LAUSR

Lipofibromatosis is a rare pediatric soft tissue tumor with predilection for the hands and feet. Previously considered to represent “infantile fibromatosis”, lipofibromatosis has distinctive morphological features, with mature adipose tissue, short fascicles of bland fibroblastic cells, and lipoblast-like cells. Very little is known about the genetic underpinnings of lipofibromatosis. Prompted by our finding of the FN1-EGF gene fusion, previously shown to be a characteristic feature of calcifying aponeurotic fibroma (CAF), in a morphologically typical case of lipofibromatosis that recurred showing features of CAF, we studied a cohort of 20 cases of lipofibromatosis for this and other genetic events. The cohort was composed of 14 males and 6 females (median age 3 years; range 1 month–14 years). All primary tumors showed classical lipofibromatosis morphology. Follow-up disclosed three local recurrences, two of which contained calcifying aponeurotic fibroma-like nodular calcifications in addition to areas of classic lipofibromatosis, and no metastases. By FISH and RNA sequencing, four cases were positive for FN1-EGF and one case each showed an EGR1-GRIA1, TPR-ROS1, SPARC-PDGFRB, FN1-TGFA, EGFR-BRAF, VCL-RET, or HBEGF-RBM27 fusion. FN1-EGF was the only recurrent fusion, suggesting that some cases of “lipofibromatosis” may represent calcifying aponeurotic fibroma lacking hallmark calcifications. Several of the genes involved in fusions (BRAF, EGFR, PDGFRB, RET, and ROS1) encode receptor tyrosine kinases (RTK), or ligands to the RTK EGFR (EGF, HBEGF, TGFA), suggesting a shared deregulation of the PI3K–AKT–mTOR pathway in a large subset of lipofibromatosis cases.