Transcript EPB PHC 6000 EPIDEMIOLOGY FALL, 1997

Migrant Studies
Migrant Studies: vary environment, keep genetics
constant:
1) Evaluate incidence of disorder among ethnicallysimilar individuals living in native environment.
2) Evaluate incidence of disorder among
comparable group of ethnically-similar
individuals who have migrated to a new
environment that has much higher or lower rates
of the disorder.
As the rates of disorder among migrants approach
those observed in the new environment,
evidence for environmental influence increases.
Migrant Studies
Examples of Findings from Migrant Studies:
With exception of stomach cancer, Japan historically
has had much lower rates of cancer than the U.S.
BUT, after just 2 generations, Japanese migrants to
the U.S. have nearly assumed the same rates of
cancer (including lower rates of stomach cancer).
European immigrants (high latitude) who migrate to
Israel (low latitude) at a young age experience a
low incidence of multiple sclerosis. However, if
the migration occurs after age 14, the “relevant”
environmental exposure may have already
occurred.
Migrant Studies
Limitations of Migrant Studies:
--- Migrants are highly selected.
--- Age at migration varies – prior causal exposure
in the native environment may have already
occurred.
--- The extent to which migrants retain cultural
and lifestyle elements of their original
environment can make it difficult to evaluate
environmental influences of the new
environment.
Genetic Marker Studies
Genetic Marker Studies: vary environment, keep
genetics constant:
1) Examine links between genetic markers and
specific disorders or traits.
2) Genetic markers are a measurable human
trait controlled by a single gene with known
chromosomal location. Must be polymorphic
with at least two alleles having a gene frequency
of at least 1 percent.
Genetic Marker Studies
Markers Used to Study Allele/Disease Associations
1) Analysis of gene products or phenotypic
expression:
--- Blood groups
--- Human leukocyte antigens (HLAs) – extensive
investigations have been/being conducted
--- Protein polymorphisms
2) Analysis of DNA polymorphisms
--- Allelic variants of genes
--- Restriction fragment length polymorphisms
--- Variable tandem repeats
Note: The “marker” may or may not be causally
involved in development of the disease.
Genetic Marker Studies
Genetic Marker Studies: vary environment, keep
genetics constant
3) The fact that alleles at different loci, but very
close in proximity, tend to be transmitted
together beyond that expected by chance alone
is known as “linkage disequilibrium.”
4) Two types of genetic marker studies:
a) Association studies
b) Linkage studies
Genetic Marker Studies
a) Association studies: Select disease cases
and controls using case-control design.
Compare the frequency of the genetic marker of
interest (e.g. specific allele) between cases and
controls.
Unrelated individuals from different families in
the general population should be selected for
both cases and controls. Ethnicity is strong
potential confounder – controls should be
selected from same ethnicity as cases.
Genetic Marker Studies
a) Association studies
Allele
Prevalence Prevalence Prevalence
Ratio
(Cases)
(Controls)
1
0.50
0.55
0.91
2
0.40
0.43
0.93
3
0.10
0.02
5.0
Genetic Marker Studies
b) Linkage studies: Assess the association of
general markers and disease within families.
--- Linkage is defined as the tendency of genes
to be inherited together as a result of their
location on the same chromosome.
Genetic Marker Studies
Linkage (background):
--- Genes co-exist on chromosomes.
22 pairs of non-sex chromosomes (autosomes)
1 pair of sex chromosomes (XX: F, XY: M)
--- In meiosis, 2 pairs of chromosomes segregate; one
chromosome of each pair is transmitted to gamete
--- Chromosomes segregate independently.
--- However, chromosomes do not always stay intact;
crossing over (recombination) occurs.
--- Frequency of crossover between 2 loci on same
chromosome depends on their distance apart.
Genetic Marker Studies
Recombination:
No recombination
Chromosome A
Loci
1
2
Linkage
Recombination
Break
3
Genetic Marker Studies
Lod Score Method: Requires knowledge of the
mode of inheritance. Based on estimation of the
“recombination fraction” – the proportion of all
gametes that are recombined between two loci
of interest – the lower the probability, the greater
the likelihood of linkage.
A likelihood ratio test is an odds ratio comparing
the probability that linkage is present versus not
present (null probability value of 0.5).
Genetic Marker Studies
Lod Score Method --- Assumptions:
a) 2 traits of interest are inherited as simple, single
locus Mendelian traits.
b) Relations between genotype and phenotype are
known (e.g. dominance and penetrance).
c) Gene frequencies of various alleles are known.
d) For a family to be informative for linkage, one
parent must be heterozygous for both traits of
interest (e.g. Aa, Bb).
Genetic Marker Studies
The Lod Score is the log of the odds ratio.
For monogenetic diseases that have Mendelian
patterns of inheritance:
>3
evidence of linkage
< -2
evidence against linkage
> -2 to < 3 inconclusive
However, Lod Scores required to confirm or
reject linkage for complex disorders (e.g.
psychiatric disorders) need to be of greater
magnitude due to complex patterns of
inheritance.
Gene-Environment Interaction
Gene-Environment Interactions:
a) Statistical: Coefficient of the product term of 2
or more risk factors (e.g. extent to which the
coefficient departs from a multiplicative model).
b) Biological: Co-participation between 2 factors in
the same causal mechanism to disease
development (e.g. extent to which coefficient
departs from an additive model).
Gene-Environment Interaction
Genetic-Environment Interactions:
* Two types of genetic markers used in geneenvironment interaction studies:
a) Markers based on gene products such
as specific blood groups, HLA antigens,
serum proteins, and enzyme systems
b) Markers based on direct analysis of DNA.
Gene-Environment Interaction
Study Designs:
a) Traditional (e.g. case-control, cohort) --- these
have many of the same concerns as nongenetic-based studies (e.g. selection of external
control subjects).
Using an Internal Control Group
b) Case-only study
c) Case-parental control study
d) Twin studies
Gene-Environment Interaction
Example: Traditional Cohort Study:
Exposure (E) Genotype (G)
0
0
0
1
1
0
1
1
Disease Risk
I
IRG
IRE
IRGE
RR
1.0
RG
RE
RGE
Compare product of RR’s for 1 factor present
versus both present (null value = 1.0)
H0:
H1:
RGE / (RG x RE) = 1.0
RGE / (RG x RE) = 1.0
Gene-Environment Interaction
Hypothetical Example:
(Exposure):
Epstein-Barr virus
(Genotype):
HLA-DR4
(Outcome):
Rheumatoid arthritis
Exposure (E) Genotype (G)
0
0
0
1
1
1
0
1
Disease Risk
0.0029
RR
1.0
0.0042
0.0038
0.0097
In the above example, is there evidence
of gene-environment interaction on the
development of rheumatoid arthritis?
Gene-Environment Interaction
Hypothetical Example:
(Exposure):
Epstein-Barr virus
(Genotype):
HLA-DR4
(Outcome):
Rheumatoid arthritis
Exposure (E) Genotype (G)
0
0
0
1
1
H0:
1
0
1
RGE / (RG x RE) = 1.0
3.38 / (1.45 x 1.31) =
3.38 / 1.90 = 1.78
Disease Risk
0.0029
RR
1.0
0.0042
0.0038
0.0098
1.45
1.31
3.38
Since 1.78 = 1.0, then
there is evidence of
G-E interaction.
Gene-Environment Interaction
Case-Only Study (internal controls):
•
•
•
Primary goal: assess gene-environment interaction.
Valid under assumption of independence between
distribution of exposure and genotype in the population
(see below).
Cannot assess effects of exposure or genotype alone.
ORCA = RGE / (RE * RG) * ORCO
ORCA = RGE / (RE * RG)
H0: ORCA = 1
H1: ORCA > 1 (mult.); ORCA < 1 (additive)
Example of Case-Control Study:
Exposure (E)
0
0
Genotype (G)
0
1
Cases
36
7
Controls
167
34
OR
1.0
1.0
1
1
0
1
13
13
69
11
0.9
5.5
Example of Case-Only Study:
Genotype (G)
Exposure (E)
0
1
0
A
B
1
C
D
ORCA = AD / BC =
(36 * 13) / (7 * 13) =
468 / 91 = 5.1
Gene-Environment Interaction
Case-Parental Control Study (internal controls):
•
Primary goal: assess gene-environment interaction.
•
Cannot assess independent effect of exposure.
•
Permits assessment of effect of genotype with and
without exposure.
•
Parents of case subjects are used as controls – must
have genotype information on the parents.
•
Genotype of case is compared to genotype of a fictitious
control formed by the non-transmitted allele from each
parent.
Gene-Environment Interaction
Case-Parental Control Study:
Parental non-transmitted
alleles
Exposure: Absent
Absent
Present
Odds ratio (unexp.)
Exposure: Present
Absent
Present
Odds ratio (exposed)
Susceptibility genotype (cases)
Absent
Present
T0
V0
U0
W0
1.0
U0 / V0
T1
V1
U1
W1
1.0
U1 / V1
Can compare the two odds ratios to assess presence of
interaction
Gene-Environment Interaction
Twin Studies:
• Involves 2 measures of relative risk (RR)
RRE = Relative risk for disease among exposed
vs. unexposed co-twins, stratified by zygosity:
Zygosity
MZ
Risk
Unexposed
RR
1.0
MZ
Exposed
???
DZ
DZ
Unexposed
Exposed
1.0
???
If effect of exposure is constant,
no interaction with genotype
Gene-Environment Interaction
Twin Studies:
Zygosity
MZ
MZ
Risk
Unexposed
Exposed
RR
1.0
2.7
DZ
DZ
Unexposed
Exposed
1.0
2.6
In the above example:
(a)Is exposure related to risk of disease?
(b)Does genotype interact with exposure on
risk of disease?
Gene-Environment Interaction
Twin Studies:
Zygosity
MZ
MZ
Risk
Unexposed
Exposed
RR
1.0
2.7
DZ
DZ
Unexposed
Exposed
1.0
2.6
(a) Since exposed persons are at overall higher risk of
disease than unexposed persons, the exposure is
related to risk of disease?
(b) Since the effect of exposure on risk of disease is
similar by zygosity (RR of 2.7 vs. 2.6), genotype
does not interact with the exposure.
Gene-Environment Interaction
Twin Studies:
RRz = Relative risk for disease among MZ vs. DZ
co-twins, stratified by exposure status:
Exposure Status
Risk
RR
Unexposed
Unexposed
DZ
MZ
1.0
???
Exposed
Exposed
DZ
MZ
1.0
???
If effect of genotype is constant, no interaction
with exposure