Cancer cells subvert ZNF93 to escape surveillance by L1 epigenetic sentinels and control APOBEC3B [RNA-seq_2]

The primate-restricted KRAB zinc finger protein (KZFP) ZNF93 represses a subset of L1 transposable elements (TEs) that propagated in the human ancestral genome some 20 to 12 million years ago and have all become transposition-incompetent due to inactivating mutations. Here, we demonstrate that these L1 integrants encode a highly genotoxic endonuclease, and that ZNF93 also represses other full-length L1 integrants via indirect effects as well as APOBEC3B. Consistent with the exquisite sensitivity of single-stranded DNA to attack by both the L1 endonuclease and the cytidine deaminase, ZNF93 expression peaks in S phase and correlates with cell proliferation. ZNF93 is induced in most cancers, and ZNF93-depleted cells exhibit defects in DNA synthesis and activation of the replication and DNA damage checkpoints. These results suggest a model whereby ZNF93 is a major guardian of genome integrity subverted by cancer cells to escape L1-mediated epigenetic surveillance and achieve a proper balance of APOBEC3B-induced tumor heterogeneity and overall genome stability.