Recombinant carcinoembryonic antigen as a reporter gene for molecular imaging.
OBJECTIVE
Reporter genes can provide a way of noninvasively assessing gene activity in vivo. However, current reporter gene strategies may be limited by the immunogenicity of foreign reporter proteins, endogenous expression, or unwanted biological activity. We have developed a reporter gene based on carcinoembryonic antigen (CEA), a human protein with limited normal tissue expression.


METHODS
To construct a CEA reporter gene for PET, a CEA minigene (N-A3) was fused to the extracellular and transmembrane domains of the human Fc gamma RIIb receptor. The NA3-Fc gamma RIIb recombinant gene, driven by a CMV promoter, was transfected in Jurkat (human T cell leukemia) cells. Expression was analyzed by flow cytometry, immunohistochemistry (IHC), and microPET imaging.


RESULTS
Flow cytometry identified Jurkat clones stably expressing NA3-Fc gamma RIIb at low, medium, and high levels. High and medium NA3-Fc gamma RIIb expression could also be detected by Western blot. Reporter gene positive and negative Jurkat cells were used to establish xenografts in athymic mice. IHC showed staining of the tumor with high reporter gene expression; medium and low N-A3 expression was not detected. MicroPET imaging, using an anti-CEA (124)I-labeled single-chain Fv-Fc antibody fragment, demonstrated that only high N-A3 expression could be detected. Specific accumulation of activity was visualized at the N-A3 positive tumor as early as 4 h. MicroPET image quantitation showed tumor activity of 1.8 +/- 0.2, 15.2 +/- 1.3, and 4.6 +/- 1.2 percent injected dose per gram (%ID/g) at 4, 20, and 48 h, respectively. Biodistribution at 48 h demonstrated tumor uptake of 4.8 +/- 0.8%ID/g.


CONCLUSIONS
The CEA N-A3 minigene has the potential to be used as a reporter gene for imaging cells in vivo.
Kenanova
Vania
V
Crump Institute for Molecular Imaging, Department of Molecular and Medical Pharmacology, David Geffen School of Medicine, University of California Los Angeles, 700 Westwood Plaza, Los Angeles, CA 90095, USA.
Barat
Bhaswati
B
Olafsen
Tove
T
Chatziioannou
Arion
A
Herschman
Harvey R
HR
Braun
Jonathan
J
Wu
Anna M
AM
D000818:Animals; D002272:Carcinoembryonic Antigen / Q000032:analysis* / Q000235:genetics* / Q000276:immunology; D002460:Cell Line; D058851:GPI-Linked Proteins; D017930:Genes, Reporter; D006801:Humans; D007128:Immunoglobulin Fragments / Q000276:immunology; D007519:Isoantigens / Q000235:genetics; D051379:Mice; D009369:Neoplasms / Q000000981:diagnostic imaging / Q000235:genetics / Q000378:metabolism; D049268:Positron-Emission Tomography; D017452:Receptors, IgG / Q000235:genetics; D011994:Recombinant Proteins / Q000032:analysis / Q000235:genetics* / Q000276:immunology; D013601:T-Lymphocytes / Q000276:immunology / Q000378:metabolism; D014057:Tomography, X-Ray Computed
D002272:Carcinoembryonic Antigen; C428200:FCGR3B protein, human; D058851:GPI-Linked Proteins; D007128:Immunoglobulin Fragments; D007519:Isoantigens; D017452:Receptors, IgG; D011994:Recombinant Proteins; C459404:neutrophil-specific antigen NA3, human