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 B₁₂-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 B₁₂ 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 B₁₂-independent methionine synthase enzyme.

Results: We report evidence that methionine dependence is caused by low MTR activity secondary to a B₁₂ deficiency. High levels of the B₁₂ 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 B₁₂-independent methionine synthase enzyme in cancer cells restored growth on homocysteine and normalized the SAM:SAH ratio, while overexpression of the B₁₂-dependent human enzyme had no effect.

Conclusion: Methionine dependence in cancer can be driven by low MTR activity secondary to B₁₂ 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.