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Naturally Occurring Genetic Variation Influences the Severity of Drosophila Eye Degeneration Induced by Expression of a Mutant Human Insulin Gene

Dominant negative mutations in the insulin gene are the second most common cause of permanent neonatal diabetes. However, variation in severity and penetrance of neonatal diabetes, as in other complex genetic diseases, cannot be accounted for by known disease mutations. In a novel approach to this problem, we have utilized the genetic tools available in Drosophila to model the effects of the C96Y mutation, a cysteine to tyrosine mutation in the insulin protein that can cause permanent neonatal diabetes in humans. This mutation, which disrupts a disulfide bridge in the proinsulin molecule, has been shown to lead to partial protein unfolding and aggregation in the endoplasmic reticulum. It is thought to induce beta cell death in humans and mice through endoplasmic reticulum stress-mediated apoptosis. We employed the UAS/GAL4 system to create a stable fly laboratory stock expressing human insulin with the C96Y mutation (INSC96Y) in the Drosophila eye. We crossed these transgenic INSC96Y flies to 36 isogenic lines derived from a wild population. The F1 flies displayed a disrupted eye development phenotype, with both reduced eye size and irregularity in ommatidia, which varied between lines and with sex. By backcrossing F1 offspring to their wild parents, we were able to analyze the effects of naturally occurring genetic variation on the INSC96Y phenotype in flies both heterozygous and homozygous for wild third chromosomes. We observed significant differences between lines in terms of eye size, eye shape, the presence or absence of lesions, and degree of ommatidial structure present, indicating that an approach that takes into account quantitative variation in the genetic background can yield valuable insights into the phenotypic severity and penetrance of a complex genetic disease.