A somatic mutation is a genetic alteration that occurs in a cell after conception. These changes can happen in any cell of the body, from skin to liver, but are not present in the germ cells (sperm and egg). Because they are not in germ cells, these mutations affect only the individual and are not passed on to their children. The altered cell can then pass the mutation on to all cells derived from it.
Somatic Versus Germline Mutations
To understand a somatic mutation’s impact, it is helpful to distinguish it from a germline mutation. The body’s somatic cells comprise all tissues and organs, while germline cells are the reproductive cells—the egg and sperm—that create offspring. This distinction determines how mutations are passed through generations.
A somatic mutation only affects a subset of cells within an individual and cannot be inherited. In contrast, a germline mutation occurs in a sperm or egg cell. Because these cells form a new organism, the mutation is incorporated into every cell of the offspring’s body. This makes the mutation heritable and is why germline mutations are often associated with genetic syndromes that affect the entire body.
How Somatic Mutations Occur
Somatic mutations can arise from both external and internal factors throughout a person’s life. A common external cause is exposure to environmental mutagens. For instance, ultraviolet (UV) radiation from sunlight can damage DNA in skin cells, and certain chemicals, like those in tobacco smoke, can also induce mutations.
Internal factors also contribute to somatic mutations. The process of cell division is not perfect, and errors can happen when a cell’s DNA is copied. Although cells have DNA repair mechanisms to fix these mistakes, some errors can escape. The accumulation of these uncorrected errors contributes to changes in a cell’s genetic makeup over time.
The Spectrum of Effects
The consequences of a somatic mutation are varied, depending on the gene altered, the cell type, and when the mutation occurs. Many mutations have no discernible effect on a person’s health. They may happen in a non-functional region of DNA or be repaired by the cell.
Some mutations can result in visible but harmless traits through mosaicism. This occurs when a mutation arises early in development, leading to an individual having distinct populations of cells with different genetic information. This can manifest as a patch of skin with a different color or two different colored eyes.
The accumulation of somatic mutations over a lifetime is also a contributing factor to aging. As cells divide and are exposed to environmental factors, the number of mutations adds up. This genetic wear and tear can gradually impair the function of tissues and organs, setting the stage for diseases that become more common with age.
The Link to Cancer
Cancer is a disease driven by the accumulation of somatic mutations within a specific lineage of cells. These mutations disrupt the controls that govern cell growth and division, leading to the uncontrolled proliferation that forms a tumor. This process requires multiple mutations in several genes over time.
Two main classes of genes are implicated in cancer. Proto-oncogenes normally function to stimulate cell division. When mutated, they can become oncogenes, which are like a car’s accelerator pedal being stuck down, causing cells to grow and divide continuously.
The other class of genes are tumor suppressor genes, which act as the brakes on cell division, repair DNA mistakes, or tell cells when to die. A mutation that inactivates a tumor suppressor gene is akin to the brakes failing, allowing a cell to ignore signals to stop dividing.
For a cell to become cancerous, it generally needs to sustain mutations in both proto-oncogenes and tumor suppressor genes. This “multiple hit” requirement explains why the risk of cancer increases with age, as there is more opportunity for these mutations to accumulate.