Succinate dehydrogenase-deficient tumors: diagnostic advances and clinical implications

Plexiform fibromyxoma (plexiform angiomyxoid myofibroblastic tumor) of stomach: an unusual presentation as a fistulating abscess

Plexiform fibromyxoma (plexiform angiomyxoid myofibroblastic tumor) is a rare benign mesenchymal tumor of stomach. The plexiform growth of bland-looking spindly cells in a richly vascularized fibromyxoid stroma is distinctive. The described cases are solid tumors associated with ulceration, with the patients presenting with symptoms related to the ulcer or mass effect of the tumor. We report an unusual case presenting as a fistulating abscess. A 42-year-old woman presented with abdominal pain, fever, and elevated white cell count. Computed tomography scan revealed a 12-cm cavitating mass in the gastric antrum, with fistulation to the gastric lumen through an ulcer. Histologic examination showed transmural involvement of the stomach by plexiform islands of fibromyxoid tumor with interspersed delicate capillaries. There was a pseudocyst-like component. The unusual presentation therefore broadens the clinical and pathologic spectrum of this rare tumor type.

Loss of expression of SDHA predicts SDHA mutations in gastrointestinal stromal tumors

Gastrointestinal stromal tumors (GISTs) are usually driven by mutations in KIT or PDGFRA, although 15% of GISTs in adults and >90% in children lack such mutations. The majority of gastric KIT/PDGFRA wild-type GISTs show distinctive morphological and clinical features and loss of expression of succinate dehydrogenase (SDH) B. Only a small subset of SDHB-deficient GISTs carries loss-of-function mutations in SDHB, SDHC, or SDHD. Because of the complexity of its locus (15 exons) and the presence of three pseudogenes, SDHA is rarely analyzed. Recently, mutations in SDHA were shown to lead to loss of expression of SDHA in a small group of paragangliomas. We sought to determine whether immunohistochemistry for SDHA could identify GISTs with SDHA mutations. Tumors (n=33) with pathological features of SDH-deficient GIST were analyzed for expression of SDHA and SDHB by immunohistochemistry, and SDHA exons were sequenced from tumors lacking SDHA expression. Exons harboring somatic mutations were examined in DNA from corresponding normal tissue. All 33 tumors showed loss of SDHB expression. A total of 9 out of 33 (27%) tumors also lacked expression of SDHA. SDHA-deficient GISTs affected five men and four women (median age 38 years). SDHA expression was intact in the 24 remaining tumors, including those with known SDHB (n=3) or SDHC (n=2) mutations. Nonsense (n=8) or missense (n=1) mutations in SDHA were identified in all SDHA-deficient tumors. Heterozygous mutations were also found in DNA from normal tissues from six patients with available material. Somatic loss of the second allele has been found in seven tumors, five by loss of heterozygosity, one by a 13-bp deletion, and one by a missense mutation. Loss of SDHA expression in GIST reliably predicts the presence of SDHA mutations, which represent a relatively common cause of SDH-deficient GIST in adults. Immunohistochemistry for SDHA can be used to select patients for SDHA-specific genetic testing.