Comprehensive genetic analysis identifies a pathognomonicNAB2/STAT6fusion gene, nonrandom secondary genomic imbalances, and a characteristic gene expression profile in solitary fibrous tumor

Solitary fibrous tumor (SFT) is a mesenchymal neoplasm displaying variable morphologic and clinical features. To identify pathogenetically important genetic rearrangements, 44 SFTs were analyzed using a variety of techniques. Chromosome banding and fluorescence in situ hybridization (FISH) showed recurrent breakpoints in 12q13, clustering near the NAB2 and STAT6 genes, and single nucleotide polymorphism array analysis disclosed frequent deletions affecting STAT6. Quantitative real-time PCR revealed high expression levels of the 5'-end of NAB2 and the 3'-end of STAT6, which at deep sequencing of enriched DNA corresponded to NAB2/STAT6 fusions. Subsequent reverse-transcriptase PCR (RT-PCR) analysis identified a NAB2/STAT6 fusion in 37/41 cases, confirming that this fusion gene underlies the pathogenesis of SFT. The hypothesis that the NAB2/STAT6 fusions will result in altered properties of the transcriptional co-repressor NAB2--a key regulator of the early growth response 1 (EGR1) transcription factor - was corroborated by global gene expression analysis; SFTs showed deregulated expression of EGR1 target genes, as well as of other, developmentally important genes. We also identified several nonrandom secondary changes, notably loss of material from 13q and 14q. As neither chromosome banding nor FISH analysis identify more than a minor fraction of the fusion-positive cases, and because multiple primer combinations are required to identify all possible fusion transcripts by RT-PCR, alternative diagnostic markers might instead be found among deregulated genes identified at global gene expression analysis. Indeed, using immunohistochemistry on tissue microarrays, the top up-regulated gene, GRIA2, was found to be differentially expressed also at the protein level.