KMT2A-MLLT3 AML Masquerading as JMML may Disguise Fatal Leukemia

Pitfalls in Diagnosis: JMML versus KMT2A Rearranged Juvenile AML

Background

Lysine methyltransferase 2A (KMT2A) rearrangements are commonly found in juvenile acute myeloid leukaemia (AML). Although distinct diseases, there is a known clinical overlap between KMT2A-rearranged AML and juvenile myelomonocytic leukaemia (JMML). Both occur in infancy or early childhood and present with abnormal monocytosis. Case Report. We report a case of a 20-month-old girl, who presented with lethargy, recurrent infections, bruising, and marked hepatosplenomegaly. JMML was suspected after initial work-up, revealing an abnormal monocytosis without blast excess on immunophenotyping. The additional cytogenetic and molecular diagnostics, revealing a KMT2A rearrangement, was decisive for the confirmation of AML.

Conclusion

This case highlights the challenges of diagnosing KMT2A-rearranged monocytic AML and the importance of careful morphological assessment in partnership with cytogenetic and molecular diagnostics to distinguish between KMT2A-rearranged AML and JMML. Moreover, the emerging role of molecular monitoring in AML is highlighted.

Characteristics of RAS pathway mutations in juvenile myelomonocytic leukaemia: a single-institution study from Korea

Juvenile myelomonocytic leukaemia (JMML), a rare clonal haematopoietic disorder of childhood, is characterised as a myelodysplastic/myeloproliferative neoplasm. Despite ground-breaking genetic discoveries, JMML remains difficult to diagnose given its diverse clinical features and disease course. A total of 24 patients with JMML were diagnosed and treated at a single institution, and their genetic profiles and association with clinical and laboratory characteristics were analysed. In all, 22 of the patients received allogeneic haematopoietic stem cell transplantation after myeloablative conditioning, mostly from a haploidentical family donor. RAS pathway mutations were identified in 88% of patients: PTPN11 [nine (38%)], NRAS [nine (38%)], KRAS [two (8%)], NF1 [five (21%)] and CBL [one (4%)]. Secondary mutations were found in 25% of patients: SETBP1, JAK3, ASXL1, GATA2, KIT, KDM6A, and BCOR. Six patients showed cytogenetic abnormalities, including three with monosomy 7. The estimated 5-year event-free survival (EFS) and overall survival (± standard error) of the entire cohort were 58·9 (10·9)% and 73·5 (10·8)% respectively. NRAS (+) patients had a higher 5-year EFS than NRAS (-) patients [72·9 (16·5)% vs. 52·5 (13·1)%, P = 0·127]. NRAS (+) patients had a better 5-year EFS than PTPN11 (+) patients [41·7 (17·3)%, P = 0·071]. Our study revealed the genetic characteristics of Korean JMML patients with RAS pathway and secondary mutations.