Novel modes of MPL activation in triple-negative myeloproliferative neoplasms

The identification of a somatic mutation associated with myeloid malignancy is of diagnostic importance in myelo-proliferative neoplasms (MPNs). Individuals with no mu-tation detected in common screening tests for variants JAK2, CALR, and MPL are described as 'triple-negative' and pose a diagnostic challenge if there is no other evi-dence of a clonal disorder. To identify potential drivers that might explain the clinical phenotype, we used an extended sequencing panel to characterise a cohort of 44 previously diagnosed triple-negative MPN patients for canonical mu-tations in JAK2, MPL and CALR at low variant allele fre-quency (found in 4/44 patients), less common variants the JAK-STAT signalling pathway (12 patients), or other variants in recurrently mutated genes from myeloid ma-lignancies (18 patients), including hotspot variants of tential clinical relevance in eight patients. In one patient with thrombocytosis we identified biallelic germline MPL variants. Neither MPL variant was activating in cell prolif-eration assays, and one of the variants was not expressed on the cell surface, yet co-expression of both variants led to thrombopoietin hypersensitivity. Our results highlight the clinical value of extended sequencing including germline variant analysis and illustrate the need for detailed func-tional assays to determine whether rare variants in JAK2 MPL are pathogenic.