Inhibition of SSH2 causes synthetic lethality in FLCN-null tumour cells

BHD syndrome is an autosomal dominantly inherited disease, caused by mutations in the FLCN gene and characterised by skin lesions; lung cysts and predisposition to pneumothoraces; and predisposition to kidney cancer. While loss of just one FLCN allele is believed to be sufficient for the skin and lung symptoms to develop, it seems that somatic loss of the wild type FLCN allele is required to develop kidney cancer (Vocke et al., 2005), making these kidney cancer cells genetically distinct from surrounding healthy tissue. The phenomenon of synthetic lethality describes the situation where the inhibition of a gene or protein, using a drug or other intervention, is lethal to cancer cells with a known genetic defect, but not in healthy tissue lacking this defect (Reinhardt et al., 2009). Given that the kidney tumours found in BHD patients are genetically distinct from surrounding healthy tissue, synthetic lethality may prove a tractable method of treating these tumours. Indeed, a recent study by Lu et al. aimed to find genes that were synthetically lethal with respect to FLCN.

Using an siRNA screening library, Lu et al. found that inhibition of Slingshot 2 (SSH2) lead to accumulation of cells in G1 phase; induction of the apoptosis genes Caspase3 and Caspase7 expression; and cell death specifically in the FLCN-null UOK-257 and FTC-133 cell lines, but not in isogenic cell lines heterozygous for FLCN. Thus, SSH2 is a synthetic lethal target of FLCN. SSH2 family members SSH1 and SSH3 were also tested and although individually they did not cause synthetic lethality, pairwise inhibition with SSH2 or inhibition of all three potentiated the effects of SSH2 inhibition, indicating that there is some redundancy between the three genes.

The Slingshot genes are phosphatases, known to work antagonistically with the LIM-kinases to regulate the phosphorylation status of Cofilin (Mizuno, 2013). Therefore, knockdown of the Slingshot genes with siRNAs should lead to the accumulation of phosphorylated Cofilin. While this was observed in FLCN-expressing cells, it was not observed in FLCN-null cells, suggesting that FLCN is required for normal Slingshot phosphatase activity (Lu et al., 2013). Indeed, there seemed to be interdependence in expression levels between FLCN and all three Slingshot genes, suggesting that these genes can modulate one another’s expression and the authors suggest that it is the secondary disruption of this pathway upon SSH2 inhibition that is responsible for the synthetic lethal phenotype observed in FLCN-null cells.

Traditional cancer treatments such as chemotherapy often cause unpleasant side effects as, although they predominantly target cancerous cells, they can also target healthy tissue. As synthetic lethality specifically targets genetic deficiencies present in tumours, this approach has the potential to provide targeted tumour treatment whilst reducing side-effects. Synthetic lethal genes have been found in both VHL- and HLRCC-associated kidney cancers (as discussed in blog posts here and here) as well as in more common cancers caused by mutations in genes such as p53 and MYCN (Molenaar et al., 2009; Reinhardt et al., 2007), showing that syntheticlethality is a popular field of research. Indeed, PARP inhibitors, which specifically target tumours caused by loss of the BRCA 1 or 2 genes (Ashworth, 2008) are showing great promise in Phase II clinical trials for breast and ovarian cancers (Audeh et al., 2010; Tutt et al., 2010), demonstrating that this may be an effective strategy of developing cancer therapies. Thus, if a drug can be developed that inhibits SSH2, this may prove an effective treatment for BHD-associated kidney tumours.


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