Clonal effects of the Ras oncogene revealed by somatic mutagenesis in a Drosophila cancer model

Somatic mutations of Ras, encoding a small GTPase, are detected in a wide range of human cancers. Tumor genome sequencing further reveals a cancer type-dependent mutational spectrum for the Ras gene, suggesting that tissue- and allele-specific effects underlie tumorigenic activity. Although biochemical studies have characterized the GTPase activity of several Ras variants in vitro, precisely how somatic mutations of the endogenous Ras locus differentially affect tissue growth and homeostasis remain elusive. Here we engineered the endogenous Drosophila Ras locus to create a spectrum of inducible oncogenic alleles and then compared their activities in vivo. In the developing wing primordium, somatic clones carrying the oncogenic mutation Ras G12V exhibited a weak activation of downstream MAPK signaling but did not disrupt tissue architecture. However, cell clones carrying the same Ras G12V allele in the adult midgut exhibited a growth advantage and progressively took over the tissue, resulting in intestinal barrier dysfunction. In contrast, cell clones expressing a distinct allele, Ras Q61H, formed aberrant cysts that disrupted epithelial architecture and triggered local cell death. Conversely, when we induced cell clones carrying Ras Q61H in the midgut, hyper-proliferating mutant cells rapidly expanded to occupy the entire tissue. Surprisingly, this population of rapidly expanding mutant cells was eventually eliminated from the midgut, restoring wild-type cells and normal barrier function. Thus, in the midgut, Ras G12V was ultimately more deleterious than Ras Q61H due to the regression of Ras Q61H mutant cells. These results establish a new model for somatic mutagenesis at the Ras locus and illuminate a mechanistic basis for the tissue-specific effects of oncogenic Ras variants. Further, this study provides direct evidence that allele-dependent clonal dynamics may play a critical role in the tissue-selectivity of Ras oncogenic mutations.