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Colleen Caldwell

Mentor: Maria Spies, PhD

Year Entered Into Program: 2015

Terminal Degree(s) Received: PhD 2021

Research Description

DNA repair: from single-molecule biochemistry to mechanism-based drug discovery

Telomeres are repetitive TTAGGG DNA sequences at the end of linear chromosomes that serve the vital function of preventing information loss during replication of linear chromosomes. The protection that the telomere provides is critical to maintaining the integrity of the genome. The G-rich sequence telomeres allow for the formation of G quadruplexes (G4) and require specialized cellular machinery to assist in replication of telomeric regions (3). RTEL1 (regulator of telomere length) was first identified and implicated in telomere maintenance through genetic comparison of mouse species with varied telomere length (4). RTEL1 is a member of the Fe-S cluster helicase family, which includes XPD and FANCJ, helicases that are integral to the process of genome maintenance and repair. RTEL1 has not yet been mechanistically characterized as other helicases of this family have (6). From cellular studies, RTEL1 has been implicated in telomere maintenance. Conditional deletion of RTEL1 results in increase telomere fragility and loss of telomeres, which ultimately is lethal (1,2). These studies suggest that RTEL1 plays a role in both the resolution of T loops and the disassembly of the G4, with the latter requiring the ability to bind PCNA (proliferating cell nuclear antigen) (1,2). Two important questions remain to be answered regarded RTEL1 function: what is the mechanism that underlies RTEL1 maintenance of telomeres and what are the effects of protein-protein interactions with PCNA. I hypothesize that RTEL1 is mechanistically similar to other Fe-S helicases and that interaction with PCNA alters RTEL1 activity to enable function specific to telomere maintenance. Mechanistic understanding of RTEL1 unwinding of DNA duplexes, G4s and whether it can dismantle nucleoprotein complexes is critical to relating its biochemical activities to its cellular functions.


  • Integrated DNA Technologies (IDT) Graduate Fellowship
  • Fellowship appointment on the Pharmacological Sciences Training Program (NIH T32 GM067795), University of Iowa, 2017-2019
  • Institutional support on the Pharmacological Sciences Training Program (NIH T32 GM067795), University of Iowa, 2016-2017


  1. Tibbs J, Ghoneim M, Caldwell CC, Buzynski T, Bowie W, Boehm EM, Washington MT, Tabei SMA, Spies M. KERA: analysis tool for multi-process, multi-state single-molecule data. Nucleic Acids Res. 2021 Feb 28:gkab087. doi: 10.1093/nar/gkab087. Epub ahead of print. PMID: 33660771
  2. Caldwell, C.C., & Spies, M.:  Dynamic elements of replication protein A at the crossroads of DNA replication, recombination, and repair. Critical Reviews in Biochemistry and Molecular Biology, 55(5):482-507, 2020.  PMID: 32856505
  3. Pokhrel, N.†, Caldwell, C.C.†, Corless, E.I., Tillison, E.A., Tibbs, J., Jocic, N., Tabei, S.M.A., Wold, M.S., Spies, M, and Antony, E.:  Dynamics and Selective Remodeling of the DNA Binding Domains of RPA.  Nature Structural and Molecular Biology 26(2):129-136, 2019.  PMCID: PMC6368398 † denotes equal contribution
  4. Yates, L.A., Aramayo, R.J., Pokhrel, N., Caldwell, C.C., Kaplan, J.A., Perera, R.L., Spies, M, Antony, E., and Zhang, X.:  A structural and dynamic model for the assembly of Replication Protein A on single-stranded DNA. Nature Communication 9(1):5447, 2018.  PMCID: PMC6303327  
  5. Caldwell, C.C., and Spies, M.:  Helicase SPRNTing through the nanopore. Proceedings of the National Academy of Sciences USA. 114(45):11809-11811, 2017.  PMCID: PMC5692609