Clusters of Childhood Mixed Phenotype Acute Leukemia Diagnosed By Nation-Wide Immunophenotyping in Japan

[Background] Acute leukemia of ambiguous lineage is a very rare subtype defined by WHO 2008 classification. It is classified as mixed phenotype acute leukemia (MPAL), acute undifferentiated leukemia (AUL) and natural killer lymphoblastic leukemia. Basically, these category of leukemia consist of heterogenous pathogenesis. Determination of the lineage were usually decided by major lineage markers such as myeloperoxidase (MPO), CD19, and cytoplasmic CD3. However, more detailed immunophenotyping is necessary to confirm the cell lineages, especially in case of MPAL. Japanese pediatric leukemia/lymphoma study group (JPLSG) started nation-wide immunophenotyping with unified panel including more than 50 antigens for childhood hematological malignancy. Throughout this study, we found 32 acute leukemia of ambiguous lineage, including 3 AUL and 29 MPAL cases. MPAL cases were divided into several peculiar groups by our detailed phenotyping. We report here the characteristics of pediatric AUL/MPAL cases, including clinical manifestations and peculiar immunophenotype.

[Result] In 3,229 case from January 2012 to December 2015, 32 cases were diagnosed as acute leukemia of ambiguous lineage. Gender ratio (M:F) was 18:14 and median age were 8.4 years (0.3 to 18.1) respectively. These cases were consisted of 3 AUL and 29 MPAL. According to detailed immunophenotyping, MPAL cases could be divided into 4 distinct clusters, (1) 7 T cell (T-) acute lymphoblastic leukemia (T-ALL)/acute myelogenous leukemia (AML)-M1 biphenotypic, (2) 4 early T-cell precursor (ETP)-like, (3) 8 B cell precursor (BCP-) ALL/Myelomonocytic bilineal, and (4) 10 BCP-ALL with MPO expression subtypes. T-ALL associated cases indicated relatively older age; (1) 12.3 and (2) 10.2 vs (3) 5.9 and (4) 7.5 respectively. 3 of (4) cases indicated characteristic genotypes; 1 minor bcr-abl, 1 major bcr-abl, and 1 AML1-ETO/Flt3-ITD. 4 out of 8 bilineal cases (3) were infantile ALL with MLL rearrangement, but other 4 older cases did not indicate MLL rearrangement.

[Discussion] Acute leukemia of ambiguous lineage is a rare subtype of leukemia. According to WHO 2008 classification, it is further classified as MPAL, AUL and natural killer lymphoblastic leukemia. In this study, we evaluate 32 patients of acute leukemia of ambiguous lineage. It is of note that we could not identify no natural killer lymphoblastic leukemia in this childhood leukemia cohort. We also evaluate 29 MPAL cases, which showed 4 distinct subgroups. T-ALL/AML-M1 biphenotypic cases indicated relatively older age, usually diagnosed as AML-M1 with morphologic FAB classification, and received mostly myeloid oriented therapy. ETP-ALL like cases were positive for T cell antigens such as CD2 and/or CD7 and also some myeloid antigens but lacked cytoplasmic CD3 expression. BCP-ALL related cases were also divided into two groups, BCP-ALL/Myelomonocytic bilineal and BCP-ALL with MPO expression. Bilineal leukemia often found in infantile ALL with MLL gene rearrangement. In this study, we also found 4 non-infantile bilineal leukemia without MLL gene rearrangement. These cases occasionally cause lineage-switch relapse, so diagnosis of bilineal leukemia seems to be very significant. Recent studies showed EP300-ZNF384 fusion indicate CD10 negative or low BCP-ALL with MPO expression. Our cases also tended to indicate negative or low CD10 expression. In conclusion, we identified a spectrum of immunophenotype in MPAL patients. In childhood cases, most of MPAL cases were divided into 4 distinct categories. It must be useful information before examination of genetic abnormality.