The properties of GPRA as a G-protein-coupled receptor, which might be used by a novel ligand as part of a new signaling pathway in asthma, mean that it has potential as a possible drug target.
In 2001, a genome-wide scan in asthma identified chromosome 7p as 1 of 6 possible asthma-susceptibility loci. Kere and colleagues have now narrowed the candidate region from a 20-cM to a 133-kb region, which contains 2 genes — GPRA and AAA1.
The authors genotyped 874 subjects from the Finnish Kainuu subpopulation by interspersing successive rounds of genotyping that increased the density of markers (SNPs and microsatellites) with analysis using the haplotype pattern mining (HPM) algorithm, which searches large sets of unrelated haplotypes for allele patterns that are shared between several haplotypes. Having identified a strong association of a conserved 47-kb haplotype pattern in this way, Kere and colleagues sequenced 133 kb that encompassed the 47-kb region from a homozygous asthmatic patient. Comparison with the public sequence identified 80 new polymorphisms.
Asthmatics from North-eastern Quebec and individuals with high serum immunoglobin E levels from North Karelia, Finland, also had a 133-kb haplotype pattern with the same limits, for which most SNPs were conserved. For the 3 populations combined, 7 alternative haplotypes were formed by 13 SNPs across the most-conserved region of 77 kb. Sequencing of the 133-kb region from 6 individuals who were homozygous for the remaining 6 haplotypes confirmed that they had different SNP compositions. Phylogenetic analysis showed these risk haplotypes to be related and distinct from non-risk haplotypes. By SNP-tagging the risk haplotypes, the authors confirmed that they confer risk in all three populations, which is consistent with the common-disease/common-variant hypothesis.
With the 133-kb region confirmed as a susceptibility locus, the authors looked for open reading frames. They found 2: exons 3 to 5 of a gene that they named GPRA (for G-protein-coupled receptor for asthma susceptibility) and, on the opposite strand, exons 3 to 10 of a gene that they called AAA1 (for asthma-associated alternatively spliced gene 1). Both GPRA and AAA1 are probably alternatively spliced, but AAA1 might not be a protein-coding gene (for example, in vitro translation of its longest open reading frame, which encodes only 74 amino acids, did not produce a stable polypeptide, and no recombinant protein was produced by transiently transfected cells).
GPRA's two main transcripts, A and B, encode proteins of 371 and 377 amino acids, respectively. Whereas the A isoform is mainly expressed by smooth-muscle cells, the B isoform is mainly found in epithelial cells; however, in asthma patients, the B isoform is strongly expressed by smooth-muscle cells. So, Kere and colleagues suggest that the balance between the A and B isoforms might be altered by polymorphisms in the risk haplotypes. In support of a role for GPRA in asthma pathogenesis, Gpra mRNA is significantly upregulated in a mouse model of ovabulmin-induced lung inflammation.
Although these are not the first candidate asthma-susceptibility genes, this study identifies GPRA as the strongest candidate so far. And, most encouragingly, the properties of GPRA as a G-protein-coupled receptor, which might be used by a novel ligand as part of a new signalling pathway in asthma, mean that it also has potential as a possible drug target.
Natalie Wilson
References
Laitinen, T. et al. Characterization of a common susceptibility locus for asthma-related traits. Science304, 300–304 (2004) | Article | PubMed |
