Transcript Genetics and Alzheimer’s Disease

DNA, RNA, AD, ADD, LD, FYI ASAP:
Cracking the code
Eric W. Johnson, Ph. D.
Director, Neurogenetics/Neuropharmacology Laboratory
Chief, Molecular Genetics, DNA Diagnostics Laboratory

Gene Mapping Promise and Potential:
Welcome to the Revolution!
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Traits: 1966; MIM
 1487 Known genetic traits, < 10 Mapped
Genes: 1973; HGM-I
 < 100 Genes mapped, Several chr. with NO genes at all.
Traits: 1989; OMIM, >1,000 Known genetic traits
 > 250 Disease genes mapped.
Genes: 1999; NCBI
 >60,000 Expressed Sequence Tags (ESTs) mapped
• Human Genome Project:
•Completed by spring 2000
•Really done 2001
•Really, really done 2003 or perhaps never
What are the questions that pertain
to genetic testing?
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What kind of genetic test is it?
How would the genetic test be used?
Would the genetic test help or hurt my patient?
How is the genetic test applied in this situation?
Where can I find a lab that does the test?
What is involved in ordering this genetic test?
How would a genetics consultation help?
Three kinds of genetic tests
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Cytogenetic
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Molecular
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Metabolic
copyright©1999
Children’s Health Care System
Uses of Molecular Genetic Testing
Diagnostic
 Predictive
 Carrier
 Prenatal
 Newborn Screening
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Clinical vs. Research Testing
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Clinical testing - Done for the purpose of
prevention, diagnosis, or treatment as
part of patient care. Results are reported
to the provider.
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Research testing - Done for the purpose
of understanding a condition better, or
developing a clinical test. Results are
usually not provided.
Mutation Analysis
DNA codes for RNA which codes for Protein
DNA
Gene Expression
RNA
mRNA
Protein
Protein Expression
(gene copy number)
Genetic Code/English Language
Exon
ATC G
Nucleotide bases
=
Sentence
=
Word
=
A to Z
26 Letters
Genetic Code Variations
Mutation vs. Polymorphism
Sentence
=
The cat chases the mouse.
Sentence
=
The cat chases the mouse.
POINT
MUTATION
Sentence
=
The rat chases the mouse.
Common Hispanic mutation in the CCM1/KRIT1 gene showing the common
Hispanic mutation. C >T (C742T) transition in exon 10 changes a GLN to a
premature termination codon (Q248X).
Typical / Mutant
G
G
A
A
C
C
T
T
C
C
A
A
A
A
A
A
G
G
A
A
A
A
GLN/Stop C
T
T
T
C
C
T
T
C
C
T
T
C
C
T
T
G
G
A
C
G
T
PCR-RFLP assay for the Common Hispanic CCM1/KRIT1 Mutation.
CM304 CM314 CM316 CM318
CM302 CM303
CM301
Sentence
=
The cat chases the mouse.
DUPLICATION
MUTATION
Sentence
=
The cat cat chases the mouse.
Tri Nucleotide Repeat Disorders
Other Types of Mutations:
Frameshift Mutation
Microdeletions
Mutations in non coding regions of genes
Genetic Testing is
Context Specific
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Why are you testing THIS patient at
THIS time?
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Be aware of the potential ramifications.
NATURE Vs. NURTURE
It’s all Moms fault
(with a little help from Dad)
Would genetic testing help me?
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Reduces morbidity and mortality
through close surveillance of at risk
individuals
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Eliminates need for extra
surveillance in individuals with no
increased risk
Would genetic testing hurt me?
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Psychological impact of knowing you
have a life-threatening condition (or that
you have escaped it and others that you
care for have not)
Possible insurance discrimination
associated with high risk
 Family discord because other members
who never agreed to be tested are now
known to have this familial risk
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Genetic Testing Info
http://www.genetests.org/
Focused on Disease review
Medical Genetics
Database
www.genetests.org
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Disease specific information
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Expert-authored
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Peer-reviewed
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Relevant Links
Familial Neurological Disease
Diagnosis
 Clinical Description
 Differential Diagnosis
 Management
 Genetic Counseling
 Molecular Genetics
 Resources
 References
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Clinical Diagnosis
Alzheimer Overview
Disease characteristics. Alzheimer disease (AD) is characterized by adultonset slowly progressive dementia associated with diffuse cerebral atrophy
on neuroimaging studies. It is the most common form of dementia, but less
than 5% of families with AD have early-onset familial AD (EOFAD), in
which symptoms consistently occur before the age of 65 years.
Diagnosis/testing. The diagnosis of Alzheimer disease is based on the
histological findings of ß-amyloid plaques and intraneuronal
neurofibrillary tangles. No accurate clinical diagnostic test for AD exists. A
significant association with the e4 allele of apolipoprotein E supports the
diagnosis of AD in patients with dementia and increases the risk that
asymptomatic individuals will eventually develop AD. ApoE genotyping,
however, is neither fully specific nor sensitive. Three forms of EOFAD
caused by mutations in one of three different genes (APP, PSEN1, PSEN2)
are recognized. A molecular genetic test of the PSEN1 gene (chromosomal
locus 14q) is available in clinical laboratories.
Management
The mainstay of treatment is necessarily supportive and each symptom is managed on an
individual basis. In general, affected patients eventually require assisted living arrangements
or care in a nursing home. The exact biochemical basis of AD is not well understood.
Deficiencies of the brain cholinergic system and of other neurotransmitters are present. There
are drugs that increase cholinergic activity by inhibiting acetylcholinesterase that play a role in
treatment of AD. A minority of patients show modest but useful behavioral or cognitive benefit.
The first such drug was tacrine, but this agent is also hepatotoxic [Knapp et al 1994]. There are
newer such drugs with similar pharmacologic action that are not hepatotoxic, such as Aricept
(donepezil), Exelon (rivastigmime) [Rogers et al 1996], and Galantamine [Raskind et al 2000,
Tariot et al 2000]. Nonsteroidal anti-inflammatory drugs and estrogen are also under study as
possible therapeutic agents. Antidepressant medication may improve associated depression.
Treatment trials evaluating several different therapeutic strategies are underway; these
strategies include use of anti-inflammatory agents, estrogens, nerve growth factors, and
antioxidants [Marx 1996, Farlow & Evans 1998]. Thus far, treatment of symptomatic AD with
estrogens has not proven beneficial [Mulnard et al 2000, Wang et al 2000]. There is evidence
that patients taking HMG-Coenzyme A reductase inhibitors for hypercholesteralemia have a
reduced incidence of dimentia [Wolozin et al 2000]. Immunization of a AD mouse model with bamyloid has attenuated the AD pathology and stimulated the search for a possible vaccination
approach to the treatment of human AD [Schenk et al 1999].
Molecular Genetic Testing
Ladder
2/2
3/3
4/4
2/3
2/4
3/4
Ladder
91 bp
83 bp
72 bp
48 bp
35 bp
Genetic Counseling
Mode of Inheritance: Alzheimer disease is most commonly inherited in a
mulitfactorial manner. Early-onset familial Alzheimer disease (EOFAD) is quite rare
but usually inherited in an autosomal dominant manner
Risk to Family Members: Late-Onset Familial Alzheimer Disease:
Many
families have multiple affected members, all of whom have onset of dementia after the
age of 65 or 70 years.
Parents, sibs and offspring of a proband:
Having two, three, or
more affected family members probably raises the risk to other first-degree relatives in
excess of that noted above for sporadic cases, although the exact magnitude of the risk is
not clear. Heston et al (1981) found a 35-45% risk of dementia in persons with a sib with
onset of AD at less than age 70 years of age and an affected parent. Bird et al (1993) also
reported preliminary data suggesting that offspring of parents with conjugal AD (i.e.,
both parents affected) had an increased risk of dementia.
Interpreting genetic
testing will
require vigilance!
Genes Associated with Alzheimer’s Disease
Chromosome #
Cellular Protein
Function
1q31-q42
Presenilin 2 (PS2)
Protease –
intramembrane
proteolysis
14q24.3
Presenilin 1 (PS1)

-secretase
Protease –
intramembrane
proteolysis
19
21q21.2
12
APOE4
Amyloid precursor
protein (APP)
Gene(s) not identified
Major serum protein
Membrane
glycoprotein
?
What if a genetic test
is not available?
Most people are seeking information
and coping strategies as much as
they are test results
THINK RESEARCH
Genetic Counseling and Testing
Primary
Care
Genetics
Researcher
Genetics
Professional
Patient
http://www.genetests.org/
Location
7q21.2
Symbol
CCM1, CAM,
KRIT1
Disorder
KREV interaction trapped 1,
Cerebral cavernous malformations 1
Common Hispanic Mutation:
KRIT1, Exon6 742C>T / 248Q>X