Clinical history, traditional microscopy, and immunohistochemistry are usually sufficient to establish the diagnosis of GIST. However, in tumors where the diagnosis remains uncertain, real-time polymerase chain reaction (RT-PCR) testing for KIT or PDGFRA gene mutations may be useful. A number of academic centers offer this type of testing, as do private reference laboratories.
In what is more likely to be the future role of molecular testing in GISTs, there has been an increasing trend, primarily at large cancer treatment centers, to employ routine testing for specific mutations to guide initial tumor management. This trend has largely been driven by phase II–III results of clinical trials that show that response to imatinib may be dependent on the specific KIT
mutation. Some PDGFRA
mutant GISTs show at least partial response to imatinib; however, the most common PDGFRA
mutation in GISTs (D842V) confers a complete resistance to the drug.54,68
mutant GISTs, a mutation in exon 11 was associated with a higher response rate (67%–83%) than a mutation in exon 9 (35%–48%). Conversely, primary resistance to imatinib was also associated with the specific KIT
mutation, in particular primary point mutations in exons 13 and 17. KIT
exon 11 mutant GISTs were the least likely (0%–5%) to show primary resistance. GISTs with neither KIT
mutations showed the least treatment response (0%–39%) and the highest primary resistance (23%) to imatinib. Despite these trends, sometimes GISTs with identical mutations in KIT
will respond differently to imatinib. Explanations for this are unclear, but may involve variable plasma levels of imatinib between patients and perhaps additional mutations in other genes. Most of the well-validated data for genotype-response correlations is based on imatinib, which is the first-line treatment for GIST; however, it is clear that other members of the ever increasing family of TKIs that target KIT, PDGFRA, and other receptors will have differing efficacies for various mutation types.34,44,69–71
Whereas initially the great majority of GISTs are often highly responsive to treatment with TKIs, acquired resistance is a vexing problem affecting the majority of patients. Mechanisms of resistance most commonly include secondary (acquired) mutations in the KIT
kinase domain and rarely KIT
genomic amplifications or activation of alternative oncogenes.3,68,72
mutations are most commonly single nucleotide substitutions affecting codons in the ATP binding pocket (exons 13 and 14) and the kinase activation loop (exons 17 and 18). These secondary mutations can be detected in up to 83% of patients.69,73
Recent in vitro and in vivo studies demonstrated that sunitinib, a TKI used after imatinib failure, is only effective against secondary mutations located in the ATP binding pocket but not against secondary mutations in the kinase activation loop.74,75
Liegl et al recently demonstrated the substantial inter- and intra-lesional heterogeneity in TKI-resistant mutations in patients treated with imatinib alone or imatinib and sunitinib. In 67% of patients, 2–5 different secondary mutations were detected in separate metastases, and in 34% of patients, two secondary KIT
mutations were even seen within a single metastasis.72
These findings emphasize that testing of secondary KIT
resistance mutations in a biopsy specimen will not aid in demonstrating the whole spectrum of resistance mutations. Thus, there are currently no clear recommendations and indications for resistance testing.
Currently, there are no established guidelines for routine KIT
mutational testing. Irrespective of their mutational status, most GISTs are treated with imatinib as first-line therapy; however, this may change in the future. The National Comprehensive Cancer Network (NCCN) and European Organisation for Research and Treatment of Cancer (EORTC) suggest obtaining mutational testing in GISTs that are unresectable or metastatic at presentation, are in young patients, have epithelioid morphology, and have primary resistance to imatinib. Recent prospective clinical trial data shows that patients with KIT
exon 9 mutations respond more poorly than those with exon 11 mutations to 400 mg of daily oral imatinib, but that this difference in response is ameliorated when exon 9 mutant patients receive 800 mg imatinib daily. This higher dose generally does not improve response for the standard, imatinib-sensitive exon 11 mutations.68
This type of data argues strongly for the relevance of genotyping; although, it is currently not known whether genotyping up front to allow immediate higher dosing offers advantages over dose escalation with initial resistance, as has been the current practice. As our understanding of the relationship between genotype and response to various TKIs increases, genotyping will likely become increasingly relevant for therapeutic selection.