Background: Alzheimer’s disease (AD) is by far the most common cause of dementia in aging persons. There are two forms of the disease, late-onset AD and early-onset (familial) AD. Disease symptoms are identical to other forms of senile dementia, and diagnosis had been possible only at autopsy by the detection of protein clusters called amyloid plaques in the cerebrum. The disease is multifactorial and inheritance patterns are complex. Some forms of familial Alzheimer’s disease appear to be inherited as autosomal dominant traits, while others are recessive. Genetic testing is generally not recommended for late-onset AD because this disease can occur in the absence of inherited factors. One gene (APOE) that is associated with an increased risk of late-onset AD is the subject of the exercise linked directly below.
Late-onset AD: Click here for an exercise exploring the nature of the mutation associated with late-onset AD, using the NCBI Genome Data Viewer and Structure tools. This will enable you to examine AD-associated SNPs and also study the three-dimensional structure of the protein encoded by the APOE gene.
Early-onset (familial) AD: Cases A and B below deal with early-onset AD, using a historical technique (Southern blotting). DNA sequencing is currently used to test for mutations associated with early-onset AD. Mutations in at least four genes have been linked to Alzheimer’s disease. One of these is the amyloid precursor protein (APP) gene, which encodes the b-amyloid peptide found in the cerebral plaques of Alzheimer’s patients. The function of APP is not yet known, but certain APP point mutations are associated with inheritance of late-onset Alzheimer’s disease in some families. Two examples which can be detected by RFLP analysis are the codon 693 Glutamic acid to Glycine mutation and the codon 717 Valine to Isoleucine mutation. The 693 mutation results in the loss of a MboII site, while the 717 mutation results in the gain of a BclI site.
Procedure: To test for the 693 Gly mutation, digest the DNA with MboII and perform a Southern blot using the APP probe. To test for the 717 Ile mutation, digest the DNA with BclI and then use the APP probe. Compare the test samples to the control samples, and use the results to determine the genotype of each individual. Small fragments are generated with the MboII digestion, so use a shorter run time (30 minutes).
Case A: Martha, age 71, has been exhibiting increasingly severe symptoms of senile dementia and has been hospitalized for testing. She is in good health otherwise. Her three children – Sam (age 43), Joan (age 41) and Robert (age 38) – want to find out the cause of the dementia and determine the prognosis for Martha’s future condition. They are also concerned that Martha may have a form of familial Alzheimer’s disease and want to know if they are at risk. The physician decides initially to test Martha for two mutations, 693 Gly and 717 Ile, in the amyloid precursor protein (APP) gene which are associated with inherited Alzheimer’s disease.
DNA samples:
- Martha (mother)
- Sam (son)
- Joan (daughter)
- Robert (son)
- Control normal APP gene
- Control with 693 mutation
- Control with 717 mutation
Results of Southern blot analysis for Case A – MboII digestion
Results of Southern blot analysis for Case A – BclI digestion
- Does Martha have either of these two APP mutations?
- Did any of Martha’s children inherit an APP mutation?
- What conclusions can you draw regarding Martha’s diagnosis?
- What can you tell Martha’s children about their risk for Alzheimer’s disease
- What issues are raised by this type of testing?
Case B: Lisa, age 17, and her cousin Jen age 18, were half-listening to music in the den and half-listening to their mothers discuss Grandma Eloise and her older sister Florence. Lisa and Jen loved Eli and Flo dearly but even they could tell something wasn’t quite right about their increasingly odd behavior. The teens moved into the kitchen to join the conversation. “Is Grandma’s erratic behavior and forgetfulness Alzheimer’s or just senile dementia commonly associated with old age?” They decide to talk to Eloise and Florence about DNA testing. The mothers also wonder about their risk for Alzheimer’s disease and decide to be tested.
DNA samples:
- Eloise (grandmother)
- Florence (Eloise’s older sister)
- Lisa’s mother
- Jen’s mother
- Control with 693 mutation
- Control with 717 mutation
- Control normal APP gene
Results of Southern blot analysis for Case B – MboII digestion
Results of Southern blot analysis for Case B – BclI digestion
- What conclusions can you draw from the results?
- What is the molecular basis of this disease, and why does this result in the observed gel patterns?
- What options are available to the family?
- What issues are raised by this type of testing?