biostudies-literatureHusseiny MMAEVetenskapsrådetCancerfonden34https://www.ebi.ac.uk/biostudies/studies/S-EPMC12220533biostudies-literatureUnknown13(1)<h4>Background</h4>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.<h4>Methods</h4>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 rescuCancer & metabolismThe role of B&lt;sub&gt;12&lt;/sub&gt; deficiency and methionine synthase in methionine-dependent cancer cells.PMC1222053320 0974 PjF2020-01631Husseiny MMAENilsson RfalseThe role of B&lt;sub&gt;12&lt;/sub&gt; deficiency and methionine synthase in methionine-dependent cancer cells.<h4>Background</h4>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.<h4>Methods</h4>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 rescu2025-01-01T00:00:00Z2025 Jul2026-07-15T08:59:39.948Z2026-07-03T03:11:10.335ZS-EPMC122205334060501610.1186/s40170-025-00405-2