  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 < 10-5 including one SNP in an intron of the gene sulfateless
(sfl) which exceeded a conservative genome-wide significance threshold of P =
0.05 level (-log10 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.
