Distinct clinical, laboratory, molecular, and pathologic features of systemic mastocytosis involving the gastrointestinal tract.
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OBJECTIVE: Gastrointestinal (GI) symptoms are common in systemic mastocytosis (SM). We hereby report the clinicopathologic and molecular features of GI mastocytosis. METHODS: The study includes 104 patients with suspected SM. Of 258 GI biopsy -specimens, 33 (20 patients, 19%) were mastocytosis-positive, with 19 (16 patients) initially missed. RESULTS: Tryptase (n = 29) was weakly positive/negative in two-thirds of the mastocytosis-positive GI biopsy specimens. Additional immunohistochemistry (n = 23) showed positive mast cell expression for CD30 (n = 2), CD123 (n = 7), and PD-L1 (n = 12), as well as increased PD-1-positive cells (n = 4). Sixty patients had positive bone marrow (BM) for SM (SM-BM+), including 19 with positive GI biopsy specimens (SM-GI+). Almost all (43/44) SM-BM- patients were SM-GI-. The SM-GI vs SM-BM status showed high positive predictive value (95%) and specificity (98%). Patients with SM with mastocytosis-positive vs mastocytosis-negative GI biopsy specimens had higher serum tryptase levels (P = .04). KIT D816V mutation was detected in 7 of 13 and 12 of 13 mastocytosis-positive GI biopsy specimens using allele-specific polymerase chain reaction and droplet digital polymerase chain reaction (ddPCR), respectively. CONCLUSIONS: GI mastocytosis is highly predictive of SM in BM; however, most SM-BM+ patients are SM-GI-. The SM-BM- patients are very unlikely to be SM-GI+. Because GI neoplastic mast cells are often tryptase negative, diagnostic evaluation should include CD117 and CD25 immunohistochemistry, as well as KIT D816V mutation analysis by ddPCR as needed.