Age as a core disease modifier: Distinct clinical, molecular and prognostic landscapes of essential thrombocythaemia in adolescents and young adults.

Essential thrombocythaemia (ET) in adolescents and young adults (AYA, 15-39 years) is a distinct entity with an incompletely defined prognosis. In this multicentre retrospective study, 1728 ET patients from 29 centres across China were stratified into AYA (n = 328) and non-AYA (≥40 years, n = 1400) cohorts. We compared their clinical profiles, genomic landscapes, long-term outcomes and risk factors for progression to post-ET myelofibrosis (MF). AYA patients had fewer cardiovascular risks and lower thrombosis rates, but higher rates of extreme thrombocytosis. Molecularly, AYA patients were enriched for calreticulin (CALR) mutations, whereas Janus kinase 2 (JAK2) predominated in older patients. The burden of non-driver mutations (tet methylcytosine dioxygenase 2 [TET2], DNA methyltransferase 3A [DNMT3A], ASXL transcriptional regulator 1 [ASXL1], SH2‑B adaptor protein 3 [SH2B3]) was lower in AYA patients. Consequently, AYA patients achieved superior long-term outcomes across all key survival endpoints, including overall, myelofibrosis-free and leukaemia-free survival. Analysis of post-ET MF progression risks identified age-specific patterns: CALR mutations are enriched in younger patients and show an age-specific association with MF progression. AYA-ET constitutes a unique clinicomolecular subtype with a favourable prognosis, supporting age-stratified management. The enrichment of CALR mutations and their specific link to MF progression in young patients underscore the urgent need for targeted therapies against CALR-mutant clones.