The application of 5-bromodeoxyuridine in the management of CNS tumors

DNA Distortion Caused by Uracil-Containing Intrastrand Cross-Links

Four uracil-containing intrastrand cross-links have been detected in human cells upon UV irradiation of 5-bromouracil-containing DNA, namely 5'-G[8-5]U-3', 5'-U[5-8]G-3', 5'-A[8-5]U-3', and 5'-A[2-5]U-3'. These lesions feature unique composition and connectivity compared with other intrastrand cross-links reported in the literature. For the first time, structural information obtained using molecular dynamics (MD) simulations reveal that all four lesions distort the DNA helix, which can involve an extrahelical location of the cross-link, changes in the helical interactions of the complementary nucleotides, or disruption of hydrogen bonding in the flanking base pairs up to two positions from the cross-linked site; however, the degree of distortion varies between the cross-links, being affected by the sequence, nucleobase-nucleobase connectivity, and the purine involved. Most importantly, the relative distortion of the damaged DNA provides the first structural explanation for the observed abundances of the four uracil-containing cross-links. Furthermore, the highly distorted conformations suggest that these lesions will likely have severe implications for DNA replication and repair processes in cells.

Formation mechanism and structure of a guanine-uracil DNA intrastrand cross-link

The formation and structure of the 5'-G[8-5]U-3' intrastrand cross-link are studied using density functional theory and molecular dynamics simulations due to the potential role of this lesion in the activity of 5-halouracils in antitumor therapies. Upon UV irradiation of 5-halouracil-containing DNA, a guanine radical cation reacts with the uracil radical to form the cross-link, which involves phosphorescence or an intersystem crossing and a rate-determining step of bond formation. Following ionizing radiation, guanine and the uracil radical react, with a rate-limiting step involving hydrogen atom removal. Although cross-link formation from UV radiation is favored, comparison of calculated reaction thermokinetics with that for related experimentally observed purine-pyrimidine cross-links suggests this lesion is also likely to form from ionizing radiation. For the first time, the structure of 5'-G[8-5]U-3' within DNA is identified by molecular dynamics simulations. Furthermore, three conformations of cross-linked DNA are revealed, which differ in the configuration of the complementary bases. Distortions, such as unwinding, are localized to the cross-linked dinucleotide and complementary nucleotides, with minimal changes to the flanking bases. Global changes to the helix, such as bending and groove alterations, parallel cisplatin-induced distortions, which indicate 5'-G[8-5]U-3', may contribute to the cytotoxicity of halouracils in tumor cell DNA using similar mechanisms.

Ethynyldeoxyuridine (EdU) suppresses in vitro population expansion and in vivo tumor progression of human glioblastoma cells

Thymidine analogs (TAs) are synthetic nucleosides that incorporate into newly synthesized DNA. Halogenated pyrimidines (HPs), such as bromodeoxyuridine (BrdU), are a class of TAs that can be detected with antibodies and are commonly used for birthdating individual cells and for assessing the proliferative index of cell populations. It is well established that HPs can act as radiosensitizers when incorporated into DNA chains, but they are generally believed not to impair normal cell function in the absence of secondary stressors. However, we and others have shown that HP incorporation leads to a sustained suppression of cell cycle progression in mammalian cells, resulting in cellular senescence in somatic cells. In addition, we have shown that HP incorporation results in delayed tumor progression in a syngeneic rat model of glioma. Here we examine ethynyldeoxyuridine (EdU), a newly developed and alkylated TA, for its anti-cancer activity, both in vitro and in vivo. We show that EdU, like HPs, leads to a severe reduction in the proliferation rate of normal and transformed cells in vitro. Unlike HPs, however, EdU incorporation also causes DNA damage resulting in the death of a substantial subset of treated cells. When administered over an extended time as a monotherapy to mice bearing subcutaneous xenografts of human glioblastoma multiforme tumors, EdU significantly reduces tumor volume and increases survival without apparent significant toxicity. These results, combined with the fact that EdU readily crosses the blood-brain barrier, support the continued investigation of EdU as a potential therapy for malignant brain tumors.

Protocols for studying adult neurogenesis: insights and recent developments

The first evidence that neurogenesis occurs in the adult brain was reported in rodents in the early 1960s, using [(3)H]-thymidine autoradiography. In the 1980s and 90s, the advent of new techniques and protocols for studying cell proliferation in situ, and particularly bromodeoxyuridine labeling, helped to confirm that neurogenesis occurs in the adult brain and neural stem cells reside in the adult CNS, including in humans. Bromodeoxyuridine labeling is currently the method most commonly used for studying neurogenesis in the adult brain. However, this procedure is not without limitations, and controversies. In this article, I will review recent protocols for studying adult neurogenesis, particularly new protocols for studying cell kinetics and cell proliferative history, using halopyrimidines. I will review these techniques, and discuss their implications for the field of adult neurogenesis.

Emerging concepts in glioma biology: implications for clinical protocols and rational treatment strategies

Glioblastoma multiforme (GBM), the most common primary central nervous system neoplasm, is a complex, heterogeneous disease. The recent identification of stem cells in murine tumor xenografts that were capable of recapitulating the tumor phenotype adds a new dimension of complexity to the already challenging treatment of patients with GBMs. Although specific cellular and genetic changes are commonly associated with GBM, the mechanism by which those changes occur may have a significant impact on treatment outcome. Of the many bioinformatics techniques developed in recent years, gene expression profiling has become a commonly used research tool for investigating tumor characteristics, and the development of rationally targeted molecular therapies has also accelerated following the initial success of specifically designed inhibitors in the treatment of malignancies. Despite these advances in research techniques and targeted molecular therapies, however, limited clinical impact has been achieved in the treatment of infiltrative malignancies such as GBMs. Thus, further extension in survival of patients with GBMs may require use of multiple analyses of tumors to develop tailored therapies that reflect the inter- and intratumoral heterogeneity of this disease. In this review, the authors briefly consider the potential use of expression profiling combined with mutation analysis in the development of treatment modalities to address the heterogeneity of this complex tumor phenotype.

Techniques to assess the proliferative potential of brain tumors

Assessment of brain tumor proliferative potential provides important prognostic information that supplements standard histopathologic grading. Many laboratories rely on mitotic figures to quantify the proliferative potential of brain tumors, but this conventional cellular proliferative index is subject to inter-observer variability and not consistently predictive for low-and high-grade tumors. Recent advancements in technology have made it possible to use proliferative indices as a standard supplement in pathology laboratories. Non-invasive tumor tissue measurements of cell proliferation can be performed using- bromodeoxyuridine labeling index (BrdU LI), flow cytometry (FCM), MIB-1 antibody to the Ki-67 antigen (MIB-1), proliferating cell nuclear antigen (PCNA), and argyrophilic nucleolar organizing regions (AgNOR). Each of these assays has been described in the literature with respect to its ability to predict tumor grade or outcome. At the present time MIB-1 and AgNOR are the simplest and most reliable of these techniques. In addition, advances in our understanding of the genetic alterations associated with proliferation promise to provide more specific markers of proliferative potential. Beyond the pathology laboratory, radiographic studies such as positron emission tomography (PET), single photon emission computed tomography (SPECT), and most recently magnetic resonance spectroscopy (MRS) have been used as follow-up measures, assessing response to treatment and tumor recurrence, rather than as predictors of response to treatment. These radiographic tools, however, have the potential to provide an assessment of tumor proliferation without the need for invasive measures. In this article, we present a review of the current techniques utilized to understand the proliferative potential of brain tumors.

Delivery systems for small molecule drugs, proteins, and DNA: the neuroscience/biomaterial interface

Manipulation of cellular processes in vivo by the delivery of drugs, proteins or DNA is of paramount importance to neuroscience research. Methods for the presentation of these molecules vary widely, including direct injection (either systemic or stereotactic), osmotic pump-mediated chronic delivery, or even implantation of cells engineered to indefinitely secrete a factor of interest. Biomaterial-based delivery systems represent an alternative to more traditional approaches, with the possibility of increased efficacy. Drug-releasing biomaterials, either as injectable microspheres or as three-dimensional implants, can deliver a molecule of interest (including small molecule drugs, biologically active proteins, or DNA) over a more prolonged period of time than by standard bolus injection, avoiding the need for repeated administration. Furthermore, sustained-release systems can maintain therapeutic concentrations at a target site, thus reducing the chance for toxicity. This review summarizes applications of polymer-based delivery of small molecule drugs, proteins, and DNA specifically relevant to neuroscience research. We detail the fabrication procedures for the polymeric systems and their utility in various experimental models. The biomaterial field offers unique experimental tools with downstream clinical application for the study and treatment of neurologic disease.