Further demonstration of the diversity of chromosomal changes involving 2p23 in ALK-positive lymphoma: 2 cases expressing ALK kinase fused to CLTCL (clathrin chain polypeptide-like).

Anaplastic lymphoma kinase (ALK)-positive lymphomas are characterized by expression of a hybrid protein, comprising the cytoplasmic portion of the ALK tyrosine kinase fused to a partner protein. This hybrid kinase is often encoded by the nucleophosmin (NPM) NPM-ALK fusion gene resulting from the (2;5)(p23;q35) chromosomal translocation. However, the ALK gene at 2p23 may also be involved in 2 variant translocations, namely t(1;2)(q25;p23) and t(2;3)(p23;q21), which create the TPM3-ALK and TFG-ALK fusion genes, respectively. We report here 2 lymphomas with an unusual finely granular cytoplasmic ALK staining pattern, clearly different from the pattern observed in ALK-positive lymphomas carrying NPM-ALK or its variants. A cloned complementary DNA sequence from 1 of these 2 lymphomas contained the ALK gene fused to the second clathrin heavy chain gene (also referred to as clathrin heavy polypeptide-like gene) (CLTCL). The distinctive granular cytoplasmic staining pattern for ALK was likely to be due to binding of the fusion protein to clathrin-coated vesicles. The CLTCL gene is constitutively expressed in lymphoid cells and therefore presumably contributes an active promoter for the CLTCL-ALK gene. The fusion protein had a molecular weight (250 kd) that differs from all known ALK products, and it was autophosphorylated in an in vitro kinase assay, confirming that it is constitutively active and hence capable of contributing to malignant transformation. These 2 cases, therefore, represent a hitherto undescribed mechanism of ALK activation in lymphoma and further illustrate the diversity of fusion partners for the ALK gene.