Recently we demonstrated that the t(11;18)(q21;q21) associated with extranodal marginal zone B cell lymphomas of MALT type results in the expression of a chimeric transcript fusing 5 API2 on chromosome 11 to 3 MLT on chromosome 18. Here we report the development of an RT-PCR approach for the detection of the API2-MLT fusion transcript and its application for the analysis of 58 cases of gastric lymphoma. Initially nested PCR amplification was combined with Southern analysis using internal API2 and MLT probes. A genuine API2-MLT fusion transcript of variable length was demonstrated in 11 out of 58 cases. Sequence analysis revealed that in all cases the breakpoint on chromosome 11 occurred between exons 7 and 8 of the API2 gene. In contrast, the breakpoints on chromosome 18 appeared to be heterogeneous as fusions to bp 814, 1123, and 1150, respectively, of MLT were observed. These observations allowed us to work out a highly sensitive diagnostic test for the API2-MLT fusion on an ABI Prism 7700 sequence detector that confirmed the results of our initial approach. The API2-MLT fusion was found in 48% of gastric marginal zone cell lymphomas of MALT type that did not contain a large cell component and it was lacking in all other lymphomas of the stomach.

The t(11;18)(q21;q21) between the inhibitor of apoptosis API2 and the MLT gene is a distinct feature of marginal zone B‐cell lymphomas of MALT‐type. Hitherto the chimeric API2‐MLT transcripts are all “in‐frame” and predominantly fuse exon 7 of API2 to different MLT exons. Recurrent chromosomal translocations are common in lymphoid neoplasms and might represent by‐products of the rearrangement processes generating antigen receptor diversity. The genomic structure of the MLT gene was determined to facilitate amplification of the genomic breakpoint junctions from 5 MALT‐type lymphomas with t(11;18). Their sequence analysis showed scattering of the chromosome 11 breakpoints in intron 7 of API2 whereas rearrangements in MLT occurred in intron 2, 4, 7, or 8, respectively. Sequences around the junctions did not display recognition signal sequences mediating lymphocytic V(D)J recombination or other sequence motifs associated with recombination. The breakpoints occurred in a copy of an AluSx repeat in three cases, but interchromosomal Alu‐mediated homologous recombination could be ruled out as the repeat resided only on one of the participating chromosomes. The t(11;18) was associated with a deletion in 4 out of 5 cases, ranging in size from 53 bp up to more than 200 kb. These deletions were observed on one or sometimes both derivative chromosomes that might indicate the susceptibility of these regions for breakage. Our data suggest that the API2‐MLT fusion might result from a non‐homologous end joining event after multiple double‐strand breaks. The clustering of breaks in intron 7 of API2 and the consistent “in frame” API2‐MLT fusions could therefore reflect certain functional constraints crucial for clonal outgrowth. © 2000 Wiley‐Liss, Inc.

The translocation of chromosome 11, long arm, region 2, band 1, to chromosome 18, long arm, region 2, band 1 (t(11;18)(q21;q21)) represents a recurrent chromosomal abnormality in extranodal marginal zone B-cell lymphoma (MZBCL) of mucosa-associated lymphoid tissue (MALT) type and leads to a fusion of the apoptosis inhibitor-2 (API2) gene on chromosome 11 and the MALT lymphoma-associated translocation (MLT) gene on chromosome 18. A 2-color fluorescence in situ hybridization (FISH) assay, which can be used for the detection of t(11;18) in interphase nuclei and metaphase chromosomes on fresh and archival tumor tissue, was developed. The P1 artificial chromosome (PAC) clone located immediately telomeric to the MLT gene and the PAC clone spanning the API2 gene were differentially labeled and used to visualize the derivative chromosome 11 resulting from t(11;18), as evident by the overlapping or juxtaposed red and green fluorescent signals. The assay was applied to interphase nuclei of 20 cases with nonmalignant conditions and 122 B-cell non-Hodgkin's lymphomas (NHLs). The latter group comprised 20 cases of nodal follicle center cell lymphoma and diffuse large B-cell NHL, 10 cases of gastric diffuse large B-cell lymphoma, 10 cases of hairy cell leukemia, and 82 cases of MZBCL (41 extranodal from various locations, 19 nodal, and 22 splenic MZBCL) including 35 cases with an abnormal karyotype, 2 of which revealed t(11;18). By interphase FISH, t(11;18) was detected in 8 gastrointestinal low-grade MALT-type lymphomas including the 2 cytogenetically t(11;18)+ cases. In the 8 t(11;18)+ cases, the FISH results were confirmed by reverse transcriptase–polymerase chain reaction (RT-PCR) usingAPI2 and MLT specific primers. Our results indicate that t(11;18)(q21;q21) specifically characterizes a subgroup of low-grade MZBCL of the MALT-type and that the FISH assay described here is a highly specific and rapid test for the detection of this translocation.