Histidine catabolism is a major determinant of methotrexate sensitivity

The chemotherapeutic drug methotrexate inhibits the enzyme DHFR (dihydrofolate reductase)(1), which generates tetrahydrofolate (THF), an essential cofactor in nucleotide synthesis(2). Depletion of THF causes cell death by suppressing DNA and RNA production(3). While methotrexate is widely used as an anti-cancer agent and the subject of over a thousand ongoing clinical trials(4), its high toxicity often leads to the premature termination of its use, diminishing its potential efficacy(5). To identify genes that modulate the response of cancer cells to methotrexate, we performed a CRISPR/Cas9-based screen(6,7). This screen yielded FTCD, which encodes an enzyme (formimidoyltransferase cyclodeaminase) needed for the catabolism of the amino acid histidine(8), a process not previously linked to methotrexate sensitivity. In cultured cancer cells, depletion of multiple genes in the histidine catabolism pathway dramatically decreased sensitivity to methotrexate. Mechanistically, histidine catabolism drains the cellular pool of THF, which is particularly detrimental to methotrexate-treated cells. Moreover, expression of the rate-limiting enzyme in histidine catabolism is associated with methotrexate sensitivity in cancer cell lines and with survival rate in patients. In vivo dietary supplementation of histidine increased flux through the histidine degradation pathway and enhanced the sensitivity of leukemia xenografts to methotrexate. Thus, the histidine degradation pathway significantly influences the sensitivity of cancer cells to methotrexate and may be exploited to improve methotrexate efficacy through a simple dietary intervention.