Genetics & Dementia

Much of the genetic etiology of dementia is still unknown. Certain dementias have well-defined genetic causes.

For example Huntington’s Disease is due to a single abnormal dominant gene on chromosome 4. If someone has Huntington's Disease, there is a 50% risk of passing the gene causing disease to children.

Other dementias like Lewy Body Dementia seem to have little hereditary basis. Still others have various etiologies including heredity.

Whereas about 70% of Alzheimer’s disease appears sporadically, about 5-10% of Alzheimer’s disease seems to be inherited in an autosomal dominant pattern like Huntington’s disease. This familial early-onset (usually before 65 years of age) Alzheimer’s disease has been associated with 3 genes. For more specific information about the genetics of Alzheimer’s Disease, click here. Having any one of these genes will cause Alzheimer’s disease and will impart a 50% risk to children. Additionally, about 20-25% of Alzheimer’s disease can be attributed to susceptibility genes that do not cause the disease but increase risk of getting Alzheimer’s disease at an early age. Researchers expect to find others genes that both increase and decrease susceptibility.

There is even less known about the genetics of frontotemporal dementia. Approximately 40% of patients with frontotemporal dementia have a positive family history of a similar dementia while 60% seem to occur sporadically with no apparent hereditary link. Four genes have been discovered as genetic causes of FTD. Two genes on Chromosome 17q21, microtubule-associated protein tau (MAPT) and progranulin (PGRN) account for about 20% of familial cases, two other genes representing a much smaller number of cases, valosin-containing protein (VCP) and charged multivesicular body protein 2B (CHMP2B), have also been discovered as a genetic cause of FTD. For more specific information about the genetics of frontotemporal dementia, click here.

The prion diseases Creutzfeldt-Jakob Disease, Gerstmann-Straussler-Scheinker, and Fatal Familial Insomnia represent other disease with varied etiologies. Whereas 85% seems to occur sporadically, approximately 15% is inherited. Additionally, there may be some genetic susceptibility factors. For more specific information about the genetics of prion diseases, click here. Research into the genetics of dementia is helping to clarify not only the inheritance but the biochemistry of these diseases. This knowledge may lead to the discovery of new treatment and prevention.

Family History
Determining the correct diagnosis for a dementia entails consideration of many factors. One of these is family history. Looking carefully at family, medical and social history can help to support or eliminate a particular diagnosis. Before coming for a clinical evaluation, patients are advised to research their family’s medical history.

The following questions should be asked of blood relatives:

1. Have you or anyone in the family had any of the following?
• Memory problems
• Speech problems
• Changes in personality: apathy, disinhibition, criminal behavior, change in dress, change in interests
• Psychiatric or mood disorder: anxiety, depression, obsessive-compulsive disorder, schizophrenia, suicide attempt etc.
• Addictions
• Muscle or nerve problems: weakness, neuropathy
• Movement problems: jerking, tremor, unsteadiness
• Seizures
• Vision/hearing loss
• Migraines
• Incontinence
• Heart problems, strokes, hypertension
2. If anyone had any of these symptoms, at what age did the symptoms develop?
3. What was the age and cause of death for all deceased family members?
4. Where did your ancestors originate?

Genetic Testing
When patients come to the UCSF Memory and Aging Center, they will be asked about family history. If a possible familial condition exists, they will meet with the genetic counselor to discuss the genetics of the diagnosis, the risk to other family members, and the available clinical or research testing. The benefits and limitations of genetic testing will be discussed so that patients and their families can determine whether such testing is right for them.

Different kinds of genetic testing exist. When the genetics of a disease are well understood, clinical testing is usually available. An example of such a disease is Huntington’s disease. A clinical test means that a blood sample is drawn and sent to a clinical laboratory that analyzes it for the known gene mutations (alterations). The lab generates a report stating that the person either has a gene mutation or not. This type of testing can be used for diagnostic purposes or as a presymptomatic test to determine if a relative has inherited the disease causing gene.

For diseases such as Frontotemporal dementia in which the genetics is not well understood, research genetic testing may be available. Research genetic testing also involves analyzing a blood sample. However, since the genetic cause is unknown, the research may or may not result in any definitive answers. Our patients often wish to be involved in research to help future generations even though they realize that they might not have any answers for themselves.

References

• Chow TW, Miller BL, Hayashi VN, Geschwind DH. Inheritance of Frontotemporal Dementia. Arch Neurol. 1999;56:817-22.
• Prusiner, Stanley B. Shattuck Lecture – Neurodegenerative Disease and Prions. NEJM. 2001;344:1516-26.
• St. George-Hyslop, Peter H. Molecular Genetics of Alzheimer’s disease. Biol Psychiatry. 2000;47:183-99.

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