Molecular Subtypes ofKIT/PDGFRAWild-Type Gastrointestinal Stromal Tumors

IMPORTANCE: Wild-type (WT) gastrointestinal stromal tumors (GISTs), which lack KIT and PDGFRA gene mutations, are the primary form of GIST in children and occasionally occur in adults. They respond poorly to standard targeted therapy. Better molecular and clinical characterization could improve management. OBJECTIVE: To evaluate the clinical and tumor genomic features of WT GIST. DESIGN, SETTING, AND PARTICIPANTS: Patients enrolled in an observational study at the National Institutes of Health starting in 2008 and were evaluated in a GIST clinic held once or twice yearly. Patients provided access to existing medical records and tumor specimens. Self-referred or physician-referred patients younger than 19 years with GIST or 19 years or older with known WT GIST (no mutations in KIT or PDGFRA) were recruited; 116 patients with WT GIST were enrolled, and 95 had adequate tumor specimen available. Tumors were characterized by immunohistochemical analysis (IHC) for succinate dehydrogenase (SDH) subunit B, sequencing of SDH genes, and determination of SDHC promoter methylation. Testing of germline SDH genes was offered to consenting patients and families. MAIN OUTCOMES AND MEASURES: For classification, tumors were characterized by SDHA, B, C, or D (SDHX) mutations and other genetic and epigenetic alterations, including presence of mutations in germline. Clinical characteristics were categorized. RESULTS: Wild-type GIST specimens from 95 patients (median age, 23 range, 7-78 years; 70% female) were classified into 3 molecular subtypes: SDH-competent (n = 11), defined by detection of SDHB by IHC; and 2 types of SDH-deficient GIST (n = 84). Of SDH-deficient tumors, 63 (67%) had SDH mutations, and in 31 of 38 (82%), the SDHX mutation was also present in germline. Twenty-one (22%) SDH-deficient tumors had methylation of the SDHC promoter leading to silencing of expression. Mutations in known cancer-associated pathways were identified in 9 of 11 SDH-competent tumors. Among patients with SDH-mutant tumors, 62% were female (39 of 63), median (range) age was 23 (7-58) years, and approximately 30% presented with metastases (liver 12 of 58, peritoneal 6 of 58, lymph node 15 of 23). SDHC-epimutant tumors mostly affected young females (20 of 21; median range age, 15 8-50 years), and approximately 40% presented with metastases (liver 7 of 19, peritoneal 1 of 19, lymph node 3 of 8). SDH-deficient tumors occurred only in the stomach and had an indolent course. CONCLUSIONS AND RELEVANCE: An observational study of WT GIST permitted the evaluation of a large number of patients with this rare disease. Three molecular subtypes with implications for prognosis and clinical management were identified.