Three-Way Translocations and Insertions Are the Molecular Basis for Complex MLL Rearrangements.

The human MLL gene on 11q23 is frequently involved in chromosomal rearrangements including almost all human chromosomes. These rearrangements are associated with high-risk acute leukemias. Today more than 60 MLL fusion partner genes which represent the MLL recombinome have been characterized at the molecular level. Interestingly, more than 20% of all MLL translocations are highly complex. Here we distinguish between two types of complex rearrangements: Type I: A complex rearrangement is a genetic prerequisite due to the transcriptional orientation of the involved genes, for example t(10;11) with MLL and AF10, respectively; Type II: A complex rearrangement is not prerequisite, although identified in leukemia patients. More than 500 MLL rearrangements have been analyzed at the Diagnostic Center of Acute Leukemia so far and here, we present data on 60 complex translocations that were resolved at the molecular level. Three-way chromosomal translocations, insertions and combinations thereof are the molecular basis for most of these complex MLL rearrangements which in most cases create 3 or more fusions. However, type I is mainly based on insertions and type II is based on 3 way translocations. Within these complex rearrangements, the 5′ part of the MLL gene is in general fused in frame to one of the most frequent partner genes. But the 3′ part of the MLL gene is fused in 36 cases (60%) to a novel gene such as, for example, APBB1IP or CMAH. We defined this heterogeneous group of novel translocation partner genes as “reciprocal MLL fusion genes” as they had not been identified as fusion partners to the 5′ part of the MLL gene before. These fusions are only in frame in four cases; the remaining fusions are out of frame or the genes have an opposite transcriptional orientation. I.e. these genes show genetic haplo-insufficiency and thus may also cooperate in promoting leukemogenesis. For the remaining 40% the 3′ part of the MLL gene is fused to a chromosomal area which does not encode any gene. In many complex aberrations additional genes like for example RREB1 or OS9 are fused mostly out of frame to other genes or non-coding chromosomal areas of these complex rearrangements. These genes are also haplo-insufficient and may also contribute to the leukemogenic process. These results demonstrate that LDI-PCR is a potent tool to identify highly complex and cryptic rearrangements as both MLL fusion alleles are analyzed at the same time and reveal immediately if a balanced or more complex rearrangement is present. Whether the pathobiology of type II complex rearrangements is different in comparison to their not complex counterparts has still to be proven. Supported in part by grants Ma 1876/7-1, /8-1 and /9-1 from the DFG, grant N1KR-S12T13 from the BMBF and grant 102362 from the Deutsche Krebshilfe.