SUMMARY Lineage mapping has identified both proliferative and quiescent intestinal stem cells, but the molecular circuitry controlling stem cell quiescence is incompletely understood. By lineage mapping, we show Lrig1, a pan-ErbB inhibitor, marks predominately non-cycling, long-lived stem cells located at the crypt base that, upon injury, proliferate and divide to replenish damaged crypts. Transcriptome profiling of Lrig1+ colonic stem cells differs markedly from highly proliferative, Lgr5+ colonic stem cells; genes up-regulated in the Lrig1+ population include those involved in cell cycle repression and response to oxidative damage. Loss of Apc in Lrig1+ cells leads to intestinal adenomas and genetic ablation of Lrig1 results in heightened ErbB1-3 expression and duodenal adenomas. These results shed light on the relationship between proliferative and quiescent intestinal stem cells, and support a model in which intestinal stem cell quiescence is maintained by calibrated ErbB signaling with loss of a negative regulator predisposing to neoplasia.

KRAS is the most frequently mutated oncogene. The incidence of specifi c KRAS alleles varies between cancers from different sites, but it is unclear whether allelic selection results from biological selection for specifi c mutant KRAS proteins. We used a crossdisciplinary approach to compare KRAS G12D , a common mutant form, and KRAS A146T , a mutant that occurs only in selected cancers. Biochemical and structural studies demonstrated that KRAS A146T exhibits a marked extension of switch 1 away from the protein body and nucleotide binding site, which activates KRAS by promoting a high rate of intrinsic and guanine nucleotide exchange factorinduced nucleotide exchange. Using mice genetically engineered to express either allele, we found that KRAS G12D and KRAS A146T exhibit distinct tissue-specifi c effects on homeostasis that mirror mutational frequencies in human cancers. These tissue-specifi c phenotypes result from allele-specifi c signaling properties, demonstrating that context-dependent variations in signaling downstream of different KRAS mutants drive the KRAS mutational pattern seen in cancer. SIGNIFICANCE: Although epidemiologic and clinical studies have suggested allele-specifi c behaviors for KRAS , experimental evidence for allele-specifi c biological properties is limited. We combined structural biology, mass spectrometry, and mouse modeling to demonstrate that the selection for specifi c KRAS mutants in human cancers from different tissues is due to their distinct signaling properties.

Doublecortin like kinase 1 (DCLK1) is an understudied kinase that is upregulated in a wide range of cancers, including pancreatic ductal adenocarcinoma (PDAC). However, little is known about its potential as a therapeutic target. We leveraged chemoproteomic profiling and structure-based design to develop the first selective, in vivo -compatible chemical probe of the DCLK1 kinase domain, DCLK1-IN-1. We demonstrate activity of DCLK1-IN-1 against clinically relevant patient-derived PDAC organoid models and use a combination of RNA sequencing, proteomics and phosphoproteomics analysis to reveal that DCLK1 inhibition modulates proteins and pathways associated with cell motility in this context. DCLK1-IN-1 will serve as a versatile tool to investigate DCLK1 biology and establish its role in cancer.