Molecular Genetic Characterization of 3'-Deletion of MLL Gene in Infant Acute Leukemia.

MLL gene rearrangements are associated with unfavorable outcome in infant acute lymphoblastic leukemia (ALL) and have intermediate prognosis in infant acute myeloid leukemia (AML). Application of fluorescence in-situ hybridization (FISH) allows detecting not only conventional MLL rearrangements, but also concurrent 3'-deletion of MLL gene. However, detailed characteristics of infant leukemia carrying 3' MLL deletion remain unclear.

To investigate molecular genetic features of MLL-rearranged infant acute leukemia with concurrent 3' MLL deletion.

64 patients (27 boys and 37 girls) aged from 1 day to 11 months (median 6.6 months) including 44 ALL patients, 18 AML patients, 1 patient with acute bilineage leukemia and 1 patient with acute undifferentiated leukemia were enrolled in the current study. Chromosome banding analysis was done according to standard procedure. FISH analysis using LSI MLL Dual Color, Break Apart Rearrangement Probe (Abbott Molecular, USA) was performed on at least 200 interphase nuclei and on all available metaphases. Presence of MLL rearrangements was detected by FISH, reverse-transcriptase PCR. In 29 cases long-distance inverse PCR was additionally performed. In case of MLL rearrangement presence standard FISH pattern was defined as simultaneous detection of 3 different fluorescent signals: 1 fused (orange) signal, 1 green signal derived from 3' part of MLL gene, 1 red signal from 5' end of MLL (1F1G1R). MLL rearrangements with concurrent 3' MLL deletion led to 1F1R FISH pattern formation due to lack of green signal.

FISH revealed MLL rearrangements in 73% of ALL cases that was higher than frequency of 11q23 translocations detected by conventional cytogenetics — 55%. In MLL-positive cases we found 38 patients (81%) with standard FISH pattern, 7 ones (15%) with concurrent 3'-deletion of MLL gene and 2 (4%) with complex MLL rearrangements. Among patients with 3' MLL deletions there were 1 case with 5' MLL duplication (1F2R) and 1 case with 5' MLL triplication (1F3R). Frequency of 3'-deletions were similar in ALL and AML patients (13% and 15%, respectively). We did not find more than one FISH pattern in bone marrow blast cells of each patient with 3' MLL deletion. In this cohort of patients all blast cells carried concurrent 3'-deletion of MLL gene. Moreover, percentage of blast cells carrying MLL rearrangements did not differ significantly between patients with standard FISH pattern (median 97%, range 22–100%) and 3'-deletion (median 83%, range 13–99%) (p=0.206). 3'-deletion of MLL was not associated with breakpoint position in MLL gene and type of translocation partner gene. MLL translocation partner genes detected in patients with 3' deletions were as follows AF4(n=2), MLLT3(n= 3), MLLT10(n=2). None of the patients with 3'-deletions had reciprocal fusion gene. Initial patients' characteristics (age, sex, WBC count, immunophenotype, CNS-status, type of MLL partner gene) and treatment response parameters (day 8 peripheral blood blast cell count, day 15 bone marrow status, day 36 remission achievement, minimal residual disease status at time point 4) did not differ significantly between 2 groups. Although cumulative incidence of relapse was lower in patients with 3'-deletion as compared to patients with standard FISH pattern (0.31±0.04 and 0.55±0.01, respectively), difference between these two groups was not statistically significant (p=0.359).

In our work we characterized rare subgroup of infant MLL-rearranged acute leukemia carrying concurrent 3' MLL deletion. Our data provide additional information of molecular genetic features of acute leukemia in children younger than one year.

No relevant conflicts of interest to declare.