change in the cell that renders it susceptible to further neoplastic changes. A promoter takes advantage of these changes to carry the transformation of the cell into a tumour cell forward. A promoter is said to be ineffective if the cell has not been exposed to an initiator. An initiator alone that is not followed by a promoter is also ineffective in inducing neoplasia. The interval between initiation and promotion may be short or long.