It is common to think of genetic diseases as simple and straightforward. Many people have a mental model of one gene mapping precisely to one illness, the “gene for” the condition. While this is the case for a handful of conditions, it’s generally much more complex than that. Each organ, each function, each body part is the domain of a number of genes scattered across the chromosomes of each cell, and any of those genes going awry could result in disease.
Now researchers say new mapping techniques may be able to help find more of the different genes responsible for a particular inherited condition than ever before. The team applied these techniques to Crohn’s disease, an autoimmune disease affecting about one in every thousand people. They analyzed the entirety of the human genome and found the most complete yet set of gene locations for any illness. Previously, 71 genes were associated with the condition. The newest analysis confirmed only 66 of those genes—but also found an additional 78 that had not previously been discovered, and identified a further 56 areas that appear likely to be associated with the condition.
Crohn’s disease is more than just genetics—it appears to result from interactions between an inherited genetic tendency and life experiences—but these genes explain a lot about how the disease is passed down, and may shed some light on how life experiences bring out the illness. The most likely theory is called the hygiene hypothesis. It proposes that exposure to parasites and microbes in childhood places demands on the developing immune system, but in the absence of these triggers, the immune response focuses on whatever is handy, including healthy tissue in the body itself. In Crohn’s disease, that means the small intestine, particularly a section called the terminal ileum, which becomes inflamed and constricted.
In addition, one recent study found a link between certain genes associated with Crohn’s disease and specific types of gut flora, normally harmless microorganisms that line the inside of the intestines and are an important part of the digestive process. The study found that different kinds of flora appear to be involved in the development of the disease depending on which genes the patient has. The better mapping may be able to help clarify this link.