A new variant anaplastic lymphoma kinase (ALK)-fusion protein (ATIC-ALK) in a case of ALK-positive anaplastic large cell lymphoma.

Anaplastic lymphoma kinase (ALK)-positive lymphomas ("ALKomas") constitute a distinct molecular and clinicopathological entity within the heterogeneous group of CD30-positive large cell lymphomas. In 80-85% of cases tumor cells express a Mr 80,000 hybrid protein comprising the nucleolar phosphoprotein nucleophosmin (NPM) and the ALK. We report here the cloning and expression of a novel ALK-fusion protein from an ALK-positive lymphoma. This case was selected for molecular investigation because of (a) the absence of NPM-ALK transcripts; (b) the atypical staining patterns for ALK (cytoplasm-restricted) and for NPM (nucleus-restricted); and (c) the presence of a Mr 96,000 ALK-protein differing in size from NPM-ALK. Nucleotide sequence analysis of ALK transcripts isolated by 5'-rapid amplification of cDNA ends revealed a chimeric mRNA corresponding to an ATIC-ALK in-frame fusion. ATIC is a bifunctional enzyme (5-aminoimidazole-4-carboxamide ribonucleotide transformylase and IMP cyclohydrolase enzymatic activities) that catalyzes the penultimate and final enzymatic activities of the purine nucleotide synthesis pathway. Expression of full-length ATIC-ALK cDNA in mouse fibroblasts revealed that the fusion protein (a) possesses constitutive tyrosine kinase activity; (b) forms stable complexes with the signaling proteins Grb2 and Shc; (c) induces tyrosine-phosphorylation of Shc; and (d) provokes oncogenic transformation. These findings point to fusion with ATIC as an alternative mechanism of ALK activation.