In the blog last week, I introduced synthetic lethality as a potential strategy for cancer therapy which targets tumour cells whilst having no effect on healthy tissue. Another synthetic lethality paper has recently been published in Nature, this time targeting fumarate hydratase (FH)-deficient cells (Frezza et al., 2011). Mutations in FH cause Hereditary Leiomyomatosis Renal Cell Carcinoma (HLRCC), a syndrome related to BHD (see this previous blog post).
FH is a component of the TCA-cycle that catalyses the hydration of fumarate into malate. The TCA-cycle occurs in the mitochondria and is a major source of NADH. Until now, it has been unclear how FH-deficient cells, which lack a functional TCA-cycle, are able to survive. This new study by Frezza et al. identifies an alternative pathway by which FH-deficient cells can generate NADH. Furthermore, this pathway suggests a novel potential therapy for HLRCC.
The authors began by analysing TCA-cycle metabolites in Fh-/- cells. These cells were found to have an accumulation of fumarate, an increase in succinate and a decrease in malate and citrate. As expected, the steady-state levels of mitochondrial NADH were also lower. However, experiments showed that Fh-/- cells were still capable of generating and oxidising NADH.
In silico modelling was then used to reconstruct metabolic networks and to predict genes that are synthetically lethal with Fh. 24 genes were predicted to be synthetic lethal and 18 of these belong to the haem metabolism pathway, suggesting this pathway is crucial to the survival of FH-deficient cells. The haem pathway uses TCA-derived carbon to generate haem. This use of a TCA-cycle component allows continued flux through the cycle and therefore some generation of NADH.
Three of the predicted synthetic lethal genes were showed to be upregulated in Fh-/- cells. These were haem oxygenase 1 (Hmox1), biliverdin reductase B (Blvrb) and UDP-glucoronyl transferase 2b34 (Ugt2b34). Silencing of Hmox1 by short hairpin RNAs caused a reduction in the growth of Fh-/- cells, with little effect on control cells. To confirm that Hmox1 is synthetic lethal for Fh-/- cells, the Hmox1 inhibitor zinc protoporphyrin (ZnPP) was tested. As with Hmox1 silencing, ZnPP reduced the growth of Fh-/- cells and it also caused a decrease in mitochondrial NADH, whereas only a small change was observed in control cells.
Not only does this study identify a metabolic pathway that allows the survival of FH-deficient cells, it also indicates that inhibition of haem metabolism might be an effective therapy for HLRCC. It would be interesting to identify synthetic lethal genes in FLCN-deficient cells as a starting point for developing therapies for BHD syndrome.
- Frezza C, Zheng L, Folger O, Rajagopalan KN, Mackenzie ED, Jerby L, Micaroni M, Chaneton B, Adam J, Hedley A, Kalna G, Tomlinson IP, Pollard PJ, Watson DG, Deberardinis RJ, Shlomi T, Ruppin E, & Gottlieb E (2011). Haem oxygenase is synthetically lethal with the tumour suppressor fumarate hydratase. Nature PMID: 21849978