PET imaging of gene expression
Introduction
Positron emission tomography (PET) is an established imaging modality that has evolved over the past 30+ years and is now widely used in the clinic, particularly in oncology to define the extent of disease (for staging prior to more invasive procedures) and to identify recurrent disease. An advantage of PET imaging is the ability to obtain specific information about physiological, biochemical and molecular processes in the body. This information is quantitative (based on radiotracer principles), can be presented in three-dimensional space, and can be obtained repeatedly (sequentially) over time in the same subject. A more detailed description of PET imaging technology has been provided in previous chapters. A point that will be emphasised in this chapter is that PET is well suited to image the expression of “marker”/“reporter” transgenes. Recent studies have shown that it is possible to image endogenous molecular events using PET. This advance has been largely due to the development of a new class of reporter constructs, complimentary radiolabelled molecules (probes) and novel imaging paradigms.
“Molecular imaging” is a term that was developed in the 1990s, with roots that go back to in situ visualization of target molecules and biological processes (in situ optical imaging) 1, 2, 3, 4. Molecular imaging has evolved and become a more broadly defined term over the past decade; it includes studies that were previously described as “gene imaging” and now relates to many aspects of biology. Advances over the past 5–10 years have included non-invasive in vivo molecular imaging in animals. Although it may appear somewhat presumptuous to imply that current non-invasive imaging technologies (magnetic resonance, PET, gamma camera, etc.) can image molecular events that occur within cells, it has already been shown that it is possible to image transcriptional regulation of endogenous gene expression [5]. Needless to say, current PET, gamma camera, magnetic resonance and optical technologies that are used to image animals and patients do not visualise individual cells, much less molecules. What is so exciting about this emerging new field relates to the novel imaging paradigms that are being developed. These paradigms can be successful within the inherent spatial resolution limits of existing imaging systems, because some degree of tissue (cell) homogeneity within the resolution elements (pixels) of the resultant images can be achieved.
Section snippets
Imaging strategies
Two molecular imaging strategies—“direct” and “indirect”— will be described, and examples of each will be discussed. “Direct” imaging of endogenous genes and molecules can be defined in terms of a probe-target interaction, whereby the resultant image of probe localisation and magnitude (image intensity) is directly related to its interaction with the target molecule, epitope or enzyme. Indirect molecular-gene imaging is a little more complex in that it may involve multiple components. One
Monitoring gene therapy
A non-invasive, clinically applicable method for imaging the expression of successful gene transduction in target tissue or specific organs of the body would be of considerable value for monitoring and evaluating gene therapy in human subjects [36]. The reporter transgene(s) can be driven by any promoter/enhancer sequence of choice [37]. The promoter can be “constitutive” (leading to continuous transcription), or it can be inducible (leading to controlled expression). The promoter can also be
Issues for the future
Molecular imaging has its roots in both molecular biology and cell biology as well as in imaging technology (nuclear, magnetic resonance, optical, etc.). These disciplines have now converged to provide a well-established foundation for exciting new research opportunities and for translation into clinical applications. The development of versatile and sensitive assays that do not require tissue samples would be of considerable value for in vivo studies and the monitoring of molecular-genetic and
References (49)
- et al.
Reporter gene expression for monitoring gene transfer
Current Opinion in Biotechnology
(1997) - et al.
Rapid and quantitative assessment of cancer treatment response using in vivo bioluminescence imaging
Neoplasia
(2000) - et al.
An adenovirus with enhanced infectivity mediates molecular chemotherapy of ovarian cancer cells and allows imaging of gene expression
Mol. Ther.
(2001) Reporter gene technologythe future looks bright
Biochemical Pharmacology
(1999)- et al.
A general approach to the non-invasive imaging of transgenes using cis-linked herpes simplex virus thymidine kinase
Neoplasia
(1999) - et al.
Starting at the beginning, middle, and endtranslation initiation in eukaryotes
Cell
(1997) - et al.
Clinical evaluation of adenoviral-mediated p53 gene transfer: review of INGN 201 studies
Semin Oncol
(2001) - et al.
Imaging TCR-dependent NFAT-mediated T-cell activation with positron emission tomography in vivo
Neoplasia
(2001) Photon counting imagingapplications in biomedical research
Biotechniques
(1989)- et al.
CCD imaging of luciferase gene expression in single mammalian cells
Journal of Bioluminescence & Chemiluminescence
(1990)