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Molecular analysis of MLL-1/AF4 recombination in infant acute lymphoblastic leukemia
1Department of Pediatrics, Hematology and Oncology, University of Giessen, Germany.
Insights
A MLL-1/AF4 rearrangement occurs in about 50% of infant acute lymphoblastic leukemia (ALL) cases. The reciprocal AF4/MLL-1 mRNA transcript is rarely detected, even in positive MLL-1/AF4 cases.
Area of Science:
- Pediatric Oncology
- Molecular Biology
- Genetics
Background:
- Infantile acute lymphoblastic leukemia (ALL) is a rare and aggressive hematologic malignancy.
- The MLL gene rearrangements are common in infant ALL, but the specific fusion partners and their clinical significance are still being investigated.
Purpose of the Study:
- To investigate the frequency and molecular characteristics of MLL-1/AF4 rearrangement in infants with ALL.
- To determine the presence of the reciprocal AF4/MLL-1 mRNA transcript in these patients.
Main Methods:
- RT-nested PCR was used to detect MLL-1/AF4 rearrangement and the reciprocal AF4/MLL-1 transcript in ten infant ALL cases.
- PCR products were analyzed for size variants, and sequencing was performed for detailed characterization of splicing variants.
- The MV411 cell line with a known t(4;11) translocation served as a positive control.
Main Results:
- A MLL-1/AF4 rearrangement was detected in 5 out of 10 (50%) infant ALL patients.
- Various splicing variants of the MLL-1/AF4 rearrangement were identified, with different fusion points between MLL-1 and AF4.
- The reciprocal AF4/MLL-1 mRNA transcript was not detected in any of the infant ALL patients, including those with MLL-1/AF4 rearrangement, but was found in the control cell line.
Conclusions:
- MLL-1/AF4 rearrangement is a frequent event in infant ALL, occurring in approximately 50% of cases.
- Alternative splicing of the MLL-1/AF4 fusion transcript is common, leading to diverse molecular subtypes.
- The reciprocal AF4/MLL-1 transcript is a rare event in infant ALL, suggesting a unidirectional nature of this specific rearrangement in this patient population.
Abstract:
We examined ten cases of acute lymphoblastic leukemia (ALL) in infants (less than 1 year of age) by RT-nested PCR for a MLL-1/AF4 rearrangement. Five patients revealed a positive result. The specific PCR product differed in size from approximately 380-670 bp indicating various splicing variants in the MLL-1/AF4 rearrangement. Three patients had a fusion between exon 6 of the MLL-1 gene and codon 362 of the known AF4 cDNA sequence. Moreover, in two patients more than one specific PCR product was detected, possibly due to alternative splicing. In the first case, sequencing of these products revealed a hybrid mRNA consisting of MLI-1 exon 7 or exon 8, respectively, fused to the AF4 gene at codon 348. In the second case with alternative splicing, again, exon 7 or 8 of the MLL-1 gene were fused to the AF4 gene as in case 1. The AF4 sequence involved in this patient, however, started at codon 362. The AF4 break was, therefore, identical to the three MLL-1/AF4 positive patients as described above. Moreover, we investigated all ten patients for the reciprocal mRNA transcript AF4/MLL-1 by a similar PCR approach. In none of these patients, including the five MLL-1/AF4 positive cases was a specific PCR product obtained. However, in the MV411 cell line bearing a t(4;11), which served as a positive control in our MLL-1/AF4-PCR assay, the reciprocal AF4/MLL-1 mRNA was detected. Our results indicate that a MLL-1/AF4 rearrangement occurs in about 50% of infants with ALL. In contrast, the reciprocal hybrid mRNA can only rarely be detected, if at all.