IL-15/IL-15Rα-Fc fusion protein XmAb24306 potentiates activity of CD3 bispecific antibodies through enhancing T cell expansion

Insufficient quantity of functional T cells is a likely factor limiting clinical activity of T cell bispecific antibodies, especially in solid tumor indications. We hypothesized that XmAb24306 (efbalropendekin alfa), a lymphoproliferative interleukin (IL)-15/IL-15 receptor α (IL-15Rα) Fc-fusion protein, may potentiate the activity of T cell dependent (TDB) antibodies. Activation of human peripheral T cells by cevostamab, an anti-FcRH5/CD3 TDB, or anti-HER2/CD3 TDB resulted in upregulation of IL-2/15Rβ (CD122) receptor subunit in nearly all CD8+ and majority of CD4+ T cells, suggesting that TDB treatment may sensitize T cells to the IL-15. XmAb24306 enhanced T cell bispecific antibody induced CD8+ and CD4+ T cell proliferation and expansion. In vitro combination of XmAb24306 with cevostamab or anti-HER2/CD3 TDB resulted in significant enhancement of tumor cell killing, which was reversed when T cell numbers were normalized, suggesting that T cell expansion is the main mechanism for the observed benefit. Pre-treatment of immune competent mice with a mouse-reactive surrogate of XmAb24306 (mIL-15-Fc) resulted in significant increase of T cells in blood, spleen and in tumors and converted transient anti-HER2/CD3 TDB responses to complete durable responses. In summary, our results support the hypothesis where the number of tumor infiltrating T cells is rate limiting for the activity of solid tumor targeting TDBs. Upregulation of CD122 by TDB treatment and the observed synergy with XmAb24306 and T cell bispecific antibodies supports clinical evaluation of this novel immunotherapy combination.

Abstract 2080: LAG3-targeted IL15/IL15Rα-Fc (LAG3 x IL15) fusion proteins for preferential TIL expansion

The IL2Rβ/γc binding human cytokines IL2 and IL15 aid in the activation, proliferation, and survival of T and NK cells, and their therapeutic potential has been well established in animal models and human trials. However, therapeutic approaches utilizing these cytokines have suffered from low tolerability, fast clearance, and limited therapeutic window due to extensive activity on peripheral lymphocytes. Conversely, higher drug concentration and prolonged exposure are desirable to allow lymphocyte activation and proliferation at the tumor site, but this can be difficult to achieve due to dose-limiting toxicities associated with this axis. IL15 functions as a stabilized heterodimeric complex with membrane-bound IL15Rα on the surface of monocytes and DCs, which is presented in trans to lymphocytes expressing IL2Rβ/γc. We hypothesized that we could selectively target tumor-reactive TILs by combining a reduced potency IL15/IL15Rα heterocomplex with an immune checkpoint(CP)-targeting arm to bias binding and activation to CP-positive TILs, potentially improving therapeutic index. Lymphocyte activation gene 3 (LAG3) was chosen as the CP targeting-arm due to its frequent co-expression with PD1, bias to CD8+ T cells, ability to easily combine with anti-PD1 agents, and recent promising results with anti-LAG3 agents in the clinic.

First, potency-reduced IL15/IL15Rα were created by engineering amino acid substitutions in IL15 - at the IL2Rβ/γc interface - that reduced in vitro potency by at least 1000-fold. We then designed LAG3 x IL15 fusion proteins containing single-chain IL15/IL15Rα and LAG3-targeting arms attached to a heterodimeric-Fc region, relying on targeting avidity to recover potency on LAG3+ cells. In vitro proliferation of lymphocytes in human PBMCs, stimulated with sub-optimal concentrations of anti-CD3 to induce LAG3 expression, was monitored by CFSE dilution or by counting Ki67+ cells after incubation with LAG3 x IL15 for 4 days. In vivo activity was evaluated using humanized mouse models by measuring the extent of human leukocyte expansion. Lead LAG3 x IL15 were evaluated for pharmacodynamic activity, pharmacokinetics, and tolerability in non-human primates.

LAG3 x IL15 showed >500-fold selectivity compared to a non-targeted IL15 in an in vitro proliferation assay of lymphocytes stimulated for induced LAG3 expression. In vitro potency was greatest on effector memory CD8 T cells. LAG3 x IL15 were 3-fold more potent on CD8 T cells compared to CD4 T cells and had very weak activity on NK cells, consistent with minimal LAG3 expression on this population. In mouse models, treatment with LAG3 x IL15 promoted significantly increased numbers of T cells. Moreover, LAG3 x IL15 combined productively with anti-PD1 to promote additional T cell expansion. These results demonstrate that LAG3 x IL15 show a promising profile of selective IL15 delivery to TIL with minimal peripheral activity.

Citation Format: Matthew J. Bernett, Suzanne Schubbert, Michael Hackett, Lukasz J. Ochyl, Lizett E. Scott, Christine Bonzon, Rumana Rashid, Kendra N. Avery, Irene W. Leung, Nicole Rodriguez, Connie Ardila, Umesh S. Muchhal, Norman J. Barlow, Rena Bahjat, John R. Desjarlais. LAG3-targeted IL15/IL15Rα-Fc (LAG3 x IL15) fusion proteins for preferential TIL expansion [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2080.

872 PD1 x TGFbR2 and CD5 x TGFbR2 bispecifics selectively block TGFbR2 on target-positive T cells, promote T cell activation, and elicit an anti-tumor response in solid tumors

Background

TGFbeta production by solid tumors and their microenvironment is a major mechanism used by tumors to avoid immunosurveillance. Blockade of TGFbeta has been shown to promote an anti-tumor response; however, systemic blockade of TGFbeta has also been associated with toxicity. We hypothesized that a T cell-targeted TGFbR2 bispecific antibody could selectively block the suppressive activity of TGFbeta on T cells and enhance their anti-tumor activity while avoiding toxicity associated with systemic blockade.

Methods

We engineered bispecific antibodies that simultaneously engage PD1 (activated) or CD5 (pan T) and block TGFbR2 using Xencor’s XmAb® platform. The anti-TGFbR2 arm was tuned for optimal activity by introducing affinity-modulating amino acid substitutions. The activity of TGFbR2 bispecifics was evaluated in vitro using a signaling assay to measure phosphorylated SMAD (pSMAD) by flow cytometry with exogenous TGFbeta in unactivated and activated PBMC. In vivo activity was evaluated by monitoring the engraftment of human PBMC in NSG mice (huPBMC-NSG). Anti-tumor activity was assessed in huPBMC-NSG mice engrafted with established human cancer cell lines.

Results

TGFbR2 bispecifics were confirmed to bind PD1 or CD5 and block binding of TGFbeta to TGFbR2. In vitro, we found that T cells from serum-deprived PBMC exhibited robust induction of pSMAD in response to TGFbeta, and TGFbR2 bispecifics selectively inhibited pSMAD induction in target-positive T cells as demonstrated by over a 100-fold potency increase compared to an untargeted anti-TGFbR2 control. Additionally, we saw an enhancement of potency when evaluating activity in target-high T cells versus target-low or -negative immune cells. Intriguingly, CD5-targeted TGFbR2 bispecifics allowed for the targeting of a broader population of T cells compared to PD1-targeting while still conferring potent selectivity against target-negative cells. In vivo, treatment of huPBMC-NSG mice with TGFbR2 bispecifics promoted superior T cell engraftment. Furthermore, TGFbR2 bispecific treatment of huPBMC-NSG mice containing established MDA-MB-231 triple-negative breast cancer tumors promoted an anti-tumor response that was augmented with PD1 blockade.

Conclusions

PD1 x TGFbR2 and CD5 x TGFbR2 bispecific antibodies were engineered to selectively block TGFbR2 on target-positive T cells and evaluated in vitro and in vivo. These observations are compelling and suggest that development of these bispecifics is warranted for the treatment of human malignancies.

Abstract 1639: Combination of PD1 blockade and T cell costimulation by bispecific antibodies promotes human T cell activation and proliferation

Tumor infiltrating lymphocytes (TILs) express multiple checkpoint receptors, in contrast to lymphocytes found in the periphery (Matsuzaki et al PNAS 2010, Fourcade et al Cancer Res 2012, Gros et al JCI 2014). Checkpoint blockade has demonstrated increased clinical response rates relative to other treatment options; however, many patients fail to achieve a response to checkpoint blockade. We sought to identify an additional therapeutic modality to stack with checkpoint blockade that could increase patient response rate. We hypothesized that engagement of T cell costimulatory receptors in combination with checkpoint blockade could further increase T cell activation and proliferation. The combination of checkpoint blockade with costimulation could be accomplished using a bispecific antibody format, with the potential benefits of reduced cost and more selective targeting of TILs to improve safety.

Antibodies binding to immune checkpoint PD1 and a T cell costimulatory receptor were assembled in a bispecific antibody platform with substitutions in the Fc domain to suppress effector function. PD1 x costimulation (PD1 x costim) bispecific antibodies were evaluated in vitro by measuring antibody binding and de-repression of Staphylococcal enterotoxin B (SEB) stimulated PBMCs. IL2 and IFNγ production was measured by immunoassay. In vivo activity was evaluated using a mouse model in which human PBMCs are engrafted into NSG mice (huPBMC-NSG) and the extent of T cell engraftment is monitored by flow cytometry.

We produced PD1 x costim bispecific antibodies that bound PD1 and a T cell costimulatory receptor monovalently. The bispecifics bound to SEB-stimulated T cells more tightly than monovalent controls, indicating that a single bispecific molecule was capable of avid simultaneous co-engagement of both PD1 and a costimulatory receptor. The bispecifics enhanced IL2 and IFNγ production in an in vitro SEB stimulation assay relative to control (p < 0.001, n = 19 donors). IL2 and IFNγ production was superior to anti-PD1 or anti-costim antibodies alone (p < 0.001), indicating productive and synergistic combination of checkpoint blockade plus costimulation. Treatment of huPBMC-NSG mice with checkpoint bispecifics promoted enhanced T cell engraftment relative to control. Engraftment levels promoted by bispecifics were superior to those found for anti-PD1 treatment alone. For example, in one run of the model, while anti-PD1 treatment alone promoted a 2-fold increase in human CD45+ cells, a PD1 x costim bispecific antibody induced a 16-fold increase.

Combination of checkpoint blockade and costimulation with bispecific antibodies is feasible and promotes strong T cell activation in vitro and in vivo. Compelling activity suggests clinical development is warranted for the treatment of human malignancies.

Citation Format: Gregory L. Moore, Michael Hedvat, Matthew J. Bernett, Rajat Varma, Suzanne Schubbert, Christine Bonzon, Kendra N. Avery, Rumana Rashid, Alex Nisthal, Liz Bogaert, Irene W. Leung, Seung Y. Chu, Umesh S. Muchhal, John R. Desjarlais. Combination of PD1 blockade and T cell costimulation by bispecific antibodies promotes human T cell activation and proliferation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1639. doi:10.1158/1538-7445.AM2017-1639

Methods for PTEN in Stem Cells and Cancer Stem Cells

PTEN (phosphatase and tensin homologue) is the first tumor suppressor identified to have phosphatase activity and its gene is the second most frequently deleted or mutated tumor-suppressor gene associated with human cancers. Germline PTEN mutations are the cause of three inherited autosomal dominant disorders. Phosphatidylinositol 3,4,5,-triphosphate (PIP3), the product of the PI3 kinase, is one of the key intracellular targets of PTEN's phosphatase activity, although PTEN's phosphatase-independent activities have also been identified. PTEN is critical for stem cell maintenance, which contributes to its controlled tumorigenesis. PTEN loss leads the development of cancer stem cells (CSCs) that share properties with somatic stem cells, including the capacity for self-renewal and multi-lineage differentiation. Methods to isolate and functionally test stem cells and CSCs are important for understanding PTEN functions and the development of therapeutic approaches to target CSCs without having adverse effects on normal stem cells. Here, we describe protocols for the isolation and functional analysis of PTEN deficient embryonic stem cells, hematopoietic stem cells and leukemia-initiating cells (LICs), neural stem cells, and prostate stem cells and CSCs.

Targeting the MYC and PI3K pathways eliminates leukemia-initiating cells in T-cell acute lymphoblastic leukemia

Disease relapse remains the major clinical challenge in treating T-cell acute lymphoblastic leukemia (T-ALL), particularly those with PTEN loss. We hypothesized that leukemia-initiating cells (LIC) are responsible for T-ALL development and treatment relapse. In this study, we used a genetically engineered mouse model of Pten(-/-) T-ALL with defined blast and LIC-enriched cell populations to demonstrate that LICs are responsible for therapeutic resistance. Unlike acute and chronic myelogenous leukemia, LICs in T-ALL were actively cycling, were distinct biologically, and responded differently to targeted therapies in comparison with their differentiated blast cell progeny. Notably, we found that T-ALL LICs could be eliminated by cotargeting the deregulated pathways driven by PI3K and Myc, which are altered commonly in human T-ALL and are associated with LIC formation. Our findings define critical events that may be targeted to eliminate LICs in T-ALL as a new strategy to treat the most aggressive relapsed forms of this disease.

Abstract B12: Co-inhibition of PI3K and Myc pathways is synthetic lethal to PTEN null T-ALL leukemia-initiating cells

T-cell acute lymphoblastic leukemia (T-ALL) is a common hematological malignancy associated with poor prognosis and significant risk of relapse. Loss of PTEN function due to mutations or deletions is common in primary T-ALL samples and is associated with resistance to therapeutic agents. We previously developed a VEC-Cre+;Ptenloxp/loxp (Pten null) T-ALL model to investigate the molecular and cellular mechanisms underlying T-ALL pathogenesis and resistance to therapy. Pten-null T-ALL develops after acquisition of a T cell receptor α/δ-c-myc translocation that causes c-Myc over-expression, and the leukemia-initiating cells, or leukemia stem cells (LSCs), are enriched in a c-kitmidCD3+ subpopulation. The goal of our current study is to develop improved therapeutic approaches to target LSCs and eliminate T-ALL. We hypothesize that small molecule inhibitors that target molecular lesions and pathways required for the development and survival of Pten null LSCs and T-ALL may be potent in eliminating LSCs and abolishing T-ALL. Rapamycin, an inhibitor of the PI3K downstream kinase mTOR, can suppress leukemia development in Pten null pre-leukemic mice, but is insufficient to eliminate LSCs and T-ALL. Therefore, we are investigating the therapeutic utility of combinational therapies that co-target multiple deregulated pathways that are critical for LSC formation and T-ALL survival. We found that combination treatment of Pten null T-ALL mice with rapamycin and VX-680, an Aurora kinase inhibitor shown to have synthetic lethality with Myc over-expression, causes robust elimination of leukemia blasts and significantly diminishes the LSC population. Functional studies for leukemia-initiating-cell activity using transplantation of treated bone marrow into NOD-SCID-IL2rγ-/-(NSG) recipients showed that dual treatment reduced leukemia-initiating-cell activity in bone marrow by at least 100-fold in comparison to single agent alone. Importantly, a large portion of Pten null T-ALL LSCs are actively cycling (~50% in S-phase), which may contribute to the mechanism underlying the efficacy of VX-680. We have developed a T-ALL in vitro culture system derived from primary Pten null T-ALL mice to conduct more detailed evaluation of the cellular and biochemical effects of rapamycin and VX-680, perform mechanistic analysis, and screen additional cell cycle-directed and Myc-targeted agents. We found that Pten null T-ALL is highly sensitive to killing by the CDK1 inhibitor purvalanol and JQ1, a bromodomain inhibitor reported to down-regulate c-myc expression that is suitable for use in vivo. Rapamycin and JQ1 combination treatment of Pten null T-ALL mice causes a significant reduction in both the leukemia blast and LSC populations, similar to the effects of rapamycin and VX-680 combination therapy. Our results highlight the importance of understanding the mechanisms underlying T-ALL development and survival and show potent synthetic lethality of PI3K and Myc pathway co-inhibition in PTEN null T-ALL LSCs. Importantly, such targeted combination therapy may offer an improved therapeutic approach to treat human T-ALL.

Citation Format: Suzanne Schubbert, Anjelica Cardenas, Christopher Wei, Lingda Ma, Consuelo Garcia, Wei Guo, Hong Wu. Co-inhibition of PI3K and Myc pathways is synthetic lethal to PTEN null T-ALL leukemia-initiating cells. [abstract]. In: Proceedings of the AACR Precision Medicine Series: Synthetic Lethal Approaches to Cancer Vulnerabilities; May 17-20, 2013; Bellevue, WA. Philadelphia (PA): AACR; Mol Cancer Ther 2013;12(5 Suppl):Abstract nr B12.

Proceedings from the 2009 genetic syndromes of the Ras/MAPK pathway: From bedside to bench and back

The RASopathies are a group of genetic syndromes caused by germline mutations in genes that encode components of the Ras/mitogen-activated protein kinase (MAPK) pathway. Some of these syndromes are neurofibromatosis type 1, Noonan syndrome, Costello syndrome, cardio-facio-cutaneous syndrome, LEOPARD syndrome and Legius syndrome. Their common underlying pathogenetic mechanism brings about significant overlap in phenotypic features and includes craniofacial dysmorphology, cardiac, cutaneous, musculoskeletal, GI and ocular abnormalities, and a predisposition to cancer. The proceedings from the symposium "Genetic Syndromes of the Ras/MAPK Pathway: From Bedside to Bench and Back" chronicle the timely and typical research symposium which brought together clinicians, basic scientists, physician-scientists, advocate leaders, trainees, students and individuals with Ras syndromes and their families. The goals, to discuss basic science and clinical issues, to set forth a solid framework for future research, to direct translational applications towards therapy and to set forth best practices for individuals with RASopathies were successfully meet with a commitment to begin to move towards clinical trials.

De novo HRAS and KRAS mutations in two siblings with short stature and neuro-cardio-facio-cutaneous features

Mutations in genes involved in Ras signalling cause Noonan syndrome and other disorders characterised by growth disturbances and variable neuro-cardio-facio-cutaneous features. We describe two sisters, who presented with dysmorphic features, hypotonia, retarded growth and psychomotor retardation. The patients were initially diagnosed with Costello syndrome, an autosomal recessive inheritance was assumed. Remarkably, however, we identified a germline HRAS mutation (G12A) in one sister and a germline KRAS mutation (F156L) in her sibling. Both mutations had arisen de novo. The F156L mutant K-Ras protein accumulated in the active, guanosine triphosphate-bound conformation and affected downstream signalling. The patient harbouring this mutation was followed for three decades, and her cardiac hypertrophy gradually normalised. However, she developed severe epilepsy with hippocampal sclerosis and atrophy. The occurrence of distinct de novo mutations adds to variable expressivity and gonadal mosaicism as possible explanations of how an autosomal dominant disease may manifest as an apparently recessive condition.

De novo HRAS and KRAS mutations in two siblings with short stature and neuro-cardio-facio-cutaneous features

Mutations in genes involved in Ras signalling cause Noonan syndrome and other disorders characterised by growth disturbances and variable neuro-cardio-facio-cutaneous features. We describe two sisters, 46 and 31 years old, who presented with dysmorphic features, hypotonia, feeding difficulties, retarded growth and psychomotor retardation early in life. The patients were initially diagnosed with Costello syndrome, and autosomal recessive inheritance was assumed. Remarkably, however, we identified a germline HRAS mutation (G12A) in one sister and a germline KRAS mutation (F156L) in her sibling. Both mutations had arisen de novo. The F156L mutant K-Ras protein accumulated in the active, guanosine triphosphate-bound conformation and affected downstream signalling. The patient harbouring this mutation was followed for three decades, and her cardiac hypertrophy gradually normalised. However, she developed severe epilepsy with hippocampal sclerosis and atrophy. The occurrence of distinct de novo mutations adds to variable expressivity and gonadal mosaicism as possible explanations of how an autosomal dominant disease may manifest as an apparently recessive condition.

Mutation analysis in Costello syndrome: functional and structural characterization of the HRAS p.Lys117Arg mutation

Costello syndrome is a mental retardation syndrome characterized by high birth weight, postnatal growth retardation, coarse face, loose skin, cardiovascular problems, and tumor predisposition. De novo heterozygous missense mutations in HRAS codon 12 and 13 disturbing the intrinsic GTP hydrolysis cause Costello syndrome. We report a patient with typical Costello syndrome and a novel heterozygous missense mutation in codon 117 (c.350A>G, p.Lys117Arg) of the HRAS gene, resulting in constitutive activation of the RAS/MAPK pathway similar to the typical p.Gly12Ser and p.Gly12Ala mutations. Recombinant HRAS p.Lys117Arg demonstrates normal intrinsic GTP hydrolysis and responsiveness to GTPase-activating proteins, but the nucleotide dissociation rate is increased 80-fold. Consistent with the biochemical data, the crystal structure of the p.Lys117Arg mutant indicates an altered interaction pattern of the side chain that is associated with unfavorable nucleotide binding properties. Together, these data show that a RAS mutation that only perturbs guanine nucleotide binding has similar functional consequences as mutations that impair GTP hydrolysis and causes human disease.

Biochemical and functional characterization of germ line KRAS mutations

Germ line missense mutations in HRAS and KRAS and in genes encoding molecules that function up- or downstream of Ras in cellular signaling networks cause a group of related developmental disorders that includes Costello syndrome, Noonan syndrome, and cardiofaciocutaneous syndrome. We performed detailed biochemical and functional studies of three mutant K-Ras proteins (P34R, D153V, and F156L) found in individuals with Noonan syndrome and cardiofaciocutaneous syndrome. Mutant K-Ras proteins demonstrate a range of gain-of-function effects in different cell types, and biochemical analysis supports the idea that the intrinsic Ras guanosine nucleotide triphosphatase (GTPase) activity, the responsiveness of these proteins to GTPase-activating proteins, and guanine nucleotide dissociation all regulate developmental programs in vivo.

Hyperactive Ras in developmental disorders and cancer

Ras genes are the most common targets for somatic gain-of-function mutations in human cancer. Recently, germline mutations that affect components of the Ras-Raf-mitogen-activated and extracellular-signal regulated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) pathway were shown to cause several developmental disorders, including Noonan, Costello and cardio-facio-cutaneous syndromes. Many of these mutant alleles encode proteins with aberrant biochemical and functional properties. Here we will discuss the implications of germline mutations in the Ras-Raf-MEK-ERK pathway for understanding normal developmental processes and cancer pathogenesis.

Deregulated Ras signaling in developmental disorders: new tricks for an old dog

Ras proteins regulate cell proliferation, survival and differentiation and are constitutively activated by somatic point mutations in many cancers. Previous studies of neurofibromatosis type 1 and Noonan syndrome also implicated hyperactive Ras in developmental disorders. Recently, germline mutations in H-RAS and K-RAS and in genes encoding other molecules in the Ras-Raf-MEK-ERK cascade were shown to underlie cases of Noonan, cardio-facio-cutaneous, and Costello syndromes. These disorders share phenotypic traits that include abnormal facial features, heart defects, and impaired growth and development. Many of these germline, disease-associated mutations encode novel Ras, Raf and MEK proteins. These studies underscore a crucial role of Ras signaling in human development.

Germline Mutations in Components of the Ras Signaling Pathway in Noonan Syndrome and Related Disorders

Ras proteins control a variety of critical cellular processes, and somatic mutations in RAS genes (and other members of signaling networks regulated by Ras) are common in human malignancies. Ras proteins are guanosine triphosphate (GTP)-binding proteins that cycle between active GTP-bound and inactive guanosine diphosphate (GDP) bound conformations. Cancer-associated Ras mutations typically alter amino acids G12, G13 or Q61. These mutant Ras proteins display impaired GTPase activity and are resistant to GTPase activating proteins (GAPs). We and others recently discovered novel germline KRAS mutations in individuals diagnosed with Noonan or cardio-facio-cutanous (CFC) syndrome, two clinically overlapping disorders characterized by short stature, distinct facial anomalies, heart defects, and other developmental abnormalities. We found that the mutant K-Ras proteins encoded by NS-associated alleles have less pronounced biochemical defects than known Ras oncoproteins, which likely explains why these mutations are tolerated in the germline. Together with the recent findings of mutations in other members of the Ras signaling cascade in CFC syndrome and in Costello syndrome, another clinically related disorder, it is now clear that Noonan-like features are common phenotypic consequences of systemic deregulation of the Ras pathway. The discovery of germline mutations in this group of related genetic disorders underscores the pivotal role of the degree and duration of Ras activation in cell fate decisions during embryonic development and morphogenesis.

Germline KRAS mutations cause Noonan syndrome

Noonan syndrome (MIM 163950) is characterized by short stature, facial dysmorphism and cardiac defects. Heterozygous mutations in PTPN11, which encodes SHP-2, cause approximately 50% of cases of Noonan syndrome. The SHP-2 phosphatase relays signals from activated receptor complexes to downstream effectors, including Ras. We discovered de novo germline KRAS mutations that introduce V14I, T58I or D153V amino acid substitutions in five individuals with Noonan syndrome and a P34R alteration in a individual with cardio-facio-cutaneous syndrome (MIM 115150), which has overlapping features with Noonan syndrome. Recombinant V14I and T58I K-Ras proteins show defective intrinsic GTP hydrolysis and impaired responsiveness to GTPase activating proteins, render primary hematopoietic progenitors hypersensitive to growth factors and deregulate signal transduction in a cell lineage-specific manner. These studies establish germline KRAS mutations as a cause of human disease and infer that the constellation of developmental abnormalities seen in Noonan syndrome spectrum is, in large part, due to hyperactive Ras.

Functional analysis of leukemia-associated PTPN11 mutations in primary hematopoietic cells

PTPN11 encodes the protein tyrosine phosphatase SHP-2, which relays signals from growth factor receptors to Ras and other effectors. Germline PTPN11 mutations underlie about 50% of Noonan syndrome (NS), a developmental disorder that is associated with an elevated risk of juvenile myelomonocytic leukemia (JMML). Somatic PTPN11 mutations were recently identified in about 35% of patients with JMML; these mutations introduce amino acid substitutions that are largely distinct from those found in NS. We assessed the functional consequences of leukemia-associated PTPN11 mutations in murine hematopoietic cells. Expressing an E76K SHP-2 protein induced a hypersensitive pattern of granulocyte-macrophage colony-forming unit (CFU-GM) colony growth in response to granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 3 (IL-3) that was dependent on SHP-2 catalytic activity. E76K SHP-2 expression also enhanced the growth of immature progenitor cells with high replating potential, perturbed erythroid growth, and impaired normal differentiation in liquid cultures. In addition, leukemia-associated SHP-2 mutations conferred a stronger phenotype than a germline mutation found in patients with NS. Mutant SHP-2 proteins induce aberrant growth in multiple hematopoietic compartments, which supports a primary role of hyperactive Ras in the pathogenesis of JMML.

Mutations in PTPN11 implicate the SHP-2 phosphatase in leukemogenesis

The PTPN11 gene encodes SHP-2 (Src homology 2 domain-containing protein tyrosine Phosphatase), a nonreceptor tyrosine protein tyrosine phosphatase (PTPase) that relays signals from activated growth factor receptors to p21Ras (Ras) and other signaling molecules. Mutations in PTPN11 cause Noonan syndrome (NS), a developmental disorder characterized by cardiac and skeletal defects. NS is also associated with a spectrum of hematologic disorders, including juvenile myelomonocytic leukemia (JMML). To test the hypothesis that PTPN11 mutations might contribute to myeloid leukemogenesis, we screened the entire coding region for mutations in 51 JMML specimens and in selected exons from 60 patients with other myeloid malignancies. Missense mutations in PTPN11 were detected in 16 of 49 JMML specimens from patients without NS, but they were less common in other myeloid malignancies. RAS, NF1, and PTPN11 mutations are largely mutually exclusive in JMML, which suggests that mutant SHP-2 proteins deregulate myeloid growth through Ras. However, although Ba/F3 cells engineered to express leukemia-associated SHP-2 proteins cells showed enhanced growth factor-independent survival, biochemical analysis failed to demonstrate hyperactivation of the Ras effectors extracellular-regulated kinase (ERK) or Akt. We conclude that SHP-2 is an important cellular PTPase that is mutated in myeloid malignancies. Further investigation is required to clarify how these mutant proteins interact with Ras and other effectors to deregulate myeloid growth.