MLL Rearrangements in Childhood Acute Lymphoblastic Leukemia: Clinical and Genetic Features of a Distinct Disease Entity

MLL rearrangements in childhood ALL and specifically that of the t(4;11)(q21;q23) translocation, have been associated with unfavorable prognosis; however, MLL rearrangements in children represent an extremely heterogeneous group, whose prognostic diversity needs further refinement. Aim of this study is to present the epidemiological, clinical, immunophenotypic and genetic features as well as the outcome of our cohort patients with ALL and MLLrearrangements.

167 ALL pts with a median age of 5.1 yrs (range 0.5-17.5), have been retrospectively recorded and analysed. Pts were consecutively diagnosed and treated on BFM based protocols in a single Center during the years 2000-2013. Evaluation upon diagnosis included conventional cytogenetics, i-FISH and RT-PCR for the detection of MLLrearrangements in 11q23 chromosomal region and FCM for the detection of MRD. Αdditionally, this study was designed to assess the predictive value of an MTT (3-[4,5-dimethylthiazol-2,5-diphenyl] tetrazolium bromide) in vitro assay for the evaluation of leukemic cell resistance/sensitivity to cytotoxic drugs and to compare these results with clinical and laboratory parameters in cases of MLL-positive childhood ALL. Thus, the chemoresistance of leukemic cells was ascertained by means of an MTT assay in all 11 pts positive for MLL rearrangements and in vitro drug resistance to prednisolone (PDN), asparaginase (ASP), vincristine (VCR), daunorubicin (DNR), etoposide (VP-16) and cytosine arabinoside (ARA-C) was evaluated.

Eleven (6/11 boys) out of the 167 pts (6.6%) were positive for 11q23 rearrangements, with a median age of 2.3 yrs (0.49-14.72). 2/11 (18.2%) presented with mediastinal, 4/11(36.4%) with CNS and 1/11(9.1%) with testicular infiltration. The median WBC was 109210/μl (4300-700000/μl), while 8/11 pts presented with WBC> 50000/μl. Immunophenotype revealed 5/11 pts with pre pre B-ALL (including 2 infant-ALL), 2/11 pre B-ALL, 2/11 B-common-ALL and 2/11 pts with T-III cortical ALL while 1/11 patient presented with mature type ALL of B-cell origin. The median DNA index was 1.0 (0.96-1.26). Cytogenetics revealed complex karyotype and t(2;9)(p13;p13-q21) in 1/11 patient, t(4;11)(q21;q23) in 2/11, atypic t(11;19)(q23;p13.3) in 2/11 and t(10;11)(p12;q23) in 1/11 patient, while no cytogenetic abnormalities were detected in 4/11 children (failure in 1/11). I-FISH analysis demostrated MLL rearrangement in all pts, with additional concomitant aberrations del9p21(1/11), 3x9q34, 3x11q23 and del 3΄MLL (1/11), 5-10x21q22(+AML1) and del5'MLL (1/11).

8/11 pts were treated according to ALL BFM protocols (BFM 95/2000/ALLIC-2009), 2/11 according to Interfant protocols (Pilot 98/06) and 1/11 according to the NHL-BFM 2004 (R4 therapy group). 10/11 pts were stratified and treated according to the High Risk Arm. The results of the MTT assay revealed that all patients bearing MLL rearrangements were resistant to asparaginase, while 6/11 pts were also resistant to PDN. Infant ALLs and t(4;11)(q21;q23) cases that were associated with the most unfavourable prognosis, were all bearing extremely chemoresistant leukemic cells to all the cytotoxic drugs administered and evaluated by the MTT assay.

Among the pts treated with ALL protocols, 4/10 were poor prednisone responders on day 8 of remission-induction therapy, 2/10 had M2/M3 bone marrow on day 15, while all achieved CR on day 33. 7/10 pts had detectable FC-MRD(>0.01%) on day 15 (2 with MRD>10%), while 5/10 pts had detectable MRD on day 33. 5/11 pts underwent HSCT (4 in CR1).

Within a median follow up period of 26.7 months (1.8-152.5), overall survival (OS) and event-free survival (EFS) was 72.7% and 63.6%, respectively.

In our study, MLL rearrangements are associated with unfavorable clinical, hematological and immunophenotypic features and have a dismal impact on outcome even in cases missing the classic t(4;11)(q21;q23) translocation. Longer follow-up and a larger number of patients is needed, in order to clarify whether the presence of the rearrangement itself constitutes a distinct entity with adverse prognostic significance or it moderates survival due to concomitant lesions and their biological features, such as 1.resistance to glucocorticoids and asparaginase as revealed in our cohort, or 2. mutations and/or overexpression of the FLT3 gene and 9p21 deletions, which are involved in the biology of the disease and adversely modify prognosis.