20.2:
Cancers Originate from Somatic Mutations in a Single Cell
Every cancer phenotype starts as a single mutation event in one of its ancestor cells. During the subsequent cell division cycles, the daughter cells develop additional mutations to gain oncogenicity.
There are over 570 genes that are frequently mutated in cancer, and the majority of them are somatic mutations. That means that such mutations are not inherited but developed in the body's somatic cells during an organism's lifetime.
However, a single mutation is not sufficient to turn a healthy cell into a cancerous cell. At least five to six independent and rare genetic alterations in succession over a period of time can result in malignancy. This multi-step process of cancer development is explained by multi-hit theory.
Consider a group of healthy somatic cells. In rare instances, one of the cells may acquire a random mutation in one of its cell cycle regulator genes – gene X, allowing it to divide slightly faster than the neighboring cells.
Suppose the mutation is left unrepaired by the DNA repair enzymes. In that case, the progenies of this cell will carry the same mutated gene X. Over time, one of these clonal cells may acquire another random mutation in a different gene, Y.
This new mutation might allow the daughter cells to accumulate random mutations at a rate faster than their healthy neighbors and other clonal cells.
Subsequently, a third mutation in gene Z, another cancer-critical gene, may allow cells to escape terminal differentiation and apoptosis. Such mutant cell types continue to grow to form a mass of abnormal cells.
Further, a random mutation in metabolic pathway genes may allow cells to increase metabolism to fuel rapid growth and tumor formation.
For instance, mutations in the cancer-critical genes, such as APC, c-Myc , K-Ras, and p53, are frequently observed in patients with colon carcinoma.
20.2:
Cancers Originate from Somatic Mutations in a Single Cell
Cancer arises from mutations in the critical genes that allow healthy cells to escape cell cycle regulation and acquire the ability to proliferate indefinitely. Though originating from a single mutation event in one of the originator cells, cancer progresses when the mutant cell lines continue to gain more and more mutations, and finally, become malignant. For example, chronic myelogenous leukemia (CML) develops initially as a non-lethal increase in white blood cells, which progressively increases for several years before turning into a lethal condition.
Mutations in the genome can arise due to errors during DNA replication or DNA damage caused by carcinogens - agents that can trigger cancer-causing mutations in the gene. Such mutations can either be a point mutation; for example, exposure to carcinogen aristolochic acid causes A to T base substitutions. The mutations can also be indels, large-scale chromosomal deletions, and translocation, as seen in breast cancer BRCA1 and BRCA2 gene mutations. The CML cells have the Philadelphia chromosome, where a part of the chromosomal segment is translocated between chromosomes 9 and 22.
The evolution of cancer cells from normal cells is described as an example of Darwinian evolution, where each cancer-critical mutation is naturally selected and fixed in the population. The development of cancer typically requires a substantial number of independent, rare genetic mutations and epigenetic changes in a single cell's lineage. This may be the reason for the increased incidence of cancer as a function of age. For instance, the median age for breast cancer diagnosis in patients is 61 years; for colorectal cancer, it is 68 years, and for lung cancer, it is 70 years.
Suggested Reading
- Molecular Cell Biology, Lodish, 8th edition, Pages 1036-1037.
- Molecular Biology of Cell, Alberts, 6th edition, Pages 1093-1094