Chromosome Banding Analysis, FISH and RT-PCR Performed in Parallel in Hypereosinophilic Syndrome Establishes the Diagnosis of Chronic Eosinophilic Leukemia in 22% of Cases: A Study on 40 Patients.
Authors: Schoch Claudia , Reiter Andreas , Bursch Sabina , Schnittger Susanne , Hiddemann Wolfgang , Kern Wolfgang , Haferlach Torsten 
Published: April 2020

Abstract
Hypereosinophilia is a common biological finding. When thorough evaluation of a patient with chronic hypereosinophilia fails to reveal an underlying disease the diagnosis of idiopathic hypereosinophilic syndrome (HES) is considered. In order to make a diagnosis of chronic eosinophilic leukemia (CEL) evidence for clonality or an increase in blasts in the blood or bone marrow is necessary according to the definition of the WHO classification. Clonality can be proven by cytogenetic or moleculargenetic aberrations. Recently, a fusion of the FIP1L1 gene to the PDGFRA gene was identified in patients with HES/CEL, which results from an approximately 800 kb interstitial chromosomal deletion on 4q12 including the cysteine-rich hydrophobic domain 2 (CHIC2) locus. FIP1L1-PDGFRA is a constitutively activated tyrosine kinase that transforms hematopoietic cells and is a therapeutic target for imatinib mesylate. A FISH assay using differentially labelled probes for CHIC2, FIP1L1, and PDGFRA can be used in a routine diagnostic setting to identify patients with FIP1L1-PDGFRA-rearrangement as well as other PDGFRA-rearrangements. Therefore, we analyzed 40 cases with hypereosinophilia which had been sent to our laboratory for diagnostic work-up with FISH and in 37 cases in addition with chromosome banding analysis. Patients characteristics were: male: n=27, female: n=13, median age: 60 years (range: 19–89), median WBC count: 14.7 G/l (range: 6–91), median percentage of eosinophils 50% (range: 20%–70%). Clonal chromosome aberrations were observed in 6 cases (15%) (+8 (n=2), t(8;9)(p21;p24), -Y, del(6)(q24), ins(9;4)(q34;q12q31)). Four cases (10%) showed a CHIC2 deletion in FISH. A FIP1L1-PDFGRA fusion transcript was confirmed by RT-PCR in all cases. All four patients were male and 33, 33, 60, and 63 years old. Three showed a normal karyotype while trisomy 8 was observed in one patient. In a fifth case of a 71-year old female patient a split of the usually colocalized signals of FIP1L1 and PDGFRA was observed. This patient showed an insertion of material of the long arm of chromosome 4 into the long arm of chromosome 9. One PDGFRA signal was localized on the derivative chromosome 9 while the FIP1L1 signal remained on chromosome 4, clearly suggestive of a yet unknown rearrangement of PDGFRA. Treatment with imatinib resulted in a complete cytogenetic and molecularcytogenetic remission in this patient. In conclusion: 1) FISH screening for the detection of a PDGFRA-rearrangement is feasible in a routine diagnostic work-up and allows the detection of FIP1L1-PDGFRA-rearrangement as well as of other PDGFRA-rearrangements 2) in 9/40 cases with HES clonality was proven by a combination of chromosome banding analysis and FISH/RT-PCR establishing the diagnosis of CEL 3) a complete diagnostic work-up of patients with HES using a combination of chromosome banding analysis, FISH and RT-PCR is recommended for classification and rational therapeutic decisions.