SLFN12

SLFN12 is a ribonuclease whose cytotoxic activity is conditionally switched on by molecular glue compounds (velcrins, including DNMDP, anagrelide, nauclefine, OPB-171775 and BAY 2666605) that drive its assembly with PDE3A into a butterfly-shaped heterotetramer, defining a drug-inducible mechanism of cancer cell death (PMID:34272366, PMID:34707099). The glue binds the catalytic-domain pocket of PDE3A and remodels a surface that recruits the short C-terminal helix (E552-I558) of SLFN12 through hydrophobic contacts; this interface is required for complex formation and for compound-induced killing (PMID:34272366, PMID:34707099). Complex formation activates SLFN12 by two coupled effects: it stabilizes cytoplasmic SLFN12 and promotes its dephosphorylation at Ser368 and Ser573, both of which are required to unleash full nucleolytic activity and cell death (PMID:35104454). The activated RNase selectively cleaves tRNALeu(TAA) — recognition depending on sequences in the variable loop and acceptor stem — which downregulates this tRNA, stalls ribosomes at Leu-TTA codons, and globally inhibits protein synthesis (PMID:36302897). The ensuing translational collapse triggers GCN2 kinase and its downstream integrated stress response as an effector arm of the apoptotic outcome (PMID:38864850). Independently of the velcrin axis, SLFN12 overexpression dampens CHK1/CHK2 phosphorylation and sensitizes triple-negative breast cancer cells to DNA-damaging agents (PMID:35430566).

1. 2021 High

   Established that SLFN12 is itself an RNase and that its activity is gated by PDE3A binding within a molecular-glue-stabilized heterotetramer, identifying the structural basis of the drug-inducible complex.

   Evidence Cryo-EM/X-ray structure, in vitro RNase assays, interface mutagenesis and viability assays of the PDE3A-SLFN12-DNMDP complex

   PMID:34272366 PMID:34707099

   Open questions at the time

   • The endogenous physiological trigger for complex formation in the absence of synthetic glues was not defined
   • The catalytic residues of the SLFN12 RNase active site were not pinpointed
2. 2022 High

   Identified the physiological substrate as tRNALeu(TAA) and connected its cleavage to ribosome pausing and global translation arrest, providing the molecular basis of apoptosis initiation.

   Evidence In vitro substrate cleavage with tRNA mutagenesis, ribosome profiling, and global translation assays in velcrin-treated cancer cells

   PMID:36302897

   Open questions at the time

   • Whether additional RNA species are cleaved in vivo was not resolved
   • The structural basis of tRNALeu(TAA) selectivity at the active site was not determined
3. 2022 High

   Defined the post-translational switch controlling SLFN12 activation, showing that PDE3A binding triggers dephosphorylation at Ser368/Ser573 and stabilization that are jointly required for nuclease-dependent killing.

   Evidence Phosphosite mapping, Ser368/Ser573 mutagenesis, in vitro RNase assays and cell viability readouts

   PMID:35104454

   Open questions at the time

   • The phosphatase responsible for SLFN12 dephosphorylation was not identified
   • How dephosphorylation mechanistically relieves the RNase was not structurally resolved
4. 2022 Medium

   Placed SLFN12 upstream of the CHK1/CHK2 checkpoint as a sensitizer to DNA-damaging chemotherapy, suggesting a velcrin-independent role.

   Evidence Lentiviral overexpression, pCHK1/pCHK2 western blots, pharmacological CHK1/2 inhibition and viability assays in TNBC lines

   PMID:35430566

   Open questions at the time

   • Whether RNase activity underlies the checkpoint effect was not tested
   • The direct molecular link between SLFN12 and CHK1/2 regulation was not established
5. 2024 Medium

   Mapped GCN2/integrated stress response as the effector pathway downstream of SLFN12-driven translational arrest mediating anticancer activity in vivo.

   Evidence Pathway and pharmacodynamic analysis in PDX GIST models treated with the complex inducer OPB-171775

   PMID:38864850

   Open questions at the time

   • The precise sensing step linking tRNA loss to GCN2 activation was not detailed
   • Contribution of GCN2 relative to other apoptotic routes was not quantified
6. 2024 Medium

   Demonstrated translational applicability by showing a brain-penetrant velcrin engages the same tRNALeu cleavage mechanism to drive glioblastoma regression.

   Evidence Viability, apoptosis, translation assays and orthotopic xenograft regression with BAY 2666605 in glioblastoma models

   PMID:39166256

   Open questions at the time

   • Tumor-type determinants of SLFN12/PDE3A dependence were not delineated
   • Resistance mechanisms to velcrin treatment were not addressed

• The endogenous, drug-independent physiological function and regulation of SLFN12 remains unresolved.
• No native signal that triggers PDE3A-SLFN12 complex formation has been identified
• The phosphatase and kinase regulating Ser368/Ser573 are unknown
• Whether the CHK1/2 sensitization role depends on RNase activity is untested