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Cell Therapy

EXG34217 and Telomere Biology Disorders with Bone Marrow Failure

EXG34217 is an autologous cell therapy for telomere biology disorders with bone marrow failure that uses Elixirgen Therapeutics’ proprietary ZSCAN4 technology to extend the telomeres of the patients. Elixirgen Therapeutics will proceed with its Phase I/II, open label, single center clinical trial to assess the safety and tolerability of EXG34217 at Cincinnati Children’s Hospital Medical Center.

EXG34217’s process was designed for the most safety and least burden possible to the patients. First, the patients’ CD34+ hematopoietic stem cells are mobilized, and then collected using standard apheresis. Next, the CD34+ cells are treated with a human ZSCAN4 protein-expressing vector in a closed, sterile tubing system outside the patient’s body for 24 hours. Finally, those cells are re-infused back to the patient.

What is ZSCAN4?

ZSCAN4 (zinc finger and SCAN domain containing 4) was originally identified by Minoru Ko’s lab at the National Institutes of Health (NIH) as a gene expressed specifically at the 2-cell stage of mouse preimplantation embryos (Falco et al., 2007). The ZSCAN4 protein contains a SCAN domain for putative protein-protein interactions as well as four zinc finger domains for putative DNA binding.


EXG34217 uses Elixirgen Therapeutics’ proprietary ZSCAN4 technology to extend the telomeres of the patients’ cells.

Subsequent studies have revealed that ZSCAN4 expression is transient and restricted to critical timings and cell types: high expression in preimplantation embryos in mouse (Falco et al., 2007); infrequent but high expression in tissue stem cells in mouse and human (Ko et al., 2013); expression in meiotic prophase I in both male and female mice (Ishiguro et al., 2016); and high and specific expression in 1-5% of mouse embryonic stem cells (ESCs) (Falco et al., 2007).

Endogenous expression of ZSCAN4 protein is transient in mouse ESCs; however, when it is expressed it localizes on telomeres and is associated with rapid telomere extension through telomere recombination and upregulation of meiosis-specific homologous recombination genes (Zalzman et al., 2010). Telomere lengthening by ZSCAN4 is not associated with increased telomerase activity, nor is it reduced by eliminating telomerase activity (Zalzman et al., 2010). ZSCAN4 knockdown studies in mice have demonstrated that removing Zscan4 shortens telomeres, increases karyotype abnormalities and spontaneous sister chromatid exchange, and slows down cell proliferation until crisis (Zalzman et al., 2010). It has also been demonstrated that these infrequent yet dynamic changes in telomeres are accompanied by transient opening and closing of permanently silenced heterochromatin, including the pericentromeric region involved in chromosome segregation and telomeric regions (Akiyama et al., 2015), and blocking of global de novo synthesis of proteins (Hung et al., 2013). These data suggest that ZSCAN4 plays a critical role in the repair of telomeres and other heterochromatic regions of the genome by being induced to be expressed only when these genomic regions are severely damaged or vulnerable to possible damages (reviewed in Ko, 2016). Importantly, it has been shown that transient and ectopic expression of ZSCAN4 as a doxycycline-inducible transgene can rapidly extend telomeres (Zalzman et al., 2010) and increase the quality and developmental potential of mouse ESCs (Amano et al., 2013).

EXG34217 uses Elixirgen Therapeutics’ proprietary ZSCAN4 technology to extend the telomeres of the patients’ cells.

Although the initial work to elucidate the biological function of ZSCAN4 was performed with the mouse ZSCAN4c gene in mouse cells, further studies by Elixirgen Therapeutics have demonstrated that these data are translatable to humans. In general, the human ZSCAN4 protein has similar biological activity in mouse and human cells. Initial experiments with synthetic mRNAs and Sendai virus vectors encoding human ZSCAN4 demonstrated an increase in the number of cells with normal karyotypes in mouse embryonic stem cells as well as in non-immortalized primary human fibroblast cells derived from people with Down Syndrome (Trisomy 21) or Edwards syndrome (Trisomy 18) (Amano et al., 2015).

  1. Zscan4: a novel gene expressed exclusively in late 2-cell embryos and embryonic stem cells. Falco G, Lee SL, Stanghellini I, Bassey UC, Hamatani T, Ko MS. Dev Biol. 2007 Jul 15;307(2):539-50. [PMID:17553482][http://www.sciencedirect.com/science/article/pii/S0012160607008792]
  2. An in situ hybridization-based screen for heterogeneously expressed genes in mouse ES cells. Carter MG, Stagg CA, Falco G, Yoshikawa T, Bassey UC, Aiba K, Sharova LV, Shaik N, Ko MS. Gene Expr Patterns. 2008 Feb;8(3):181-98. [PMID:18178135][https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2238805/]
  3. Zscan4 regulates telomere elongation and genomic stability in ES cells. Zalzman M, Falco G, Sharova LV, Nishiyama A, Thomas M, Lee SL, Stagg CA, Hoang HG, Yang HT, Indig FE, Wersto RP, Ko MS. Nature. 2010 Apr 8;464(7290):858-63. [PMID:20336070][https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2851843/]
  4. Zscan4 transiently reactivates early embryonic genes during the generation of induced pluripotent stem cells. Hirata T, Amano T, Nakatake Y, Amano M, Piao Y, Hoang HG, Ko MS. Sci Rep. 2012;2:208. [PMID:22355722][https://www.nature.com/articles/srep00208]
  5. Inflammation increases cells expressing ZSCAN4 and progenitor cell markers in the adult pancreas. Ko SB, Azuma S, Yokoyama Y, Yamamoto A, Kyokane K, Niida S, Ishiguro H, Ko MS. Am J Physiol Gastrointest Liver Physiol. 2013 Jun 15;304(12):G1103-16. [PMID:23599043][http://ajpgi.physiology.org/content/304/12/G1103.long]
  6. Repression of global protein synthesis by Eif1a-like genes that are expressed specifically in the two-cell embryos and the transient Zscan4-positive state of embryonic stem cells. Hung SS, Wong RC, Sharov AA, Nakatake Y, Yu H, Ko MS. DNA Res. 2013 Aug;20(4):391-402. [PMID:23649898][https://academic.oup.com/dnaresearch/article-lookup/doi/10.1093/dnares/dst018]
  7. Zscan4 restores the developmental potency of embryonic stem cells. Amano T, Hirata T, Falco G, Monti M, Sharova LV, Amano M, Sheer S, Hoang HG, Piao Y, Stagg CA, Yamamizu K, Akiyama T, Ko MS. Nat Commun. 2013;4:1966. [PMID:23739662][https://www.nature.com/articles/ncomms2966]
  8. Correction of Down syndrome and Edwards syndrome aneuploidies in human cell cultures. Amano T, Jeffries E, Amano M, Ko AC, Yu H, Ko MS. DNA Res. 2015 Oct;22(5):331-42. [PMID:26324424][https://academic.oup.com/dnaresearch/article-lookup/doi/10.1093/dnares/dsv016]
  9. Transient bursts of Zscan4 expression are accompanied by the rapid derepression of heterochromatin in mouse embryonic stem cells. Akiyama T, Xin L, Oda M, Sharov AA, Amano M, Piao Y, Cadet JS, Dudekula DB, Qian Y, Wang W, Ko SB, Ko MS. DNA Res. 2015 Oct;22(5):307-18. [PMID:26324425][https://academic.oup.com/dnaresearch/article-lookup/doi/10.1093/dnares/dsv013]
  10. Emergence of undifferentiated colonies from mouse embryonic stem cells undergoing differentiation by retinoic acid treatment. Sharova LV, Sharov AA, Piao Y, Stagg CA, Amano T, Qian Y, Dudekula D, Schlessinger D, Ko MS. In Vitro Cell Dev Biol Anim. 2016 May;52(5):616-24. [PMID:27130680][https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4884469/]
  11. Zygotic Genome Activation Revisited: Looking Through the Expression and Function of Zscan4. Ko MS. Curr Top Dev Biol. 2016;120:103-24. [PMID:27475850][http://www.sciencedirect.com/science/article/pii/S0070215316301016]
  12. Expression analysis of the endogenous Zscan4 locus and its coding proteins in mouse ES cells and preimplantation embryos. Ishiguro KI, Nakatake Y, Chikazawa-Nohtomi N, Kimura H, Akiyama T, Oda M, Ko SB, Ko MS. In Vitro Cell Dev Biol Anim. 2017 Feb;53(2):179-190. [PMID:27699651][https://link.springer.com/article/10.1007%2Fs11626-016-0097-y]
  13. Zscan4 is expressed specifically during late meiotic prophase in both spermatogenesis and oogenesis. Ishiguro KI, Monti M, Akiyama T, Kimura H, Chikazawa-Nohtomi N, Sakota M, Sato S, Redi CA, Ko SB, Ko MS. In Vitro Cell Dev Biol Anim. 2017 Feb;53(2):167-178. [PMID:27699653][https://link.springer.com/article/10.1007%2Fs11626-016-0096-z]

Covid-19 Vaccine: EXG-5003

EXG-5003 is an RNA vaccine expressing the receptor binding domain (RBD) of the SARS-CoV-2 spike protein. EXG-5003 is a temperature-controllable, intradermally-injected, srRNA (self-replicating RNA) vaccine designed with unique features that have safety and dose sparing benefits.

  1. 1.
    Temperature Control

    EXG-5003 is active at normal skin temperature (where it is injected, intradermally) but inactive at normal core body temperature. This improves the safety of the vaccine by ensuring that the srRNA is only active at the injection site.

  2. 2.

    Both intradermal injection and srRNA are more immunogenic, and therefore more dose sparing than mRNA only.

  3. 3.
    Vaccine targets RBD

    Adverse effects (e.g. liver toxicity) have been reported in vaccines targeting the full spike protein in SARS-CoV. Targeting only the RBD avoids such effects to further increase the safety profile of EXG-5003.

EXG-5003 was designed with unique features that have potential dose sparing and safety benefits. Using both intradermal injection and srRNA technology may result in improved immunogenicity, and may allow the vaccine candidate to be more dose sparing than mRNA (messenger RNA) alone. Additionally, EXG-5003's novel and proprietary temperature control may significantly enhance the vaccine's safety profile.

Elixirgen Therapeutics concluded its pre-IND meeting with the FDA on May 18, 2020, and has announced the start of dosing in a Phase 1/2 clinical trial in Japan on May 25, 2021.