Genetics of Asthma 

Updated: Aug 05, 2015
  • Author: Scott T Weiss, MD, MS; Chief Editor: Karl S Roth, MD  more...
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Asthma-Susceptibility Genes

Genetic association studies have identified and replicated susceptibility genes for asthma in 3 major types of studies:

  • Candidate gene studies
  • Positional cloning using linkage studies
  • Genome-wide association studies (GWAS)

Candidate gene studies

Candidate gene studies represent the most common form of genetic association study performed to find asthma-susceptibility genes. These studies focus on genes thought to contribute to the pathobiology of a disease and are therefore unlikely to identify novel biology. Rather, they emphasize what is already known. Although they only minimally advance the field, studies such as these are the most commonly performed scientific investigation of asthma-susceptibility genes.

Fifty-three genes have been identified and replicated in more than one study, and 20 genes have been replicated in more than 10 separate investigations. [1] These genes are involved in a variety of biologic processes: Th2 inflammation, T-reg cell function, the HLA locus/immunity, and IgE response of B cells. However, as expected, the knowledge that resulted from these candidate gene studies has been limited to understanding molecular mechanisms related to known disease pathobiologic processes.

Linkage studies

Linkage studies, which identify genetic markers related to asthma using statistical association, are uniquely suited to isolating regions of chromosomes that demonstrate association signals between asthma and novel genetic markers.

Fine genotype mapping in the region is then used to identify the actual gene(s) responsible for the linkage peak. By 2003, 6 novel genes for asthma were identified using this approach: ADAM33, VDR, DPP10, PHF11, HLA-G, and GPR154. [2] These genes are thought to be involved in atopy, bronchial hyperresponsiveness, elevated IgE levels, and other asthma-related traits, although the effect of each gene on asthma susceptibility seems to be small. Also, because this method limits investigation to the specified chromosomal regions, it is less likely to fully elucidate complex disorders such as asthma.

Genome-wide association studies

Genome-wide association studies (GWAS) survey single nucleotide polymorphisms (SNPs), or specific genetic variations, across the entire genome. As all SNPs in the genome correlate with each other, one could conceivably type as few as 500,000 SNPs to obtain information on most of the 13 million SNPs across the genome. Thus, unlike the candidate gene studies, GWAS are adequately powered to discover common genetic variations associated with complex disorders such as asthma. [3]

The initial GWAS in asthma incorporated a unique approach by combining gene expression with genetic association to find ORMDL3, a novel locus on chromosome 17q21. [4] Other, small GWAS subsequently identified several novel genes for asthma. [5, 6, 7, 8, 9] However, because most of these genes explained only a small amount of the variability in the asthma phenotype, larger studies were needed to identify additional genes.

To accomplish this goal, 2 large consortia comprising more than 20,000 patients each were formed: one in Europe, known as GABRIEL, which included white patients from across Europe, and one in the United States, known as EVE, which included a more ethnically diverse patient base from across North America.

GABRIEL investigators identified 10 genes (IL1RL1/IL18, TSLP, IL33, SMAD3, HLA-DQ, ORMDL3, IL2RB, SLC22A5, IL13, and RORA), 6 of which were confirmed by EVE investigators (IL1RL1/IL18, TSLP, IL33, SMAD3, HLA-DQ and ORMDL3).{Re9} [10] All of the genes identified seemed to support a role for disordered inflammatory/immunologic responses, but the precise mechanism and impact of each gene on asthma susceptibility has yet to be elucidated.

In one study, 16 SNPs in all 3 ORMDLs were associated with asthma, with 14 SNPs in ORMDL3. Baseline expression of ORMDL1 and ORMDL2 was higher in peripheral blood mononuclear cells from patients with asthma. [11]

Through GWAS, a novel pharmacogenomic locus was identified regarding improved montelukast response in people with asthma. In this study, 28 SNP associations from the discovery GWAS were replicated, and of these, rs6475448 achieved genome-wide significance. Patients from all 4 studies who were homozygous for rs6475448 showed increased ΔFEV1 from baseline in response to montelukast. [12]

Next:

Clinical Considerations

With each successive wave of new technology, more genes are being identified. The next steps in the research agenda are to determine the actual functional variants in these genes and to figure out how the novel genes are involved in asthma pathogenesis.

Data from current studies have already resulted in a rethinking of asthma as being less a Th2 disease and being more a disease of T-reg cell dysfunction, but further research is required to determine how all of the different genes interact in the pathobiology of the disease.

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