Transcript Document

Disorders with Complex Genetics
Signs & Symptoms:
• Memory loss for recent events
• Progresses into dementia  almost total memory loss
• Inability to converse, loss of language ability
• Affective/personality disturbance (fatuous, hostile)
• Death from opportunistic infections, etc.
Confirmation of Diagnosis:
• Neuronal (amyloid, b amyloid, Ab amyloid,
Ab 42) plaques
• Neurofibrillary tangles
• Brain Atrophy
Neuronal (Ab 42) Plaques in Alzheimer’s
Disease
From http://www.rnw.nl/health/html/brain.html
Neurofibrillary Tangles in Alzheimer’s Disease
From http://www.rnw.nl/health/html/brain.html
Plaques and neurofibrillary tangles
From Department of Pathology, Virginia Commonwealth University
http://www.hosppract.com/genetics/9707gen.htm
Following are from the NIA, Alzheimer’s Disease
Education and Referral Center, Alzheimer’s Disease:
Unraveling the Mystery (www.niapublications.org/
pubs/unraveling/01.htm ff.)
Amyloid precursor protein (APP) is membrane protein that sits in the membrane and extends outward. It is though to
be important for neuronal growth, survival, and repair.
Enzymes cut the APP into fragments, the most important of which for AD is called b-amyloid (beta-amyloid) or
Ab.
Beta-amyloid is “sticky” so the fragments cling together along with other material outside of the cell, forming the
plaques seen in the AD brain.
Microtubules are like railroad tracks that transport nutrition and other molecules. Tau-proteins act as “ties” that
stabilize the structure of the microtubules. In AD, tau proteins become tangled, unstabilizing the structure of the
microtubule.
Brain Atrophy in AD
WRONG!
http://abdellab.sunderland.ac.uk/lectures/Neurodegeneration/References/Brain_Neurons_AD_Normal.html
Classification:
(1) FAD v SAD: Familial AD versus Sporadic AD
• No complete consensus
• Usually FAD = at least 1 first degree relative affected
• Sometimes 2 second degree relatives
(2) Early v Late Onset:
• Early onset (EOAD) = usually before 65
• Early onset correlated with FAD
• LOAD = late onset AD
Breakthrough:
(1) Down’s Syndrome
• Have AD brain pathology in later life
(2) Pedigrees with dominant-like transmission:
• Studied these first
• Concentrated on chromosome 21
Alzheimer’s Disease, Type 1:
•Several mutations in AAP gene on chromosome 21
•Most common = Val717Iso
•Produce abnormal beta amyloid fragment
•15%-20% of early onset, familial AD
•Autosomal dominant
http://ghr.nlm.nih.gov/condition=alzheimerdisease
http://perso.wanadoo.fr/alzheimer.lille/APP/APPmutations.html
Alzheimer’s Disease, Type 3:
•Mutations (> 130) in the presenilin1 gene on chromosome
14
•Most mutations lead to amino acid substitution
•Overproduction of the beta amyloid fragment
•30% - 70% of early onset, familial AD
•Autosomal dominant
Alzheimer’s Disease, Type 4:
•Mutations in the presenilin2 gene on chromosome 1
•2 alleles: Asn141Iso and Met239Val
•Overproduction of the beta amyloid fragment
•< 5% of early onset, familial AD (only a few
families world wide)
•Autosomal dominant
Alzheimer’s Disease, Type 2:
•Epsilon 4 (e4, AKA E4) allele of the Apolipoprotein E
(ApoE) gene on chromosome 19 confers risk
•Epsilon 2 (e2, AKA E2) allele of the Apolipoprotein E gene
on chromosome 19 confers protection
•Mechanism unclear; ApoE is a very low density lipoprotein
that transports cholesterol
•Most cases are late onset, familial
•Susceptibility Locus
Prevalence of APOE genotypes in
Alzheimer’s disease (AD) and controls.
Genotype:
Controls
AD
E2/E2
1.3%
0%
E2/E3
12.5%
3.4%
E2/E4
4.9%
4.3%
E3/E3
59.9%
38.2%
E3/E4
20.7%
41.2%
E4/E4
0.7%
12.9%
Jarvik G, Larson EB, Goddard K, Schellenberg GD, Wijsman EM (1996) Influence of apolipoprotein E genotype on the transmission of Alzheimer disease in a
community-based sample. Am J Hum Genet 58:191-200
http://www.hosppract.com/genetics/9707gen.htm
AD: The Great Unknown:
What is causing the majority of AD cases?
1. Unknown Mendelian forms (probably not)
2. Unknown major loci (probably not)
3. Phencopies
4. Multifactorial-threshold
Current theory: Multifactorial, involving
several pathways.
• Protein accumulation:  placques & tangles
• Inflammation: Unregulated activation of glia
• Lipid distribution: Lipid membrane site of APP
cleavage.
From Sleegers et al. (2010) Trends in Genetics, 26, 84-94, p. 87
Nature Genetics 41, 1088 - 1093 (2009)
Cases with no known etiology:
(theoretical extremes)
Mendelian/
Phenocopy
Disease (Genetic)
Heterogeneity
Multifactorial/
Threshold
CDCV
Common disease/
common variant
Heterogeneity
Mendelian/Phenocopy
• Many rare alleles with high penetrance (“Mendelian”
forms of the disorder).
• Almost no person will get two or more of these AD
alleles.
• Non familial cases due to phenocopies.
Multifactorial Threshold Model
• Many common alleles with “low” penetrance.
• Most people will have several risk alleles.
• Risk alleles are additive (multiplicative).
• Many additive environmental factors.
• Genes and environment  liability.
• Once liability reaches a certain value (i.e., the
threshold) a disease process begins.
HGSS: Carey: Figure 6.1
Animal Models
Human APP
gene
Mice gratia http://www.kidscolorpages.com/mouse.htm
Human ApoE
gene
Human Presenilin
gene
Figure 1. Development of the Transgenic Mouse Model of Alzheimer's Disease.
The transgene consists of the human APP gene containing a mutation causing a rare form of early-onset familial Alzheimer's disease (Val717Phe).
The transgene, whose expression is driven by the platelet-derived growth factor (PDGF) promoter, is microinjected into mouse eggs and implanted in a
pseudopregnant female mouse. After the progeny are screened for the presence of the transgene, they are bred and their offspring are analyzed for pathologic
features characteristic of Alzheimer's disease. The brains of the transgenic PDAPP (PDGF promoter expressing amyloid precursor protein) mice have abundant
-amyloid deposits (made up of the A peptide), dystrophic neurites, activated glia, and overall decreases in synaptic density.
From NEJM Volume 332:1512-1513
From McGowan, Erikson & Hutton (2006), Trend in Genetics, 22: 281-289.