A Subset of Malignant Mesotheliomas in Young Adults Are Associated With Recurrent EWSR1/FUS-ATF1 Fusions

Abstract

Malignant mesothelioma (MM) is a rare, aggressive tumor often associated with asbestos exposure and characterized by complex genetic abnormalities, including deletions of chromosome 22. A gene fusion involving EWSR1 and YY1 gene on 14q32 has been reported in 2 patients over the age of 60 with peritoneal MM. However, the incidence of EWSR1 rearrangements in MM and the spectrum of its fusion partners remain unknown. We recently encountered 2 MM cases with EWSR1-ATF1 fusions and sought to investigate the prevalence and clinicopathologic features associated with this abnormality. As both index cases occurred as intra-abdominal tumors in young adults, we searched our files for pleural and peritoneal MM occurring in adults younger than age of 40. All cases were tested by fluorescence in situ hybridization using custom bacterial artificial chromosomes probes for EWSR1, FUS, and ATF1 genes. When available, immunohistochemistry for BAP1 was performed. A total of 25 MM from patients aged 40 or less were screened, either from peritoneum (n=13) or pleura (n=12), with a median age of 31 (range: 7 to 40 y). Two additional ATF1-rearranged tumors were identified at pleural and peritoneal sites with EWSR1 and FUS as fusion partners, respectively, for a total of 4 cases (16%, 4/25). The fusion-positive cases displayed classic epithelioid morphology, immunoreactivity for cytokeratins and WT1, and negativity for S100. BAP1 expression was retained in the 3 fusion-positive cases with available material, and in 80% (12/15) of the fusion-negative cases. Our results expand the spectrum of tumor types harboring EWSR1/FUS-ATF1 gene fusions to include a subgroup of conventional epithelioid MM. Other features of this unique MM subset include young age at presentation, lack of asbestos exposure and retained BAP1 expression.

Figure 1. Histologic features of EWSR1-ATF1 fusion positive mesotheliomas

Medium power view of peritoneal index case (MM1) exhibiting focal psammoma bodies (A). The tumor displayed a conventional epithelioid morphology with focal papillary architecture, as well as abundant eosinophilic cytoplasm and open chromatin (B, C). MM2 showed a predominantly solid growth (D), with only focal papillary architecture (E). MM3 exhibited a predominant round cell phenotype with scant cytoplasm (F). All cases were immunohistochemically positive for AE1/AE3 (G, MM2), WT1 (H, MM2) and focal desmin expression was observed in a single case (I, MM2). Immunohistochemical expression of BAP1 was retained in the 3 fusion positive cases tested (J, MM2).

Figure 2. Fluorescence in situ hybridization showing EWSR1

(A, red, centromeric; green, telomeric) and ATF1 (B, red, centromeric; green, telomeric) break-apart signals (arrows) in MM3. C) FISH fusion assay in MM2 illustrates the come-together signals (arrows) between centromeric EWSR1 (red) and telomeric ATF1 (green), confirming the fusion result detected by next-generation sequencing assay (MSK-IMPACT). D) Large deletion or monosomy of 22q12 showing only one copy of EWSR1 (red, centromeric, green telomeric) in a fusion-negative pleural mesothelioma from a 37 years-old patient.

Figure 3

Representation of the EWSR1-ATF1 fusion in Integrated Genome Viewer for MM2, where each bar represents a single sequenced read. The sets of reads (on top) map to EWSR1 on chromosome 22, corresponding paired end reads map to ATF1 on chromosome 12, supporting a somatic fusion involving both genes. Upper panel: tumor; lower panel: normal blood control.