Mature B‐cell acute lymphoblastic leukaemia associated with a rare MLL‐FOXO4 fusion gene

Abstract

A 6-week-old female infant presented with pallor, hepatosplenomegaly and a leucocyte count of 329 9 10/l (91% blasts). Light microscopy of the peripheral blood film showed blasts with rounded nuclei and scanty cytoplasm. No intracytoplasmic vacuoles were present (top left). Immunophenotyping by flow cytometry using peripheral blood (bottom panels) showed expression of CD10, surface membrane immunoglobulin (mu heavy chain and kappa light chain) with no expression of nuclear terminal deoxynucleotidyl transferase or CD34, indicating mature B-cell leukaemia. Cytogenetic studies showed an abnormal karyotype, 46,XX, t(X;11)(q13;q23), suggesting translocation between the MLL gene on chromosome 11 and the FOXO4 gene on chromosome X (top middle). Interphase fluorescence in situ hybridization using dual-colour break-apart probes showed the presence of an MLL gene rearrangement (top right). She was treated with a protocol designed for precursor B-cell acute lymphocytic leukaemia. However, she succumbed to sepsis in the fifth week of induction therapy.