- What is asthma?
- Can asthma run in families?
- When asthma runs in families, is the reason genes or environment?
- What genes are involved in asthma?
- What environmental factors are involved in asthma?
- When asthma runs in families, why don’t all family members have it?
- There is no asthma in my family. Does this mean I will never get it?
- How will discoveries about DNA help people and families with asthma?
What is asthma?
Asthma is a disorder of the airways in the lungs. Most people with asthma have little trouble breathing from day to day. Once in a while, however, asthma will flare up, causing a person to cough, wheeze, and feel short of breath. The airways are responsible for these symptoms.
Here’s what happens to an airway during an asthma attack:
- Muscle in the airway wall squeezes the airway narrower.
- The wall of the airway swells, further narrowing the air passage.
- The inside lining of the airway produces mucus, which can further block the air passage.
- Medication can prevent or reverse an asthma attack, but severe, untreated attacks can be fatal.
The role of inflammation
Asthma is also classified as a disease of the immune system. People with asthma have a smoldering immunological reaction in their airways called “inflammation.” The more inflammation in the airways, the more sensitive the airways, and the more likely an asthma attack. (Think of it this way: The more glowing embers in a fireplace, the more likely that paper thrown in will catch fire.)
Modern treatments for asthma are designed to reduce the inflammation in the airways (put out the embers), so asthma attacks never develop. Other treatments are designed to control an attack after it starts (put out the fire).
6-year-old Tiffany is visiting her friend Becky and discovers that Becky’s cat has had kittens. Tiffany knows that she’s allergic to cats; in fact, that’s why she can’t have one of her own. But she can’t resist playing with the cute little balls of fur. The kittens begin chasing each other around the room while Tiffany strokes the mother cat.
Soon Tiffany’s eyes begin to itch and tear and her nose starts to run. Of course, she rubs her eyes and wipes her nose on the back of her hand, breathing in even more cat dander. Tiffany is about to have her first asthma attack.
Tiffany senses something is wrong when she begins coughing and hears tiny whistling sounds coming from her chest. As the air passages in her lungs narrow, it becomes harder and harder for her to breathe. She tells Becky’s mother, who recognizes an asthma attack, and takes Tiffany to an emergency room. There, she is treated, and after awhile, can breathe normally again.
Jeff has been looking forward to a week of skiing in Colorado as long-awaited break from his desk job in Louisiana. He’s confident that his exercise program has prepared his 32-year-old body for whatever challenges he’ll face on the mountain. But, even as he is driving from the airport to the resort, the dry, cold Colorado mountain air has started irritating the airways in his lungs.
The next day, Jeff finds his exercise program has paid off. He gets a couple good runs twisting and turning through the moguls, and feels strong. But on the next run, the exertion and the cold dry air will give Jeff a new experience — an asthma attack.
Halfway down the slope Jeff stops to catch his breath. His chest feels tight and he hears wheezing as he breathes. He skiis a bit more, but his chest only feels tighter. A member of the ski patrol comes over and quickly recognizes an asthma attack in progress. The ski patrol gets Jeff down the mountain and into the clinic where he can be treated. It wasn’t the best vacation he ever had….
Can asthma run in families?
- Yes. We know this because of studies like one performed in Arizona in 1984 . Researchers looked at parents and children in 344 families. They found that:
In families where neither parent had asthma 6% of the children had asthma.
- In families where one parent had asthma 20% of the children had asthma.
- In families where both parents had asthma 60% of the children had asthma.
When asthma runs in families, is the reason genes or environment?
As in most human disease, the answer is “both.” Asthma has strong genetic components as well as environmental components. So, it’s a matter of nature and nurture, not nature versus nurture.
Studies of twins provide the clearest evidence for genes and environment both having a role. For example, in 1995 researchers looked at 325 pairs of twins
They found 84 pairs in which at least one twin had asthma.
If asthma were governed only by genes, then every time one identical twin has asthma, the other should have it, too. (Because identical twins have the same genes.) In other words, the “concordance rate” should be 100%. The table shows, however, that instead of a 100% concordance, there is a 59% concordance between identical twins. This shows that asthma has an environmental component.
If asthma were purely an environmental condition, then genes should make no difference at all. The concordance rate would be the same for identical twins and non-identical twins. The table shows, however, that the concordance rate is more than twice as high in identical twins (59%) as in non-identical twins (24%). This shows that asthma has a genetic component.
Other researchers have used statistical techniques to calculate how much of asthma is genetic and how much is environmental. A study of asthmatic twins in Finland, for example, showed that when one parent has asthma, genes explain 87% of the concordance difference between their twin children. But, when neither parent has asthma, the environment could explain 100% of the concordance difference between their twin children.
Most researchers would agree with the following statements:
- When asthma runs in a family, genes are a stronger contributor than environment.
- When asthma does not run in a family, genes are less strong, and environment is stronger.
These conclusions sound like pure common sense. But the human body is very complicated, and understanding comes slowly. The scientific community is hopeful, however, that new techniques to analyze the DNA in genes will rapidly advance our understanding of asthma and other diseases.
What genes are involved in asthma?
There is no such thing as a single “asthma gene.” Instead, the combined effects of several weak genes add up to produce susceptibility to asthma. Some genes increase the susceptibility to asthma, and some lower it. Finding all the relevant genes will take time.
- eta-2 adrenergic receptor
Variations in this gene influence the severity of asthma, not susceptibility to asthma. This gene also influences the response of asthma to different medicines.
- interleukin genes
The interleukins are a family of molecules that help control inflammation. The genes for at least 5 of the interleukins are found in a small region on the longer arm of chromosome 5. These genes influence susceptibility to asthma.
- HLA class II
The HLA complex on the short arm of chromosome 6 contains many genes, divided into several classes. Some studies show that HLA class II genes are involved in allergy to dust mites and ragweed pollen; others do not. The DPB1*0401 allele (variant) of the HLA-DP gene protects against asthma. During epidemics of asthma in Barcelona, Spain, caused by blowing soybean dust, people with the DRB1*13 allele of the HLA-DRB gene were more likely to have an asthma attack.
TNF-alpha contributes to inflammation and increases the sensitivity of airways. Some variants of this gene (and the genes that control it) lead to increased production of TNF-alpha and asthma.
- genes related to IgE
IgE is an important molecule in allergies. Therefore, it is important in asthma, too. Many variations of IgE molecules exist in all of us, each assembled from the products of several different genes. At least some of these genes are on chromosome 11.
There is substantial evidence that genes on the long arm of chromosome 12 are related to asthma. However, scientists have not yet determined which gene(s) it is.
- TcR alpha
The results on this gene have been conflicting.
- C-C chemokine receptor 5 (CCR5)
A study of 425 Scottish children showed that children with one variant of this gene were 64% less likely to have asthma than other children. Interestingly, the same variant of the CCR5 gene lowers the risk of becoming infected with HIV-1, one of the viruses that causes AIDS.
What environmental factors are involved in asthma?
Not surprisingly, many of the environmental factors in asthma relate to air: tobacco smoke, pollution, chemical fumes in certain jobs, and tiny creatures called “dust mites” that cling to dust particles. As Jeff noticed in Colorado, even the temperature and humidity of the air can be a factor.
Other environmental contributors to asthma are not associated with air, for example:
- the amount of salt in the diet (more salt, more asthma).
- lung infections (can trigger asthma in susceptible people).
- a dog in the home (makes asthma less likely for non-rural children in later life).
- brothers and sisters at home (younger children in a family are less likely to get asthma, and having an older brother is an especially effective prevention).
When asthma runs in families, why don’t all family members have it?
It helps to frame this question a little differently…. All members of a family do not have the same height, weight, and face. So, it makes sense that they don’t all have the same conditions and diseases — or the same susceptibility to various diseases.
Here again, it’s genetic and environmental differences that explain differences in our appearance and health. Some family members will inherit genes that predispose to asthma, and others will not. Some family members will be exposed to environmental agents that trigger disease, and others will not.
There is no asthma in my family. Does this mean I will never get it?
No. Anyone can develop asthma. Studies as far back as the 1920s showed that 40-60% of people with asthma or allergies do not have a family history of asthma. But if someone in your family does have asthma, you are at greater risk.
Given all our talk about the importance of genes in asthma, why might a person with no family history of asthma develop it? There are four possible reasons. (We’ll limit our discussion to just the parents and their child.)
One of the parents may have undiagnosed asthma. Sometimes a person can have such mild or atypical disease that it is never recognized. Sometimes a person will die for some other reason before developing asthma.
Although each parent may not have enough genetic susceptibility to develop asthma, it’s possible that a certain combination of their genes does.
Although each parent may not have enough genetic susceptibility to develop asthma, it’s possible that a new variant found only in the child’s DNA results in more susceptibility to the disease.
Although each parent may not have enough genetic susceptibility to develop asthma, it’s possible that the child received an unusually large exposure to environmental agent(s) that contribute to asthma.
How will discoveries about DNA help people and families with asthma?
To learn about asthma we need to go deeper into the history.
How long has asthma been around? No one is sure, of course, but historians may have identified a word used by ancient Egyptians to mean asthma several millennia ago.
The ancient Egyptians wrote their words in pictures, called hieroglyphics. Deciphering hieroglyphics is difficult. The hieroglyphic to the right shows the word that seems to mean asthma. It includes symbols for a jar stand, a wick of twisted flax, a couple less obvious things, and some sort of lizard. Somehow, the combination of these symbols once reminded people of “asthma.”
But why these symbols? Scholars often debate why the Egyptians used particular symbols in a word. In looking at the asthma hieroglyphic, one scholar sees the lizard as a reference to the wheezing noise a chameleon makes when he deflates his lungs in anger!
Our word “asthma” comes from the Greek word for “panting.”
One of the more interesting populations with asthma is found on the island of Tristan da Cunha. Of all inhabited islands on our planet, it is the most remote. A small speck in the middle of the southern Atlantic Ocean, the nearest human habitation is on another island 1200 miles away. The mainland (South Africa) is over 2000 miles away. It was the only place on earth not visited by the great influenza epidemic of 1918.
Just after World War II, a physician in the British Navy visited the island. He wrote that 97 of the 222 islanders had asthma — an astonishing 44% of the population. The reason for this is simple — the islanders are, biologically, one family. The first settlers on the island were William Glass, his wife Susanna, and two other men. The year was 1816. New blood was scarce: over the next 130 years, only 8 new women arrived. New men arrived as a result of shipwreck: 3 from 1835-1849, and 2 in 1892.
Thus, in 1946, all 222 persons were descended from just 8 men and 9 women.
Everyone is therefore related to everyone else on the island. Breeding between cousins is unavoidable. The British physician wrote:
There is no doubt as to the hereditary nature of the [asthma] condition. Whole families are involved and the asthmatic trait can be traced in the pedigrees through the generations. The evidence seems to point to Glass [and his wife] as the originators of the condition. The islanders themselves are of the opinion that these two persons were the original sufferers.
Asthma on Tristan da Cunha is one example of what geneticists call a “founder effect.” William and Susanna Glass founded a population that has remained geographically isolated. There has been little new blood on the island to dilute the effect of the Glass asthma genes. Thus, the large amount of asthma in the population can be traced to the founders.
In 1961, the islanders were evacuated to Great Britain because a volcano on the island was erupting. Most returned two years later. Today there are about 300 islanders.