Page 167 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

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Molecular Diagnostics

ANNE C. AVERY, KARA MAGEE, MELISSA C. PAOLONI, AND CHAND KHANNA

Goals of Molecular Diagnostic Testing in that have been treated. The best example of this is detection of
Oncology the bcr-abl fusion gene, which can allow oncologists to detect
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as few as 1:10 neoplastic cells in the peripheral blood of people
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Molecular diagnostic testing in oncology is performed to achieve with chronic myelogenous leukemia. Tumor-specific primers
one of several goals: (1) to determine whether a patient has can- that recognize the unique immunoglobulin genes found in both
cer, typically in circumstances in which visual examination of tis- canine and human B-cell lymphomas have been used to quantify
sue by cytology or histology cannot distinguish a reactive from a tumor burden and monitor disease in both dogs and people with
neoplastic process; (2) to establish a prognosis; and (3) to guide lymphoma. 1,4,5
treatment. Some of these goals currently are realized in veterinary The previous examples describe testing for single genetic altera-
oncology; the polymerase chain reaction for antigen receptor rear- tions that are known to be shared by most tumors of the same
rangement (PARR) assay is used to confirm hematopoietic malig- type. Increasingly, the field of oncology is moving toward person-
nancy in both canine and feline patients, and the presence of c-kit alized, or precision, medicine (PMED). The goal of PMED is to
gene mutations can inform the prognosis in canine mast cell dis- identify genetic mutations and activated signaling pathways that
ease. Increasingly, global analysis of oncogenes and gene expres- are found in an individual’s cancer, even when such changes have
sion is being used in human oncology, and advances in technology not yet been described in a particular cancer type. (PMED is dis-
and ongoing veterinary research will make such testing available, cussed at the end of this chapter.)
affordable, and informative in veterinary medicine within the next For the purposes of this chapter, molecular diagnostics refers to
few years. the analysis of genes and gene expression. The goal of this chapter
The presence of a particular mutation or chromosomal abnor- is to review several molecular techniques useful in the diagnosis
mality can help subclassify a tumor. For example, in people, leuke- and classification of cancer. Advanced molecular methodologies
mia/small cell lymphoma (CLL/SCL) and mantle cell lymphoma and diagnostics likely will continue to improve, become increas-
(MCL) are both neoplasms of mature B cells, with a similar (but ingly inexpensive, simpler to use, and more broadly available to
not identical) immunophenotype. However, MCL almost always veterinarians over the next few years.
has a rearrangement between the immunoglobulin heavy chain
locus (IgH) and the CCND1 gene (encoding cyclinD1), whereas Genomic Dysregulation in Cancer
this rearrangement is very rare in CLL/SCL. The prognosis for
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and treatment of these two diseases are quite different, so the dis- Most molecular diagnostics target genomic dysregulation that
tinction is important to make. may exist in cancer cells. Such dysregulation may occur at the level
Molecular diagnostic testing also can help guide therapy. This of the copy number of a gene; a point mutation in that gene that
may be best illustrated by the development of tyrosine kinase changes its function; epigenetic modification of deoxyribonucleic
inhibitors (TKIs). These drugs inhibit signaling through tyro- acid (DNA) that changes the level of expression; or large-scale
sine kinase receptors, such as KIT, platelet-derived growth fac- changes to chromosomes that remove genes from their normal
tor receptor (PDGFR), and epidermal growth factor receptor regulatory environment. Different methods are required for iden-
(EGFR). Tumors with mutations in these receptors that result in tifying each of these types of changes.
their constitutive activation may respond well to TKIs, whereas
those without such mutations may require different kinds of Methods for Analyzing Genes
therapy. Thus testing for mutations in these genes has become
commonplace in human medicine (e.g., EGFR in small cell lung DNA represents the genetic code of all species. This code consists
carcinoma, stem cell factor [SCF] receptor [c-kit] in gastrointesti- of a series of continuous nucleic acid sugar strands linked through
nal stromal cell tumors). Similarly, mast cell tumors in dogs that hydrogen bonds. This series of nucleic acids takes on a tertiary
harbor a c-kit mutation may respond better to TKIs than those folded structure through modification by binding proteins called
without the mutation. 2 histones. The folded and wrapped DNA strand is packaged within
Oncogenes and chromosomal translocations uniquely distin- the chromosomes of the cell. The earliest techniques used to assess
guish neoplastic from normal tissue. As such, sensitive detection the genetic changes of cancer defined gains, losses, or structural
of mutations can be used to quantify residual disease in patients changes in chromosomes, referred to as cytogenetics. Subsequently,

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