References http://genesdev.cshlp.org/content/17/5/545.full.html#related-urls Article cited in: http://genesdev.cshlp.org/content/17/5/545.full.html#ref-list-1 This article cites 228 articles, 79 of which can be accessed free at: service Email alerting click here top right corner of the article or Receive free email alerts when new articles cite this article sign up in the box at the Collections Topic (33 articles) Molecular Physiology and Metabolism  (98 articles) Cancer and Disease Models  Articles on similar topics can be found in the following collections

/pdf/molecular-imaging-in-living-subjects-seeing-fundamental-58mg35grjk.pdf

Molecular imaging in living subjects: seeing fundamental biological processes in a new light

The imaging of specific molecular targets that are associated with cancer should allow earlier diagnosis and better management of oncology patients. Positron emission tomography (PET) is a highly sensitive non-invasive technology that is ideally suited for pre-clinical and clinical imaging of cancer biology, in contrast to anatomical approaches. By using radiolabelled tracers, which are injected in non-pharmacological doses, three-dimensional images can be reconstructed by a computer to show the concentration and location(s) of the tracer of interest. PET should become increasingly important in cancer imaging in the next decade.

http://pfeifer.phas.ubc.ca/refbase/files/Gambhir-NatRev-2002-2-683.pdf

Molecular imaging of cancer with positron emission tomography.

Diseases are biological processes, and molecular imaging with positron emission tomography (PET) is sensitive to and informative of these processes. This is illustrated by detection of biological abnormalities in neurological disorders with no computed tomography or MRI anatomic changes, as well as even before symptoms are expressed. PET whole body imaging in cancer provides the means to ( i ) identify early disease, ( ii ) differentiate benign from malignant lesions, ( iii ) examine all organs for metastases, and ( iv ) determine therapeutic effectiveness. Diagnostic accuracy of PET is 8–43% higher than conventional procedures and changes treatment in 20–40% of the patients, depending on the clinical question, in lung and colorectal cancers, melanoma, and lymphoma, with similar findings in breast, ovarian, head and neck, and renal cancers. A microPET scanner for mice, in concert with human PET systems, provides a novel technology for molecular imaging assays of metabolism and signal transduction to gene expression, from mice to patients: e.g., PET reporter gene assays are used to trace the location and temporal level of expression of therapeutic and endogenous genes. PET probes and drugs are being developed together—in low mass amounts, as molecular imaging probes to image the function of targets without disturbing them, and in mass amounts to modify the target9s function as a drug. Molecular imaging by PET, optical technologies, magnetic resonance imaging, single photon emission tomography, and other technologies are assisting in moving research findings from in vitro biology to in vivo integrative mammalian biology of disease.

https://www.pnas.org/content/pnas/97/16/9226.full.pdf

Positron emission tomography provides molecular imaging of biological processes.

The complete genomic sequence of the modified vaccinia ankara strain : comparison with other orthopoxviruses

https://www.cell.com/trends/pharmacological-sciences/pdf/S0165-6147(05)00020-9.pdf

Emerging role of homo- and heterodimerization in G-protein-coupled receptor biosynthesis and maturation

We are developing assays for noninvasive, quantitative imaging of reporter genes with positron emission tomography (PET), for application both in animal models and in human gene therapy. We report here a method to improve the detection of lower levels of PET reporter gene expression by utilizing a mutant herpes simplex virus type 1 thymidine kinase (HSV1-sr39tk) as a PET reporter gene. The HSV1-sr39tk mutant was identified from a library of site-directed mutants. Accumulation (net uptake) of the radioactively labeled substrates [8-3H]penciclovir ([8-3H]PCV), and 8-[18F]fluoropenciclovir (FPCV) in C6 rat glioma cells expressing HSV1-sr39tk is increased by a factor of ≈2.0 when compared with C6 cells expressing wild-type HSV1-tk. The increased imaging sensitivity of HSV1-sr39tk when FPCV is used is also demonstrated in vivo both with tumor cells stably transfected with either HSV1-tk or HSV1-sr39tk, and after hepatic delivery of HSV1-tk or HSV1-sr39tk by using adenoviral vectors. The use of HSV1-sr39tk as a PET reporter gene and FPCV as a PET reporter probe results in significantly enhanced sensitivity for imaging reporter gene expression in vivo.

https://www.pnas.org/content/pnas/97/6/2785.full.pdf

A mutant herpes simplex virus type 1 thymidine kinase reporter gene shows improved sensitivity for imaging reporter gene expression with positron emission tomography

Cancer suicide gene therapy affords the prospect of using the most optimal genes available because the source of the therapeutic gene is often irrelevant. Currently, there are numerous preclinical and clinical trials to develop tumor ablative therapies that use viral, yeast, or bacterial genes. One such gene, the herpes simplex virus type 1 (HSV-1) thymidine kinase (TK) is widely used as a suicide gene in combination with ganciclovir. In the study reported here, a restricted set of random sequences (semi-random) was introduced into the active site of HSV-1 TK, and the resulting variants were selected on the basis of their ability to confer increased ganciclovir or acyclovir sensitivity to Escherichia coli. Sequence analysis demonstrated that functional mutants contained three to five amino acid substitutions that are unique and novel combinations. On the basis of enzyme assay results, three mutants were identified for further analysis in vitro. These three mutants conferred substantial increased sensitivity to both ganciclovir and acyclovir when compared with IC50s of wild-type TK expressing rat C6 glioma cells. One mutant, SR39, was further evaluated in a xenograft tumor model in nude mice. Expression of SR39 in tumors was shown to prevent tumor growth at prodrug dosages that did not affect wild-type HSV-1 TK-expressing tumors. The use of any of these mutants as a suicide gene should provide a more effective and safer alternative to wild-type TK, because lower, less immunosuppressive doses of ganciclovir will be necessary for tumor ablation, and the use of acyclovir may now be possible.

https://cancerres.aacrjournals.org/content/canres/61/7/3022.full.pdf

Herpes Simplex Virus-1 Thymidine Kinase Mutants Created by Semi-Random Sequence Mutagenesis Improve Prodrug-mediated Tumor Cell Killing

Herpes Simplex Virus type 1 (HSV-1) thymidine kinase (TK) is currently the most widely used suicide agent for gene therapy of cancer. Tumor cells that express HSV-1 thymidine kinase are rendered sensitive to prodrugs due to preferential phosphorylation by this enzyme. Although ganciclovir (GCV) is the prodrug of choice for use with TK, this approach is limited in part by the toxicity of this prodrug. From a random mutagenesis library, seven thymidine kinase variants containing multiple amino acid substitutions were identified on the basis of activity towards ganciclovir and acyclovir based on negative selection in Escherichia coli. Using a novel affinity chromatography column, three mutant enzymes and the wild-type TK were purified to homogeneity and their kinetic parameters for thymidine, ganciclovir, and acyclovir determined. With ganciclovir as the substrate, one mutant (mutant SR39) demonstrated a 14-fold decrease in K(m) compared to the wild-type enzyme. The most dramatic change is displayed by mutant SR26, with a 124-fold decrease in K(m) with acyclovir as the substrate. Such new "prodrug kinases" could provide benefit to ablative gene therapy by now making it feasible to use the relatively nontoxic acyclovir at nanomolar concentrations or ganciclovir at lower, less immunosuppressive doses.

/pdf/characterization-of-herpes-simplex-virus-type-1-thymidine-1b9qpi0krk.pdf

Characterization of herpes simplex virus type 1 thymidine kinase mutants engineered for improved ganciclovir or acyclovir activity.

Cloning, Characterization, and Modeling of Mouse and Human Guanylate Kinases

Herpes Simplex Virus type 1 (HSV-1) thymidine kinase (TK) is currently the most widely used suicide agent for gene therapy of cancer. Tumor cells that express HSV-1 thymidine kinase are rendered sensitive to prodrugs due to preferential phosphorylation by this enzyme. While ganciclovir (GCV) is the prodrug of choice for use with TK, this approach is limited in part by the toxicity of this prodrug. From a random mutagenesis library of over a million mutant thymidine kinases, ten thymidine kinase variants were identified on the basis of activity towards ganciclovir and acyclovir (Black ME, Newcomb TG, Wilson H-MP and Loeb LA: Proc. Natl. Acad. Sci. U.S.A. 93: 3525-3529, 1996). Six mutants described here contain three to six amino acid changes and render mammalian cells more sensitive to acyclovir (ACV) including one that demonstrates an 8.5-fold reduction in IC50 compared to wild-type TK. These novel enzymes could provide benefit to ablative gene therapy by now making it feasible to use the relatively non-toxic acyclovir at nanomolar concentrations.

Mark S. Kokoris

Papers

A mutant herpes simplex virus type 1 thymidine kinase reporter gene shows improved sensitivity for imaging reporter gene expression with positron emission tomography

Herpes Simplex Virus-1 Thymidine Kinase Mutants Created by Semi-Random Sequence Mutagenesis Improve Prodrug-mediated Tumor Cell Killing

Characterization of herpes simplex virus type 1 thymidine kinase mutants engineered for improved ganciclovir or acyclovir activity.

Cloning, Characterization, and Modeling of Mouse and Human Guanylate Kinases

In vitro evaluation of mutant HSV-1 thymidine kinases for suicide gene therapy.