Characterization of Three New Fusion Genes Generated by Disruption of PDGFRB in Eosinophilia-Associated Chronic Myeloproliferative Neoplasms
Authors: Erben Philipp , Gosenca Darko , Walz Christoph , Metzgeroth Georgia , Haferlach Claudia , Müller Martin C. , Hochhaus Andreas , Cross Nicholas C.P. , Reiter Andreas 
Published: October 2019

Abstract
Fusion genes involving imatinib-sensitive (PDGFRA, PDGFRB) and imatinib-resistant (FGFR1 and JAK2) tyrosine kinases (TK) have been identified in a substantial proportion of patients with eosinophilia-associated myeloproliferative neoplasms (Eos-MPNs). They result in constitutive activation of the corresponding TK moiety by dimerization domains of the partner gene or loss of the autoinhibitory WW-like domain within the juxtamembrane region. We here present three new fusion genes with involvement of PDGFRB. Two male patients (51 and 42 years old) presented with chromosomal aberrations involving chromosome bands 5q31-33; t(5;17)(q33-35;q11.2) and t(5;20)(q33;p12). In patient #1, LDI-PCR (Walz et al., Haematologica2007:92,163) identified an in-frame fusion between myosin XVIIIA (MYO18A) exon 40 and PDGFRB exon 10. Activation is likely to occur through dimerization as the autoinhibitory WW-like domain of PDGFRB is fully retained in the fusion protein. In patient #2, 5′-RACE-PCR of mRNA identified an in-frame fusion between D-tyrosyl-tRNA deacylase 1 (DTD1) exon 4 and a truncated PDGFRB exon 12. DTD1 potentially lacks known dimerization motifs suggesting that the disruption of the autoinhibitory WW-like domain region solely contributes to enhanced TK activity. Male patient #3 (42 years old) had a dry tap due to marked myelofibrosis and cytogenetic analysis could only be performed after centrifugation of bone marrow biopsy cells. Four metaphases were obtained which all showed a normal karyotype. In Eos-MPN with normal, low quality or missing karyotype, we routinely perform quantitative RT-PCR for 3′-sequences of PDGFRA and PDGFRB which are retained in all known fusion genes. Overexpression of mRNA was shown in all samples with variable PDGFRA (5 different fusion genes in 50 samples) or PDGFRB (5 different fusion genes in 8 samples) fusion genes as compared to samples from HES or reactive eosinophilia (ratio PDGFRA/ABL1 0.73 vs. 0.0066 vs. 0.0064, p<0.0001; ratio PDGFRB/ABL1: 196 vs. 5.8 vs. 5.85, p<0.0001). Patient #3 revealed overexpression of PDGFRB similar to controls with known PDGFRB fusion genes. 5′-RACE-PCR revealed an in-frame fusion between squamous cell carcinoma antigen recognized by T-cells 3 (SART3) exon 16 and PDGFRB exon 11. In this patient, dimerization motifs of the partner gene and disruption of the WW-like domain potentially contribute to enhanced TK activity. All three fusion genes were confirmed by RT-PCR. Reciprocal fusion genes were also amplified by RT-PCR in all three cases. Clinical follow-up is available from patients #1 and #3 which both achieved rapid and sustained complete hematologic remission during treatment with imatinib. Patient #1 also achieved complete cytogenetic remission while RT-PCR remains positive in both patients after 19 and 8 months on imatinib. We conclude that Eos-MPNs need a careful and systematic diagnostic work-up with inclusion of quantitative RT-PCR for mRNA overexpression of TK genes in patients without informative cytogenetic analysis. This might lead to the identification of further potential candidates eligible for treatment with TK inhibitors.