Page 275 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

254 PART III Therapeutic Modalities for the Cancer Patient

Normal cells (Intact p53) that the optimal approach for patients will be a combination of
approaches, including a combined approach with more conven-
tional treatments.
VetBooks.ir No effect Rescue of the Cancer Cell Through Gene

Replacement or Repair Technologies
Early cancer gene therapy strategies focused on the ablation of
oncogenes or the replacement of defective tumor suppressor
genes. One of the most studied genes in cancer development
1
has been the tumor suppressor gene p53, acting as a genomic
guardian for the cell and being “switched on” when a cell is
E1b deleted Ad5 exposed to DNA damaging agents. The p53 gene product
Viral oncolysis causes the cell to either stop dividing or undergo apoptosis,
depending on the degree of damage. In many cancers (50% of
human cancers), this gene is defective and the second allele is
Tumor cells (mutated p53) missing. Damaged cells fail to stop dividing and can accumu-
late further damaging events, which can allow selection for a
• Fig. 15.3 Conditionally replicating adenovirus. The ONYX-015 vector is malignant phenotype. A number of studies have addressed this
an E1b-deleted Adenovirus that conditionally replicates in cells with a non- by attempting to replace the defective p53 gene with its normal
functional p53 gene. p53 protein has the potential to shut down cell cycling 31
when infected with wild-type adenovirus but is prevented from doing so counterpart ; however, problems associated with this approach
through the actions of the product of viral E1b. E1b-deficient viruses can- include:
not replicate in normal cells with p53 intact. However, in cells that have no • The current technology is unable to efficiently deliver a normal
functional p53 protein, viral replication can proceed and cause cell lysis. p53 gene to every cancer cell in a tumor mass.
• Cancer is a multigenetic abnormality, and the delivery of one
correct gene to a tumor cell may still not have the desired phe-
2. Transcriptional targeting. This strategy exploits unique notypic effect.
gene expression in specific cell types once the vector has Gene replacement for cancer therapy proved to be disappoint-
entered the cell. 26–30 Although every gene is represented in ing clinically, but gene repair is possible, at least in the laboratory
every cell of the body, expression of any one gene requires setting. Gene repair has been achieved using lentiviral-mediated
specific transcription factors that may be unique to a par- zinc finger nucleases and exploiting endogenous repair mecha-
ticular cell or tissue type. Certain genes have been identi- nisms. In addition, newly developed gene editing approaches
fied that are expressed in cancer cells but are not expressed such as Transcriptional Activator-Like Nucleases (TALENS),
in normal cells (e.g., telomerase) or are expressed only in a and Clustered Regularly Interspaced Short Palindromic Repeats
33
specific tissue type (e.g., prostate-specific antigen [PSA]). By (CRISPR) provide novel ways to manipulate the genome. At
using the promoter sequences for these genes to drive trans- present, these technologies are laboratory based and are being
gene expression, targeted expression in cancer cells only (e.g., used to develop newly engineered cancer models. Their util-
using the promoter for telomerase) or to a specific tissue type ity in actual “gene repair” treatments is very far from clinical
(e.g., to the prostate using the promoter for PSA) can be exploitation.
achieved (see Fig. 15.2).
3. Replication-competent oncolytic viruses. Progress has been Gene Silencing in Cancer Cells
made in the development of viruses that conditionally repli-
cate in cancer cells. 31,32 One of the first examples to be used Gene silencing usually refers to the delivery and use of small inter-
in clinical trials was the ONYX-015 vector, an E1b-deleted fering double-stranded RNA (siRNA) molecules to cancer cells to
adenovirus that conditionally replicates in cells with a non- ablate the deleterious effects of activated oncogenes. In the cell,
functional p53 gene. p53 protein has the potential to shut exogenously delivered siRNA is directed to the RNA-induced
down cell cycling when infected with wild-type adenovirus but silencing complex (RISC). This complex is then directed to the
is prevented from doing so through the actions of the product target mRNA of the offending gene. By degradation of mRNA,
of viral E1b. E1b-deficient viruses cannot replicate in normal expression of the target gene is suppressed, which is known as
cells with p53 intact; however, in cells that have no functional posttranscriptional gene silencing (PTGS). 34
p53 protein, viral replication can proceed and cause cell lysis Proponents of RNA interference (RNAi)-based cancer therapy
(Fig. 15.3). Many other conditionally replicating viruses are have argued a high efficiency and potential low cost compared
being developed that rely on specific cancer cell defects (e.g., with the other methods of gene therapy, 34,35 and high specific-
reoviruses that conditionally replicate in cells with intact Ras ity compared with other modalities of cancer therapy such as
signaling pathways) or are transcriptionally targeted. Replica- chemotherapy. The major advantage of RNAi is the potential
tion-competent viruses are described in more detail in the text to target multiple genes of various cellular pathways involved in
36
that follows. tumor progression. Simultaneous inhibition of multiple genes
is an effective approach to treat cancer and to reduce the pos-
Gene Therapy Strategies for Cancer sibility of multidrug resistance. RNAi suffers from some of the
same issues as conventional gene delivery, in that its efficiency is
In the following sections, the broad approaches that can be applied dependent on an efficient delivery system. siRNA can be deliv-
to cancer treatments are outlined. In reality, experience tells us ered directly to tumors but systemic delivery is vulnerable to