TRACKING OF THE PRELEUKEMIC PERIOD IN SECONDARY ACUTE LEUKEMIAS REVEALS MULTIPOTENT PROGENITOR AS A TARGET OF MLL REARRANGEMENTS

Background:

Secondary leukemias are often characterized by breakpoints in the MLL gene.  The MLL gene is an extremely promiscuous translocation partner, as more than 70 MLL partner genes have been identified so far. The most frequent partner genes (AF4, AF9, ENL, AF10, AF6, ELL) account for >80% of MLL translocated leukemias.

In some patients presenting with secondary acute leukemia (sAL) after primary leukemia, archived bone marrow (BM) samples collected during the follow-up of the primary disease might be available, thus providing us with material for backtracking of the sAL markers in the prediagnostic period.

Aims:

We considered MLL rearranged sAL cases as a suitable model to study presence and dynamics of the (pre-)leukemic cells in the preleukemic period.

Methods:

We have collected BM samples from 5 patients monitored for primary ALL (n=3) or AML (n=2), who developed sAL after variably long period of remission (1,25 to 5,5 years). The sAL cases presented as ALL (n=2) or AML (n=3).

Leukemospecific clonal markers (Ig/TCR and MLL gene rearrangements) were investigated in BM of both original and secondary diagnoses using real time PCR. In addition, to better characterize the (pre)leukemic clone, FISH and SNP array were performed in some cases. In an attempt to backtrack the sAL cells at different timepoints preceding the sAL, methods of qPCR and FISH were employed.

Results:

The sAL were characterized by the presence of either typical (2 sAML cases with MLL/AF9 fusion) or rare (2 sALL cases with MLL/MAML2 and MLL/FOXO3A which we described previously and 1 sAML case harboring a novel, so far undescribed MLL/ME2 fusion) MLL rearrangements.

In all 3 patients, who presented with the atypical MLL partner, we backtracked the presence of MLL fusion in BM samples which were taken several months prior to the sAL (20, 23 and 9 months in MLL/FOXO3A, MLL/MAML2 and MLL/ME2, respectively). In both ALL patients, the leukemic clone was defined also by an incomplete IgH rearrangement, which seemed to be specific for the overt disease in both cases, as they were not detected in the MLL positive specimens preceding the sAL diagnosis.

In these patients the MLL positive cells comprised unexpectedly large proportion of bone marrow in the preleukemic period (approx. 10% in the MLL/MAML2 case and even >90% in MLL/FOXO3A case). Interestingly, the observed high levels of MLL rearranged cells in these two samples (taken 15 and 16 months before the sALL diagnosis) were followed by a remarkable decrease of the MLL positivity levels before the onset of sALL. The high percentage of preleukemic cells (not restricted to lymphoid lineage) during clinically silent period suggests the involvement of a multipotent cell originally affected by MLL translocation.

In 2 sAML patients, harboring typical MLL/AF9 translocation, the fusion gene was not backtracked in any of the prediagnostic samples (the last ones taken 11 and 12 months before the diagnosis).

Summary / Conclusion:

Our data suggest, that sAL – as well as de-novo leukemia - is a multiple-step process with protracted latency between the first (MLL rearrangement) and finally transforming hit leading to the active disease. Moreover, the preleukemic clone can reach unexpectedly high levels even during a clinically silent period with normal BM morphology. Finally, our data suggest that typical and rare MLL fusions possibly drive different dynamics of the processes preceding the overt disease.

Supported by grants IGA-MZ NT/12428-5 and GAUK 618212.

Keywords: Leukemia, MLL, Second malignancy

Abstrakta v časopise Haematologica 2013, Suppl1

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