The effects of R683S (G) genetic mutations on the JAK2 activity, structure and stability

Malignant JAK-signaling: at the interface of inflammation and malignant transformation

The JAK pathway is central to mammalian cell communication, characterized by rapid responses, receptor versatility, and fine-tuned regulation. It involves Janus kinases (JAK1, JAK2, JAK3, TYK2), which are activated when natural ligands bind to receptors, leading to autophosphorylation and activation of STAT transcription factors [1, 2]. JAK-dependent signaling plays a pivotal role in coordinating cell communication networks across a broad spectrum of biological systems including development, immune responses, cell growth, and differentiation. JAKs are frequently mutated in the aging hematopoietic system [3, 4] and in hematopoietic cancers [5]. Thus, dysregulation of the pathway results in various diseases, including cancers and immune disorders. The binding of extracellular ligands to class I and II cytokine receptors initiates a critical signaling cascade through the activation of Janus kinases (JAKs). Upon ligand engagement, JAKs become activated and phosphorylate specific tyrosine residues on the receptor, creating docking sites for signal transducer and activator of transcription (STAT) proteins. Subsequent JAK-mediated phosphorylation of STATs enables their dimerization and nuclear translocation, where they function as transcription factors to modulate gene expression. Under physiological conditions, JAK-signaling is a tightly regulated mechanism that governs cellular responses to external cues, such as cytokines and growth factors, ensuring homeostasis and maintaining the functional integrity of tissues and organs. Highly defined regulation of JAK-signaling is essential for balancing cellular responses to inflammatory stimuli and growth signals, thus safeguarding tissue health. In contrast, dysregulated JAK-signaling results in chronic inflammation and unrestrained cellular proliferation associated with various diseases. Understanding the qualitative and quantitative differences at the interface of physiologic JAK-signaling and its aberrant activation in disease is crucial for the development of targeted therapies that precisely tune this pathway to target pathologic activation patterns while leaving homeostatic processes largely unaffected. Consequently, pharmaceutical research has targeted this pathway for drug development leading to the approval of several substances with different selectivity profiles towards individual JAKs. Yet, the precise impact of inhibitor selectivity and the complex interplay of different functional modules within normal and malignant cells remains incompletely understood. In this review, we summarize the current knowledge on JAK-signaling in health and disease and highlight recent advances and future directions in the field.

Integrative Insights into Philadelphia-like B-Cell Acute Lymphoblastic Leukemia: A Genetic and Molecular Landscape

Philadelphia-like chromosome acute lymphoblastic leukemia (Ph-like ALL) is a new subtype of B-ALL that was discovered in 2009 and recognized in the 2016 revision of the World Health Organization criteria under the classification of myeloid neoplasms and acute leukemia. This new subtype has an extremely poor prognosis compared to that for other subtypes of ALL, with a 41% five-year overall survival (OS) rate. Ph-like ALL is chemoresistant, with a high minimum residual disease (MRD) level after induction therapy, and it is associated with a high relapse rate. Clinical trials are currently being conducted to study the effectiveness of specific tyrosine kinase inhibitors against different genetic alterations in Ph-like ALL patients and the effect of allogeneic hematopoietic cell transplants (allo-HCT) on treatments. This review summarizes the current findings on Ph-like ALL, focusing on its molecular landscape and clinical implications.

Clinical features and prognosis of pediatric acute lymphocytic leukemia with JAK-STAT pathway genetic abnormalities: a case series

The objective of this study is to explore the clinical features and outcomes of pediatric patients with acute lymphoblastic leukemia (ALL) harboring JAK-STAT signaling pathway genetic abnormalities. This retrospective case series examined the clinical data of pediatric patients diagnosed with ALL harboring JAK-STAT pathway genetic abnormality at the Children's Hospital of the Capital Institute of Pediatrics between January 2016 and January 2022. Bone marrow next-generation sequencing was used to reveal the JAK pathway abnormalities. Descriptive statistics were used. From 432 children with ALL during the study period, eight had JAK-STAT pathway genetic abnormalities. Regarding immunotyping, there were four patients with common-B cell types and one with pre-B cell type. The three patients with T-ALL had early T-cell precursor(ETP) type, pre-T cell type, and T cell type. Gene mutations were more common than fusion genes. There was no central nervous system involvement in eight patients. All patients were considered at least at intermediate risk before treatments. Four patients underwent hematopoietic stem cell transplantation (HSCT). One child had a comprehensive relapse and died. The child had a severe infection and could not tolerate high-intensity chemotherapy. Another child relapsed 2 years after HSCT and died. Disease-free survival was achieved in six children. JAK-STAT pathway genetic abnormalities in pediatric Ph-like ALL are rare. Special attention should be paid to treatment-related complications, such as infection and combination therapy (chemotherapy, small molecule targeted drugs, immunotherapy, etc.) to reduce treatment-related death and improve long-term quality of life.

Molecular basis of JAK2 H608Y and H608N mutations in the pathology of acute myeloid leukemia

Risk-stratification of acute myeloid leukemia (AML) based on (cyto)genetic aberrations, including hotspot mutations, deletions and point mutations have evolved substantially in recent years. With the development of next-generation sequence technology, more and more novel mutations in the AML were identified. Thus, to unravel roles and mechanism of novel mutations would improve prognostic and predictive abilities. In this study, two novel germline JAK2 His608Tyr (H608Y) and His608Asn (H608N) mutations were identified and the molecular basis of these mutations in the leukemiagenesis of AML was elucidated. Our results indicated that JAK2 H608Y and H608N mutations disrupted the hydrogen bond between Q656 and H608 which reduced the JH2 domain's activity and abolished interactions between JH1 and JH2 domains, forced JAK2 into the active conformation, facilitated the entrance of substrates and thus caused JAK2 hyperactivation. Further studies suggested that JAK2 H608Y and H608N mutations enhanced the cell proliferation and inhibited the differentiation of Ba/F3 and MV4-11 cells via activating the JAK2-STAT5 signaling pathway. Moreover, rescue experiments demonstrated that mutations repaired the hydrogen bond between Q656 and H608 displayed opposite results. Thus, this study revealed the molecular basis of JAK2 H608Y and H608N mutations in the pathology of AML.

[Detection of activating mutations in RAS/RAF/MEK/ERK and JAK/STAT signaling pathways]

ISSUE

The study of activating mutations (NRAS,KRAS,FLT3,JAK2,CRLF2genes) of RAS/RAF/MEK/ERK and JAK/STAT signaling pathways in B-cell acute lymphoblastic leukemia (B-ALL) in adult patients which are included in Russian multicenter clinical trials.

MATERIALS AND METHODS

Within the multicenter study there were 119 adult patients included withde novoB-ALL. The study was considered as prospective and retrospective. The group withBCR-ABL1-negative B-ALL consisted of up to 93 patients (45 male and 48 female, at the age of 17 to 59, the median age 31), they were treated according to the protocols ALL-2009, ALL-2016. The median follow-up lasted for 19 months (1119). The group withBCR-ABL1-positive B-ALL with up to 26 patients (10 male and 16 female, at the age of 23 to 78, the median age 34 years) was included in the study as well. The treatment was carried out according to the protocols ALL-2009 and ALL-2012 in combination with tyrosine kinase inhibitors. The median follow-up lasted for 23 months (4120). The molecular analysis of activating mutations inNRAS,KRASgenes (RAS/RAF/MEK/ERK signaling pathway) andJAK2,CRLF2genes (JAK/STAT signaling cascade) was performed via Sanger sequencing. The internal tandem duplications (ITDs) inFLT3gene were studied by fragment analysis. The evaluation of CRLF2 expression was fulfilled via flow cytometry.

RESULTS

Activating mutations inNRAS,KRAS,FLT3genes were found in 22 (23.6%) patients withBCR-ABL1-negative B-ALL. In total, 23 mutations were revealed in theNRAS(n=9),KRAS(n=12), andFLT3(n=2) genes, according to statistics that was significantly more frequent than withBCR-ABL1-positive B-ALL, these genes mutations were not identified in patients (p=0.007). The frequency of mutations detection inKRASandNRASgenes in patients withBCR-ABL1-negative B-ALL was comparable as 12.9% (12 of 93) to 9.7% (9 of 93), respectively (p=0.488). One patient was simultaneously revealed 2 mutations in theKRASgene (in codons 13 and 61).FLT3-ITD mutations were detected in 3.5% (2 of 57) cases ofBCR-ABL1-negative B-ALL. In patients withBCR-ABL1-positive B-ALLFLT3-ITD mutations were not assessed. Violations in the JAK/STAT signaling cascade were detected in 4 (4.3%) patients withBCR-ABL1-negative B-ALL. They were represented by the missense mutations ofJAK2gene (n=3) and the overexpression of CRLF2 (n=2); in one patient were detected the overexpression of CRLF2 and a mutation inJAK2gene simultaneously. No mutations were found inCRLF2gene. In patients withBCR-ABL1-positive B-ALL noJAK2mutations were detected. As long as analyzing demographic and clinical laboratory parameters between groups of patients with and without mutations, there were no statistically significant differences obtained. In the analyzed groups of patients, long-term therapy results did not differentiate according to the mutations presence inNRAS,KRAS,FLT3,JAK2genes. Also, substantive differences were not shown in the rate of the negative status achievement of the minimum residual disease between patients with and without activating mutations in the control points of the protocol (on the 70th, 133rd and 190th days).

CONCLUSION

NRAS,KRAS,FLT3,JAK2activating mutations do not affect the long-term results of the therapy and the rate of the negative status achievement of the minimum residual disease in patients withBCR-ABL1-negative B-ALL treated by the Russian multicenter clinical trials.

Genetic Signature of Acute Lymphoblastic Leukemia and Netherton Syndrome Co-incidence-First Report in the Literature

The aim of the following case report is to provide a description of acute lymphoblastic leukemia (ALL) in a patient with Netherton syndrome (NS). A 15-year-old male with NS was referred with suspicion of acute leukemia. Severe anemia, leukocytosis, thrombocytopenia, and elevated CRP level were demonstrated in pre-hospital laboratory tests. Physical examination revealed generalized ichthyosiform erythroderma. ALL was diagnosed on the basis of bone marrow biopsy. The patient was initially classified as CNS3 status. No signals indicating fusion of BCR/ABL1, ETV6, and RUNX1 genes and MLL gene rearrangement were found in the cytogenetic analysis. The patient was qualified for chemotherapy and treated according to ALL IC-BFM 2009 protocol for high-risk ALL. During induction therapy, severe skin toxicity occurred (WHO grade III), which prompted the modification of treatment down to intermediate-risk strategy. In the course of reinduction therapy, severe chemotherapy-induced adverse drug reactions occurred, including progression of skin toxicity to WHO grade IV. The patient achieved complete remission. In view of life-threatening toxicities and the confirmed complete remission, intensive chemotherapy regimen was discontinued and maintenance treatment was started. Because of the baseline CNS3 status, the patient received cranial radiotherapy. Whole exome sequencing (WES) was used to identify disease-associated mutations. WES revealed two germline mutations: a novel premature termination variant in SPINK5 (p.Cys510^*), along with a novel potentially pathogenic variant in NUP214 (p.Arg815Gln). Somatic mutations were known pathogenic variants of JAK2 (p.Arg683Gly), IL17RC (p.Ala303Thr), and potentially pathogenic non-synonymous variants of TTN (p.Gly1091Arg and p.Pro17245Leu), ACTN2 (p.Ile143Leu), TRPV3 (p.Arg729^*), and COL7A1 (p.Glu2842fs) genes. Currently, the patient continues maintenance chemotherapy, with stable status of skin lesions and no features of ALL relapse. To our knowledge, this is the first report of ALL in a patient with NS. As has been presented, in such patients, optimal treatment according to the current protocols is extremely difficult. WES was used to confirm the diagnosis of Ph-like ALL in our patient. The detection of JAK2 gene mutation offers the possibility of therapy personalization. A specific signature of rare germline variants and somatic mutations can be proposed as a factor predisposing to the co-incidence of ALL and NS.

Roles of T875N somatic mutation in the activity, structural stability of JAK2 and the transformation of OCI-AML3 cells

Activating mutations in JAK2 have been described in patients with various hematologic malignancies including acute myeloid leukemia (AML) and myeloproliferative neoplasms. However, mechanism of these mutations in JAK2's activity, structural stability and pathology of AML remains poorly understood. The JAK2 T875N somatic mutation has been detected in about 5.2% of AML patients. But the structural basis and mechanism of JAK2 T875N mutation in the pathology of AML is still unclear. Our results suggested that JAK2 T875N mutation disrupted the T875 and D873 interaction which destroyed the compact structure of JH1 domain, forced it into the active conformation, facilitated the entrance of substrate and thus led to JAK2 hyperactivation. Mutations (T875N, T875A, D873A and D873G) disrupted the T875 and D873 interaction enhanced JAK2's activity, decreased its structural stability and JH2 domain's activity which further enhanced JAK2's activity, while mutations (T875R, D873E, T875R/D873E) repaired this interaction displayed opposite results. Moreover, JAK2 T875N mutation enhanced the activity of JAK2-STAT5 pathway, promoted the proliferation and transformation of OCI-AML3 cells. This study provides clues in understanding structural basis of T875N mutation caused JAK2 hyperactivation and its roles in the pathology of AML.

Prognostic gene alterations and clonal changes in childhood B-ALL

Genomic profiles of leukemia patients lead to characterization of variations that provide the molecular classification of risk groups, prediction of clinical outcome and therapeutic decisions. In this study, we examined the diagnostic (n = 77) and relapsed (n = 31) pediatric B-cell acute lymphoblastic leukemia (B-ALL) samples for the most common leukemia-associated gene variations CRLF2, JAK2, PAX5 and IL7R using deep sequencing and copy number alterations (CNAs) (CDKN2A/2B, PAX5, RB1, BTG1, ETV6, CSF2RA, IL3RA and CRLF2) by multiplex ligation proximity assay (MLPA), and evaluated for the clonal changes through relapse. Single nucleotide variations SNVs were detected in 19% of diagnostic 15.3% of relapse samples. The CNAs were detected in 55% of diagnosed patients; most common affected genes were CDKN2A/2B, PAX5, and CRLF2. Relapse samples did not accumulate a greater number of CNAs or SNVs than the cohort of diagnostic samples, but the clonal dynamics showed the accumulation/disappearance of specific gene variations explained the course of relapse. The CDKN2A/2B were most frequently altered in relapse samples and 32% of relapse samples carried at least one CNA. Moreover, CDKN2A/2B alterations and/or JAK2 variations were associated with decreased relapse-free survival. On the other hand, CRLF2 copy number alterations predicted a better survival rate in B-ALL. These findings contribute to the knowledge of CDKN2A/2B and CRLF2 alterations and their prognostic value in B-ALL. The integration of genomic data in clinical practice will enable better stratification of ALL patients and allow deeper understanding of the nature of relapse.

Disruption of R867 and Y613 interaction plays key roles in JAK2 R867Q mutation caused acute leukemia

Janus tyrosine kinase 2 (JAK2) mediates downstream signaling of cytokine receptors in all hematological lineages, constitutively active somatic JAK2 mutations were important for the leukemogenesis of acute leukemia (AL). The JAK2 R867Q somatic mutation is detected in a subset of AL patients. However, roles of JAK2 R867Q mutation in the pathogenesis of AL remain unclear. In this study, homology modeling analysis showed that loss of interaction between R867 and Y613 disrupted the JAK2 JH1/JH2 domain's interactions was responsible for its activation. JAK2 R867Q and mutations (R867A and R867G) abolished this interaction caused JAK2 constitutive activation. While, mutations (R867K, Y613E, R867K/Y613E) repairing this interaction reduced JAK2 R867Q mutation's activity. Furthermore, our studies showed that abolished R867 and Y613 interaction disrupted JH1/JH2 domains' interactions and led to JAK2 constitutive activation. More importantly, mutations (R867Q, R867A and R867G) disrupted this interaction enhanced the activity of JAK2-STAT5 pathway and the proliferation of Ba/F3 and MV4-11 cells. Further study showed that JAK2 R867Q mutation promoted the expression of proliferation marker and inhibited the differentiation marker of Ba/F3 and MV4-11 cells. Thus our studies provide clues in understanding the pathogenesis of JAK2 R867Q mutation in AL.

Loss of K607 and E877 interaction is a key reason for JAK2 K607N mutation caused acute myeloid leukemia

Oncogenic activation of tyrosine kinase signaling pathway is recurrent in human leukemia. The acquired Janus kinase 2 (JAK2) K607N somatic mutation was detected in about 6.8% of acute myeloid leukemia (AML) patients. However, roles of JAK2 K607N mutation in the leukemogenesis of AML remain unclear. In this study, loss of interaction between K607 and E877 was identified as key reasons for JAK2 K607N mutation constitutive activation. JAK2 K607N and mutations (K607A, K607G and E877A) abolished the K607 and E877 interaction caused JAK2 constitutive activation. While, mutations (K607R, E877D) repairing this interaction reduced K607N mutation's activity. Furthermore, our studies showed that disruption of K607 and E877 interaction abolished JH1/JH2 domains' interactions and led to JAK2 constitutive activation. More importantly, JAK2 K607N and mutations disrupted this interaction enhanced JAK2-STAT5 pathway activation and the proliferation of Ba/F3 cells. Thus our studies provide clues in understanding the leukemogenesis of JAK2 K607N mutation in AML.

Effects of JAK2 V556F mutation on the JAK2's activity, structural stability and the transformation of Ba/F3 cells

Although roles of somatic JAK2 mutations in clonally myeloproliferative neoplasms (MPNs) are well established, roles of germline JAK2 mutations in the pathogenesis of MPNs remain unclear. Recently, a novel activating, germline JAK2 F556V mutation was identified and involved in the pathogenesis of MPNs, but, its pathogenesis mechanism was still unknown. In this study, homology models of JAK2 demonstrated that F556 located between two threonine residues which interacted with ATP phosphate groups by hydrogen bonds, Thr555 with the γ-phosphate and Thr557 with the β-phosphate in the active site of JAK2's JH2 domain. Moreover, the hydrogen bond between Thr557 and Arg715 played vital roles in sustaining the structural conformation of JH2's active site and JH1-JH2 domains' interactions. When F556 was replaced by other amino acids except Trp, the hydrogen bond, JH2 domain's structural conformation and JH1-JH2 domains' interactions disrupted for changing the helix between β2 and β3 strands which finally caused JAK2 activation. Mechanistic and functional studies showed that JAK2 F556V mutation disrupted JAK2 JH2 domain's activity, caused JAK2-STAT5 pathway activation and promoted the proliferation of BaF3 cells. Thus, our results herein may provide clues to understand the pathogenesis mechanism of JAK2 F556V mutation in the MPNs.

Roles of germline JAK2 activation mutation JAK2 V625F in the pathology of myeloproliferative neoplasms

Janus tyrosine kinase 2 (JAK2) mediates downstream signaling of cytokine receptors in all hematological lineages, constitutively active somatic JAK2 mutations play key roles in the pathology of myeloproliferative neoplasms (MPNs). Recently, germline JAK2 mutations are also associated with triple-negative MPNs. A novel germline mutation JAK2 V625F is reported to be involved in a subset of MPNs patients. However, the pathogenesis of this mutation caused MPN is still unclear. In this study, the homology models of JAK2 V625F showed that the newly formed interaction between F625 and Y613 disrupted the JAK2 JH1-JH2 domain interactions was responsible for its activation, when F625 and Y613 interaction was disrupted, its activity significantly decreased. While, when this interaction was repaired whether by forming hydrogen bond or salt bond, it would cause JAK2 activation. Biochemical studies also demonstrated that JAK2 V625F mutation led to JAK2-STAT5 pathway activation and promoted the proliferation of BaF3 cells. Thus, our results herein provide clues to understand the mechanism JAK2 V625F mutation caused MPNs and give information for the development of JAK2 mutation specific inhibitors.

New Challenges in Targeting Signaling Pathways in Acute Lymphoblastic Leukemia by NGS Approaches: An Update

The identification and study of genetic alterations involved in various signaling pathways associated with the pathogenesis of acute lymphoblastic leukemia (ALL) and the application of recent next-generation sequencing (NGS) in the identification of these lesions not only broaden our understanding of the involvement of various genetic alterations in the pathogenesis of the disease but also identify new therapeutic targets for future clinical trials. The present review describes the main deletions, amplifications, sequence mutations, epigenetic lesions, and new structural DNA rearrangements detected by NGS in B-ALL and T-ALL and their clinical importance for therapeutic procedures. We reviewed the molecular basis of pathways including transcriptional regulation, lymphoid differentiation and development, TP53 and the cell cycle, RAS signaling, JAK/STAT, NOTCH, PI3K/AKT/mTOR, Wnt/β-catenin signaling, chromatin structure modifiers, and epigenetic regulators. The implementation of NGS strategies has enabled important mutated genes in each pathway, their associations with the genetic subtypes of ALL, and their outcomes, which will be described further. We also discuss classic and new cryptic DNA rearrangements in ALL identified by mRNA-seq strategies. Novel cooperative abnormalities in ALL could be key prognostic and/or predictive biomarkers for selecting the best frontline treatment and for developing therapies after the first relapse or refractory disease.

JAK2 aberrations in childhood B-cell precursor acute lymphoblastic leukemia

JAK2 abnormalities may serve as target for precision medicines in pediatric B-cell precursor acute lymphoblastic leukemia (BCP-ALL). In the current study we performed a screening for JAK2 mutations and translocations, analyzed the clinical outcome and studied the efficacy of two JAK inhibitors in primary BCP-ALL cells. Importantly, we identify a number of limitations of JAK inhibitor therapy. JAK2 mutations mainly occurred in the poor prognostic subtypes BCR-ABL1-like and non- BCR-ABL1-like B-other (negative for sentinel cytogenetic lesions). JAK2 translocations were restricted to BCR-ABL1-like cases. Momelotinib and ruxolitinib were cytotoxic in both JAK2 translocated and JAK2 mutated cells, although efficacy in JAK2 mutated cells highly depended on cytokine receptor activation by TSLP. However, our data also suggest that the effect of JAK inhibition may be compromised by mutations in alternative survival pathways and microenvironment-induced resistance. Furthermore, inhibitors induced accumulation of phosphorylated JAK2^Y1007, which resulted in a profound re-activation of JAK2 signaling upon release of the inhibitors. This preclinical evidence implies that further optimization and evaluation of JAK inhibitor treatment is necessary prior to its clinical integration in pediatric BCP-ALL.

Mutations in TP53 and JAK2 are independent prognostic biomarkers in B-cell precursor acute lymphoblastic leukaemia

BACKGROUND

In B-cell precursor acute lymphoblastic leukaemia (B-ALL), the identification of additional genetic alterations associated with poor prognosis is still of importance. We determined the frequency and prognostic impact of somatic mutations in children and adult cases with B-ALL treated with Spanish PETHEMA and SEHOP protocols.

METHODS

Mutational status of hotspot regions of TP53, JAK2, PAX5, LEF1, CRLF2 and IL7R genes was determined by next-generation deep sequencing in 340 B-ALL patients (211 children and 129 adults). The associations between mutation status and clinicopathological features at the time of diagnosis, treatment outcome and survival were assessed. Univariate and multivariate survival analyses were performed to identify independent prognostic factors associated with overall survival (OS), event-free survival (EFS) and relapse rate (RR).

RESULTS

A mutation rate of 12.4% was identified. The frequency of adult mutations was higher (20.2% vs 7.6%, P=0.001). TP53 was the most frequently mutated gene (4.1%), followed by JAK2 (3.8%), CRLF2 (2.9%), PAX5 (2.4%), LEF1 (0.6%) and IL7R (0.3%). All mutations were observed in B-ALL without ETV6-RUNX1 (P=0.047) or BCR-ABL1 fusions (P<0.0001). In children, TP53mut was associated with lower OS (5-year OS: 50% vs 86%, P=0.002) and EFS rates (5-year EFS: 50% vs 78.3%, P=0.009) and higher RR (5-year RR: 33.3% vs 18.6% P=0.037), and was independently associated with higher RR (hazard ratio (HR)=4.5; P=0.04). In adults, TP53mut was associated with a lower OS (5-year OS: 0% vs 43.3%, P=0.019) and a higher RR (5-year RR: 100% vs 61.4%, P=0.029), whereas JAK2mut was associated with a lower EFS (5-year EFS: 0% vs 30.6%, P=0.035) and a higher RR (5-year RR: 100% vs 60.4%, P=0.002). TP53mut was an independent risk factor for shorter OS (HR=2.3; P=0.035) and, together with JAK2mut, also were independent markers of poor prognosis for RR (TP53mut: HR=5.9; P=0.027 and JAK2mut: HR=5.6; P=0.036).

CONCLUSIONS

TP53mut and JAK2mut are potential biomarkers associated with poor prognosis in B-ALL patients.

Recurrent mutation of JAK3 in T-cell prolymphocytic leukemia

T-cell prolymphocytic leukemia (T-PLL) is an aggressive post-thymic T-cell malignancy characterized by the recurrent inv(14)(q11q32)/t(14;14)(q11;q32) or t(X;14)(q28;q11) leading to activation of either the TCL1 or MTCP1 gene, respectively. However, these primary genetic events are insufficient to drive leukemogenesis. Recently, activating mutations in JAK3 have been identified in other T-cell malignancies. Since JAK3 is essential for T-cell maturation, we analyzed a cohort of 32 T-PLL patients for mutational hot spots in the JAK3 gene using a step-wise screening approach. We identified 14 mutations in 11 of 32 patients (34%). The most frequently detected mutation in our cohort was M511I (seen in 57% of cases) previously described as an activating change in other T-cell malignancies. Three patients carried two mutations in JAK3. In two patients M511I and R657Q were simultaneously detected and in another patient V674F and V678L. In the latter case we could demonstrate that the mutations were on the same allele in cis. Protein modeling and homology analyses of mutations present in other members of the JAK family suggested that these mutations likely activate JAK3, possibly by disrupting the activation loop and the interface between N and C lobes, increasing the accessibility of the catalytic loop. In addition, four of the 21 patients lacking a JAK3 point mutation presented an aberrant karyotype involving the chromosomal band 19p13 harboring the JAK3 locus. The finding of recurrent activating JAK3 mutations in patients with T-PLL could enable the use of JAK3 inhibitors to treat patients with this unfavorable malignancy who otherwise have a very poor prognosis.