Copper is an essential cofactor for all organisms, and yet it becomes toxic if concentrations exceed a threshold maintained by evolutionarily conserved homeostatic mechanisms. How excess copper induces cell death, however, is unknown. Here, we show in human cells that copper-dependent, regulated cell death is distinct from known death mechanisms and is dependent on mitochondrial respiration. We show that copper-dependent death occurs by means of direct binding of copper to lipoylated components of the tricarboxylic acid (TCA) cycle. This results in lipoylated protein aggregation and subsequent iron-sulfur cluster protein loss, which leads to proteotoxic stress and ultimately cell death. These findings may explain the need for ancient copper homeostatic mechanisms.

The chemotherapeutic drug methotrexate inhibits the enzyme dihydrofolate reductase¹, which generates tetrahydrofolate, an essential cofactor in nucleotide synthesis². Depletion of tetrahydrofolate causes cell death by suppressing DNA and RNA production³. Although methotrexate is widely used as an anticancer agent and is the subject of over a thousand ongoing clinical trials⁴, its high toxicity often leads to the premature termination of its use, which reduces its potential efficacy⁵. 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—that is required for the catabolism of the amino acid histidine⁸, a process that has not previously been linked to methotrexate sensitivity. In cultured cancer cells, depletion of several genes in the histidine degradation pathway markedly decreased sensitivity to methotrexate. Mechanistically, histidine catabolism drains the cellular pool of tetrahydrofolate, 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 leukaemia xenografts to methotrexate. The histidine degradation pathway markedly influences the sensitivity of cancer cells to methotrexate and may be exploited to improve methotrexate efficacy through a simple dietary intervention.