The human body is composed of trillions of cells, each containing DNA, the instruction manual for all cellular activities. While this genetic blueprint is stable, it is not static. Over a person’s life, changes can occur in the DNA of individual cells, which are known as somatic changes or mutations.
A somatic change is a genetic modification that happens in a body cell after conception and is acquired during a lifetime. As cells divide, these alterations can be passed on to all future cells that descend from the affected one. This means an individual can accumulate a variety of these changes in different tissues and organs over time.
Somatic vs. Germline Cells
To understand the impact of somatic changes, it is important to distinguish between the two categories of cells in the body: somatic and germline. Somatic cells encompass all cells not involved in reproduction, including skin, muscle, liver, and brain cells that form the body’s tissues and organs.
Germline cells are the reproductive cells, specifically the egg and sperm, responsible for passing genetic information from one generation to the next. This distinction determines the heritability of genetic changes. Somatic changes occur only in somatic cells, affect only the individual, and cannot be passed on to offspring.
A useful analogy is to think of DNA as a book. A somatic change is like a typo in one printed copy, altering only that specific copy. In contrast, a change in a germline cell is like a typo in the original manuscript before printing. Every copy made from that manuscript will contain the error.
Causes of Somatic Changes
Somatic changes arise from a combination of internal and external factors. These causes can be grouped into spontaneous errors that happen inside our cells and damage from environmental exposures.
One internal source of somatic changes is errors during DNA replication. Every time a cell divides, it must make a copy of its DNA. While this process is highly accurate, mistakes can happen, and the cellular machinery might insert the wrong base or skip a section. Although cells have repair systems to fix these errors, some can slip through and become permanent changes in that cell’s lineage.
External factors, called mutagens, also play a role. These are elements from the environment that can directly damage DNA. Ultraviolet (UV) radiation from sunlight, for example, can cause DNA damage in skin cells. Exposure to certain chemicals, such as those in tobacco smoke or industrial pollutants, can also induce mutations, as can some viruses that insert their genetic material into a host’s DNA.
Somatic Changes and Human Health
The majority of somatic changes that occur are harmless. Many happen in non-coding regions of the DNA, or the cell’s repair mechanisms successfully correct them. The gradual accumulation of these changes over a lifetime is a factor in the aging process, contributing to the general decline in tissue and organ function.
Sometimes, these changes can have more serious consequences for health, with a strong link to the development of cancer. Certain genes are responsible for regulating the cell cycle, telling cells when to grow, divide, and die. If a somatic mutation occurs in one of these regulatory genes, it can disrupt this controlled process.
Mutations in tumor suppressor genes, for example, can remove the normal “brakes” on cell division. Conversely, mutations in genes known as proto-oncogenes can turn them into oncogenes, which act like a stuck accelerator pedal, promoting constant growth. When mutations accumulate in several of these genes within a single cell, it can lead to the uncontrolled proliferation of cells, forming a tumor. This is why cancer risk increases with age, as there is more time for a cell to acquire the necessary mutations. The accumulation of somatic mutations has also been associated with other conditions, including certain neurodegenerative diseases.
Somatic Mosaicism
Somatic mosaicism is a condition in which an individual has two or more genetically distinct sets of cells within their body. This situation arises when a somatic mutation occurs very early in embryonic development. After the mutation, every cell that descends from that altered cell will carry the change, while the rest of the cells in the body remain unaffected. This creates a “mosaic” pattern of normal and mutated cells throughout an individual’s tissues.
The timing of the mutation is an important factor; the earlier it happens in development, the greater the proportion of cells that will be affected. Depending on which tissues contain the mutated cells and the nature of the gene altered, somatic mosaicism can lead to genetic conditions that appear in patches or affect only specific parts of the body. In some instances, it can result in a milder form of a genetic disorder compared to if the mutation were present in every cell.