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  • 1. IS 35 January 2007 Information sheet Genetics and dementia When we have children, we Introduction pass on only one copy of Many people with dementia each of our genes. are concerned that their There are millions of disease may have been combinations of genes that inherited and that they may we could inherit, and the pass it on to their children. effect of each gene is not yet Family members of people known, although scientists with dementia are also are rapidly expanding our sometimes concerned that knowledge. There can be they might be more likely to different versions of the develop dementia same gene which may work themselves. This differently or not as well. information sheet outlines This can cause a genetic the present state of condition or disease and knowledge about the some gene faults may lead inherited risk of dementia. to a person developing Genes and dementia dementia. Genes are the unique set of However, the vast majority instructions inside our bodies of cases of dementia are not which make each of us an caused by an inherited individual. There are many genetic fault. Dementia is so thousands of different genes, common that having one or each carrying a different two close relatives with instruction. dementia in itself is not evidence of a family link. We have two copies of each gene. One copy is inherited from each of our parents. Page 1
  • 2. Genetics and dementia The genetic factors people (under the age of 65) associated with rather than older people. dementia Only a very small number of families are affected. These The genetic factors particular genetic faults can associated with Alzheimer’s result in people developing disease and other forms of Alzheimer’s disease in their dementia can be 30s and 40s. However, it is summarised as follows: important to remember that • There is no single gene most cases of early onset responsible for all cases Alzheimer’s disease are not of dementia inherited. • Genetic factors only On average, half of the directly cause the children of a person with one disease in a very small of these rare familial number of families with Alzheimer’s disease genetic dementia faults inherit the faulty gene. Almost all those who inherit • Among cases without a it develop Alzheimer’s family link, there is disease at a comparatively often a genetic early age. People who do component to the not inherit the faulty gene disease; however, cannot pass it on to their inherited factors alone children. do not explain why some people develop it The genetic factors for and others do not. familial early onset Alzheimer’s disease Early onset Alzheimer’s disease • A small number of families worldwide have We use the term Familial a genetic fault in a gene Alzheimer’s disease in called amyloid precursor families where a genetic fault protein (APP), which directly causes the disease. affects production of the Familial Alzheimer’s disease protein amyloid. usually affects younger Amyloid build-up in the Page 2
  • 3. Genetics and dementia brain has been linked to such as other illnesses, diet, Alzheimer’s disease. levels of activity and random chance, are probably more • A slightly larger number significant in the of families carry a fault development of Alzheimer’s in a gene called disease in later life. See presenilin-1 leading to Further reading section early onset familial below for more information Alzheimer’s disease. about risk factors and • A very small group of dementia. families has a fault in a Genetic factors for late gene called presenilin-2 onset Alzheimer’s causing early onset disease familial Alzheimer’s disease. The best known genetic factor for late onset Late onset Alzheimer’s Alzheimer’s disease is a gene disease called apolipoprotein E Late onset Alzheimer’s (ApoE). It comes in three disease (over 65) is not forms ApoE2, ApoE3 and inherited in the same way as ApoE4. We all have two some cases of early onset copies of the gene, which Alzheimer’s disease. Many may be the same version as factors combine to alter a each other or different. person’s risk of developing The ApoE risk is different late onset Alzheimer’s from how familial disease so that some Alzheimer’s disease is develop it in later life and inherited. Having one or two others do not. Genetic and copies of ApoE4 increases environmental factors are the chance of developing the both involved. Most of these disease, but does not make factors are not fully it certain. One or two copies understood. We do know of ApoE3 are associated with that having a close family an average risk, and the risk member with the condition of developing Alzheimer’s increases risk – but only by a disease for people with two small amount. Other factors Page 3
  • 4. Genetics and dementia copies of ApoE2 is reduced. Down’s syndrome Some other factor, not yet understood, must also People with Down’s contribute. syndrome are at particular risk of developing Some researchers think that Alzheimer’s disease ApoE4 does not affect whether a person will get the disease but, rather, when Huntington’s disease they get it, causing people with ApoE4 to develop the Huntington’s disease is a disease before people with progressive hereditary ApoE2. Because ApoE disease caused by a testing cannot accurately particular gene. The predict who will develop the symptoms of Huntington's disease it is generally not disease usually develop available to patients and when people are 30-50 years their families except as part old, although they can start of a research study. much earlier or much later and can vary from person to Vascular dementia person, even in the same There are no established family. The course of the direct genetic causes for disease also varies for each vascular dementia but the person and dementia can ApoE gene described above occur at any stage. is a risk factor for vascular Other dementias dementia as well as Alzheimer’s disease. There Other forms of dementia can are known genes that be inherited. Some people contribute to some of the with fronto-temporal risk factors for vascular dementia or Pick’s disease dementia, such as high have a very strong family cholesterol levels, high blood history. In some of these pressure and diabetes. cases, a genetic change has been found in the TAU gene. These inherited forms of dementia are rare. Page 4
  • 5. Genetics and dementia There are other very rare information and an causes of inherited dementia opportunity to discuss the with changes in different risk to yourself and other genes, such as the PRP gene family members. in inherited CJD and the NOTCH gene in a rare For a few families it may be disorder called CADASIL. A possible to identify a gene few families worldwide have change that is responsible an inherited dementia, the for the disease in that family cause of which has yet to be but for most families this will discovered. not be the case. Genetic testing and However, if such a change is counselling for people found in your family this at risk of familial raises the possibility of dementias testing to see if you too have the change. This sort of Anyone who is worried about testing is called predictive inheriting a form of dementia testing and is currently and who has a relative with available to people with the condition should speak Huntington’s disease in the first to their GP. Although family, for example. Before scientists are discovering having a predictive test you more about the genetics of will be offered extensive late onset Alzheimer’s counselling to make sure it is disease there are no the right decision for you. approved tests for this condition. However, if you The pros and cons of have more than one close genetic testing family member affected by A genetic test might: early onset dementia, and particularly if your family • Identify people who members first showed signs might benefit from of the disease between the Alzheimer’s drugs ages of 30 and 50, you may be referred to a regional • Help people plan for the clinical genetics department. future Here you will be given more Page 5
  • 6. Genetics and dementia • Help genetic researchers your risk. Free information understand the disease leaflet. better and so lead to improved treatment Alzheimer Scotland (2006). Good for You Good for Your • Lead to problems Brain: the evidence on risk getting a mortgage or reduction and dementia. life insurance in the £5.00 future (although in March 2005 the government announced Available from Alzheimer a continuation of the Scotland, 22 Drumsheugh moratorium on Gardens, Edinburgh EH3 insurance companies 7RN. Tel: 0131 243 1453 or using genetic visit information to www.goodforyourbrain.org November 2011) Acknowledgements • Raise anxiety without offering any useful We are grateful to the treatment. Alzheimer’s Society for permission to reproduce, Testing for genes which are with amendments, the text risk factors, like ApoE4, but of their factsheet Genetics not predictive as in familial and dementia. Thanks also Alzheimer's disease, is not to Professor Veronica van done. This sort of test is not Heyningen, MRC Human helpful because a higher risk Genetics Unit, Western does not necessarily mean General Hospital, Edinburgh someone will develop the and Dr Mary Porteous, illness and a lower risk does Department of Clinical not mean they won’t, and Genetics, Western General there is no treatment to offer Hospital, Edinburgh for someone at high risk. comments and amendments. Further reading Alzheimer Scotland (2006). Dementia – how to reduce Page 6
  • 7. Genetics and dementia Alzheimer Scotland 22 Drumsheugh Gardens, Edinburgh EH3 7RN Telephone: 0131 243 1453 Fax: 0131 243 1450 Email: alzheimer@alzscot.org Alzheimer Scotland - Action on Dementia is a company limited by guarantee and is recognised as a charity by the Inland Revenue. Registered in Scotland No. 149069. Scottish Charity No. SC022315. Page 7 Find us on the internet at www.alzscot.org

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