X chromosome insertion in the MLL gene in a case of childhood acute myeloblastic leukemia

Using bacterial artificial chromosomes in leukemia research: the experience at the university cytogenetics laboratory in Brest, France

The development of the bacterial artificial chromosome (BAC) system was driven in part by the human genome project in order to construct genomic DNA libraries and physical maps for genomic sequencing. The availability of BAC clones has become a valuable tool for identifying cancer genes. We report here our experience in identifying genes located at breakpoints of chromosomal rearrangements and in defining the size and boundaries of deletions in hematological diseases. The methodology used in our laboratory consists of a three-step approach using conventional cytogenetics followed by FISH with commercial probes, then BAC clones. One limitation to the BAC system is that it can only accommodate inserts of up to 300 kb. As a consequence, analyzing the extent of deletions requires a large amount of material. Array comparative genomic hybridization (array-CGH) using a BAC/PAC system can be an alternative. However, this technique has limitations also, and it cannot be used to identify candidate genes at breakpoints of chromosomal rearrangements such as translocations, insertions, and inversions.

MLL–SEPT6 fusion transcript with a novel sequence in an infant with acute myeloid leukemia

The MLL gene at 11q23 is a site of frequent rearrangement in acute leukemia with multiple fusion partners. A relatively uncommon rearrangement, associated with infant AML-M4, fuses the MLL and SEPT6 genes. SEPT6, located at Xq24, is a member of a family of mammalian septins involved in diverse functions such as cytokinesis, cell polarity, and oncogenesis. We describe the case of an infant with acute myelogenous leukemia who showed cytogenetic evidence of rearrangement between 11q23 and Xq24 regions. Fluorescence in situ hybridization analysis suggested a possible break in the MLL gene, and molecular analysis using reverse transcriptase-polymerase chain reaction followed by sequencing confirmed the expression of an MLL-SEPT6 fusion transcript with a novel sequence. The findings emphasize the importance of combined cytogenetic and molecular analyses in the workup of acute leukemia, especially in those leukemias that occur infrequently.

Screening by fluorescence in situ hybridization for MLL status at diagnosis in 239 unselected patients with acute myeloblastic leukemia

A large number of abnormalities involving the MLL gene have been associated with hematological malignancies, including acute myeloblastic leukemias (AML). Given the overall unfavorable prognosis of AML with an MLL abnormality, its reliable and accurate detection is needed for informed treatment decision. We therefore investigated the occurrence of MLL abnormalities in 239 unselected consecutive AML patients, using conventional cytogenetic and fluorescent in situ hybridization (FISH) analyses. FISH analysis for MLL was performed using a commercial dual-color probe. Of the 239 patients, 30 (12.6%) showed MLL abnormalities under FISH analysis, 10 (4.2%) showed a split signal indicating the disruption of the MLL gene by translocation or insertion, and 20 (8.4%) showed overrepresentation of the MLL gene without evidence of rearrangement. MLL abnormalities were more frequently found in AML-M5 and M4, mainly as rearrangements, and in AML-M2, mainly as overrepresentation. Our results are in agreement with those reported in other studies. All M2, M4, and M5 AML patients without known recurrent translocations, such as t(8;21) and inv(16), should be investigated by FISH with an MLL probe because it allows the detection of MLL rearrangement that would go undetected by conventional cytogenetics and because it has the ability of detecting multiple copies of the MLL gene in, for example, marker chromosomes and double minutes.