STAT3 as a Common Pathway for Anaplastic Lymphomas: Something to Target in ALK-Negative Types?

STAT3 has already been identified as a central player in several inflammation-related malignancies, and various mechanisms, such as chronic cytokine stimulation, constitutive receptor activation, and downstream signaling, have been shown to dysregulate this transcription factor. The role of the JAK/STAT3 pathway is already recognized in ALK⁺ anaplastic large-cell lymphoma (ALCL), where translocations yield a chimeric ALK gene that activates STAT3 and maintains the malignant phenotype. No such mechanism has previously been demonstrated in ALK-negative lymphoma, which lacks these translocations. However, this study from an international collaborative group led by Dr. Giorgio Inghirami and Dr. Raul Rabadan of the University of Torino, Weill Cornell Medical College, New York University, and Columbia University, gives important new clues as to both the pathogenesis and potential targets for intervention.

Exome sequencing was used to investigate the frequency of somatic mutations and copy number variation. Although mutations were evenly distributed across the chromosomes, it was apparent that JAK1 and STAT3 genes were recurrently mutated. This was confirmed by targeted sequencing of 88 cases of ALK-negative systemic ALCL, among which nonsynonymous mutations in JAK1 or STAT3 were found in 18 percent, with 38 percent of cases exhibiting lesions in both genes. Conversely, no mutations in these genes were seen in ALK-positive ALCL or in other types of peripheral T-cell lymphoma. The cases with mutations were all found to express nuclear phosphorylated STAT (pSTAT) by immunohistochemistry, with some nonmutant cases also showing this pattern, suggesting that alternative mechanisms of STAT3 upregulation may also exist. Similar increases in pSTAT were observed in cell lines transfected with mutant STAT3 expression constructs, an effect heightened by exposure to interleukin-6. Injection of mutant STAT3 cells led to metastatic engraftment and early death in immunodeficient mice, whereas mice remained healthy after injection of the wild-type equivalent. Mutant STAT3-expressing cell lines showed upregulation of the transcription factor ATF3 and its downstream targets. Similar observations were made using constructs with mutations in JAK1 of the type found in ALCL, but there was no synergy between the STAT3 and JAK1 mutations, which were lethal when present together in a single cell.

Further investigation of the molecular determinants of malignancy in ALCL were sought by whole-transcriptome RNA sequencing, which revealed that in some ALK-negative cases there were fusion transcripts involving tyrosine kinases such as ROS1 and TYK2, with partners which could provide dimerization domains such as NFkB2. This was shown to permit transphosphorylation, which in turn led to phosphorylation of JAK2/JAK3 and STAT3. Transfection into mouse fibroblasts showed that these chimeric transcripts could produce transformation in vitro and tumors in vivo, albeit less efficiently than NPM-ALK chimeras of the type found in ALK⁺ ALCL.

Investigating the potential for targeted therapy based on these findings, the authors tested inhibitors of the pathway for their effects in cell lines and preclinical models. Although the STAT3 inhibitor niclosamide showed limited activity in cells bearing mutations, an ALK-negative ALCL-derived xenograft showed growth inhibition with clinically relevant concentrations of the JAK1/JAK2 inhibitor ruxolitinib. In the cells bearing translocations such as NFkB2-ROS1, crizotinib did not significantly affect signaling, but a specific anti-ROS1 small molecule did prevent transphosphorylation.