Gastrointestinal Stromal Tumors (GISTs), c-KIT Mutation Analysis

Formalin-fixed, paraffin-embedded (FFPE) tissue or five unstained slides from a paraffin block in 10-μM sections and a matching H&E reference slide

Formalin-fixed, paraffin-embedded (FFPE) block or five unstained slides from paraffin block in 10-μM sections and a matching H&E reference slide

2 mm x 2 mm tumor area with ≥50% tumor

Slides or blocks

c-KIT is a proto-oncogene that encodes a type III transmembrane tyrosine kinase. c-KIT and its ligand stem cell factor have a key role in survival, proliferation, differentiation, and functional activation of cells.

Genomic DNA is purified from the specimen provided. Exons 9, 11, 13, and 17 of c-KIT gene coding are subjected to PCR amplification and bidirectional sequencing in duplicate to identify sequence variations. This assay has a sensitivity to detect approximately 10% of cells containing the c-KIT mutations in a background of nonmutant cells. This assay will not detect the mutation below the sensitivity of this assay.

This test was developed, and its performance characteristics determined, by LabCorp. It has not been cleared or approved by the US Food and Drug Administration (FDA). The FDA has determined that such clearance or approval is not necessary.

Polymerase chain reaction (PCR) and DNA sequencing

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumor of the gastrointestinal tract, located mostly in the stomach (60%) and small intestine (35%). Approximately 80% of GISTs have a mutation in c-KIT and 5% to 10% of GISTs have a mutation in PDGFRA. PDGFRA mutations are mutually exclusive with c-KIT mutations but activate similar signal transduction pathways that support GIST oncogenesis. The location of c-KIT and PDGFRA mutations in GISTs is associated with the site of origin, histological phenotype, and treatment response to tyrosine kinase inhibitors (TKI, such as imatinib and sunitinib). Patients with mutations in c-KIT exon 11 have been shown to have significantly better response rates to imatinib treatment when compared with patients who have the c-KIT exon 9 mutations or no mutation. Patients with mutations in c-KIT exon 9 may benefit from dose escalation depending on tolerance. Secondary mutations usually occur in c-KIT kinase domains in patients after imatinib treatment resulting in resistance to this drug. Most known mutations in the PDGFRA gene are associated with imatinib response with the exception of D842V mutation. In a subset of intestinal high-risk GISTs lacking c-KIT/PDGFRA mutations, 7% have a mutation in BRAF. Kinase inhibitors targeting BRAF may be effective therapeutic options in this molecular GIST subset.