Page 290 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

CHAPTER 15 Molecular/Targeted Therapy of Cancer 269

be impossible, and thus this review is designed to present a brief drugs for the evaluation of methylation inhibition in veterinary
overview of some of the more promising and well developed, cancer patients.
Recent genome-wide studies have demonstrated significant
“druggable” targets that have been discovered recently, with an
VetBooks.ir emphasis on those for which in vitro or in vivo data in veterinary changes in patterns of specific gene methylation in canine lym-
phomas, some of which appear capable of providing prognostic
species are available.
information. 352,353 Multiple in vitro studies have demonstrated
DNA Methylation selective growth inhibition and/or induction of apoptosis in canine
and feline tumor cells treated with demethylating agents. 354–358
In addition to the information encoded within the genome Importantly, a phase I clinical trial of the demethylating agent
sequence, epigenetic changes are of great importance in the modi- 5-azacitidine has been performed in dogs with urothelial carci-
fication and maintenance of gene expression. These changes take nomas. Objective responses were reported, but there was no cor-
place through a number of mechanisms, including polymerase relation between gene methylation changes after treatment and
enzyme modulation, microRNAs, chromatin condensation, and clinical outcome. 359
DNA methylation. Mammalian DNA is methylated at cytosines Two potential problems exist regarding the wide clinical imple-
within CpG dinucleotide sequences. During tissue differentia- mentation of DNMT inhibitors for cancer treatment. As discussed
tion, methylation pattern is one governor of tissue-specific gene earlier, induction of long-term genome-wide hypomethylation
expression and thus phenotype. 325–327 could decrease chromosome stability leading to potentially tumor-
Two different methylation-related phenomena have been identi- igenic chromosome rearrangements. 327 Demethylation could also
fied in cancer. Tumor DNA in dogs and other mammals is glob- trigger the reactivation of genes promoting a more aggressive or
ally hypomethylated, 328,329 specifically in pericentromeric satellite metastatic phenotype. 327 In support of this theory, treatment of
sequences. This may lead to decreased genome stability and an nonmetastatic breast cancer cells with 5-azacytidine was shown to
increase in the incidence of oncogenic chromosome defects. Indeed, upregulate expression of urokinase-like plasminogen activator, an
the purposeful induction of genomic hypomethylation by reduction enzyme important in tumor invasion and metastasis, leading to
in germline DNA methyltransferase-1 (DNMT1) levels in genetically enhanced metastatic potential. 360
engineered mice is associated with a high incidence of T-cell lympho-
mas displaying trisomy 15. 330 Cancer cells also acquire sequence- Histone Deacetylase
specific promoter hypermethylation and transcriptional repression
in normally unmethylated regions, several of which have been shown Another critical determinant of gene expression is the condensa-
to be associated with known tumor suppressor genes, including Rb, tion of chromatin in the form of heterochromatin, which results
p16, p73, and the von Hippel–Lindau protein (VHL), 325,326,331–334 in transcriptional silencing. This is accomplished by a number
or other important tumor-associated genes, such as E-cadherin, of pathways, one of which is the acetylation and deacetylation
estrogen, retinoic acid receptors, and P- glycoprotein. 335,336 In addi- of histones, controlled by histone acetyltransferases and histone
tion, downregulation of expression of cytokines, tumor antigens, deacetylases (HDACs). The HDACs specifically maintain chro-
and/or antigen presentation machinery has been demonstrated to matin in a condensed form, and can associate with specific tran-
be regulated by promoter methylation in some cancers, which could scription factors resulting in transcription repression. Histone
contribute to tumor immune avoidance. 337–339 acetylation reduces electrostatic charge interactions between his-
The methylation of DNA is controlled by four known DNMTs, tones leading to chromatin decondensation. Histone acetylation
of which DNMT1 may be the most important in cancer. 325–327 A may be key in regulating the expression of genes associated with
variety of agents can inhibit DNMT function. The two best stud- cellular proliferation, differentiation and survival, both in devel-
ied are 5-azacytidine (Vidaza, Celgene) and 5-aza-deoxycitidine opment and carcinogenesis. 361,362 Induction of HDAC expres-
(decitabine, Dacogen, Otsuka), nucleoside analogs that incorpo- sion, leading to transcriptional repression, is a common feature in
rate into DNA and inhibit DNMT activity, but allow replication human cancers such as colon cancer, 363 and negatively regulates
to proceed. A large number of single-agent human clinical trials the expression of multiple tumor suppressor genes, including p53
with these agents have been reported, and significant activity has and VHL. 364 Certain HDAC isoforms are capable of acetyating
been demonstrated in hematopoietic neoplasia, leading to the US nonhistone proteins, such as DNMT1, tubulin, and p53, which
FDA approval of 5-azacytidine and decitabline for the treatment can alter protein stability, intracellular trafficking, and protein–
of myelodysplastic syndrome. 340,341 Encouraging response rates to protein or protein–DNA interactions. 365–367 Differential expres-
the nucleoside analog decitabine have also been seen in patients sion of certain HDAC isoforms has been associated with outcome
with imatinib-refractory chronic myelogenous leukemia. 342,343 in a variety of human tumors, with HDACs 2 and 6 studied most
Results in advanced solid tumors have been generally disap- completely. 368
pointing 344–346 ; however, studies in combination with standard Pharmacologic inhibition of HDAC can affect multiple
antineoplastic therapy and other targeted agents are ongoing. 347 facets of the malignant phenotype. HDAC inhibition inhib-
Interestingly, the commonly used cardiac medications procain- its colon carcinogenesis in the APC mouse model. 363 Angio-
amide and hydralazine also possess demethylating activity, 335, genesis can be inhibited through upregulation of VHL and
348 and clinical trials have demonstrated alterations in promoter subsequent inhibition of hypoxia-inducible factor-1alpha
methylation and reactivation of silenced genes after administra- (Hif-1α) function and vascular endothelial growth factor pro-
tion of well-tolerated doses of hydralazine to human cervical can- duction 364,369,370 ; decreased expression of other proangiogenic
cer patients. 349 Hydralazine–valproic acid (VPA) combinations factors, such as basic fibroblast growth factor, angiopoietin-2,
have demonstrated activity in myelodysplastic syndrome and and Tie-2 369,370 ; inhibition of endothelial nitric oxide synthase
cutaneous T-cell lymphoma in early human trials. 350, 351 Procain- and endothelial cell proliferation and tube formation 371,372 ;
amide and hydralazine have long track records of use in veterinary and inhibition of the commitment of endothelial progenitor
medicine, and as such could serve as inexpensive and available cells to the endothelial lineage. 373 Inhibition of HDACs can