The MLL Recombinome of Adult ALL - Results from the GMALL Study Group.

The pro B and the CD10-negative pre B immunophenotypes are found in approximately 12% and 2–3% of adult acute lymphoblastic leukemia cases, respectively. Both are characterized by a high prevalence of MLL aberrations. MLL fusion genes define a high risk group of ALL patients and they have proven useful as target structures for assessing minimal residual disease. It has been a matter of debate whether MLL aberrations are of prognostic significance within the group of pro B ALL patients. To better characterize the MLL recombinome in adult ALL we have prospectively and retrospectively investigated BCR-ABL-negative pro B (N=152) and CD10-negative pre-B ALL (N=31) patient samples obtained within the framework of the GMALL study group between 2001 and 2007. We used RT-PCR to detect MLL-AF4, MLL-ENL and MLL-AF9 and other MLL fusion genes. An inverse long PCR method was used to identify the exact chromosomal breakpoints in the MLL gene or unknown MLL fusions.

Results: RT-PCR analysis revealed a MLL-AF4 fusion in 20/31 (64.5%) and a MLL-ENL fusion in 2/31 (6.5%) of CD10-negative pre-B cases. A MLL-AF4 fusion was detected in 101/151 (67.3%) and a MLL-ENL fusion in 9/151 (6.0%) of pro B cases. Additionally, two pro B patients showed a MLL-AF9 and MLL-CXXC6 fusion, respectively. The relative frequency of MLL aberrations was higher (50/68 = 73.5%) in those pro B patients with immunophenotypic coexpression of myeloid markers than in those without myeloid coexpression (41/84 = 48.8%). The chromosomal breakpoints in the MLL gene and the respective partner gene were determined on the genomic level in 91 patients. The distribution of chromosomal breakpoints in the MLL gene was the following: 1 in intron 7, 1 in intron 8, 33 in exon/intron 9, 27 in exon/intron 10, 28 in intron/exon 11, 1 in intron 12. These data illustrate the spectrum and relative frequency of MLL fusion genes in adult pro B and CD10-negative pre B ALL. The chromosomal breakpoint data provide insight into the molecular mechanisms leading to MLL aberrations in adult ALL. Chromosomal breakpoint sequences are powerful molecular markers for assessing MRD in individual patients as has been shown in the context of the GMALL MRD evaluation trials.