Acute Bilineal Leukemia Is a Very Rare Entity in Childhood
Authors: Mejstrikova Ester , Slamova Lucie , Fronkova Eva , Volejnikova Jana , Muzikova Katerina , Domansky Jiri , Sterba Jaroslav , Zapletal Ondrej , Zemanova Zuzana , Lizcova Libuse , Vodickova Elena , Zuna Jan , Trka Jan , Stary Jan , Hrusak Ondrej 
Published: October 2019

Abstract
Abstract 4871
Recent WHO 2008 classification introduced a new category named Mixed Phenotype Acute Leukemia (MPAL) for leukemias in which primary lineage cannot be determined by morphology, cytochemistry and/or flow cytometry (FC). Acute bilineal leukemia (ABL) is a subtype of MPAL and is defined by presence of distinct myeloid and lymphoid clonal populations simultaneously at diagnosis. No epidemiological data on ABL have been published so far and there are also few data on the origin of distinct leukemic clones. We examined the incidence and biology of ABL cases among children with primary acute leukemia in the period between 1996 and 2011 in the Czech Republic. Morphology and FC were centrally evaluated in all patients. In total 1065 patients were diagnosed (919 ALL, 146 AML); out of them 3 patients were classified as ABL. Two cases had simultaneous presence of distinct B-cell precursor (BCP) and myeloid clones (BCP-My) at diagnosis, one patient had discrete T ALL and myeloid population (T-My). All ABL patients were screened for immunoglobulin (Ig) and T-cell receptor (TCR) clonality, BCR-ABL, MLL gene rearrangements and FLT3-ITD. All three patients had detectable clonality in lymphoid-specific Ig/TCR rearrangements. In pt1 (BCP-My), FLT3-ITD abnormality and in pt3 (BCP-My), BCR-ABL fusion gene were found. Both patients with BCP-My ABL had Ikaros (IKZF1) gene deletion. In pt2 complex karyotype with MLL gene translocation was identified. Using high speed cell sorting we evaluated the presence or absence of previously mentioned changes in separated subpopulations. In patients with BCP-My ABL, identical clonal Ig rearrangements were found in both lymphoid and myeloid clones. Also FLT3-ITD and BCR-ABL aberrations were present in both clones of respective patients. In pt2 (T-My) TCR gene rearrangement was absent in myeloid population. All three patients achieved complete remission (CR) by lymphoid-directed induction treatment followed by switch to myeloid-oriented blocks according Interfant 99, resp. 2006 protocol in pt 1 and 2 and ALL treatment combined with tyrosine kinase inhibitor (TKI) in pt3. Finally all patients underwent allogeneic hematopoietic stem cell transplantation (SCT) in first CR. Pt1 relapsed after SCT as “typical” cALL. However, the plasticity was maintained and the myeloid clone reappeared at day 28 of relapse therapy. Pt2 relapsed as AML with undetectable TCR gene rearrangements. Pt3 is in complete remission 23 months after SCT with detectable low-level MRD and mixed chimerism with repeated lowering MRD after re-administered TKI dasatinib.

Conclusions:
ABL is an extremely rare entity in childhood accounting for less than 0.3 % of all acute leukemia cases. Surprisingly, the same genetic changes can be identified in both clonal populations making the “true” ABL even rarer. FC in combination with morphology is the basic method for identifying ABL and should be followed by detailed genetic analysis. The prognosis of ABL patients in our cohort was poor, despite SCT preceded by the application of treatment modalities targeting both lymphoid and myeloid clones.

Disclosures:
No relevant conflicts of interest to declare.