Sugimoto et al. (J Cell Sci, 2009) showed that non-transformed human cells resist re-replication after overexpression of any single replication licensing factor (Cdt1, ORC1 or CDC6), whereas cancer-derived cells such as HeLa re-replicate in response to Cdt1 alone, and co-expression of Cdt1+ORC1+CDC6 synergistically induces re-replication even in normal fibroblasts. Their discussion enumerated five unresolved issues. Since 2009, three of the five have been substantially resolved at the molecular level: the human ORC1 CDK phosphorylation sites and the SKP2-cyclin A-CDK2-dependent destruction mechanism were identified (Hossain et al., 2021); the mechanism by which Cdt1 overexpression activates DNA damage checkpoints without measurable re-replication was established (excess CDT1 directly inhibits the CMG helicase at replication forks, causing fork stalling and collapse; Ratnayeke et al., 2023; Tsuyama et al., 2026); and the CDC6 restraint mechanism was decomposed into contributing export, proteolysis and protein-interaction controls (Pelizon et al., 2000; Yim et al., 2013; Walter et al., 2016; Zhou et al., 2020). The surviving open core is the context-dependence question: which variables — for example, checkpoint competence and residual ATR/ATM activity, expression vector and expression kinetics, and cell type and oncogenic background (p53 pathway status, CRL4-Cdt2/SCF-Skp2 degradative capacity, geminin abundance) — causally determine whether licensing-factor deregulation produces overt re-replication and whether the S-phase checkpoint restrains it. The problem is to identify these determinants causally in matched human cell backgrounds.

Background

DNA replication in eukaryotic cells is restricted to once per cell cycle by a licensing mechanism. In late mitosis and G1, the origin recognition complex (ORC, a six-subunit DNA-binding complex) together with the AAA+ ATPase CDC6 and the loading factor Cdt1 deposits the MCM2-7 helicase hexamer onto replication origins, forming the pre-replicative complex (pre-RC). Once origins fire in S phase, re-assembly of pre-RCs on already-replicated DNA must be prevented; failure results in re-replication, a form of replication stress that produces DNA damage, copy-number aberrations, aneuploidy and genome instability. Mammalian cells deploy several overlapping inhibitory mechanisms during S and G2: the licensing factor Cdt1 is degraded by two ubiquitin ligases (the CRL4-Cdt2 complex, recruited by the DNA-sliding clamp PCNA at replication forks, and SCF-Skp2, a cyclin A-CDK2-associated ubiquitin ligase complex) and is inhibited by the protein geminin; CDC6 is regulated by CDK-mediated phosphorylation, nuclear export and ubiquitin-dependent proteolysis; and ORC1, the largest ORC subunit, is destroyed as cells enter S phase through a mechanism involving cyclin A-CDK2 and SCF-Skp2 (the human ORC1 CDK phosphorylation sites and the destruction mechanism were only molecularly defined in 2021). CDKs (cyclin-dependent kinases, the master S-phase kinases) therefore both drive replication and repress re-licensing. Surveillance of these processes involves the checkpoint kinases ATR (ataxia-telangiectasia and Rad3-related, activated by replication stress and single-stranded DNA) and ATM (ataxia-telangiectasia mutated, activated by DNA double-strand breaks), plus the p53 pathway. In 2009, Sugimoto et al. asked how these redundant barriers operate in non-cancerous human cells, using telomerase-immortalized human foreskin fibroblasts (HFF2/T) and another non-transformed line: overexpressing Cdt1, ORC1 or CDC6 individually induced no detectable re-replication, whereas Cdt1+ORC1+CDC6 together caused strong re-replication with excess MCM loading — showing that normal cells are protected by redundancy, and revealing for the first time a role for ORC1 regulation in re-replication prevention. Their study also exposed unexplained context dependence. First, cancer-derived HeLa cells re-replicate after Cdt1 overexpression alone, while non-transformed fibroblasts do not, and the basis of this differential sensitivity (oncogenic lesions, p53 status, degradative capacity for licensing factors, checkpoint competence) was unknown. Second, the contribution of the ATR-mediated S-phase checkpoint remained without a causal account: with adenovirus-driven Cdt1/CDC6 overexpression, shRNA-mediated inhibition of ATR — but not of ATM — induced robust re-replication in transformed cell lines (A549, T98G) as well as in non-transformed primary fibroblasts (IMR90, WI38), and Liu et al. (2007) proposed that specific defects in the ATR-mediated S-phase checkpoint pathway present in certain tumor cell lines contribute to their susceptibility, but which variables — vector type and expression kinetics, residual checkpoint protein levels, cell type — govern when the ATR-mediated checkpoint is the load-bearing restraint on re-replication was left open. Later work has clarified the intrinsic anti-re-replication biochemistry of the licensing factors themselves (ORC1 destruction sites and mechanism; direct inhibition of the active CMG helicase (the Cdc45-MCM-GINS helicase that drives fork elongation) by CDT1 at forks; CDC6 export, degradation and interaction controls), but the determinants of when and in which cells licensing deregulation is converted into overt re-replication remain without a causal account.

Problem Statement

Identify, causally, the variables that determine whether deregulation of replication licensing factors produces overt DNA re-replication in human cells, and whether the ATR/ATM-mediated S-phase checkpoint restrains it — the context-dependence left unexplained by Sugimoto et al. (2009). Concretely: (1) given that shRNA-mediated inhibition of ATR — but not of ATM — induces robust re-replication when Cdt1 or Cdt1/Cdc6 is overexpressed from adenoviral vectors, in transformed cell lines (A549, T98G) as well as in non-transformed primary fibroblasts (IMR90, WI38) (Liu et al., 2007), and that the proposal that specific defects in the ATR-mediated checkpoint pathway contribute to the susceptibility of certain tumor cell lines remains untested — which factors (for example, vector type and expression kinetics, residual checkpoint-protein levels, and cell type) govern whether the ATR-mediated S-phase checkpoint is the load-bearing restraint of re-replication; and (2) why do cancer-derived cells such as HeLa undergo re-replication after overexpression of Cdt1 alone while telomerase-immortalized human fibroblasts (HFF2/T) do not — i.e., which oncogenic lesions or cellular capacities (p53-pathway status, CRL4-Cdt2 and SCF-Skp2 degradative capacity, geminin abundance, checkpoint competence) are the decisive sensitizers to single-factor deregulation, and how do they combine to explain the synergy of multi-factor (Cdt1+ORC1+CDC6) deregulation. Both parts (1) and (2) must be answered; a response that establishes determinants for only one of them does not resolve the task. For each determinant it invokes, the answer must state the causal evidence, whether that evidence establishes the determinant as necessary, as sufficient, or as contributory, and which determinant(s) it judges decisive in the relevant cell-background comparison.

References

  1. Sugimoto N, Yoshida K, Tatsumi Y, Yugawa T, Narisawa-Saito M, Waga S, Kiyono T, Fujita M (2009). Redundant and differential regulation of multiple licensing factors ensures prevention of re-replication in normal human cells. Journal of Cell Science 122(8):1184-1191. DOI: 10.1242/jcs.041889. URL: https://doi.org/10.1242/jcs.041889
  2. Tatsumi Y, Sugimoto N, Yugawa T, Narisawa-Saito M, Kiyono T, Fujita M (2006). Deregulation of Cdt1 induces chromosomal damage without rereplication and leads to chromosomal instability. Journal of Cell Science 119(15):3128-3140. DOI: 10.1242/jcs.03031. URL: https://doi.org/10.1242/jcs.03031
  3. Liu E, Lee AY, Chiba T, Olson E, Sun P, Wu X (2007). The ATR-mediated S phase checkpoint prevents rereplication in mammalian cells when licensing control is disrupted. Journal of Cell Biology 179(4):643-657. DOI: 10.1083/jcb.200704138. URL: https://doi.org/10.1083/jcb.200704138
  4. Vaziri C, Saxena S, Jeon Y, Lee C, Murata K, Machida Y, Wagle N, Hwang DS, Dutta A (2003). A p53-dependent checkpoint pathway prevents rereplication. Molecular Cell 11:997-1008. DOI: 10.1016/s1097-2765(03)00099-6. URL: https://doi.org/10.1016/s1097-2765(03)00099-6
  5. Méndez J, Zou-Yang XH, Kim SY, Hidaka M, Tansey WP, Stillman B (2002). Human origin recognition complex large subunit is degraded by ubiquitin-mediated proteolysis after initiation of DNA replication. Molecular Cell 9:481-491. DOI: 10.1016/s1097-2765(02)00467-7. URL: https://doi.org/10.1016/s1097-2765(02)00467-7
  6. Wang B, Song J (2019). Structural basis for the ORC1-Cyclin A association. Protein Science 28(9):1727-1733. DOI: 10.1002/pro.3689. URL: https://doi.org/10.1002/pro.3689
  7. Hossain M, Bhalla K, Stillman B (2021). Multiple, short protein binding motifs in ORC1 and CDC6 control the initiation of DNA replication. Molecular Cell 81(9):1951-1969.e6. DOI: 10.1016/j.molcel.2021.03.003. URL: https://doi.org/10.1016/j.molcel.2021.03.003
  8. Iwahori S, Kohmon D, Kobayashi J, Tani Y, Yugawa T, Komatsu K, Kiyono T, Sugimoto N, Fujita M (2014). ATM regulates Cdt1 stability during the unperturbed S phase to prevent re-replication. Cell Cycle 13(3):471-481. DOI: 10.4161/cc.27274. URL: https://doi.org/10.4161/cc.27274
  9. Ratnayeke N, Baris Y, Chung M, Yeeles JTP, Meyer M (2023). CDT1 inhibits CMG helicase in early S phase to separate origin licensing from DNA synthesis. Molecular Cell 83(1):26-42.e13. DOI: 10.1016/j.molcel.2022.12.004. URL: https://doi.org/10.1016/j.molcel.2022.12.004
  10. Tsuyama T, Takayama N, Tanaka R, Arai Y, Yamaguchi Y, Nawata Y, Azuma Y, Tada S (2026). Overexpression of CDT1 inhibits cell cycle progression at S phase by interacting with the mini-chromosome maintenance complex and causes DNA damage. FEBS Open Bio 16(2):352-364. DOI: 10.1002/2211-5463.70127. URL: https://doi.org/10.1002/2211-5463.70127
  11. Hathaway C, Le T, Hatoyama Y, Kanemaki M, Dungrawala H (2026). CDT1 acts with the replisome to remodel replication forks. Science Advances 12(33):eaee6914. DOI: 10.1126/sciadv.aee6914. URL: https://doi.org/10.1126/sciadv.aee6914
  12. Rona G, Pagano M (2023). CDT1, a licensing factor that limits rereplication. Molecular Cell 83(1):1-3. DOI: 10.1016/j.molcel.2022.11.019. URL: https://doi.org/10.1016/j.molcel.2022.11.019
  13. Jiang W, Wells NJ, Hunter T (1999). Multistep regulation of DNA replication by Cdk phosphorylation of HsCdc6. Proceedings of the National Academy of Sciences USA 96(11):6193-6198. DOI: 10.1073/pnas.96.11.6193. URL: https://doi.org/10.1073/pnas.96.11.6193
  14. Pelizon C, Madine MA, Romanowski P, Laskey RA (2000). Unphosphorylatable mutants of Cdc6 disrupt its nuclear export but still support DNA replication once per cell cycle. Genes & Development 14:2526-2533. DOI: 10.1101/gad.176300. URL: https://doi.org/10.1101/gad.176300
  15. Yim H, Park JW, Woo SU, Kim ST, Liu L, Lee CH, Lee SK (2013). Phosphorylation of Cdc6 at serine 74, but not at serine 106, drives translocation of Cdc6 to the cytoplasm. Journal of Cellular Physiology 228(6):1221-1228. DOI: 10.1002/jcp.24275. URL: https://doi.org/10.1002/jcp.24275
  16. Walter D, Hoffmann S, Komseli ES, Rappsilber J, Gorgoulis V, Sorensen CS (2016). SCF(Cyclin F)-dependent degradation of CDC6 suppresses DNA re-replication. Nature Communications 7:10530. DOI: 10.1038/ncomms10530. URL: https://doi.org/10.1038/ncomms10530
  17. Zhou Y, Pozo PN, Oh S, Stone HM, Cook JG (2020). Distinct and sequential re-replication barriers ensure precise genome duplication. PLoS Genetics 16:e1008988. DOI: 10.1371/journal.pgen.1008988. URL: https://doi.org/10.1371/journal.pgen.1008988
  18. Klotz-Noack K, McIntosh D, Schurch N, Pratt N, Blow JJ (2012). Re-replication induced by geminin depletion occurs from G2 and is enhanced by checkpoint activation. Journal of Cell Science 125(Pt 10):2436-2445. DOI: 10.1242/jcs.100883. URL: https://doi.org/10.1242/jcs.100883
  19. Yoshida K, Sugimoto N, Iwahori S, Yugawa T, Narisawa-Saito M, Kiyono T, Fujita M (2010). CDC6 interaction with ATR regulates activation of a replication checkpoint in higher eukaryotic cells. Journal of Cell Science 123(Pt 2):225-235. DOI: 10.1242/jcs.058693. URL: https://doi.org/10.1242/jcs.058693
  20. Lee HO, Zacharek SJ, Xiong Y, Duronio RJ (2010). Cell type-dependent requirement for PIP box-regulated Cdt1 destruction during S phase. Molecular Biology of the Cell 21:3639-3653. DOI: 10.1091/mbc.E10-02-0130. URL: https://doi.org/10.1091/mbc.E10-02-0130
  21. Petropoulos M, Champeris Tsaniras S, Nikou S, Maxouri S, Dionellis VS, Kalogeropoulou A, Karamichali A, Ioannidis K, Danalatos IR, Obst M, Naumann R, Delinasios GJ, Gorgoulis VG, Roukos V, Anastassiadis K, Halazonetis TD, Bravou V, Lygerou Z, Taraviras S (2023). Cdt1 overexpression drives colorectal carcinogenesis through origin overlicensing and DNA damage. Journal of Pathology 259(1):10-20. DOI: 10.1002/path.6017. URL: https://doi.org/10.1002/path.6017
  22. Petrakis TG, Komseli ES, Papaioannou M, Vougas K, Polyzos A, Myrianthopoulos V, Mikros E, Trougakos IP, Thanos D, Branzei D, Townsend P, Gorgoulis VG (2016). Exploring and exploiting the systemic effects of deregulated replication licensing. Seminars in Cancer Biology 37-38:3-15. DOI: 10.1016/j.semcancer.2015.12.002. URL: https://doi.org/10.1016/j.semcancer.2015.12.002