MIXED LINEAGE LEUKEMIAS / HYBRID LEUKEMIAS

Epidemiology : biphenotypic acute leukemias may affect adults or children, particularly infants under 2 years old.
It is likely that the cases referred to as myeloid antigen-positive acute lymphoblastic leukemia (My+ ALL) and lymphoid antigen-positive acute myeloid leukemia (Ly+ AML) include a miscellaneous group that encompasses true biphenotypic leukemias as well as others which are ALL or AML with atypical expression of a single marker from another lineage.

A scoring system has been proposed aimed at distinguishing cases of bona fide biphenotypic acute leukemia from those with aberrant expression of a marker from another lineage, eg. Ly+AML and My+ALL. This system is based on the number and degree of specificity of the markers (lymphoid and myeloid) expressed by the leukemic cellsref. The table below shows the markers regarded as most specific as follows : Scoring system for the definition of acute biphenotypic leukemias. Biphenotypic acute leukemia is established when the score from 2 separate lineages is > 2.
scoring points
lineages
B-lymphoid
T-lymphoid
myeloid
2 CD79a (mb-1) 
CD22
cyt IgM
cyCD3 > sCD3 (TcRb) MPO (demonstrated by cytochemical or immunological methods)
1 CD19
CD10
CD20
CD2
CD5
CD13
CD33
CD65
0.5 TdT
CD24
TdT
CD7
CD1a
CD11b
CD11c, 
CD14
CD15
CD64
CD117
According to this scoring system, a case is considered biphenotypic when point values are > 2 for the myeloid and 1 of the lymphoid lineages. Cases involving both lymphoid lineages (B and T) are very rare; cases with markers for the 3 lineages (triphenotypic) are also rare. It is likely that in the future other markers with a high degree of lineage specificity will be incorporated into the scoring system, for instance markers recognizing the a/b and g/d chains of the TcR for the T-lymphoid lineage or CD117 (c-kit), which seems to be specific for myeloid cells. Some of these markers are currently being evaluated by the EGIL group. Symptoms & signs : they may present as de novo or, rarely, they become apparent during a relapse following anti-AML or ALL therapy. The WBC is often high and most cases have a varying proportion of circulating blasts (Zomas A, Modak S, Pinkerton R, et al. Childhood biphenotypic leukaemia: six-year experience from a single centre [abstract]. Br J Haematol 1995; 90:67.)
Therapy : there are no uniform criteria about whether to treat these cases as ALL or AML when they are diagnosed only by standard morphology and cytochemistry, or whether to use an approach which combines drugs that are effective for ALL and AMLref, followed by bone marrow or mobilized peripheral stem cell transplantation in complete remissionref.
Prognosis : extensive data on response to therapy and clinical outcome are not available; however, our impression based on cases treated at the RMH and from single cases reported in the literature is that of a poor outcome in both children and adults. This may be related to the underlying chromosome abnormalityref.
Differential diagnosis :
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