Pitfalls of Using Polymerase Chain Reaction-based Assays for JAK2 and CALR Exon 9 Variant Testing in Myeloproliferative Neoplasms: Knowing when to Go the Extra Mile!

Objectives: The BCR::ABL1 negative myeloproliferative neoplasms are sequentially tested for JAK2 p.V617F, followed by CALR exon 9 pathogenic variants. Historically, these variants were thought to be mutually exclusive. However, recent reports indicate coexisting JAK2 p.V617F and CALR exon 9 somatic variants.

Methods: Analysis of JAK2 p.V617F and CALR exon 9 variant was performed by polymerase chain reaction (PCR)-based assays. Subsequent testing was performed on the Genexus integrated sequencer (ThermoFisher) using the Oncomine myeloid assay GX v2.

Results: CALR exon 9 variants were positive in 3 cases, while 2 were positive for JAK2 p.V617F on PCR-based assays. Next-generation sequencing confirmed the JAK2 P.V617F status in all cases. CALR variants resulting in in-frame deletions were identified in 2 cases at a variant allele frequency of 52.16% and 50.91%, while the third case had an intronic CALR variant c.-48G>A at a variant allele frequency of 51.1%. Thus, CALR variants in all 3 cases were interpreted as potentially germline. Of the 228 cases that underwent JAK2 p.V617F and CALR cotesting in the past 2 years, only these 2 cases were positive for both JAK2 p.V617F and CALR exon 9 variants.

Conclusions: These cases highlight the importance of understanding the pitfalls of molecular techniques in current practice.