Author:

Keywords:

T-cell acute lymphoblastic leukemia, phosphorylation, tumor suppressor, PTPN2

Abstract:

Cancer originates from cells that have undergone oncogenic transformation allowingthem to escape from their usual control mechanisms. Transformed cells carrytypical alterations in their genome that are most often acquired throughout acells life span due to exposure to irradiation, infectious reagents, and othercarcinogens. As a consequence neoplastic cells display uncontrolled cell growthand/or can spread unlimited through the human body. Leukemia is a cancer of theblood or the bone marrow. When leukemia occurs, the bone marrow produces anexcess of leukemia cells that don’t serve their purpose, and outnumber theregular cells preventing them from doing their work properly. T-cell acute lymphoblasticleukemia (T-ALL) is a type of leukemia of a particular white blood cell type,called T-cells. T-ALL arises from clonal expansion of a developing T-cell thathas undergone stepwise alteration at distinct stages of differentiation. Todayit is known that multigenetic events collaborativelydrive the leukemogenic process in T-ALL. Typically, accumulated mutationssimultaneously disturb multiple pathways thereby providing the cell a facet offunctional advantages such as enhanced proliferation and/or reduced sensitivityto death signals and inhibit its normal maturation. Despite major improvements in ourunderstanding of the molecular genetics of T-ALL, the underlying mechanismsthat lead to the abnormal proliferation and enhanced survival of the leukemiccells remain largely unknown. Therefore our objective was to decipher noveltumor promoters or suppressors with a role in the pathogenic proliferation/survivalof leukemic T-cells.In this work, we identified the protein tyrosinephosphatase PTPN2 as novel tumor suppressor in T-ALL. ArrayCGH analysis revealed an acquired microdeletion at chromosome 18p11 in 6 % ofT-ALL cases. Thedeleted region was only about 125 kb in size and restricted to the PTPN2 gene locus. Deletion of PTPN2 was stronglyassociated with the oncogenic cortical T-ALL subtype which is defined by ablock in differentiation at the CD4+CD8+CD3- stagedue to ectopic expression of homeobox transcription factor TLX1. Eight caseswith deletion of PTPN2 also expressed the oncogenic fusion kinase NUP214-ABL1. Functional studies confirmed a suppressive effectof PTPN2 on T-cell function. Using RNA interference techniques, we demonstratedthat the phosphatase acts as modulator of JAK1-associated cytokine receptorsignaling. Downregulation of PTPN2 protein levels enhanced the responsiveness ofleukemic cells to cytokines which was associated with enhanced proliferationand reduced sensitivity to kinase inhibitors. We also identified the oncogenicNUP214-ABL1 fusion as novel bona-fide substrate of PTPN2. We were ableto demonstrate a direct interaction between the fusion kinase and PTPN2 andalso showed a modulatory effect on NUP214-ABL phosphorylation levels due to alterationsin PTPN2 expression. Our genetic and functional data indicate that loss of PTPN2 enhances theoncogenic properties of NUP214-ABL1, and suggest that loss of PTPN2 could alsopotentiate other oncogenic kinases, such as JAK1 kinase, a known substrate ofPTPN2 and reported to be mutated in about 10% of T-ALLs. Weidentified two T-ALL cases where PTPN2was either deleted or mutated in the presence of an activating JAK1 mutation. In line with thesegenetic findings, knockdown of Ptpn2 facilitated cytokine independent growth ofJAK1 mutant expressing Ba/F3 cells which was accompanied by increased basal activationof the JAK/STAT pathway. Finally, cells expressing lower levels of Ptpn2 wereless sensitive to JAK inhibition. Overall, our data provide genetic andfunctional evidence for a concomitant appearance of mutation-positive JAK1 kinaseor NUP214-ABL1 with inactivation of the negative regulator PTPN2 therebypotentiating each others oncogenic capacity. Wealso investigated a role of PTPN2 astumor suppressor gene in Hodgkin lymphoma and T-cell non-Hodgkin lymphoma.While we could not confirm a role of PTPN2 in the development of HL,inactivation of PTPN2 was evidently a recurrent genetic aberration in aparticular subtype of T-cell non-Hodgkin lymphoma (T-NHL). Thiswork provides valuable evidence for a tumor suppressor function of PTPN2 inT-cell neoplasms, namely T-ALL and T-NHL, and warrants testing of JAKinhibitors for the treatment of this specific subset of T-ALLs as well asfurther analysis of a potential negative impact of loss of PTPN2 onresponsiveness to anti-cancer treatments. Beyond the specific implications ofour findings for T-ALL, our results suggest that loss of phosphatases in cancercells with activated kinases could be a common mechanism toreinforce each others oncogenicity in the transformation process.