Genetics of Asthma
Paul E. Moore, M.D.
Vanderbilt University School of Medicine
May 9, 2013
Definition of asthma
•
•
•
•
Chronic inflammation
Airway reactivity to specific triggers
Reversible airway obstruction
Manifest as symptoms that can include
cough, wheeze, and dyspnea
Weiss, Pharmacogenomics, 2006
Phenotype
• Phenotype: observable properties of an
organism that are produced by the
interactions of the genotype and the
environment
• Asthma: collection of different phenotypes,
rather than a single disease
• Asthma is a heterogeneous phenotype
resulting from interaction between multiple
genetic and environmental influences
Wenzel, Nature Medicine, 2012
Genetics of Asthma
• Twin studies and family studies suggest a
heritable pattern.
• Some genes influence asthma development
(susceptibility) or progression (severity)
• Other genes modify response to therapy
(pharmacogenetics).
• Gene-environment interactions add to the
level of complexity.
Issues in Studying Asthma
Genetics
• Polygenic disease
• No "gold standard" to define asthma
• Inconsistent application of clinical
parameters
• Gene-environment interactions
Measurable Asthma Phenotypes
• Presence of asthma
• Asthma-related traits
– Bronchial hyperresponsiveness
– IgE levels, Skin test reactivity
• Response to asthma medication
– Physiologic improvement
– Quality of life measurements
Genetic Approaches
• Candidate-gene association studies
• Genome-wide linkage studies in families
• Genome-wide association studies
(GWAS)
Association Studies
• Hypothesis-driven
• Determine the relationship between
specific polymorphisms in candidate
genes and certain disease characteristics.
• To date, close to 200 candidate genes in
over 500 published studies have been
linked to different asthma phenotypes.
Categories of asthma
susceptibility genes
• Innate immunity and immunoregulation
• TH2-cell differentiation and effector
functions
• Epithelial biology and mucosal immunity
• Lung function, airway remodeling, and
disease severity
Vercelli D, Nat Rev Immunol, 2008
Susceptibility genes associated
with asthma phenotypes in 20+
association studies
IL13
IL4
CD14
ADRB2
TNF
HLA-DRB1
FCERIB
IL4R
5q31
5q31.1
5q31.1
5q31-32
6p21.3
6p21
11q13
16p12.1
TH2 effector function
TH2 differentiation
Innate immunity
ASM relaxation
Inflammation
Antigen presentation
Fc receptor for IgE
a-chain of IL4R,13R
Limitations of association
studies
• Selection of genes is based on available
knowledge.
• Each variant makes only a modest
contribution to overall heritability in a
polygenic disease.
Linkage studies
• Allows identification of new genes and
pathways
• Technique: statistical approach using DNA
markers, followed by positional cloning
• To date, ~20 linkage scans have been
reported in different asthma study
populations.
ADAM33
• Original study (2002): 460 Caucasian
families found linkage to chromosome 20p13
• Gene: A disintegrin and metallopeptidase
domain 33, expressed in lung fibroblasts and
smooth muscle cells
• Series of replications studies inconclusive
• Single gene with modest contribution to
overall population risk
Van Eerdewegh P, Nature, 2002
Genes identified by linkage scans
•
•
•
•
•
DPP10 (dipeptidyl-peptidase X)
PHF11 (plant homeodomain finger protein)
GPRA (G-protein coupled receptor)
HLA-G
Cytoplasmic fragile X mental retardation
protein (FMR) interacting protein 2 (CYFIP2)
• IRAKM (IL-1 receptor associated kinase)
• COL29A1 (Collagen XXIX)
• PCDH1 (Protocadherin 1)
Limitations of linkage scans
• Difficult to identify candidate gene
• Challenge to link specific locus or gene to
asthma or asthma-related traits
Genome-wide association studies
• International HapMap resource
• Availability of dense genotyping chips
• Collection of sets of large and well
characterized human samples
First GWAS: 17q21
• 317,000 SNPs in 994 patients with childhoodonset asthma and 1,243 non-asthmatics
• Multiple markers on chr 17q21 strongly
associated with childhood asthma
• Independent replication studies of the 17q21
locus showed strong association with diagnosis
of childhood asthma:
– 2,320 subjects in a cohort of German children
– 3,301 subjects in the British 1958 Birth Cohort
Moffatt MF, Nature, 2007
First GWAS: 17q21
• Independent replications in North Americans of
European ancestry, Puerto Rican, Mexican,
Japanese, and Chinese populations
• No associations yet described with AfricanAmerican populations
• In Caucasians, even stronger association with
early tobacco smoke exposure and early-onset
asthma
Bouzigon E, New Engl J Med, 2008
First GWAS: 17q21
• Transcript levels of 19 genes in EBVtransformed lymphoblastoid cell lines
• Rationale: Variation in gene transcription may
mediate disease susceptibility, and transcript
abundance may be directly modified by
polymorphisms in regulatory elements.
• ORMDL3: member of a gene family that
encodes transmembrane proteins anchored in
the endoplasmic reticulum
Moffatt MF, Nature, 2007
GWAS and Asthma
Susceptibility
• 2 large meta-analyses of asthma
susceptibility identified 4 regions that were
associated with asthma in individuals of
different ethnic backgrounds:
– loci in the ORMDL3 region of 17q21
– IL1RL/IL18R on chromosome 2q
– TSLP on 5q22
– IL33 on chromosome 9p24
Slager Clin Chest Med 2012
GWAS and Asthma Severity
• Genes that are associated with asthma
subphenotypes, such as lung function,
biomarkers levels, and asthma therapeutic
responses, can provide insight into
mechanisms of asthma severity progression.
• A joint model of risk variants in lung function
genes identified in the general population
were highly associated with lower lung
function and increased severity in asthma
populations.
Slager Clin Chest Med 2012
GWAS and Pharmacogenetics
• A pharmacogenetic genome-wide screen
identified 2 correlated genetic variants in
the GLCCI1 gene related to response to
inhaled glucocorticoids.
Slager Clin Chest Med 2012
Benefits of GWAS
• Identification of novel genes not previously
linked to asthma: ORMDL3, DENND1B, HHIP
• Suggestion of link between lung development,
affecting COPD and asthma
Limitations of GWAS
• Some genes identified by GWAS do not appear
biologically plausible
• Challenges in finding the specific mutation or
gene within a region of interest
• Single genes unlikely to be sufficient for disease
causation
• Relatively small sample size precludes results
of statistical significance
• Replication limited in non-European populations
• Heterogeneity in environmental exposures
• Challenges in bioinformatics
Gene-Environment Interactions
• Concordance rates among monozygotic
twins: 50-60%
• Allergens
• Respiratory infections
• Tobacco smoke
• Air pollution
• Diet
β2-Adrenergic Signaling Pathway
Questions about deleterious effects
of b-agonists
• Concern: whether b-agonist use is
associated with poor asthma control
• In Beta Agonist in Mild Asthma (BAGS),
AM PEF no different with regular albuterol
use
• Meta-analysis: Just 1 to 6 weeks of
regular b-agonist use significantly
reduced the bronchodilator response
Sears MR, Lancet, 1990; Drazen JM, NEJM, 1996; Salpeter SR, Ann Int Med, 2004
Questions about deleterious effects
of long-acting b-agonists
• Concern: whether long-acting b-agonist
use is associated with poor asthma control
• In Salmeterol Multi-Center Asthma
Research Trial (SMART): higher number of
asthma-related deaths or life-threatening
experiences with Serevent
• Meta-analysis: Long-acting b-agonists
increased exacerbations requiring
hospitalization and life-threatening
exacerbations.
Nelson HS, Chest, 2006; Salpeter SR, Ann Int Med, 2006
b2-Adrenergic Receptor
Liggett, AJRCCM, 1997
No association of ADRB2 polymorphisms
and the diagnosis of asthma
Amino acid
Position
16
27
Genotype
Homozygous Arg
Heterozygous
Homozygous Gly
Homozygous Gln
Heterozygous
Homozygous Glu
Frequency (%)
Normal
Asthmatic
13.5
14.6
29.2
32.2
57.3
53.1
28.0
44.0
28.0
26.0
50.0
24.0
adapted from Liggett et al., AJRCCM, 1997
Clinical Correlation
• Importance of early in vitro studies
• No consensus on which polymorphism
influences asthma severity.
• 4 studies from the Asthma Clinical
Research Network: Arg16
BAGS and BARGE: Patients homozygous for the
Arg16 allele showed worsening in AM PEF while on
regularly scheduled albuterol.
Israel E, AJRCCM, 2000; Israel E, Lancet, 2004
SOCS and SLIC:
Arg16 subjects
who received
salmeterol alone
had worsening
AM PEF.
Wechsler, AJRCCM, 2006
Effect of ADRB2 polymorphisms on
response to LABA
The Gly16
genotype had no
effect on the
percentage of
participants with
severe
exacerbations
across all treatment
groups (primary
endpoint).
Bleecker et al., Lancet, 2008
The inability to reproduce
genotype-phenotype associations
is related to a number of factors:
• Measurements of many different
parameters relative to asthma
• Coding block haplotypes may be markers
for more functionally significant haplotypes
in the promoter region.
Promoter/coding block
haplotypes
• Resequencing of a 5.3-kb region in 429
whites and 240 African Americans revealed
31 SNPs with minor allele frequency >3%.
• 4 extended haplotypes account for >90% of
the persons genotyped from 4 ethnic
backgrounds.
Drysdale et al, PNAS, 2000
Hawkins et al, AJRCCM, 2006
ADRB2 haplotype influences
spirometric response
• Genotype: ADRB2 SNPs in 99 adults
admitted for asthma exacerbation
• Primary endpoint: Change in FEV1 24 h
after admission
• Results: Complex promoter/Arg16 coding
block haplotype present only in African
Americans associated with decreased
FEV1 response
Moore et al, Clin Trans Sci, 2008
SPARE RECEPTORS
C. 25,000 receptors
A. 50,000 receptors
ASM
ASM
=spare
receptor
NO SPARE RECEPTORS
Desensitization:
reduced b2AR
number
ASM
B. 30,000 receptors:
No reduction in maximal
responsiveness;
thus, no change in
functional desensitization
ASM
D. 5,000 receptors:
Reduced maximal
effectiveness;
thus, increased
functional desensitization
Drury et al., Brit J Pharm, 1997
Nishikawa et al, AJRCMB, 1994
Rousseau et al, Eur J Pharm, 1997
Other regulatory elements of the ADRB2
• Rare genetic variations in the promoter
region
• Poly-C repeat in the 3’-untranslated
region: length associated with FEV1 in
African American.
• In vitro studies suggest that length of the
poly-C repeat alters mRNA stability and
ADRB2 expression.
Hawkins et al. , AJRCCM, 2006.
Panebra et al, AJP: Lung, 2008.
Gene-gene interactions
• Adenylyl Cyclase IX
• GRK5
• Regulation of ADRB2 signaling by Snitrosylation of GRK2 may provide a link
between NO and ADRB2 function.
Whalen EJ, Cell, 2007.
Tantisira KG, Hum Mol Gen, 2005
Zhang G, Eur Respir J, 2007.
Gene-environment interactions
• ADRB2 genotype effects may be manifest
only after specific environmental
exposures, including cigarette smoke and
viral infection.
• Children homozygous for the Arg16 allele
exposed to maternal smoking in utero: 3fold increased risk for lifetime wheezing
Moore et al, AJRCMB, 2006
Wang et al, AJRCCM, 2001
Wang et al., Pediatrics, 2008
Summary
• Asthma is a heterogeneous phenotype
resulting from interaction between multiple
genetic and environmental influences.
• Genes can influence asthma susceptibility,
severity, or response to therapy.
• Genome-wide association studies have
helped to identify genes not previously
linked with asthma.
Resources
• Vercelli D. Discovering susceptibility genes for
asthma and allergy. Nat Rev Immunol 2008;
8:169–182.
• Sleiman P and Hakonarson H. Recent
advances in the genetics and genomics of
asthma and related traits. Curr Opin Ped 2010;
22:307-312.
• Slager RE et al. Genetics of Asthma
Susceptibility and Severity. Clin Chest Med
2011;33:431-43.
• Wenzel S. Asthma phenotypes: the evolution
from clinical to molecular approaches. Nat Med
2012;18:716-25.