Toxicon 51 (2008) 473–477 Short communication Diphtheria toxin mutant CRM197 is an inhibitor of protein synthesis that induces cellular toxicity $ Jian Qiao 1 , Karim Ghani, Manuel Caruso à Le Centre de Recherche en Cance´rologie de l’Universite´Laval, L’Hoˆtel Dieu de Que´bec, Centre Hospitalier Universitaire de Que´bec, 9 rue McMahon, Que´bec, Canada G1R 2J6 Received 30 August 2007; accepted 27 September 2007 Available online 1 October 2007 Abstract Diphtheria toxin (DT) ADP-rybosylates elongation factor-2 on a modified histidine residue called diphthamide, leading to a block in protein synthesis. CRM197 is a nontoxic DT mutant commonly used as a carrier for conjugate vaccines, and may have a potential as an anti-tumor agent. We now report that CRM197 expression is indeed toxic to cells, and inhibits protein synthesis. These results should be considered for future vaccine studies, and to investigate the mechanism of CRM197-mediated anti-tumor effect. r 2007 Elsevier Ltd. All rights reserved. Keywords: Diphtheria toxin; CRM197; Elongation factor-2; ADP-rybosylation Diphtheria toxin (DT) is made of 2 chains: the A chain that contains the enzymatic activity and the B chain that mediates its entry into cells by binding to a receptor (proheparin-binding epidermal growth factor-like growth factor; proHB-EGF) (Naglich et al., 1992). After receptor-mediated endocytosis, the A fragment is translocated from the endosome into the cytoplasm, and leads to a block in protein synthesis. More precisely, the toxin catalyzes the adenosine diphosphate (ADP)-rybosylation of diph- tamide, a post-translationally modified histidine residue present on elongation factor-2 (EF-2) (Collier, 2001; Honjo et al., 1968). The A subunit of diphtheria toxin (DT-A) is attractive for cancer gene therapy (Maxwell et al., 1986), but its toxicity renders challenging the production of recombinant viruses. Deletion of direct repeat sequences (RSs) present in a retroviral genome is a common process that occurs during reverse transcription (Pathak and Temin, 1990). We originally designed a study that could take advan- tage of this property to generate recombinant viruses containing a DT-A gene that would be disrupted in retrovirus producer cells and recon- stituted upon infection. To measure the deletion percentage of a DT-A/RS vector, a well-character- ized nontoxic DT mutant, CRM197, was chosen. This DT toxin mutant has been isolated by mutation with nitrosoguanidine of corynephage b DNA followed by infection and lysogenization of ARTICLE IN PRESS www.elsevier.com/locate/toxicon 0041-0101/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.toxicon.2007.09.010 $ Ethical statement: The authors attest that all potential conflicts of interest have been disclosed and addressed in this manuscript. à Corresponding author. Tel.: +1 418 525 4444; fax: +1 418 691 5439. E-mail address: manuel.caruso@crhdq.ulaval.ca (M. Caruso). 1 Present address: Molecular Medicine Program, Mayo Clinic, Rochester, MN 55905, USA.