A Rare Exon 15 Germline Mutation Identified in a Patient With Phenotypic Manifestations Concerning for GIST-Plus Syndrome: A Case Report and Review of Literature.

Molecular alterations in are well-described as drivers of sporadic gastrointestinal stromal tumors (GISTs) and inflammatory fibroid polyps (IFPs). However, a small number of families with germline mutations in exons 12, 14, and 18 have been reported, forming the basis of an autosomal dominant inherited disorder with incomplete penetrance and variable expressivity, now referred to as -mutant syndrome or GIST-plus syndrome. Phenotypic manifestations of this rare syndrome include multiple gastrointestinal GISTS, IFPs, fibrous tumors, and other variable features. Herein, we report the case of a 58-year-old female who presented with a gastric GIST and numerous small intestinal IFPs, found to harbor a previously undescribed germline exon 15 p.G680R mutation. Somatic tumor testing was performed on the GIST, a duodenal IFP, and an ileal IFP utilizing a targeted next-generation sequencing panel, revealing additional and distinct secondary exon 12 somatic mutations in each of the 3 tumors. Our findings raise important considerations regarding mechanisms of tumor development in patients with underlying germline alterations and highlight the potential utility of expanding currently available germline and somatic testing panels to include exons outside the typical hotspot regions.