The Trinity of SPRTN Protease Regulation.

The protease SPRTN emerged as the essential enzyme for DNA-protein crosslink proteolysis repair. Biochemical and cell biological work indicated that SPRTN is a nonspecific protease. Recent and independent studies from Lou, Stingele, and Ramadan reveal that SPRTN activity is modulated via three layers of regulation that make it selective for DNA-protein crosslinks.

Mechanisms and Regulation of DNA-Protein Crosslink Repair During DNA Replication by SPRTN Protease.

DNA-protein crosslinks (DPCs) are deleterious DNA lesions that occur when proteins are covalently crosslinked to the DNA by the action of variety of agents like reactive oxygen species, aldehydes and metabolites, radiation, and chemotherapeutic drugs. Unrepaired DPCs are blockades to all DNA metabolic processes. Specifically, during DNA replication, replication forks stall at DPCs and are vulnerable to fork collapse, causing DNA breakage leading to genome instability and cancer. Replication-coupled DPC repair involves DPC degradation by proteases such as SPRTN or the proteasome and the subsequent removal of DNA-peptide adducts by nucleases and canonical DNA repair pathways. SPRTN is a DNA-dependent metalloprotease that cleaves DPC substrates in a sequence-independent manner and is also required for translesion DNA synthesis following DPC degradation. Biallelic mutations in SPRTN cause Ruijs-Aalfs (RJALS) syndrome, characterized by hepatocellular carcinoma and segmental progeria, indicating the critical role for SPRTN and DPC repair pathway in genome maintenance. In this review, we will discuss the mechanism of replication-coupled DPC repair, regulation of SPRTN function and its implications in human disease and cancer.

The protease SPRTN and SUMOylation coordinate DNA-protein crosslink repair to prevent genome instability.

DNA-protein crosslinks (DPCs) are a specific type of DNA lesion in which proteins are covalently attached to DNA. Unrepaired DPCs lead to genomic instability, cancer, neurodegeneration, and accelerated aging. DPC proteolysis was recently identified as a specialized pathway for DPC repair. The DNA-dependent protease SPRTN and the 26S proteasome emerged as two independent proteolytic systems. DPCs are also repaired by homologous recombination (HR), a canonical DNA repair pathway. While studying the cellular response to DPC formation, we identify ubiquitylation and SUMOylation as two major signaling events in DNA replication-coupled DPC repair. DPC ubiquitylation recruits SPRTN to repair sites, promoting DPC removal. DPC SUMOylation prevents DNA double-strand break formation, HR activation, and potentially deleterious genomic rearrangements. In this way, SUMOylation channels DPC repair toward SPRTN proteolysis, which is a safer pathway choice for DPC repair and prevention of genomic instability.

DNA protein crosslink proteolysis repair: From yeast to premature ageing and cancer in humans.

DNA-protein crosslinks (DPCs) are a specific type of DNA lesion consisting of a protein covalently and irreversibly bound to DNA, which arise after exposure to physical and chemical crosslinking agents. DPCs can be bulky and thereby pose a barrier to DNA replication and transcription. The persistence of DPCs during S phase causes DNA replication stress and genome instability. The toxicity of DPCs is exploited in cancer therapy: many common chemotherapeutics kill cancer cells by inducing DPC formation. Recent work from several laboratories discovered a specialized repair pathway for DPCs, namely DPC proteolysis (DPCP) repair. DPCP repair is carried out by replication-coupled DNA-dependent metalloproteases: Wss1 in yeast and SPRTN in metazoans. Mutations in SPRTN cause premature ageing and liver cancer in humans and mice; thus, defective DPC repair has great clinical ramifications. In the present review, we will revise the current knowledge on the mechanisms of DPCP repair and on the regulation of DPC protease activity, while highlighting the most significant unresolved questions in the field. Finally, we will discuss the impact of faulty DPC repair on disease and cancer therapy.