“Single-Gene” Diseases: Why Common Mutations Don’t Always Mean Illness
For decades, the search for the genetic roots of disease has focused on identifying mutations – changes in our DNA – that seemed to directly *cause* conditions like thyroid cancer, ovarian insufficiency, and certain types of diabetes. The assumption was straightforward: find the faulty gene, understand the disease. But emerging research is challenging this long-held belief, revealing a far more complex picture of inherited illness. It turns out that many of these so-called “monogenic diseases” aren’t caused by single gene mutations acting in isolation, and plenty of healthy individuals carry these mutations without ever developing the associated condition.
A Shift in Understanding Genetic Inheritance
This isn’t to say that genetics doesn’t play a role. Rather, it suggests that the relationship between genes and disease is rarely a simple one-to-one correspondence. “It kind of challenges our standard dogma,” explains Caroline Wright, a professor of genomic medicine at the University of Exeter in England. She and her colleagues are finding gene variants that appear to cause disease in clinical settings, but are surprisingly common in the general population, where they don’t necessarily lead to illness. “In much of single-gene genetics we’ve often assumed that a particular genetic cause is necessary and sufficient, and everything else is irrelevant,” Wright told Live Science. “And what we’re seeing is that that’s not necessarily true.”
This discovery has significant implications for both genetic counseling and the development of modern treatments. If a gene variant doesn’t guarantee disease, understanding *why* some people develop illness whereas others don’t becomes crucial.
The Complications of Penetrance
The foundation of modern genetics, established by Gregor Mendel in the mid-1800s, describes how offspring inherit genes from their parents. Some genes are dominant – requiring only one copy to be expressed – while others are recessive, needing two copies. Though, the reality is often more nuanced. Genes interact with each other and with environmental factors, influencing a person’s traits. “Penetrance” describes the likelihood that a person with a specific genetic combination will actually express the associated trait or disease. Some diseases were previously classified as having 100% penetrance, like Tay-Sachs disease, where individuals with two copies of a specific mutated gene invariably develop the condition.
However, conditions like Crohn’s disease and schizophrenia are considered polygenic, arising from the interplay of many genes and environmental influences. Scientists now calculate risk scores based on a person’s genetic spectrum, rather than relying on single-gene triggers. The shift in understanding has been driven by the availability of large genetic databases from healthy populations, something that wasn’t possible when gene sequencing was expensive and limited to those already diagnosed with a disease.
Researchers, like Dr. Eric Pierce at Mass Eye and Ear, point to “ascertainment bias” as a key issue in earlier studies. Focusing solely on individuals with a disease can create a skewed picture of which gene variants are truly causative. Large-scale population studies, such as the U.S. National Institutes of Health All of Us cohort and the U.K. Biobank, are helping to overcome this bias by including genetic data from millions of people, regardless of their health status.
For example, studies on inherited retinal degenerations revealed that individuals with gene variants previously thought to almost always cause severe vision loss actually experienced vision loss less than 30% of the time. Similar patterns have emerged in research on genes linked to thyroid cancer – where variants once thought to be highly impactful are now found to cause disease in only 2% to 19% of the general population – and ovarian insufficiency. A study published in *JAMA Network Open* highlights these findings.
The Role of the “Supporting Cast”
If certain genes are the “lead actors” in disease development, the rest of the genome and environmental factors represent the “supporting cast.” In families with a history of a particular disease, the lead actor genes are often shared. But so is the supporting cast, making it difficult to isolate the impact of secondary genes. Population-level studies, with their diverse genetic backgrounds, allow researchers to commence unraveling the role of these other factors.
Researchers are identifying a growing list of gene variants that appear necessary, but not sufficient, to cause disease. For instance, studies have found that variants linked to brittle bone disease (osteogenesis imperfecta) cause the condition in only 21% to 40% of individuals who carry them. Similar patterns are being observed in genes associated with rare childhood eye cancers, mitochondrial diseases, and certain forms of inherited diabetes. A 2024 study in *The American Journal of Human Genetics* details these findings.
The emerging picture is one of complex interactions, where the effect of a gene variant can be modified by other genes and environmental influences. As Anna Murray, a professor of human genetics at the University of Exeter, explains, “Just because you can demonstrate that your variant affects a process [in the lab] doesn’t actually necessarily mean that that’s what is happening in your particular cell in that system.”
What This Means for Risk Assessment and Treatment
These findings are reshaping our understanding of individual risk. Studies of patient groups can establish an upper limit on the risk associated with a genetic variant, while population studies can provide a lower limit. The challenge now is to help patients understand what this range means for their personal risk – a question researchers are actively working to answer.
As genetic screening becomes more widespread, accurate risk assessment is increasingly key. Parents undergoing IVF and screening embryos for genetic conditions, for example, may make different decisions depending on whether the risk of disease is estimated at 100% or 20%. Similarly, individuals receiving genetic counseling need to understand whether their risk is as high as suggested by studies on patient populations, or if they have protective factors that lower their risk.
This new understanding may also refine gene therapy treatments. While these therapies will continue to be important for those with disease-causing genes, understanding the broader genetic context could help improve their effectiveness. Researchers are exploring how additional genetic factors influence disease expression, potentially identifying new therapeutic targets.
Looking Ahead: Refining Genetic Understanding
Researchers are now focusing on large-scale collaborations to further investigate the interplay between genes and environment in conditions like retinal disorders and ovarian insufficiency. The goal is to move beyond identifying single genes and towards a more holistic understanding of disease development. While treatment options remain limited for many of these conditions, a deeper understanding of the underlying mechanisms could pave the way for more effective preventative and therapeutic strategies. As Michael Hayden of the University of British Columbia notes, “When therapies are available for those diseases, early treatment—and particularly early treatment for degenerative disease of the brain and the eye—is better than later, because you can’t replace neurons.”