Platelet count predicts driver mutations’ co-occurrence in low JAK2 mutated essential thrombocythemia and myelofibrosis

Essential thrombocythemia (ET) and primary myelofibrosis (PMF) are Philadelphia-negative myeloproliferative neoplasms (MPNs), commonly associated with mutations in JAK2, CALR, or MPL genes. Traditionally, these mutations were considered mutually exclusive, with JAK2 testing prioritized in diagnostic workflows. However, recent studies suggest that double-mutated (DM) cases—co-occurring JAK2 and CALR/MPL mutations—exist in 5–15% of MPNs, especially in cases with low (<5%) JAK2 allele burden (AB). This study analyzed 388 JAK2-mutated ET and PMF cases, alongside a validation cohort of 91 low JAK2 AB cases. Among low JAK2 AB cases, 30–36% were DM, with CALR being the most frequent secondary mutation. DM cases exhibited higher platelet counts and a significantly increased risk of post-diagnosis bleeding, particularly when receiving antiplatelet therapy. No significant differences were found between DM and single-mutated (SM) cases in terms of thrombosis, disease progression, or mortality. NGS analysis identified additional mutations in half of low JAK2 AB cases, most commonly in TET2, DNMT3A, ASXL1, and RUNX1, genes associated with epigenetic regulation. A platelet count ≥700 × 10⁹/L was a strong predictor of DM status, validated in an external cohort. These findings suggest that screening for DM cases is clinically relevant, particularly to guide antiplatelet therapy and improve individualized treatment decisions.