Huntington's Disease Association

Fact Sheet 3

Huntington's disease is an hereditary disorder of the central nervous system. It is caused by a faulty gene. If one of your parents or another relative has Huntington's disease then you will probably be wondering about the likelihood of developing the disease yourself.

This fact sheet explains:

how Huntington's disease is passed on
the likelihood of developing the disease
genetic testing
the Huntington's gene
having children - including fetal exclusion testing

How Huntington's disease is passed on

In each cell of your body you have 22 pairs of chromosomes (autosomes), and 2 sex chromosomes. One of each pair of autosomes comes from your father and one from your mother. Chromosomes contain genes which are the basic units of inheritance. The gene for Huntington's disease is on chromosome 4.

If you have the faulty gene you will, at some stage, develop the disease. If you inherit the 'good' gene, you won't develop the disease and cannot pass it on to your children.

The symptoms of Huntington's disease usually develop when people are between 30 and 50 years old (though they can start much earlier or much later). The faulty gene is present from conception.

As the age of onset is so variable, an individual who carries the faulty gene may die before the disease has had time to develop and the relatives, then, may not know the true extent of their risk.

The likelihood of developing the disease

If one of your parents has Huntington's disease your 'risk factor' does not remain at 50% all your life. As you get older, the likelihood of developing the disease decreases. As you can see from the table below, at the age of 50 years, nearly three- quarters of the persons who are going to develop the disease will have done so.

If you reach 70 years of age without any symptoms your risk will be very low. However, these are standardised figures and your Genetic Clinic will advise you individually of your risk.

Percentage of Huntington's disease patients who develop signs by the age of:

10 yrs       0.1%
20 yrs       2.2%
30 yrs       13.0%
40 yrs       35.3%
50 yrs       71.5%
60 yrs       95.1 %
After       60 yrs 4.9%

For technical reasons, the figures used by some genetic clinics or in other countries may vary slightly from those shown in this leaflet.

It does not make any difference to your risk factor if:

you look like your affected parent
your brothers or sisters develop the disease
you are male or female

Genetic Testing

Living with the knowledge that you are at risk can be very Worrying. You may feel that you would prefer to know for certain whether or not you have the faulty copy of the gene. A DNA test can now be carried out which will usually give you this information. In a few cases the test is still uninformative. Although the test is available, it does not mean that you should have it. You need to consider very carefully whether the test is right for you. If you do decide that it is, you are advised to sort out any life insurance before being tested. It is probable that a positive test result would make it impossible to get life insurance.

If you have only just discovered that you are at risk. then be careful not to rush into making a decision. Once you have been given your test results, you can't change your mind about whether or not you wanted to know.

Only you can make the decision about whether you want to be tested and you usually need to be over eighteen years of age before it will be performed.

Parents, partners and other family members may pressure you one way or the other, but it remains your decision. Please let the Huntington's Disease Association know about any pressure which you feel is being put on you by health care professionals, employers or insurance companies.

If one of your grandparents has/had Huntington's disease but your own parent is so far not showing any symptoms and does not wish to be tested, you will need to talk things over particularly carefully. If you take the test and find that you have the faulty gene, then it will automatically mean that your parent has as well. Trying to keep this result secret is likely to be difficult or even impossible. So you will need to discuss how any adverse effects can be minimised. Though this is a very difficult issue the general opinion at the moment is that your right to have the test is greater than the right of your parent not to know.

Testing is only available at Regional Genetics Clinics, which are located throughout the country. A list of these centres is given at the end of this leaflet. You can ask your GP to arrange an appointment for you.

However not all GPs are aware that testing is now available so you may have to explain the situation. It might be useful to give your GP a copy of this fact sheet and other information from the HDA.

Going to the Genetic Clinic does not mean that you are obliged to take the test but it does give you the chance to talk over all the implications and any other concerns you may have. Each Clinic follows an agreed counselling procedure or 'protocol' which is usually spread over at least three sessions, to help you decide. You can withdraw from the procedure at any time. According to national and international guidelines, follow-up counselling - after you have been given the test result - should be available.

If you decide to have the test done you may have two separate blood samples taken (to double check the results). Your affected parent's blood may also be tested to check the original diagnosis of Huntington's disease The DNA which is extracted from the blood is then analysed in a specialised laboratory.

Some people find waiting for the results very stressful. If you feel there is a particularly long delay, do ask the Genetic Clinic for the reason. Technically, the test can be quite complicated both to perform and interpret. The Clinic will want to allow time to ensure that a result is ready for you and this may be from 4-6 weeks after the third counselling session. In some areas there is a backlog of testing to be done as more families have come forward since the Huntington's disease gene was discovered.

The Huntington's Gene

Genes are made up of DNA (deoxyribonucleic acid). DNA itself is made up of four chemicals which are known by letters of the alphabet:

A       Adenine            G       Guanine C       Cytosine             T       Thymine

One section of the Huntington's gene contains three of these chemicals - CAG - repeated a number of times. In the faulty gene these three chemicals are repeated many times, like a 1molecular stutter'.

The normal number of CAG repeats is under 30. In between 30 and 39 CAG repeats is a 'grey area' where it is uncertain whether people will develop the disease or not. (Some laboratories use slightly different figures). Over 39 repeats means the gene is defective.

Though the test can tell whether you carry the Huntington's disease mutation, it cannot tell you when the disease itself will start to develop.

Having Children - Including Fetal Exclusion Testing

Knowing that you are at risk may affect your decisions about having a family of your own.

Some people decide never to have children at ail, whilst others go ahead on the grounds that the children are likely to have many years of normal life before developing the disease (if they get it at all).

Couples at risk to Huntington's disease are not usually allowed to adopt although they may be able to undertake fostering. IVF (in vitro fertilisation) and AID (artificial insemination by donor) may also be considered.

Your decision to have children may depend upon the results of genetic testing. If testing shows that you don't have the faulty gene, then you can't pass it on to your children.

If you do have the faulty gene then your unborn children can be tested to see if they have inherited it. If you do not know if you have the faulty gene, and do not want to take the test yourself there is a different type of prenatal test which can be performed using linkage analysis. It was used before the gene was found and direct testing was available so it is not as accurate but it does not alter your risk.

If you are considering this option do discuss it at the Genetic Clinic well before embarking on the pregnancy.

NORTHERN

Department of Human Genetics
University of Newcastle
19120 Claremont Place
Newcastle upon Tyne
NE2 4EE

YORKSHIRE

Department of Clinical Genetics
Yorkshire Regional Genetics Unit
Ashley Wing
St. James University Hospital
Beckett St
Leeds
LS9 7TF
Tel No. 0113 2837071

TRENT

Centre for Human Genetics
1 1 7 Manchester Road
Sheffield
S10 5DN
Tel No. 0114 271 7025

Department of Clinical Genetics
City Hospital
Hucknall Road
Nottingham
NG5 1PB
Tel No. 0115 962 7728

Clinical Genetics Department
Leicester Royal Infirmary
Leicester
LE1 5WW
Tel No. 0116 2585736

EAST ANGLIA

East Anglia Regional
Genetic Services
PO Box 134
Level 1
Addenbrooke's Hospital
Hills Road
Cambridge
CB2 20Q
Tel No. 01223 216446

SOUTH WESTERN

Department of Child Health
Genetic Service for Devon and Cornwall
Royal Devon and Exeter Hospital
Barrack Road
Exeter
EX2 5DW
Tel No. 01392 403151

Clinical Genetics Department
Royal Hospital for Sick Children
St Michaels Hill
Bristol
BS2 8BJ
Tel No. 0117 9285652

MERSEY

Mersey Regional Genetic Services
Royal Liverpool Childrens Hospital
Alder Hey
Eaton Road
Liverpool
Ll 2 2AP
Tei No. 0151 252 5605

Regional Genetic Service
The Women and Children Directorate
Countess of Chester Hospital
Liverpool Road
Chester
CH2 1UL
Tel No. 01244 364754

NORTH WEST THAMES

Kennedy Galton Centre for Clinical Genetics
Northwick Park Hospital
Harrow
Middx
HA1 3UK
Tel No. 0181 869 2795

NORTH EAST THAMES

Clinical Genetics Unit
Institute of Child Health
30 Guildford Street
London
WC1N lEH
Tel No. 0171 242 9789

Department of Clinical Genetics
Royal Free Hospital
Pond Street
London
NW32OG
Tel No. 01 71 794 0500 Ext 3702

SOUTH EAST THAMES

Genetic Centre
Guy's Hospital
St Thomas Street
London
SE1 3RT
Tel No. 0171 955 464819

SOUTH WEST THAMES

Regional Genetic Services
St Georges Hospital Medical School
Cranmer Terrace
London
SWI7 ORE
Tel No. 01 81 672 9944

WESSEX

Wessex Clinical Genetic Service
Princess Anne Hospital
Coxford Road
Southampton
S09 4HA
Tel No. 01703 796166

OXFORD

Department of Clinical Genetics
The Oxford Radcliffe Hospital
The Churchill
Oxford
OX3 7LJ
Tel No. 01865 226000

WEST MIDLANDS

West Midlands Clinical
Genetics Unit
Birmingham Womens Hospital
Edgbaston
Birmingham
B15 2TG
Tel No. 0121 627 2630

NORTH WESTERN

Department of Medical Genetics
St Mary's Hospital
Hathersage Road
Manchester
M13 OJH
Tel No. 0161 276 6510

Department of Clinical Genetics
Royal Manchester Children's Hospital
Pendlebury
Manchester
M274HA
Tel No. 0161 794 4696 Ext 2335

NORTHERN IRELAND

Department of Medical Genetics
Floor A
Tower Block
Belfast City Hospital
51 Lisburn Road
Belfast
BT9 7AB
Tel No. 01232 329241 Ext 2873

SCOTLAND

Duncan Guthrie Institute of Medical Genetics
Yorkhill
Glasgow
G3 8SJ
Tel No. 0141 201 0365

Department of Clinical Genetics
Western General Hospital
Crewe Road
Edinburgh
EA4 2XU
Tel No. 0131 315 2768

Medical Genetics Department
Polworth Building
Aberdeen Royal Infirmary
Foresthiii
Aberdeen
AB9 2ZP
Tel No. 01224 681818 Ext 52120

Department of Genetics
Royal Northern Infirmary
Inverness
Scotland
1V2 3VJ
Tel No. 0463 704000

Human Genetics
Dept of Pathology
Ninewells Hospital & Medical School
Dundee
SCOTLAND
DD1 9SY
Tel No. 0382 632035

WALES

Institute of Medical Genetics
University Hospital of Wales
Heath Park
Cardiff
CF4 4XW
Tel No. 01222 74747