Effect of Genetic Variation in a Drosophila Model of Diabetes-Associated Misfolded Human Proinsulin
Bin Z. He, Michael Z. Ludwig, Desiree A. Dickerson, Levi Barse, Bharath Arun, Soo Young Park, Natalia A. Tamarina, Scott B. Selleck, Patricia Wittkopp, Graeme I. Bell, Martin Kreitman
(Submitted on 23 May 2013)
The identification and validation of gene-gene interactions is a major challenge in human studies. Here, we explore an approach for studying epistasis in humans using a Drosophila melanogaster model of neonatal diabetes mellitus. Expression of mutant preproinsulin, hINSC96Y, in the eye imaginal disc mimics the human disease activating conserved cell stress response pathways leading to cell death and reduction in eye area. Dominant-acting variants in wild-derived inbred lines from the Drosophila Genetics Reference Panel produce a continuous, highly heritable, distribution of eye degeneration phenotypes. A genome-wide association study (GWAS) in 154 sequenced lines identified 29 candidate SNPs in 16 loci with P 7.62). RNAi knock-downs of sfl enhanced the eye degeneration phenotype in a mutant-hINS-dependent manner. sfl encodes a protein required for sulfation of the glycosaminoglycan, heparan sulfate. Two additional genes in the heparan sulfate (HS) biosynthetic pathway (tout velu, ttv and brother of tout velu, botv) also modified the eye phenotype, suggesting a link between HS-modified proteins and cellular responses to misfolded proteins. Finally, intronic variants marking the QTL were associated with decreased sfl expression, a result consistent with that predicted by RNAi studies. The ability to create a model of human genetic disease in the fly, map a QTL by GWAS to a specific gene (and noncoding variant), validate its contribution to disease with available genetic resources, and experimentally link the variant to a molecular mechanism, demonstrate the many advantages Drosophila holds in determining the genetic underpinnings of human disease.
Haldane's Sieve 
Thanks Joe for posting this. We were exploring the idea of using Drosophila to understand the genetic architecture underlying a complex disease trait. We did so by crossing a transgenic, Mendelian disease carrying line to a panel of wild-derived inbred lines, and asked whether the severity of the disease is dependent on the genetic background. The answer is a definite yes: the range of phenotype quantified by the size of the eye span from 10% to 80% of wildtype (the mutant human proinsulin was expressed in the eye disc during development, causing neurodegeneration. We used eye because it is dispensable in lab conditions, and easy to measure the phenotype). We then conducted a GWAS, which led to the identification of sfl, as described above, and also the HS biosynthetic pathway by genetic test. One unique advantage of our system is its ultra-high resolution in mapping: we localized the association signal to ~400bp LD block within one of the introns of sfl, allowing us to test specific hypotheses about the molecular mechanisms of the associated variants. Pyro-sequencing analysis revealed allele-specific expression difference due to the intronic variation, but also highlighted the genetic heterogeneity even within that locus, with additional cis-variants present to influence the expression level. Overall, we believe that our fly model system is a powerful complementary approach to the genetic study of complex traits. Its high mapping resolution and rich molecular/genetic toolkits allow faster and in-depth characterization of disease-associated variation, which is a unique advantage.
Pingback: Our paper: Effect of Genetic Variation in a Drosophila Model of Misfolded Human Proinsulin | Haldane's Sieve