The role of B12 deficiency and methionine synthase in methionine-dependent cancer cells

Background Human cells can synthesize methionine from homocysteine and folate-coupled methyl groups via the B 12 -dependent enzyme methionine synthase (MTR). Yet, it has been known for decades that cancer cells fail to grow when methionine is replaced by homocysteine, a phenomenon known as methionine dependence. The underlying mechanism remains unknown. Methods Cancer cell lines were cultured with homocysteine in place of methionine, and growth responses were measured. Revertant cells capable of growing in homocysteine were generated through long-term culture with high B 12 and analyzed using single-cell RNA-seq. Metabolite uptake/release was measured using isotope dilution and MTR activity was assessed using metabolic flux analysis (MFA). Functional rescue experiments were performed by overexpressing the B 12 -independent methionine synthase enzyme. Results We report evidence that methionine dependence is caused by low MTR activity secondary to a B 12 deficiency. High levels of the B 12 cofactor were required to revert methionine-dependent cancer cells to grow on homocysteine. The adapted “revertant” cells display gene expression signatures consistent with reduced invasion and metastasis. Metabolic flux analysis indicated that methionine-dependent cells do not fully activate MTR when cultured in homocysteine. High concentrations of homocysteine partially rescued growth of methionine-dependent cells. Expression of a B 12 -independent methionine synthase enzyme in cancer cells restored growth on homocysteine and normalized the SAM:SAH ratio, while overexpression of the B 12 -dependent human enzyme had no effect. Conclusion Methionine dependence in cancer can be driven by low MTR activity secondary to B 12 deficiency, at least in the cell lines studied. This mechanistic insight resolves a long-standing question in cancer metabolism and may open new avenues for exploiting the phenomenon for cancer therapy.