Frequently asked questions.

A question of genes

Asthma News • 51 • Oct 1997. National Asthma Campaign 

We know that asthma runs in families, but just what are the chances of passing it on to your children? Dr Ian Hall, National Asthma Campaign senior research fellow, answers some of your questions. It has been known for many years that asthma runs in families. However, the chance of developing asthma depends upon the balance of genetic – that is inherited – factors and environmental factors to which a person is exposed. Research studies have shown – and the National Asthma Campaign has funded much of it – that there are several genes, rather than a single 'all or none' gene which contribute to someone developing asthma. But although we can't yet predict accurately exactly who is likely to develop the condition, we can use our existing knowledge about asthma and genetics to tackle some 

I thought the house-dust mite caused all asthma. Are the genes really important?

Studies of families and twins have helped doctors to determine how much of the risk of developing asthma is caused by inherited factors and how much by the environment. The best guess is that 30-50 per cent of the risk of developing asthma is caused by inherited factors. However, just to inherit genes that increase the risk of getting asthma is not by itself enough – you must then be exposed to the important environmental stimuli that trigger asthma. In the UK the house-dust mite is probably the most important single allergen, or trigger. But, if you haven't inherited genetic influences that make the development of asthma likely, then exposure to even high amounts of house-dust mite will probably not cause you to develop asthma.

Our first child had severe asthma. What are the chances of our second child developing asthma?

If your first child has asthma, but neither you or your partner has, and there is not a strong history of asthma in your family, the chance of your second child having asthma is not much greater than the risk in the general population – that is, roughly one in seven. Even if that child does develop asthma, it does not follow that it will be severe. Studies have also shown that the more older siblings you have, the less likely you are to develop asthma and other allergies. It is thought that this is because children with older siblings are exposed to more severe infections at an early age, which might protect them against the development of allergic disease.

I have asthma but my husband doesn't. What are the chances of our children developing asthma?

Asthma is more common in families where someone already has the condition. If one parent has asthma, the chance of their child developing asthma is approximately double that of children whose parents don't have asthma. We also know that a mother has a greater risk of passing on asthma than a father, though the reasons are not clear. 

Both my husband and I have asthma. What are the chances of our child developing asthma?

As you might expect, a child runs a higher risk of developing asthma if both parents have asthma than if only one parent has, and the risk is about three times that in the general population. If both of you have asthma, the absolute risk of each of your children having asthma may be 40 per cent



I have mild asthma but my child has severe asthma. Why is this?

We don't know whether the genes that increase the likelihood of someone developing asthma also influence how severe it is. The genes that dictate this may well be different from those that determine the overall risk of developing asthma. It is common for the severity of asthma to differ considerably between individuals within a family. It certainly does not follow that asthma developing in the child of a parent with severe asthma will also be severe.

One of my children who has asthma stopped having eczema and at the same time developed hay fever. Is this just chance?

Whether a person develops asthma, eczema or hay fever – or more than one of these three – will depend on how many genes are inherited, and the timing and the way in which they are exposed to certain triggers.

My children have a mixture of asthma, eczema, hay fever and migraine – how are all these linked?

Studies in families have shown that eczema, hay fever and asthma often exist together, particularly in children. These children are often 'atopic', which means that they are sensitised to common allergens such as house-dust mite, grass pollen and cat and dog fur. In genetic terms, this can be explained by assuming that there is a group of genes that predispose a child to develop 'atopy'. Different genes probably then control whether the allergy is seen in the skin (eczema), the nose (hay fever) or the lungs (asthma). The evidence that migraine is an atopic disease is much less strong, although there is a weak link between the risk of developing migraine and asthma, eczema and hay fever.

Our family has a strong family history of asthma, and my children would like a pet. Is this sensible?

If your children already have asthma it would be sensible to avoid getting a pet, because many children with asthma are sensitised to animal allergens, particularly cat or dog fur. Given the strong family history, if your children are young and do not have asthma they are at increased risk of developing it, so it would probably still be sensible not to get a pet. If you still feel you must get a pet, choose an animal that can be kept out of doors or one that will not trigger a reaction.

My first child has severe asthma. Can antenatal screening of a foetus detect the risk of developing asthma?

Doctors cannot even consider genetic screening for asthma until we know more about the genes responsible for predisposing a person to asthma. Even then, given that the severity of asthma varies markedly between individuals, it is unlikely that we would be able to predict how severe the disease would be, even if we knew the important genes in a particular family.Most people with asthma have relatively mild symptoms that can be controlled with low doses of inhaled medication. Even if your second child were to develop asthma, it does not follow that he or she will have the severe problems that your first child has had. So, I think antenatal screening would be inappropriate even if we could say exactly which genes we should be looking for.

Doctors and other people have often told me that my children will 'grow out' of their asthma. Is this true?

Many children who have severe asthma find that it becomes much milder as they grow older. Asthma can certainly disappear in some people, although it will often come back later in life, usually in a less severe form. If, on the other hand, the child has asthma together with hay fever and eczema, they are more likely to continue having asthma into adulthood. It is important to remember that once you have asthma, you are always going to have an 'asthma tendency' and asthma can recur at any time

Can gene therapy be used to treat asthma?

Gene therapy for airway disease is an attractive option, because genes can be delivered directly to the airways using a nebuliser. However, we have effective treatments for most people with asthma and the risks and cost of gene therapy would make it inappropriate for 99.9 per cent of people with the condition. Gene therapy has been pioneered in the treatment of cystic fibrosis – but it has had its problems. In particular, the delivery of the genes has caused inflammation in the airways of some people, which cancels out any possible benefit of the therapy. It is likely, though, that better ways of introducing genes into the cells of a person's airway will be developed, and gene therapy will then be a much more powerful treatment. No medical trials of gene therapy have been carried out in people with asthma, but theoretical targets exist, particularly among the genes related to controlling reactions to allergens. Even if gene therapy for asthma were to prove possible, however, it would be suitable only for people with very severe asthma that is unresponsive to all other conventional treatments. This article is taken from 'Common questions on asthma genetics' by Dr Ian Hall (National Asthma Campaign's Asthma Journal, Volume 2, Issue 2) Gene fact file In each human cell there are 23 pairs of chromosomes. Chromosomes contain molecules called DNA. DNA holds the instructions that tell the different parts of the body how to react to different circumstances. These instructions are arranged in segments of DNA called genes. Every person has about 100,000 genes. Research funded by the National Asthma Campaign has identified one of the genes that predispose people to asthma. Research continues.