Cancer cells, Oncogene/Proto-oncogenes, and Tumore suppressors

Overview and Characteristics of Cancer

Cancer is defined as a genetic disease that results specifically from mutations occurring within somatic cells. It is primarily characterized by uncontrolled cellular proliferation, which leads to the formation of a mass known as a tumor or neoplasm. These tumors possess the capacity for progression, which involves invading neighboring tissues and spreading to more distant sites in the body, a process known as metastasizing.

The Development of Cancer (Oncogenesis)

The development of cancer, or oncogenesis, is driven by mutations in genes that regulate cell growth and programmed cell death. Cancer can occur through two primary pathways: hereditary and sporadic. When cancer is part of a hereditary cancer syndrome, the initial cancer-causing mutation is inherited through the germline and is consequently present in every cell of the body. However, most cancers are sporadic, meaning the mutations occur in a single somatic cell. This cell then divides and develops into a cancer.

Cancer-Critical Genes

Researchers have identified over 100 genes that are repeatedly altered in human cancer, termed cancer-critical genes. A cancer-critical gene is defined as any gene whose mutation contributes to the causation of cancer. These genes are categorized into two broad classes based on whether the risk of cancer arises from an excess of gene product activity or a deficiency.

The first class consists of proto-oncogenes. These are genes for which a gain-of-function mutation drives a cell toward a cancerous state. When these genes become mutated and overactive, they are referred to as oncogenes. The second class consists of tumor suppressor genes. These genes create a danger of cancer when a loss-of-function mutation occurs, removing the regulatory checks on cell growth.

Nature and Function of Proto-Oncogenes

Proto-oncogenes are a group of genes that cause normal cells to become cancerous when they undergo mutation. Under normal conditions, proto-oncogenes promote cell cycle progression and control protein products that manage cell growth and differentiation. They have been identified in various signal transduction cascades that stimulate cell division, inhibit cell differentiation, and halt cell death—processes essential for normal human development and the maintenance of tissues and organs.

Mutations in proto-oncogenes are typically dominant in nature. The resulting oncogenes exhibit increased production of proteins, leading to increased cell division, decreased cell differentiation, and the inhibition of cell death. Collectively, these phenotypes define the characteristics of cancer cells. Because of this relationship, oncogenes are currently a major molecular target for the design of anti-cancer drugs.

Mechanisms of Converting Proto-Oncogenes to Oncogenes

There are more than 40 different human proto-oncogenes known today. These genes can be converted into oncogenes through mutations that increase the expression level or the activity of the gene product. Key mechanisms include point mutations, deletions, or insertions that lead to a hyperactive gene product or occur in the promoter region to increase transcription. Additionally, gene amplification events can create extra chromosomal copies of a proto-oncogene.

Chromosomal translocations are another significant mechanism. These events can relocate a proto-oncogene to a new chromosomal site that leads to higher expression levels. Alternatively, translocations can lead to a fusion between a proto-oncogene and a second gene, resulting in a fusion protein with oncogenic activity.

Specific Examples of Proto-Oncogenes and Receptor Signaling

Many proto-oncogenes play critical roles during embryogenesis by stimulating growth and proliferation or negatively regulating cell differentiation. These activities are typically turned off once developmental processes are complete. However, if they remain high or are inappropriately reactivated later in life, cancer may result.

A number of proto-oncogenes code for cell surface receptors that bridge communication between the extracellular environment and the cell interior. These transmembrane receptors consist of an extracellular region (the antenna for signals), a transmembrane region spanning the plasma membrane, and an intracellular region that often possesses enzymatic activity or associates with internal proteins.

Cells respond to growth factors or angiogenic factors (which stimulate blood vessel formation) through the binding of extracellular ligands to these receptors. Ligand binding triggers a conformational change in the receptor, activating the intracellular domain and a signaling chain that regulates growth, proliferation, angiogenesis, or death. Examples include EGFR (epidermal growth factor receptor) and KDR (vascular endothelial growth factor receptor, involved in angiogenesis). Other examples include intracellular signaling proteins like HRAS and KRAS, as well as cell cycle regulators such as cyclin D1 ($CCND1$) and cyclin E1 ($CCNE1$).

The Philadelphia Chromosome and Chronic Myelogenous Leukemia

Oncogene activation can occur through specific chromosomal translocation events, with the Philadelphia chromosome being a prime example. In this instance, one end of chromosome 9 is exchanged with one end of chromosome 22. This results in the unregulated expression of the fusion protein BCR-ABL. This fusion protein activates a repertoire of other proteins involved in cell cycle regulation and the stimulation of cell division. The Philadelphia chromosome is specifically associated with chronic myelogenous leukemia (CML) and several other forms of leukemia.

Tumor Suppressor Genes and Growth Inhibition

Tumor suppressor genes are essential for maintaining normal cell growth control by curtailing unregulated progression through the cell cycle. Their protein products normally repress cell growth and division. Therefore, a loss of function due to mutations or other alterations removes the restraints on cell growth, leading to malignant transformation and neoplastic changes.

While some cancers, such as retinoblastoma, occur early in life and are associated with a single gene mutation ($RB1$), cancer is more often a multistep process. Cells acquire a series of mutations that collectively decrease tumor suppressor function and increase proto-oncogene function. This result is a heterogeneous tumor cell population that grows and divides without restraint and fails to respond to cell death signals.

Case Studies in Tumor Suppression: RB1 and p53

The retinoblastoma gene ($RB$) was the first tumor suppressor gene to be cloned, in 1987. Its function is to prevent excessive cell growth by inhibiting cells in the $G_1$ phase of the cell cycle. Inactivation of $RB$ occurs via phosphorylation, which allows the cell to progress into the $S$ phase. Mutant $RB$ encodes a dysfunctional protein that permits unregulated progression out of the $G_1$ phase.

The most frequently inactivated tumor suppressor gene is $p53$, often referred to as the "guardian of the genome." It encodes a protein with a molecular mass of $53\,KD$. More than half of all human cancers involve mutations in $p53$. This gene is crucial for preventing cancer because it regulates gene expression for growth control, facilitates DNA repair by sensing damage and causing $G_1$ arrest, and triggers apoptosis when DNA damage is beyond repair. A loss of $p53$ function contributes significantly to genomic instability within cells.

Genetic Nature of Mutations: Dominant vs. Recessive

Genetic mutations in cancer-critical genes confer a growth advantage, allowing for selective growth of the mutated cells. Generally, proto-oncogenes are "activated" to oncogenes; since these genes normally regulate growth, a mutation in only one copy (allele) is required to favor unregulated growth. Thus, oncogenes are dominant in action.

In contrast, tumor suppressor genes are generally "inactivated" by mutations or deletions. For a complete loss of growth control to occur, both copies of the tumor suppressor gene must be mutated or lost. Consequently, these genes act recessively at the cellular level. This is often described as the "two-knockout" mutation model: the first knockout creates a susceptible carrier, while the second knockout leads to the total inability to stop cell growth.