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Functional Evaluation of Novel STAT3 Mutations Identified in Large Granular Lymphocytic Leukemia

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Title: Functional Evaluation of Novel STAT3 Mutations Identified in Large Granular Lymphocytic Leukemia
Author(s): Kuusanmäki, Heikki
Contributor: University of Helsinki, Faculty of Medicine, Institute of Biomedicine
Language: English
Acceptance year: 2013
Recently, our research group together with the Hematology Research Unit Helsinki, found that 31 of 77 patients (40%) with T-cell large granular lymphocytic leukemia have somatic point mutations in the Src Homology 2 (SH2) domain of the STAT3 gene. LGL leukemia is a rare and indolent disease characterized by the clonal expansion of large granular lymphocytes of unknown etiology. The aim of this master's thesis study was to elucidate whether the identified Y640F and D661V mutations can cause hyperactivity of the STAT3 protein and excessive proliferation of T-cells. STAT3 is a transcription factor that is known to have a key role in cell proliferation and apoptosis, and which is activated by the phosphorylation of receptor-associated kinases. The phosphorylation of tyrosine residue 705 in STAT3 induces dimerization and localization of the STAT3 dimer to the nucleus. In several cancers STAT3 protein has been reported to be constitutively active. Furthermore, previously published data strongly support our hypothesis that the mutations identified in LGL patients might cause STAT3 to be hyperactive resulting in inhibition of apoptotic pathways in cytotoxic T cells. In order to evaluate the function of the novel mutations, expression constructs of STAT3 containing the D661V and Y640F mutations were generated. In addition, lentiviral vectors were produced to establish a T-cell line (Jurkat) with stable expression of mutant STAT3. After the successful generation of STAT3 constructs and cell line models, several functional assays were performed. Transcriptional activity of STAT3 was measured by luciferase reporter assay and immunocytochemistry was used to determine whether the mutations promote nuclear localization of STAT3. STAT3 phosphorylation was examined by immunoblotting. In addition, quantitative RT-PCR was used to detect differential expression of five STAT3 target genes from patient samples. The results, particularly the luciferase reporter assay, indicated a significant difference between the mutant and wild type STAT3 providing strong evidence that the STAT3 mutants are transcriptionally more active. The localization assay, imaged by fluorescence microscopy, showed more STAT3 D661V and Y640F protein present in the nucleus when compared to wild type STAT3. However, the proliferation rate of mutant STAT3 expressing Jurkat cells was not increased. In addition, STAT3 target gene expression levels of two patient samples did not show large differences when compared to healthy LGL cells. As a result of these findings, it can be strongly hypothesized that aberrant STAT3 signaling underlies the cause of T-cell LGL leukemia. Understanding the molecular basis of LGL leukemia is important in order to develop diagnostic and therapeutic strategies for patients suffering from the disease. Since constitutively active STAT3 is common amongst many cancers and autoimmune disorders, activating mutations could possibly be found in these diseases. More careful sequencing studies of STAT3 upstream molecules are warranted as well, and will be performed in the future from leukemic LGL samples.

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