The Cell Cycle

• Cancer develops from cells that are capable of deciding. All tissues in the body contain some cells that can divide and renew themselves

• A cell's ability to produce exact replicas of itself is an essential component of life. This process must be performed with great fidelity in order for whole organisms and species to propagate

• The molecular mechanism used to control the cell division cycle and replicate is highly organized and well conserved through evolution.
• In any proliferating mammalian cell, the replication of the entire genome and the division of the cell can be broken down into four distinct cell-cycle phases
  • G0: cell cycle arrest
  • G₁ phase: the cell undergoes biochemical changes to prepare for entry into S phase in which new DNA is synthesized, cellular contents excluding the chromosomes are duplicated.
  • S phase: each of 46 chromosomes is duplicated by the cell. The cell generates a complete copy of its genetic material and then proceeds into a second preparatory phase.
  • G₂ phase: the cell "double-checks" the duplicated chromosomes for error, making any needed repairs.
  • M phase: the replicated DNA is carefully condensed into compact chromosomes that are precisely segregated so that two daughter cells each receive a full complement of the genetic materials. Following mitosis, a proliferating cell directly reenters G₁ phase as it prepares for further replication.
• Any proliferating cell has an additional option, which is the entry into a quiescent state known as G₀. Entry into G₀ is an important decision in cellular development (such as in lymphocyte or skeletal muscle cell maturation).
• In vitro, it is largely governed by the presence or absence of growth factors or nutrients that allow the procession through G₁ into S phase
• If cellular environmental conditions are not right for a cell to proceed across the restriction point, it enters G₀ Importantly, entry into G₁ phase from G₀ may be reversible or it may be irreversible.
• Tumor suppressor genes are one of the most important regulatory mechanisms in the cell replication process. They are normal genes that slow down cell division, repair DNA mistakes, or tell cells when to die [a process known as apoptosis (programmed cell death]
• When tumor suppressor genes don't work properly, cells can grow out of control, which can lead to cancer
• A tumor suppressor gene is like the brake pedal on a car. It normally keeps the cell from dividing too quickly, just as a brake keeps a car from going too fast
• Types of tumor suppressor genes:
  • Caretaker genes: repairing DNA damage
  • Gatekeeper genes: apoptosis.
• When something goes wrong with the gene, such as a mutation, cell division can get out of control
• key player in the G₁-S checkpoint system is the retinoblastoma gene Rb
• Activation of Rb gene is done by phosphorylation of the Rb protein by cyclin D-dependent kinase.
• Inactivation of Rb by genetic alterations occurs in retinoblastoma and is also observed in other human cancers, for example, small cell lung carcinomas and osteogenic sarcomas.
• The p53 gene product is an important cell cycle checkpoint regulator at both the G₁-S and G₂-M checkpoints
• The p53 tumor suppressor gene is the most frequently mutated gene in human cancer, indicating its important role in the conservation of normal cell cycle progression. One of p53's essential roles is to arrest cells in G₁ after genotoxic damage, to allow for DNA repair prior to DNA replication and cell division
• In response to massive DNA damage, p53 triggers the apoptotic cell death pathways.

Apoptosis

• Apoptosis (sometimes called programmed cell death) is a cell suicide mechanism that enables multicellular organisms to regulate cell number in tissues and to eliminate unneeded or aging cells as an organism develops
• The apoptosis pathway involves a series of positive and negative regulators of proteases called caspases.
• In addition, apoptosis is accompanied by degradation of chromosomal DNA, producing the typical DNA ladder seen for chromatin isolated from cells undergoing apoptosis. The endonuclease responsible for this effect is called caspase­ activated DNase or CAD.
• Death receptors are cell surface receptors that transmit apoptotic signals initiated by death ligands. The death receptors sense signals that tell the cell that it is in an uncompromising environment and needs to die.
• These receptors can activate the death caspases within seconds of ligand binding and induce apoptosis within hours. Death receptors belong to the tumor necrosis factor (TNF) receptor gene superfamily and have the typical cysteine-rich extracellular domains and an additional cytoplasmic sequence termed the death domain
• The best-characterized death receptors are CD95 and TNF receptor TNFRl (also called p55 or CD120a)
• Mitochondria play a pivotal role in the events of apoptosis by at least three mechanisms:
  • release of proteins, g., cytochrome c, that trigger activation of caspases.
  • alteration of cellular redox potential
  • production and release of reactive oxygen species after mitochondrial membrane damage
• Another mitochondrial link to apoptosis is implied by the fact that Bcl-2, the anti-apoptotic factor, is a mitochondrial membrane protein that appears to regulate mitochondrial ion channels and proton pumps.
• The importance of the apoptotic pathway in cancer progression is seen when there are mutations that alter the ability of the cell to undergo apoptosis and allow transformed cells to keep proliferating rather than die.
• Such genetic alterations include the translocation of the bcl-2 gene in lymphomas that prevents apoptosis and promotes resistance to cytotoxic drugs

Molecular Genetic Alterations In Cancer Cells

• Most cancers are an acquired molecular genetic disease in which a single (or a few) clone(s) of cells accumulate cellular genetic changes that progress to the full-blown cancer phenotype
• Cancer can be characterized as a disease of genetic instability, altered cellular behavior, and altered cell-extracellular matrix interactions
• These alterations lead to dysregulated cell proliferation, and ultimately to invasion and metastasis.
• These genetic alterations include chromosomal translocations, inversions, deletions, amplifications, point mutations, and duplications or losses of whole chromosomes (aneuploidy).

Translocations

• Reciprocal translocations are typical of leuke­mias, lymphomas, and sarcomas. Although chromosomal reciprocal translocations are less common in solid tumors, they do occur
• The gene rearrangements caused by translocations have two principal effects:
  • they cause activation of proto-oncogenes by relocation to the site of active gene regulatory elements
  • they generate fusion gene products resulting from breakpoints within intrans of two genes on two different chromosomes

Chromosomal Deletions

• Certain general statements can be made about the kinds of chromosomal abnormalities seen
• The most common defects usually observed in solid tumors have been deletions in specific gene sequences, sometimes observed as a loss of a part of a banding region or the loss of heterozygosity of a specific genetic allele
• Deletion of genetic material in a cancer cell suggests loss of function that regulates cell proliferation or differentiation. More than 20 human solid tumors have been shown to have some type of chromosomal deletion.
• Some chromosome deletions appear to be specific for certain tumor types. These include deletion del(13)(q14q14) seen in retinoblastoma that results in loss of the rb tumor suppressor gene, the 11p13 deletion in Wilms' tumor.
• Deletions in the long arm of chromosome 5 (del 5q) are seen in a number of hematologic diseases, including chronic myeloproliferative disorders.
• Other chromosome deletions are observed in multiple kinds of cancer. These include deletions in the chromosome 3p13-23 region in small cell carcinoma and adenocarcinoma of the lung, renal cell carcinoma, and ovarian adenocarcinoma

Point Mutations

• Point mutations that lead to single base changes in a DNA sequence. These mechanisms are involved in chemical carcinogenesis, activation of proto-oncogenes, and loss of function of some tumor suppressor genes
• Suffice it to say here that reaction of DNA with carcinogenic chemicals or as a result of spontaneous mutations due to oxidative damage can lead to the formation of base adducts that can cause base mispairing during DNA replication or loss of an adducted nucleic acid base producing an abasic site in the DNA chain.
• Such abasic sites may then be filled with an inappropriate base during DNA repair or replication, leading to a point mutation. If this mutation is in a regulatory element of a gene, loss or alteration of regulation of gene expression can occur.

Aneuploidy

• It is an abnormal chromosome number. Changes in cell ploidy, however, are associated with a variety of tumor types in their advanced stages and may be random in the sense that no definitive pattern of chromosome number is associated with a given tumor type.
• Although leukemias and lymphomas, as noted above, often contain reciprocal translocations and point mutations, they generally remain diploid or near diploid. This is not the case for carcinomas. In the latter, dramatic gains and losses of chromosomal material (aneuploidy) frequently occur.

Genomic Imprinting

• The process by which the expression of one of the two parental genes is shut off in the embryo. Mammals inherit two complete sets of chromoso1nes, one from each parent, and thus two copies of every autosomal gene
• Both copies of parental genes may be expressed, but sometimes only one of the two parental genes is expressed. (The other allele is said to be "imprinted"

Loss Of Heterozygosity

• Deletion of genetic material is a very common event in human Indeed, it is the most frequently observed genetic abnormality in solid tumors. These deletion events often involve loss of heterozygosity (LOH) of the expression of either the maternal or paternal alleles of a gene.
• If this is accompanied by mutation of the remaining allele, as is sometimes the case for a tumor suppressor gene such as p53, an important mechanism to regulate cell proliferation and differentiation is lost. An early observation of LOH in human cancer was by Solomon et al., who showed that about 20% of human colorectal cancers had undergone allelic loss on chromosome 5q

Oncogene

• Proto-oncogenes are genes that normally help cells grow
• When a proto-oncogene mutates (changes) or there are too many copies of it, it becomes  a "bad" gene that can become permanently turned on or activated when it is not supposed to be
• When this happens, the cell grows out of control, which can lead to cancer. This bad gene is called an oncogene

VIRAL CARCINOGENESIS

• Some human cancers are considered to be caused by viral infection either directly or indirectly.
  • By ''directly," I mean that the viral gene(s) can themselves cause cells to become malignant (sometimes also requiring the loss of a tumor suppressor gene).
  • By "indirectly," I mean that viral infection may simply cause the progression of malignant cell growth by producing an immunodeficiency state (e.g, non-Hodgkin's lymphoma in HIV-infected patients) or by stimulating the proliferation of already transformed cells.
• Epstein-Barr virus has been linked to four different types of human cancer: Burkitt's lymphoma (BL), nasopharyngeal carcinoma (NPC), B-cell lymphomas in immunosuppressed individuals such as HIV-infected patients, and some cases of Hodgkin's lymphoma. The evidence is strongest for an association with BL and NPC.
• Epidemiological evidence strongly points to a link between chronic hepatitis B virus (HBV) infection and hepatocellular carcinoma (HCC).
• Only two HPV subtypes have been closely associated with cervical cancer, HPV 16 and HPV 18

ANGIOGENESIS

• Is the term used to describe development of new blood vessels from pre-existing ones. This is the process that takes place during wound healing, the reproductive cycle, and in tumors
• In growing tumors, endothelial cells that will form the rudiments of new blood vessels may proliferate 20 to 2000 times faster than normal tissue endothelium in the adult
• Initiation of the angiogenesis response is triggered by several factors. Among these are VEGF family members, angiopoietins, and factors that facilitate blood vessel formation by modulating extracellular matrix (ECM) production or differentiation of cell types involved in blood vessel
• As noted above, angiogenesis is also a normal process by which new blood vessels are formed, for example, in the development of the placenta, in the vascularization of developing organs, and in wound healing. Under these conditions, however, angiogenesis is highly regulated, being turned on for specific periods of time and then shut off
• It is an unregulated form of angiogenesis that occurs in tumors and in certain other diseases, such as arthritis, age-related macular degeneration (AMD), diabetic retinopathy (DR), and hemangioma
• There are data indicating that tumors of 1 to 2 mm in diameter can persist in tissue without a tumor-derived vasculature.
• Epithelial cancers do not develop normal vascular beds like normal tissues and depend to a large extent on the diffusion of oxygen and substrates for growth. When tumor cells are too far away from the capillary blood supply for diffusion to provide the needed nutrients the cells may die. This explains why the core of large solid tumors is often necrotic.
• As long tumors grow and progress to a more malignant cell type, however, this process becomes limiting. At that point, tumors may be stimulated to release angiogenic factors that induce capillary outgrowth from the host's surrounding normal tissues into the tumor.

Cancer Spread

• Malignant tumors spread locally to surrounding tissues and distally
• Normal cells are glued together by molecules called cadherins, tumor cells lose the normal cadherin expression detachment.
• Tumor cells attach to the basement membrane and then destruct the ECM by proteolytic enzymes (collagen is the main component of ECM) leading to the migration of tumor cells

Pathways Of Dissemination

• Local Invasion
• Seeding of body cavities and surfaces:
  • Malignant cells penetrate into a natural open field e.g peritoneal cavity.
  • Direct transplantation of tumor cells by surgery.
• Lymphatic spread: mainly carcinomas
• Hematogenous spread: mainly sarcomas (liver & lung commonly involved)