Spartan deficiency causes genomic instability and progeroid phenotypes

Abstract

Spartan (also known as DVC1 and C1orf124) is a PCNA-interacting protein implicated in translesion synthesis, a DNA damage tolerance process that allows the DNA replication machinery to replicate past nucleotide lesions. However, the physiological relevance of Spartan has not been established. Here we report that Spartan insufficiency in mice causes chromosomal instability, cellular senescence and early onset of age-related phenotypes. Whereas complete loss of Spartan causes early embryonic lethality, hypomorphic mice with low amounts of Spartan are viable. These mice are growth retarded and develop cataracts, lordokyphosis and cachexia at a young age. Cre-mediated depletion of Spartan from conditional knockout mouse embryonic fibroblasts results in impaired lesion bypass, incomplete DNA replication, formation of micronuclei and chromatin bridges and eventually cell death. These data demonstrate that Spartan plays a key role in maintaining structural and numerical chromosome integrity and suggest a link between Spartan insufficiency and progeria.

The DNA-binding box of human SPARTAN contributes to the targeting of Polη to DNA damage sites

“…Previously, we and others have shown that SPARTAN plays a critical role in the regulation of targeting Polη, in which the binding of Ub-PCNA via its PIP and UBZ domains is very important [26][27][28]. In parallel, it has been proposed that -besides Ub-PCNA binding -another targeting mechanism could be present by which SPARTAN regulates damage bypass [25,26,32]. Therefore, we tested how the DNA binding of SPARTAN affects Polη foci formation.…”

“…Both wild-type and SPARTAN A -containing cells exhibited the same cell cycle progression (Supplementary Figure 2), proving the correct localization of the SPARTAN A mutant. Additionally, since it has been described that [32] PIP and UBZ mutations do not affect cell cycle progression either, it is possible that SPARTAN's in vivo DNA-binding function is cooperative with these domains or the phenotype is not affected because this DNA-binding-site mutation does not cause a complete loss of function.…”

“…SPARTAN A localization was the same as in the wildtype cell line, therefore, we suggest that SPARTAN A localizes correctly if its DNA binding activity is suppressed. It has been shown previously that the loss of SPARTAN results in a late S or G2/M arrest off the cells [32]. Therefore, we checked the cell cycle profile of the SPARTAN A mutant to exclude the possibility that the similar localization pattern compared to the wild-type SPARTAN is due to a cell cycle abnormality.…”

“…Although the importance of the UBZ and PIP domains in the targeting of Polη is clear, defects in these domains do not completely inactivate SPARTAN function. Additionally, SPARTAN has a putative protease domain (SprT), whose mutation results in serious deficiency in SPARTAN's functions in vivo [31,32]. Recently, the DNA binding and DNA dependent protease activities of WSS1 has been described, and it was suggested to be the yeast functional homologue of human Spartan, based on the domain organization they contain [33].…”

DNA–protein crosslink repair

“…In these patients, SPRTN deficiency is causative for a progeroid (premature ageing) syndrome accompanied by early-onset hepato cellular carcinomas. Similarly, mice with reduced SPRTN levels (knocking out Sprtn is lethal) display progeroid phenotypes and genome instability 56 .…”