Cancer cells are the cells that divide and proliferate excessively out of control of the body's control mechanism. They may invade other tissues and if they are not controlled adequately they may kill the whole organism. Cancer cells differ from their normal counterparts in a number of attributes. Their shapes, the form of their nucleus, the arrays they assume within the tumor mass and others.

In many normal cells the nucleus is much smaller than its cytoplasm often only one-fifth as large, in many cancer cells the nucleus is almost as large as the whole cell, with only a small cytoplasmic rim around it. Analysis often shows increased amount of DNA per tumor cell reflecting an abnormal increase in chromosomal number and the nuclei carrying this DNA may take on a variety of unusual shapes. Significantly, an unusually high percentage of cells in a tumor are undergoing mitosis. Normal tissues by contrast usually have a low, even undetectable, percentage of dividing cells.

In summary cancer cells often lose organ or tissue specific traits. The energies of these cells are directed exclusively toward their own proliferation, they no longer focus on helping to build a functional organ or tissue. This loss of differentiated traits often signals the presence of what is termed a high grade, aggressive tumor, with poor associated prognosis for the patient.

The growth of tumor cells seems to be regulated by same internal program not by cues from the external environment that are communicated by growth factors. When a tumor cell is seeded in a petri dish for culture, its descendants, lacking contact inhibition, continue to divide without limit, over growing the surrounding monolayer of normal cells and creating the multi layered clumps of cells known as a focus. Loss of contact inhibition seems to represent the essence of the Cancer State-failure of the ability to respond to environmental cues by stopping growth.

By 1970 investigators had introduced SV40 polyoma or Rous Sarcoma viruses into culture of rodent or chicken cells and observed the resulting foci of altered cells. That was the ability to convert normal cells to cancer cells in the culture dish and it demystified the cancer process. It meant that tumors could be traced back to individual cells that had undergone specific alteration. These successes meant that the process of transformation by which a normal cell becomes a tumor cell could be studied directly. The viral model of human cancer pathogenesis focused on the so-called retroviruses.

These reverse viruses including the Rous Sarcoma virus, carry their genetic information in the form of RNA molecules that they copy into DNA upon infecting a cell in addition to their ability to pass from cell to cell. Transforming each. Some retroviruses can enter cells and hide within them in a latent state.

Upon entering a cell, the retrovirus particle copies its genomic RNA into double stranded DNA molecule by means of reverse transcription. The resulting DNA is then inserted into a site in the host cell chromosomal DNA. This process termed integration, result in a viral DNA genome-a provirus-that becomes established in the host cell chromosome and function like other cellular genes arrayed along the chromosomal DNA.

From the moment of its integration the chromosome associated provirus may be expressed issuing instruction (in the form of messenger RNA molecules). That result in cell transformation and the production of progeny virus particles. Days, weeks or years later, the expression of a latent provirus may be stimulated by some signal impinging on the cell. The provirus may begin to produce messenger RNA, and a cell that had lacked evidence of viral infection may suddenly release copious virus particles. In germ cells in ovary or testes can, as egg or sperm contributes to a new embryo, passing genes on to an organism in the next generation. In this way, a provirus passed through the germ line may be transmitted like an inherited gene. Such an endogenous provirus can actually become part of the genetic endowment of a species, no less than the other tens of thousands of genes that constitute its genome and define its uniqueness.

It is hypothesized that a version of this endogenous retrovirus model came to be called a virogen-oncogene and it led by a circuitous path to our current insight into cancer's molecular origin. They may develop the ability to direct malignant cell growth.

It is now clear that at least four human viruses are inciting factors or co-factors in the formation of specific cancers. Epstein Bar virus is an agent of Burkitt's lymphoma (Sub Sahara Africa) and Nasopharyngeal carcinoma (South East Asia) Hepatitis B virus is closely associated with hepatocellular carcinoma (world wide) and human T cell leukemia virus, a retro virus is a factor in adult T cell leukemia (Carabians, Southern Japan), (Gene & Cancer page 57).

In addition to viruses other factors such as X-rays carcinogenic mutagens, such as chemical pollutants in water and air, tobacco and others have been implicated in the cause of mutation of genes for producing cancer cells. In the following subjects I will summarize the structure of cellular genome and its changes that may occur during cancer processes.

The DNA molecule consists of two anti parallel complimentary strands of nucleotides (A-T-G-C) held together by hydrogen bonds between the basis of the nucleotides. When DNA is replicated by DNA polymerase, synthesis of the new strand is directed by complimentary base paring of the incoming nucleotide with the template strand.

During the DNA replication the two strands of the preexisting DNA molecule are separated so that each can serve as a template strand to direct the synthesis of a new strand by attaching to incoming nucleotides. The two resulting molecules thus consist of one old and one new strand.

If a cell inherit abnormal DNA with oncogenic property (or as the result of exposure to radiation or carcinogens which may cause mutation of DNA) it may produce miss-happen and distorted cells. Such cell may no longer be compelled by surface sensors to stop dividing into confine growth within tissue boundaries and to remain rooted. Thus, uninhibited and grow wild.

The nucleotides are named for the base they contain and are 1. Adenosin triphosphate (dATP). 2. Guanosine triphosphate (dGTP). 3. Cytidine triphosphate (dCTP) and 4. thymidine triphosphate (dTTP), Biology page 283.

A single strand of DNA is synthesized by linking the most proximal or alpha phosphate of the triphosphate at position 5 of an incoming nucleotide to the hydroxyl group at position 3 of the most recently added nucleotide. Synthesis is fueled by the release of the beta and gamma phosphates. Thus, the single-stranded DNA molecule is said to grow in a 5 prime (5') to 3 prine (3') direction. The resulting monophosphate nucleotides are called adenylic acid (dAMP) guanylic acid (dGMP), cytidylic acid (dCMP) and thymidylic acid (dTMP) and are abbreviated A, G, C and T.

The two complementary strands of the double-stranded DNA are held together non covalently by specific hydrogen bonds between the bases protruding from the sugar-phosphate back bone: dAMP forms two hydrogen bonds with dTMP and dGMP forms 3 hydrogen bonds with dCMP.

The precise order of the four nucleotides within genomic DNA confers the specificity of the genetic code. The chemistry of DNA and the genetic code is virtually the same for all organisms.

Thus, the molecular methods for analysis and manipulation of DNA from any source is universal (cancer - principles - practice of oncology page I). DNA polymerase is the name of the enzyme that is used in vivo to form DNA molecules. Addition of new nucleotide and synthesis of new single-stranded DNA is fueled by the release of the beta and gamma phosphatic thus the single stranded DNA molecule is said to grow in a 5 prime 5' to 3 prime 3' direction.

The polypeptide synthesis occur within the ribosomes in the cytoplasm and in addition to messenger RNA other protein carriers such as transfer RNA (tRNA) and ribosomal RNA (rRNA participate. (See ref. I page 304).

After discovery of the DNA structure evolution has devised another ingenious way of increasing complexity which is to divide the gene into several different segments and use them in different combination to make different protein. The protein-coding segment of a gene is called axons and the DNA in between are called introns. As a matter of fact introns are the non-coding system of the gene. The initial transcript of the gene is processed by a delicate piece of cellular machinery called as spliceosome which trips out all the introns and joins the exons together. Sometimes perhaps because of signals from the introns that have yet to be identified certain exons are skipped and a different protein is made.

The ability to make different proteins from the same gene is known as alternative splicing. Human genome also adds new elements to create expanded versions of proteins that they share with other organism.

Transcription is a regulatory protein which allows other genes to turn off and on (these proteins are called suppressor protein or promoting protein or gene).

Complexity of human expands geometrically because they have more genes than flies and worms and they also have a greater ability to alter proteins.

All genes located in a cell are distributed within 23 pairs of chromosomes. The distribution is not uniform but they are arranged in patches with some regions of chromosomes being gene rich and other regions being gene deprived. So the map of entire genome resembles a population map of US with urban area being dense.

Habitation and vast rural tracts occupied by these people. Repetitive sequence with an excess of nucleotides C and G (for Cytosine and Guanine) tend to be found m the neighborhood of genes while repeated sequences heavy on As and Ts (Adenosine and Thymidines) generally dominate throughout the non gene deserts. For reasons that remain unknown some chromosomes for example chromosome 19, is among the smallest of 23 chromosomes yet it is most densely packed with gene as well as non-coding sequence that lean toward Cs and Gs As the result chromosome 19 is a karyotype display very few dark bands.

The great bulk of non-coding sequence of human DNA are not foreign born but represent the offspring of bits of genetic material that long ago broke away from chromosome or part of the cells.

In the year 1947 Dr. McClintock announced his important discovery of 22 existence of transposable gene in corn. It took years that her colleagues recognized the importance of her discovery and honored her with the Noble Prize I the year 1983 at the age 81. She continued her experiments at Cold Spring Harbor Laboratory in New York until she died in 1992 (Ref. I page 363).

Transposons also called transposable gene are pieces of DNA that can move from me location to another in the cell's genome. They are responsible for combing several genes into a simple plasmid by moving the genes to that location from different plasmids. Transposons are considered RNA and protein synthesis machinery of the cell that decided to break away from chromosome or apart of the cells to go to free land. They are called "jumping genes" too. The little entrepreneurs figured out how to produce themselves and reinsert their copies and their progeny back into mother genome.

Researchers examining the landscape of human genome have identified four classes of transposoms. One class is called DNA transposoms, appear to be dead near fossils in the DNA that have lost the signals they need for effective replication. They are gradually decaying out of existence (due to evolution of human being). The second class called LTR transposoms called by Dr. Lander on the critically endangered list and may soon go extinct due to evolution of human being. Only two transposom families in the genome are considered active replicating themselves tidally in the course of transmission from parent to child. One is genome parasite called a line which are mostly located at rich region of the chromosomes far from genes and therefore far from DNA quality control. The other is ALU element, which is widely scattered along the chromosome including the line located area. This interposone preferentially clustered in the GC regions and usually they are useful to human biology. It is just possible that ALU sequence of DNA helps make us human. ALU elements are found only in higher primates and the core of ALU sequence is responsive to a large family receptor proteins the so called nuclear receptor super family (New York Times 2/13/2001).

The genomes of a cell can be influenced by other factors either suppressors or stimulants such as platelet derived growth factor (PDGF). It is now often possible to predict important properties of a protein long before any direct biochemical analysis of the protein has been undertaken by making use of the large collections of sequence of other proteins that have been stored in computerized data base. Structural similarity with other, known proteins may predict the location of an oncogenic protein within the cell, its chemical modification or its enzymatic activities (Gene & the Biology of Cancer, page 97)

In 1983 one of the first and most striking stimulatory growth factor was discovered. There were similarities between the amino acid sequence of the protein product of this stimulatory protein and the sequence of a secreted growth factor of human PDGF. The Link indicated that the deregulated production by an oncogene of a growth stimulatory factor could lead to cell transformation. The amino acid number of this newly discovered protein when computed by computer was 150 in comparison to the number of PDGF amino acid number which was 80 (see picture page 96, Ref. 4). Many of the proteins involved in cellular growth control are related to each other by sequence and function, they are encoded by members of gene families that have common evolutionary origins and may be partially redundant in their functions.

An oncogen may disrupt the carefully balanced molecular controls on cell proliferation to such an extent that malignant growth ensue.

On the other hand there are tumor suppressor genes which prevent abnormal growth and proliferation of cells so prevent cancer development. These genes are called suppressor genes, tumor suppressor genes, or even anti oncogen. They appear to be as important as proto oncogens in governing the normal cell proliferation.

Nevertheless heritable mutations are by far the exception. Rather most alteration for canca producing genes is acquired in the forms of Somatic mutations, chromosomal translocations, deletions, invasions, amplification or point mutation in the genome and chromosomes. Tumor suppressor gene products are negative growth regulators and their loss of function promotes carcinogenesis. Four basic approaches have been used for the identification of genes involved in the study of cancer:

For example in cases of HBV, the DNA of most hepatocellular carcinoma harbor integrated HBV DNA in amount varying from a single copy to multiple copies per cell. The integrated DNA is usually rearranged having undergone deletion, inversion and related modifications. It is thought that insertional mutagenesis may cause tumors. HBV infection also can trigger a host inflammatory and immunologic response that in some cases results in hepatocyte toxicity coupled with hepatocyte regeneration over 30 to 50 years of incubation between acute infection and HCC, the abnormal proliferation, regeneration of the cells increases the chances that mutations will arise in one or more hepatocyte proto oncogenes (Clinical oncology, Jan. 2000, page 8 Ref. No. 5).

The molecular biology of neoplasia revolves basically around two questions. 1. What change in DNA/gene structure triggers the cells to become neoplastic? And 2. How do changes in DNA/gene structure affect gene regulation in preneoplastic and neoplastic cells? It is interesting to know that all cells contain the genes for globin and insulin etc. but only erythroid cells make globin and only pancreatic cells make insulin.

The cause of this phenomenon is that the genes can be regulated at any of the steps in information flow from DNA to protein. A protein can be ova expressed or under expressed by amplifying or deleting its gene, respectively. Some mutations alter portion of the gene called enhancers and promoters which are responsible for assembling the proper transcription factors and cofactors to form a competent transcription complex. Mutation in transcriptional promoters reduces or eliminates transcription of the gene or disrupts the normal regulation of gene transcription.

Mutation in introns can reduce or eliminate the processing of mRNA precursors. Any change in the coding region can have multiple effects. Changing in amino acid sequence can destroy the ability of the protein to function as an enzyme by modifying its active site or its ability to fold properly. Mutation that changes any of the protein phophorylation sites to amino acids that can not be phosphorilated may result in abnormal regulation of enzymatic activity. It is important to stress that neoplasia is the consequence of mutations and that these mutations can have multiple sequels, Ref. 5 page 6).

The inactivation of tumor suppressor genes or the transformation of positively acting proto oncogenes into cancer causing oncogenes can occur via small intragenic deletion that inactivates the protein product or via point mutations. As the result, the cell would be prone to develop cancer character. The example of such an inactivation of tumor suppressor gene and development of tumor is that of inactivation of gene called P53.

This gene (P53)is a transcription factor that is induced following the DNA damage caused by ultraviolet light or chemical toxins. This gene (P53) is thought to function as a tumor suppressor by acting as a brake to allow the DNA that had been damaged to repair itself before replicating. Loss of (P53) function is associated with a broad spectrum of human cancer. In most or all of these cases the (P53) gene in the tumor is modified by point mutation in the region of the protein that are critical for P53 functions. These individuals will be susceptible for development of variety of cancers such as cancer of breast, brain tumor, sarcoma, (Ref. 5 page 7).

This (P53) is frequently deleted in these cases and mutational inactivation of the remaining allele occurs in more than 90% of the cases, further more reintroducing a wild type (P53) gene into lung cancer cells by injection into tumor dramatically blocks tumor cell growth despite multiple other mutation, (Ref. 5 page 851).

In addition to hereditary genetic factors and viruses there are evidences that indicate additional factors may be involved in carcinogenesis. These lists include chemical carcenogenesis, hormones, and radiation including X-Rays and ultraviolet lights, (Chemical Oncology, page 8).

2. There is a central enzyme in DNA that catalyses the methylation of uridin monophosphate to thymidine monophosphate. This enzyme is called "TS" or thymidiylate synthesease. Many antineoplastic drug development efforts have been expended in the search for the ideal inhibitor of this enzyme "TS". Progression of this field has resulted in development of the drug ZD 9331, which can be used in refractory cancers (Journal of Clinical Oncology, March 1, 2001)

3. PKC (Protein Kinase C family) are involved in growth of cancer cells by increasing nuclear oncogenic activities. Manipulation of various PKC izoenzymes by transfection and antisense oligonucleotide experiment indicates that they are likely to be dependent on izoenzyme expression. The inhibitor of PKC enzymes may have antitumor activity. Such an inhibitor (PKC 412) can be safely administered orally (Journal of Clinical Oncology, March 1, 2001).

4. Monocolonal antibody against a tumor may be obtained through recombinant technique. These antibodies may cause destruction of the tumor if administered either alone or in conjunction with chemotherapy and radiotherapy. These antibodies may mediate tumor destruction by both direct and indirect mechanism. Direct mechanism include binding to calcium channels resulting in increased induction of apoptosis or programmed cell death or binding to growth factor receptors resulting inhibition of ligand binding and suppression of transcription factors within the tumor cell. Monoclonal antibodies also may kill tumor cells indirectly through immunologic mechanism including antibody dependent cell mediated cytotoxisity and complement dependent cytototoxisity. (Seminars in Oncology VoL27, No.6, December 2000, page 64).

References