IS91_Genetic Inheritance and Testing

7/29/2019 IS91_Genetic Inheritance and Testing

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Genetic inheritance and testing

What does genetics mean?Our bodies are made up of thousands of different proteins that all together makeeach one of us unique. The nature of each protein is determined by a special codewhich is carried on molecules of DNA.

Every cell carries the DNA coding for all proteins in the body, but only the parts ofthe code that are relevant to that cell are then translated into proteins. The part of theDNA that carries the code for the cell is called a gene. Genes are arranged end toend along a length of DNA called a chromosome. You inherit a copy of your genesfrom each of your parents.

It may help to think of yourself as a book:

The chromosomes are the chapters. There are 22 pairs of chromosomes andone pair of sex chromosomes, so 23 chapters.

The genes are coding for proteins that dictate characteristics like hair and eyecolour or your height. The coding is like the words in the chapters.

The code in your DNA is like the letters in the words. The way these letters are

arranged can influence your risk of developing conditions such as high bloodpressure or heart disease. For example, the words BARE and BEAR have thesame letters, but theyre arranged differently so they mean different things. In thesame way, if the codes in the DNA are arranged differently, they may work indifferent ways.

What is the link between genes and heart conditions?If one of your genes has a mistake in it a bit like a spelling mistake in a word itcould lead to an abnormal protein being produced in that particular type of cell. Thesame mistake will be passed on to the next and following generations.

We inherit one copy of each gene from each of our parents. If you have a correctlyspelt gene from one of your parents, it might compensate for an incorrectly speltgene in the copy from the other parent. So, a person may not be aware that they arecarrying a gene that could lead to a particular disease or condition. This is howabnormal genes can be passed on silently (without anyone knowing) from onegeneration to another. They are called recessive genes.

If both your parents have a gene with the same mistake in it, this will result in youhaving an abnormal protein, which may cause a particular disease or condition.Sometimes, a gene with a mistake in it can produce an abnormal protein, even if the


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other copy of the gene is normal. The abnormal gene is known as a dominant gene.

Some rare heart conditions are caused by serious mistakes known as mutations in a single gene. For example, hypertrophic cardiomyopathy is caused by amutation in a dominant genethat produces an abnormal protein in the heart cells.This results in thickening of the heart wall and makes the person more prone tosudden death.

However, most heart conditions are caused by the interaction of many subtlespelling mistakes. So its like having the same word with the same meaning, butspelt slightly differently for example, COLOUR and COLOR.

What is a genetic test?A genetic test is a test that is carried out on a sample of your DNA. The DNA is takenfrom a sample of cells or from a blood sample. The test will look at the sample toinvestigate if a specific gene is spelt correctly. If a mistake is found, a test can alsobe carried out on other family members to see if they have the same gene mutation.

Genetic counsellors (specialists in genetic medicine) will explain, to people who havea genetically determined disease, how likely it is that they will pass the abnormal

gene on to their children. Brothers and sisters of the affected person can also beaffected and will also be tested.

In most single-gene defects (that is, defects that are caused by a single gene), thecondition is likely to develop if a person has a mutation. In polygenic disorders (thatis, conditions that are caused by a combination of several different defective genes),the condition or disease is more likely to be caused by the interaction betweenlifestyle behaviours such as smoking, diet or lack of physical activity and theinteraction of mutations in several different genes.

There are good, accurate genetic tests available for certain conditions such as

hypertrophic cardiomyopathy, Marfans syndrome and familial hyperlipidaemia (seebelow).

Mutations of specific genes have been identified. This allows doctors to diagnose ifyou have a condition that is caused by one of those genes. So, if you have genetictesting and you are found to have the mutation, you have the condition although itmay not be immediately obvious. How severe the condition is may vary from oneperson to another, and you can pass on the condition. If you dont have the mutation,you dont have the disease and you cannot pass it on.


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Genetic testing and screening should only be undertaken following advice from agenetic specialist and counsellor who can make sure that the right test is done andthat the results are interpreted correctly. Buying genetic tests without the appropriatesupport and guidance could give misleading and inaccurate information.

Conditions caused by single-gene defects

Hypertrophic cardiomyopathy affects about 1 in every 500 people in the UK. Inpeople with this condition, the heart muscle thickens, and this may lead to seriousabnormal heart rhythms. This condition has a dominant inheritance pattern. Thismeans that if a child has one parent with cardiomyopathy but the other parent doesnot have the condition, the child will have a 1 in 2 chance of inheriting the condition.Affected families are sometimes genetically screened using a blood test which maydetermine if someone is carrying the affected gene. The testing can help to reassurepeople who are worried about their family history.

Marfans syndrome is an inherited disease which affects the connective tissue ofdifferent parts of the body including the heart, blood vessels, skeleton and eyes. Ithas a dominant inheritance pattern, which means that the children of a person with

this syndrome will have a 1 in 2 chance of inheriting it.

Families with Marfanssyndrome may need genetic counselling and screening as they have a greater risk ofdeveloping an aneurysm (a weakening or swelling of an artery wall).Familial hyperlipidaemia occurs in about 1 in 500 adults in the UK. It is an inheriteddisorder which may lead to premature coronary heart disease. Familialhyperlipidaemia affects the way in which low density lipoproteins (LDL or harmfulcholesterol) are removed from the blood. People with this condition have anabnormally high cholesterol level which will need to be treated with medication.

If you think you may have an inherited heart condition and wouldlike to speak to someone from our genetic information serviceplease call 0300 456 8383 Mon Fri 0900 1800. Calls charged at alocal rate.


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What about coronary heart disease?Coronary heart disease is caused by variations in lots of different genes. So it is apolygenic disease (see above).

Coronary heart disease is caused by the build-up of atheroma (fatty deposits) in thecoronary arteries. Unlike the three conditions mentioned above, there is no specificgenetic test for coronary heart disease because many different factors contribute toits development. These things are called risk factors. The risk factors for coronaryheart disease are:

smoking

high blood pressure

raised cholesterol

being overweight

physical inactivity

diabetes

family history of coronary heart disease.

Many of the risk factors listed above may be due to your lifestyle for example,smoking or not taking enough physical activity. So, making lifestyle changes willreduce your risk of developing heart disease. However, some people are more

susceptible to high blood pressure or to developing diabetes and so their risk ofdeveloping heart disease may have a genetic link. For example, high blood pressureand diabetes are thought to be due to minor spelling mistakes in multiple genes. Ontheir own, they may not cause the condition, but when they occur in clusters, theyincrease the risk of the person getting the condition. This is why there are no reliablegenetic tests for these conditions.

So, while currently there is no specific genetic test for coronary heart disease,everyone should take steps to reduce their overall risk by examining their lifestyleand adjusting the factors that may increase their risk.

What about the future?In the near future, genetic screening is likely to help many families where there is afamily history of coronary heart disease or of structural heart defects. Blood testscapable of detecting genetic mutations are being developed. Those people who aregenetically prone to heart disease, high blood pressure and diabetes could then beidentified.

The British Heart Foundation is helping to fund research into the rogue genes thatmay contribute to coronary heart disease. However, the results of this research will


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not be available for several years.

For more information

British Heart Foundation booklets

Keep your heart healthyPut your heart into walkingSo you want to lose weight for good

Useful organisations

Cardiomyopathy AssociationUnit 10Chiltern CourtAsheridge RoadCheshamBucks HP5 2PXPhone: 0800 018 1024Email: [email protected]: www.cardiomyopathy.org

Heart UK7 North RoadMaidenheadBerkshire SL6 1PEHelpline: 0845 450 5988Email:[email protected]: www.heartuk.org.ukFor information on hyperlipidaemia.

Marfan Association UKRochester House5 Aldershot RoadFleet

Hampshire GU51 3NGPhone: 01252 810472Answerphone 01252 617320Email: [email protected]: www.marfan-association.org.uk

March 2009.

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IS91_Genetic Inheritance and Testing