The Molecular Story: Not all GISTs are created equal.
Are there possible therapeutic implications associated with molecular heterogenicity?
Louisiana State University Health Sciences Center
As a result of our growing knowledge of human genes and their role and function in pathology, we have
moved toward molecular characterization of human disease. Molecular pathology fills that void and
facilitates disease diagnosis. Molecular assays, once ancillary and obscure laboratory mainstays, have
now become essential to the pathologist and clinician in diagnosis of disease. Molecular methods are now
forecasted to increase to improve patient care by decreasing the turnaround times for confirming
diagnoses based on clinical observations. In many cases, molecular techniques allow identification of
genetic aberrations that would not be detected by other methods. Predictably, the range of molecular
pathology will extend beyond nuclei acid-based detect systems and will evolve to use all information
derived from the genome as well as provide information on optimizing molecularly targeted treatment.
Gastrointestinal (GI) stromal tumors, albeit historically subjected to numerous classification
schemes, are no exception to this new molecular-profiling trend. It now appears that molecular
characteristics may critically impact the ultimate biologic behavior of the tumor(s) and these molecular
endpoints may serve to add precision to an otherwise imprecise classification system. Until recently,
outside of the pathology literature, GIST remained a relatively obscure clinical entity. Progress in
molecular medicine with the development and clinical implementation of targeted molecular therapies
provided the impetus for renewed attention to this otherwise, rare, soft tissue tumor. The overwhelming
success of the tyrosine kinase inhibitor imatinib mesylate (Gleevec; Novartis) in patients with CML
parallels initial clinical trial successes in patients with metastatic GISTs.
Initially, GIST emerged as a prototypical solid tumor model destined for this new genre of molecular
medicine. Histopathologic definition of these tumors was unreliable based on conventional techniques.
Clinically, surgery was the only option for resectable tumors and even these patients did poorly.
Conventional adjuvant therapies were unsuccessful. GISTs represent an incurable malignancy for patients
with metastatic or unresectable disease. Histologic, molecular genetic, and immunohistochemistry
advances heralded a new era for GISTs and GIST patients. The common denominator in most GISTs is the KIT
protein. KIT, a type III tyrosine kinase growth factor receptor, has similarities to receptors of
macrophage colony stimulating factor and platelet derived growth factor. CD117, the epitope for KIT,
was introduced as a new, reliable phenotypic marker useful for distinguishing between GISTs versus
non-GIST spindle cell tumors of the gastrointestinal tract. CD117, the protein product of the
proto-oncogene c-kit, represents upregulated tyrosine kinase activity, a
central pathogenetic event in most GISTs. The pharmaceutical development and therapeutic implications
of protein tyrosine kinase inhibitors has refocused our attentions on GIST, a disease where uncontrolled
and constitutive activation of KIT signaling leads to uncontrolled cell proliferation and resistance to
It is now known that most overexpression of KIT/CD117 is in direct response to mutations in the c-kit gene, mapped to chromosomal region 4q11-21. Most GISTs, both benign and
malignant, carry mutations in c-kit. Supplemental information has shown
that these mutations vary among these tumors and no definitive genotype/phenotype correlations have been
established. The molecular story of GISTs is complex, however, and the spectrum of mutations and their
functional consequences are yet to be refined. Furthermore, KIT activation is still thought to occur
regardless of the presence of a kit mutation. Activating or
gain-of-function mutations in the juxtamembrane domain (exon 11) of the c-kit gene appear to cause GIST, as compared with mutations in the tyrosine kinase
domain (exon 17) that are more often associated with aggressive mastocytosis. The c-kit exon 11 mutations occur in 21 to 57% of GIST and portend a more aggressive
tumor behavior. The fact that GIST with c-kit mutations have a higher
mitotic count and more frequently have necrosis and/or hemorrhage supports the correlation of c-kit mutations with a poorer clinical outcome. C-kit mutations in exon 11, 9 and 13 were recently associated exclusively with the
spindle cell phenotype. Mutations in exon 9 may also define tumors by site in addition to prognosis.
These mutational hotspots, including exon 11 and to a lesser degree exons 9 and 13, create unique subsets
of GISTs that 1) delineate malignant forms from benign, 2) may not necessarily show a correlation with
CD117 expression, and 3) segregate poor prognoses patient subsets.
Other pathways cannot be excluded when molecularly characterizing GISTs. Mutations in kit enable the receptor to phosphorylate various substrate proteins, resulting in
activation of a signal transduction cascade, which regulates cell proliferation, apoptosis, chemotaxis
and adhesion. Aberrations of these processes may be manifested via other mechanisms, aside from
classical mutations and/or protein expression. Conventional cytogenetic analysis has revealed that GISTs
demonstrate karyotypes far less complex than those in other spindle cell tumors of comparable histologic
grade. Other mechanisms of investigating genetic changes in GISTs have focused on genomic profiling by
comparative genomic hybridization (CGH) and fluorescence in situ hybridization (FISH) studies. GISTS
have been shown to have relatively noncomplex cytogenetic profiles that show del(14q) and 22q as common
mechanisms of cytogenetic aberration. CGH showed that DNA sequence copy number changes are more
prominent in metastatic disease > malignant >benign tumors (El_Rifai, 2000). FISH and CGH have
been used to correlate loss of 14q and 22q in tumors with c-kit mutations. A cytogenetic continuum has
been suggested (Heinrich 2002) whereby benign GISTs more frequently show a normal karyotype with
occasional partial loss of chromosome 14 (14q32). Intermediate or borderline malignant lesions have a
consistent loss of chromosome 14, but show additional acquired abnormalities such as loss of 1p, 9p, 11
p, or 22q. High grade (malignant) GISTs predictably have 3 or more of the previously mentioned
aberrations. Additionally, other abnormalities, 8q or 17q gains are shown to be associated almost
exclusively with the malignant GISTs. Thus, while there is overlap in chromosomal/mutational changes,
there appears to be a spectrum of genetic events with a fairly defined repertoire of accumulated genetic
changes and tumor progression.
Will molecular profiling segregate GISTs into groups and identify which are most likely to respond to
molecular targeting? Should we be routinely testing clinically for mutations in c-kit exons that herald
a poor prognosis in these patients? Laboratory verification of mutations in c-kit may be clinically
useful as an adjunct in confirming and stratifying patients with GISTs (versus GANTs and other benign or
malignant neoplasms). The mutation, particularly the in-frame deletion in exon 11, is unique to GISTS,
both in somatic tumor cells and in leukocytes of patients with a family history of GISTs. Monitoring
c-kit mutations may be helpful in assessing residual tumor burden and monitoring patients for recurrent
disease. Should we be investigating cytogenetic abnormalities in these patients? Likely so, but
clinical outcome and other answers are only yet beginning to unravel. Inconsistencies in diagnosis,
treatment, treatment response, mutational status, chromosomal aberrations, and mutations in genes yet
undefined all contribute to our tenuous grasp on the role of molecular genetics as a predictor of
biologic behavior in GISTs.
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