KMT2A (MLL) fusions in aggressive sarcomas in young adults

To characterise unclassifiable sarcomas by use of a combined histological and molecular approach.

Using RNA sequencing, we identified in-frame fusions involving KMT2A (MLL) in two cases. Case 1 was a 20-year-old woman with a deep soft tissue mass in the thigh. The tumour consisted of monomorphic round, epithelioid and spindle cells in a highly sclerotic background that were focally immunopositive for CD34, CD31, and ERG. Case 2 was a 30-year-old woman with a tumour that affected the femur and surrounding soft tissue. The tumour consisted of monomorphic round to spindle cells that were immunopositive for BCOR, Wilms tumour 1, and NKX2-2. Both tumours were aggressive and had metastasised to the lung; both patients died within a few years. RNA sequencing identified a YAP1 (exon 5)-KMT2A (exon 4) fusion in case 1 and a VIM (exon 4)-KMT2A (exon 2) fusion in case 2, both of which were confirmed by reverse transcription polymerase chain reaction, Sanger sequencing, and fluorescence in-situ hybridisation. The fusion protein structure was different from that in acute leukaemia, suggesting a novel oncogenic mechanism.

KMT2A fusions account for a subset of aggressive unclassifiable sarcomas in young adults. Although it is presently unclear whether these sarcomas belong to a single group, the well-established role of KMT2A fusions as drivers of acute leukaemia and a recent publication regarding identification of YAP1-KMT2A in one unclassifiable sarcoma support the significance of these fusions. Further studies on additional cases are necessary to fully understand the clinicopathological and molecular aspects of KMT2A-rearranged sarcomas.