Resistance to asparaginase is a common clinical problem, whose biologic basis is poorly understood. We hypothesized, from the concept of synthetic lethality, that gain-of-fitness alterations in drug-resistant cells had conferred a survival advantage that could be exploited therapeutically. Using a genome-wide CRISPR/Cas9 screen in T-ALL, we found that negative regulators of Wnt signaling were selectively depleted upon treatment with asparaginase. Wnt pathway activation induced profound asparaginase sensitivity in distinct treatment-resistant subtypes of acute leukemia, but not in normal hematopoietic progenitors. Sensitization to asparaginase was mediated by Wnt-dependent stabilization of proteins (Wnt/STOP), which inhibits GSK3-dependent protein ubiquitination and proteasomal degradation, a catabolic source of asparagine. Inhibiting the alpha isoform of GSK3 was sufficient to phenocopy synthetic lethality with asparaginase, and pharmacologic GSK3 alpha inhibition profoundly sensitized drug-resistant leukemias to asparaginase in vitro and in vivo. These results demonstrate that synthetic lethal drug-drug interactions can improve the therapeutic index of cancer therapy.
Abstract Colorectal cancer is driven by mutations that activate canonical WNT/β-catenin signaling, but inhibiting WNT has significant on-target toxicity, and there are no approved therapies targeting dominant oncogenic drivers. We recently found that activating a β-catenin–independent branch of WNT signaling that inhibits GSK3-dependent protein degradation induces asparaginase sensitivity in drug-resistant leukemias. To test predictions from our model, we turned to colorectal cancer because these cancers can have WNT-activating mutations that function either upstream (i.e., R-spondin fusions) or downstream (APC or β-catenin mutations) of GSK3, thus allowing WNT/β-catenin and WNT-induced asparaginase sensitivity to be unlinked genetically. We found that asparaginase had little efficacy in APC or β-catenin–mutant colorectal cancer, but was profoundly toxic in the setting of R-spondin fusions. Pharmacologic GSK3α inhibition was sufficient for asparaginase sensitization in APC or β-catenin–mutant colorectal cancer, but not in normal intestinal progenitors. Our findings demonstrate that WNT-induced therapeutic vulnerabilities can be exploited for colorectal cancer therapy. Significance: Solid tumors are thought to be asparaginase-resistant via de novo asparagine synthesis. In leukemia, GSK3α-dependent protein degradation, a catabolic amino acid source, mediates asparaginase resistance. We found that asparaginase is profoundly toxic to colorectal cancers with WNT-activating mutations that inhibit GSK3. Aberrant WNT activation can provide a therapeutic vulnerability in colorectal cancer. See related commentary by Davidsen and Sullivan, p. 1632. This article is highlighted in the In This Issue feature, p. 1611
<div>Abstract<p>Colorectal cancer is driven by mutations that activate canonical WNT/β-catenin signaling, but inhibiting WNT has significant on-target toxicity, and there are no approved therapies targeting dominant oncogenic drivers. We recently found that activating a β-catenin–independent branch of WNT signaling that inhibits GSK3-dependent protein degradation induces asparaginase sensitivity in drug-resistant leukemias. To test predictions from our model, we turned to colorectal cancer because these cancers can have WNT-activating mutations that function either upstream (i.e., R-spondin fusions) or downstream (<i>APC</i> or β-catenin mutations) of GSK3, thus allowing WNT/β-catenin and WNT-induced asparaginase sensitivity to be unlinked genetically. We found that asparaginase had little efficacy in <i>APC</i> or β-catenin–mutant colorectal cancer, but was profoundly toxic in the setting of R-spondin fusions. Pharmacologic GSK3α inhibition was sufficient for asparaginase sensitization in <i>APC</i> or β-catenin–mutant colorectal cancer, but not in normal intestinal progenitors. Our findings demonstrate that WNT-induced therapeutic vulnerabilities can be exploited for colorectal cancer therapy.</p>Significance:<p>Solid tumors are thought to be asparaginase-resistant via <i>de novo</i> asparagine synthesis. In leukemia, GSK3α-dependent protein degradation, a catabolic amino acid source, mediates asparaginase resistance. We found that asparaginase is profoundly toxic to colorectal cancers with WNT-activating mutations that inhibit GSK3. Aberrant WNT activation can provide a therapeutic vulnerability in colorectal cancer.</p><p><i>See related commentary by Davidsen and Sullivan, p. 1632</i>.</p><p><i>This article is highlighted in the In This Issue feature, p. 1611</i></p></div>
