Immunohistochemical identification of SDHA-mutant gastrointestinal stromal tumors (GISTs).

Abstract

10029 Background: GISTs are most commonly driven by activating mutations in KIT or PDGFRA. However, 15% of GISTs in adults and >90% in children lack such mutations. A subset of KIT/PDGFRA wild-type GISTs shows distinctive morphologic and clinical features and loss of expression of succinate dehydrogenase (SDH) B. Only a fraction of SDHB-deficient GISTs carry loss-of-function mutations in SDHB or SDHC. Due to the complexity of its locus and the presence of several pseudogenes, SDHA is rarely analyzed. Recently, mutations in SDHA were shown to lead to loss of expression of SDHA and SDHB in paraganglionomas. We sought to determine whether SDHA IHC could identify GISTs with SDHA mutations. Methods: Tumors (n=11) with features of SDH-deficient GIST (gastric origin, epithelioid morphology, multinodular/plexiform architecture) were selected from pathology archives under an IRB-approved protocol. Expression of SDHA and SDHB was determined on tumor sections by IHC. Genomic DNA was isolated from SDHA-negative tumors and amplified using primers specific to introns flanking each of the 15 SDHA exons. Amplicons were bidirectionally sequenced and compared to genomic repository data. Exons with somatic mutations were also examined in DNA from corresponding normal tissue to determine germline status. Results: All tumors (100%) were deficient for SDHB expression by IHC. Four of 11 (36%) SDHB-deficient GISTs also lacked expression of SDHA. SDHA expression was intact in the 7 remaining tumors, including 3 with known SDHB (n=2) or SDHC (n=1) mutations. Nonsense mutations in SDHA were identified in all 4 SDHA-deficient tumors, caused by a single base pair (bp) substitution (n=3) or a single bp deletion (n=1), and heterozygous mutations were also found in DNA from normal tissue of all 4 patients. Somatic loss of the 2nd allele has thus far been found in 3 of 4 tumors; 2 by loss of heterozygosity and 1 by a 13-bp deletion. Further analysis of the 4th specimen is ongoing. Conclusions: SDHA mutations are a common cause of SDH-deficient GISTs and result in combined loss of expression of both SDHA and SDHB. Loss of SDHA expression by IHC reliably predicts SDHA mutations and can be used to select cases for SDHA genetic testing.