Pathway Dependence on the Tyrosine Kinase TYK2 and Its Mediator STAT1 In T-Cell Acute Lymphoblastic Leukemia

Abstract 3155

The often aggressive and unpredictable behavior of T-cell lymphoblastic malignancies continues to pose both major clinical challenges in children and adults. To discover oncogenic pathways downstream of critical genetic abnormalities that are characteristically deregulated in T-cell acute lymphoblastic leukemia (T-ALL), and to identify novel molecular targets for anti-leukemic agents with T-cell specificity, we carried out a genome-wide functional screen in T-ALL cell lines using a retroviral library of inducible short-hairpin RNAs (shRNAs). Among the genes that are required for the growth of T-ALL cells, we found that loss of TYK2, a JAK family tyrosine kinase, was specifically lethal in each of three T-ALL cell lines that we tested in this screen. By contrast, TYK2 knock-down did not affect the growth of cell lines from diffuse large B-cell lymphoma or multiple myeloma, indicating that TYK2 is specifically required for the growth of T-ALL cells. We confirmed by knock-down with multiple independent shRNAs that the loss of TYK2 induces apoptosis in T-ALL, whereas knock-down of other JAK proteins (JAK1, JAK2 or JAK3) had no effect. We found that the TYK2 protein is constitutively phosphorylated in many T-ALL cell lines, and that these cells are sensitive to small molecule JAK/TYK2 inhibitors, including JAK inhibitor-I, AG-490 and CEP-701. To identify upstream receptors involved in TYK2 activation, we knocked down IFNAR, IFNGR, IL10R and IL12RB, and found that loss of IFNAR1 and IFNAR2 specifically inhibit the growth of T-ALL cells, as strongly as TYK2 knock-down. In addition, targeted knock-down analysis of downstream STAT proteins revealed that loss of STAT1 also inhibits the growth of T-ALL cells, indicating that this transcription factor is involved in the TYK2 pathway and required for cell survival. In fact, STAT1 protein was constitutively phosphorylated in many T-ALL cell lines and this phosphorylation was inhibited by both TYK2 knock-down and treatment with JAK/TYK2 inhibitors. Although interferon-mediated pathways are cytostatic in many cell types, our results indicate a requirement for the IFNAR-TYK2-STAT1 pathway in promoting the growth and survival of T-ALL cells. Dependence on this pathway confers unique sensitivity of T-ALL cells to TYK2 inhibition by small molecule inhibitors, thus providing a novel therapeutic target for clinical testing in patients with this disease.