Fumarate hydratase (fumarase) is a TCA cycle enzyme that also functions as a tumor suppressor. Germline mutations in the fumarate hydratase (FH) gene predispose to hereditary leiomyomatosis and renal cell cancer (HLRCC). The oncogenic mechanism involves accumulation of fumarate, which competitively inhibits:

• A Succinate dehydrogenase, causing further TCA blockade
• B Prolyl hydroxylase (PHD), preventing HIF-1alpha hydroxylation and degradation ✓
• C Isocitrate dehydrogenase (IDH), reducing alpha-ketoglutarate production
• D ATP-citrate lyase (ACLY), reducing acetyl-CoA for lipid synthesis

Explanation

Fumarate (and succinate) are alpha-KG analogues. When they accumulate due to FH (or SDH) mutations, they competitively inhibit prolyl hydroxylase domain (PHD) enzymes, which normally hydroxylate HIF-1alpha proline residues for pVHL-mediated ubiquitin-proteasomal degradation. With PHD inhibited, HIF-1alpha accumulates, translocates to the nucleus, and drives transcription of hypoxia-response genes (VEGF, GLUT1, etc.) even in normoxia — a 'pseudohypoxic' state that promotes angiogenesis and tumorigenesis. This connects the TCA cycle to epigenetic and oncogenic regulation.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.