Rearrangements of the MALT1 gene by the t(11;18)(q21;q21) and t(14;18)(q32;q21) are the most frequent structural chromosomal abnormalities in MALT lymphomas. These translocations lead to fusions of BIRC3-MALT1 and IGH-MALT1 respectively, and activate the NF-kappaB pathway. Among 122 diffuse large B-cell lymphomas and 28 Burkitt's lymphomas screened by interphase FISH, we found two cases with a break within MALT1, but without a t(11;18) or a t(14;18). Molecular genetic analyses in one of these cases revealed a novel "in frame" fusion of exon 9 of MALT1 and exon 9 of the microtubule-associated protein 4 (MAP4) gene. The translocation was accompanied by a deletion of MALT1 sequences distal to the breakpoint including the caspase-like domain, which is essential for activation of NF-kappaB. As a result of the deletion, the reciprocal 5'MAP4-3'MALT1 transcript was not present, demonstrating that the 5'MALT1-3'MAP4 fusion represents the pathogenetically relevant transcript. Immunohistochemistry with amino-terminal and carboxy-terminal MALT1 antibodies, indicated a strong expression of the chimeric MALT1-MAP4 protein. Moreover, NF-kappaB activation was not increased in this case as shown by the levels of IkappaBalpha phosphorylation and NEMO ubiquitination. Our data demonstrate that the pathogenetic consequences of the novel MALT1-MAP4 fusion are different from those of the known MALT1-associated chromosomal rearrangements and do not involve NF-kappaB activation.
(c) 2006 Wiley-Liss, Inc.