Somatic mutations: what they are and how they affect health

Every time a cell divides, there is a chance of an error in copying the DNA. Over a lifetime, the body accumulates trillions of these mutations. Far from being a design flaw, this process is fundamental to survival. Science journalist Roxanne Cammy, a contributor to The Atlantic, documents in her book Beyond Inheritance how somatic mutations (those that occur in body cells rather than in eggs or sperm) are reshaping our understanding of cancer, aging, and several chronic diseases.

We are genetic mosaics

We each start from a single cell. By day five, we are roughly one hundred cells; in adulthood, the body contains around 37 trillion. Each cell division introduces the possibility of errors, and it is estimated that trillions of mutations occur every day. This makes us what scientists call genetic mosaics: organisms whose cells are not genetically identical to one another.

This idea has historical roots. In 1881, biologist Wilhelm Roux applied Darwinian principles to the inside of the body and proposed that cells compete with each other through natural selection. Single-cell sequencing technology developed over recent decades has confirmed and expanded that hypothesis with a level of detail unimaginable at the time.

Somatic mutations and cancer: rethinking treatment

One of the most important points in the book is the rethinking of cancer treatment. For decades, the dominant strategy has been to attack as aggressively as possible, with intensive chemotherapy and radiation aimed at eliminating all cancer cells.

The problem is that this approach places evolutionary pressure on tumor cells to develop resistance. What often compromises survival is not the initial tumor but the resistance that the treatment itself induces. An alternative perspective, grounded in Darwinian dynamics, proposes keeping drug-sensitive tumor cells as a counterweight to resistant ones, thereby prolonging disease control. In addition, many chemotherapy agents induce new mutations in the cells themselves, accelerating the very problem they are trying to solve.

The immune system and beneficial mutation

Not all mutations are dangerous. The immune system depends on them to function. B cells subject their DNA to a hypermutation process to generate a nearly unlimited variety of antibody shapes, capable of recognizing and neutralizing pathogens that do not yet exist. This is exactly what happens when we get vaccinated or recover from an infection: immune cells mutate to build more specific and lasting defenses.

People with hyper-IgM syndrome, a condition in which immune cells cannot mutate correctly, face fatal risk from common infections. Their existence confirms that mutation is not a flaw but an essential function.

The brain: unique neurons and the Alzheimer's link

Each neuron carries an average of 1,500 unique mutations. Recent research suggests that immune cells in the brain (microglia) can accumulate mutations similar to those found in cancer cells, and that this may be a causal mechanism in Alzheimer's disease. In these patients, mutated microglia would trigger deep inflammation in brain tissue, opening a new line of investigation into the origin of the disease. The same phenomenon is being studied in schizophrenia, epilepsy, and autism.

CHIP: mutations that age with us

Clonal hematopoiesis of indeterminate potential (CHIP) is a condition in which a clone of blood stem cells acquires mutations associated with higher risk of cardiovascular disease, cancer, and Alzheimer's. It affects 10 to 20 percent of people over 70 and is virtually never detected in routine care, despite the existence of tests to identify it. In men, loss of the Y chromosome, which occurs in more than 40 percent of males over 70, has also been linked to serious diseases.

Natural autocorrection: the body that repairs itself

One of the most encouraging revelations in the book is that the body also corrects itself. There are documented cases of children born with severe immunodeficiencies in which some cells acquired secondary mutations that reversed the problem and restored immune function. In patients with epidermolysis bullosa, a hereditary skin disease, fully healthy skin patches have been observed where cells had spontaneously reverted the causative mutation.

Understanding these mechanisms could open the door to natural gene therapies and preventive interventions before a disease reaches clinical manifestation.

The environment also writes in DNA

Behaviors such as smoking or sleeping poorly leave traces in the mutational profile of cells. Pesticides, pollution, and other environmental toxins induce mutations that accumulate over time. Advanced sequencing can now read those traces in DNA as if they were a record of past exposures. The reverse is also true: sleeping well and avoiding tobacco has been shown to correlate with a lower prevalence of mutant blood cells.

A new era of genetic medicine

The somatic mutation framework not only explains known diseases from a new angle but makes it possible to imagine more precise diagnostics, personalized treatments, and prevention strategies based on detecting high-risk mutations before they become disease. We are at the beginning of an era in which genetics is no longer only about inheritance but also about lived history.