Genetic Complexity in aDrosophilaModel of Diabetes-Associated Misfolded Human Proinsulin

Abstract

Drosophila melanogaster has been widely used as a model of human Mendelian disease, but its value in modeling complex disease has received little attention. Fly models of complex disease would enable high-resolution mapping of disease-modifying loci and the identification of novel targets for therapeutic intervention. Here, we describe a fly model of permanent neonatal diabetes mellitus and explore the complexity of this model. The approach involves the transgenic expression of a misfolded mutant of human preproinsulin, hINSC96Y, which is a cause of permanent neonatal diabetes. When expressed in fly imaginal discs, hINSC96Y causes a reduction of adult structures, including the eye, wing, and notum. Eye imaginal discs exhibit defects in both the structure and the arrangement of ommatidia. In the wing, expression of hINSC96Y leads to ectopic expression of veins and mechano-sensory organs, indicating disruption of wild-type signaling processes regulating cell fates. These readily measurable “disease” phenotypes are sensitive to temperature, gene dose, and sex. Mutant (but not wild-type) proinsulin expression in the eye imaginal disc induces IRE1-mediated XBP1 alternative splicing, a signal for endoplasmic reticulum stress response activation, and produces global change in gene expression. Mutant hINS transgene tester strains, when crossed to stocks from the Drosophila Genetic Reference Panel, produce F1 adults with a continuous range of disease phenotypes and large broad-sense heritability. Surprisingly, the severity of mutant hINS-induced disease in the eye is not correlated with that in the notum in these crosses, nor with eye reduction phenotypes caused by the expression of two dominant eye mutants acting in two different eye development pathways, Drop (Dr) or Lobe (L), when crossed into the same genetic backgrounds. The tissue specificity of genetic variability for mutant hINS-induced disease has, therefore, its own distinct signature. The genetic dominance of disease-specific phenotypic variability in our model of misfolded human proinsulin makes this approach amenable to genome-wide association study in a simple F1 screen of natural variation.

Effect of Genetic Variation in a Drosophila Model of Diabetes-Associated Misfolded Human Proinsulin

“…The F1 lines displayed a wide, nearly continuous, range of heritable eye-degeneration phenotypes, suggesting a polygenic basis for this genetic background variation (Park et al 2013). To investigate the genetic basis of this background variation, here we performed a genome-wide association study in a larger set of 154 DGRP lines.…”

A Bystander Mechanism Explains the Specific Phenotype of a Broadly Expressed Misfolded Protein

“…In Drosophila , expression of human insulin bearing the Akita C96Y mutation results in ER stress and cellular dysfunction. While this mutant protein caused a general degeneration phenotype in the eye, its expression in the wing imaginal discs resulted in phenocopies of Notch and crossveinless mutations [76,77]. Interestingly, a targeted deletion of the ER thiol oxidase Ero1L specifically in the wing also caused a phenocopy of Notch , by inducing misfolding of Notch protein while not affecting other secretory proteins [78].…”

Divergent forms of endoplasmic reticulum stress trigger a robust unfolded protein response in honey bees

“…S1 and Table S1). A core set of putative target genes were culled from lists of UPR target genes in Drosophila (Chow et al, 2013;Park et al, 2014), and include candidate genes from each of the following categories: chaperones (Hsc70-3, p58ipk, Gp93), disulfide bond generating enzymes (Ero1l, CaBP), and the ER-associated degradation (ERAD) pathway (Derlin1, Herp2, Hrd) ( Fig. S1 and Table S1).…”