Mutation D816V alters the internal structure and dynamics of c-KIT receptor cytoplasmic region: implications for dimerization and activation mechanisms

Management of Advanced Systemic Mastocytosis and Associated Myeloid Neoplasms

Advanced systemic mastocytosis (AdvSM) is a heterogeneous group of disorders characterized by neoplastic mast cell-related organ damage and frequently associated with a myeloid neoplasm. The 3 clinical entities that comprise AdvSM are aggressive SM (ASM), SM-associated hematologic neoplasm, and mast cell leukemia. A gain-of-function KIT D816 V mutation is the primary oncogenic driver found in about 90% of all patients with AdvSM. Midostaurin, an oral multikinase inhibitor with activity against KIT D816V, and avapritinib, an oral selective KIT D816V inhibitor are approved for AdvSM.

Salivary Cystatin D Interactome in Patients with Systemic Mastocytosis: An Exploratory Study

Mastocytosis, a rare blood disorder characterized by the proliferation of clonal abnormal mast cells, has a variegated clinical spectrum and diagnosis is often difficult and delayed. Recently we proposed the cathepsin inhibitor cystatin D-R26 as a salivary candidate biomarker of systemic mastocytosis (SM). Its C26 variant is able to form multiprotein complexes (mPCs) and since protein-protein interactions (PPIs) are crucial for studying disease pathogenesis, potential markers, and therapeutic targets, we aimed to define the protein composition of the salivary cystatin D-C26 interactome associated with SM. An exploratory affinity purification-mass spectrometry method was applied on pooled salivary samples from SM patients, SM patient subgroups with and without cutaneous symptoms (SM+C and SM-C), and healthy controls (Ctrls). Interactors specifically detected in Ctrls were found to be implicated in networks associated with cell and tissue homeostasis, innate system, endopeptidase regulation, and antimicrobial protection. Interactors distinctive of SM-C patients participate to PPI networks related to glucose metabolism, protein S-nitrosylation, antibacterial humoral response, and neutrophil degranulation, while interactors specific to SM+C were mainly associated with epithelial and keratinocyte differentiation, cytoskeleton rearrangement, and immune response pathways. Proteins sensitive to redox changes, as well as proteins with immunomodulatory properties and activating mast cells, were identified in patients; many of them were involved directly in cytoskeleton rearrangement, a process crucial for mast cell activation. Although preliminary, these results demonstrate that PPI alterations of the cystatin D-C26 interactome are associated with SM and provide a basis for future investigations based on quantitative proteomic analysis and immune validation.

KIT D816V is dimerization-independent and activates downstream pathways frequently perturbed in mastocytosis

KIT, a type III tyrosine kinase receptor, plays a crucial role in haematopoietic development. The KIT receptor forms a dimer after ligand binding; this activates tyrosine kinase activity leading to downstream signal transduction. The D816V KIT mutation is extensively implicated in haematological malignancies, including mastocytosis and leukaemia. KIT D816V is constitutively active, but the molecular nuances that lead to constitutive tyrosine kinase activity are unclear. For the first time, we present experimental evidence that the KIT D816V mutant does not dimerize like KIT wild type. We further show evidence of decreased stabilization of the tyrosine kinase domain in the KIT D816V mutant, a phenomenon that might contribute to its constitutive activity. Since the mechanism of KIT D816V activation varies from that of the wild type, we explored downstream signal transduction events and found that even though KIT D816V targets similar signalling moieties, the signalling is amplified in the mutant compared to stem cell factor-activated wild type receptor. Uniquely, KIT D816V induces infection-related pathways and the spliceosome pathway, providing alternate options for selective as well as combinatorial therapeutic targeting.

Testicular germ cell tumors: Genomic alternations and RAS-dependent signaling

Testicular germ cell tumors (TGCTs) are a common malignancy occurring in young adult men. The various genetic risk factors have been suggested to contribute to TGCT pathogenesis, however, they have a distinct mutational profile with a low rate of somatic point mutations, more frequent chromosomal gains, and aneuploidy. The most frequently mutated oncogenes in human cancers are RAS oncogenes, while their impact on testicular carcinogenesis and refractory disease is still poorly understood. In this mini-review, we summarize current knowledge on genetic alternations of RAS signaling-associated genes (the single nucleotide polymorphisms and point mutations) in this particular cancer type and highlight their link to chemotherapy resistance mechanisms. We also mention the impact of epigenetic changes on TGCT progression. Lastly, we propose a model for RAS-dependent signaling networks, regulation, cross-talks, and outcomes in TGCTs.

Immunohistochemical Staining to Identify Concomitant Systemic Mastocytosis in Acute Myeloid Leukemia with RUNX1::RUNX1T1

Systemic mastocytosis with associated hematological neoplasm (SM-AHN) poses diagnostic challenges because of the coexistence of atypical mast cell proliferation and hematological neoplasms. We assessed the presence of SM-AHN in patients with acute myeloid leukemia (AML) with RUNX1::RUNX1T1 from 2014 to 2020. Bone marrow (BM) samples were evaluated for mast cell aggregates using CD117 and CD25 immunohistochemical (IHC) staining. The KIT D816V variant burden at diagnosis and post induction was assessed using droplet digital PCR. Among 23 patients diagnosed as having AML with RUNX1::RUNX1T1, four (17.4%) were also diagnosed as having SM-AHN. No significant differences in clinical characteristics or overall survival (P=0.565) were observed between patients with or without SM-AHN, except for the presence of KIT variants (P=0.040). After induction therapy, IHC staining revealed the presence of mast cell aggregates in the BM, and the KIT D816V variant burden decreased with decreasing blast count and was similar in BM aspirates, smear slides, and sections. Concomitant SM-AHN was not infrequent in AML patients with RUNX1::RUNX1T1. This study showed the importance of CD117 and CD25 IHC staining after induction chemotherapy for SM-AHN screening, especially in patients with KIT variants.

Avapritinib for the treatment of KIT mutation-negative systemic mastocytosis

Systemic mastocytosis results from the spread of abnormal mast cells in different parts of the body, with variable clinical presentation. It is difficult to diagnose and to determine the appropriate therapy regimen. We present a case of a 53-year-old man diagnosed with KIT-negative advanced systemic mastocytosis based on the 2016 World Health Organization criteria. The patient presented with widespread symptoms that continued to worsen despite supportive therapy and traditional tyrosine kinase inhibitors. He was ultimately started on avapritinib, which reduced his tryptase level and provided symptomatic relief many years after his diagnosis.

Mutations responsible for the carbapenemase activity of SME-1

SME-1 is a carbapenemase, produced by Serratia marcescens organism and causes nosocomial infections such as in bloodstream, wounds, urinary tract, or respiratory tract infections. Treatment of such infections becomes very complex due its resistance towards penicillins, cephalosporins, monobactams, and carbapenems. Resistance to such antibiotics is of great medical concern. The misuse and overuse of these antibiotics result in the clinical mutation and production of novel β-lactamase enzymes such as SME-1, which show resistance to carbapenems. Class A contains most of the clinically significant extended spectrum of β-lactamase enzymes and carbapenemases. In this study, class A β-lactamase SME-1 sequence, structure, and binding were compared with naturally mutated class A β-lactamase enzymes and a wild-type TEM-1. This study was performed for revealing mutations, which could be responsible for the carbapenemase activity of SME-1. The dynamic characteristics of SME-1 enzymes manifest a different degree of conservation and variability, which confers them to possess carbapenemase activities. Met69Cys, Glu104Tyr, Tyr105His, Ala237Ser, and Gly238Cys mutations occur in SME-1 as compared to wild-type TEM-1. These mutated residues are present close to active site residues such as Ser70, Lys73, Ser130, Asn132, Glu166, and Asn170, which participate in the hydrolytic reaction of β-lactam antibiotics. Furthermore, these mutated residues demonstrate altered interactions with the β-lactam antibiotics (results in altered binding) and within themselves (results in active site structure alterations), which results in expanding the spectrum of activity of these enzymes. This study provides important insights into the structure and activity relationship of SME-1 enzymes. This is evident from the Ω-loop structure modification, which forms the wall of the active site and repositioning of residues involved in hydrolytic reactions, when present in the complex with meropenem in a stable state of MD simulation at 50 ns. Hence, Met69Cys, Glu104Tyr, Tyr105His, Ala237Ser, and Gly238Cys mutations could result in an altered active site structure, binding, and activity of SME-1 with meropenem and thus become resistantant against meropenem, which is a carbapenem.

Novel potential oncogenic and druggable mutations of FGFRs recur in the kinase domain across cancer types

Fibroblast growth factor receptors (FGFRs) are recurrently altered by single nucleotide variants (SNVs) in many human cancers. The prevalence of SNVs in FGFRs depends on the cancer type. In some tumors, such as the urothelial carcinoma, mutations of FGFRs occur at very high frequency (up to 60%). Many characterized mutations occur in the extracellular or transmembrane domains, while fewer known mutations are found in the kinase domain. In this study, we performed a bioinformatics analysis to identify novel putative cancer driver or therapeutically actionable mutations of the kinase domain of FGFRs. To pinpoint those mutations that may be clinically relevant, we exploited the recurrence of alterations on analogous amino acid residues within the kinase domain (PK_Tyr_Ser-Thr) of different kinases as a predictor of functional impact. By exploiting MutationAligner and LowMACA bioinformatics resources, we highlighted novel uncharacterized mutations of FGFRs which recur in other protein kinases. By revealing unanticipated correspondence with known variants, we were able to infer their functional effects, as alterations clustering on similar residues in analogous proteins have a high probability to elicit similar effects. As FGFRs represent an important class of oncogenes and drug targets, our study opens the way for further studies to validate their driver and/or actionable nature and, in the long term, for a more efficacious application of precision oncology.

Case Report: Evolution of KIT D816V-Positive Systemic Mastocytosis to Myeloid Neoplasm With PDGFRA Rearrangement Responsive to Imatinib

Systemic mastocytosis (SM) is a rare neoplasm resulting from extracutaneous infiltration of clonal mast cells (MC). The clinical features of SM are very heterogenous and treatment should be highly individualized. Up to 40% of all SM cases can be associated with another hematological neoplasm, most frequently myeloproliferative neoplasms. Here, we present a patient with indolent SM who subsequently developed a myeloid neoplasm with PDGFRA rearrangement with complete response to low-dose imatinib. The 63-year-old patient presented with eosinophilia and elevated serum tryptase level. Bone marrow analysis revealed aberrant MCs in aggregates co-expressing CD2/CD25 and KIT D816V mutation (0.01%), and the FIP1L1-PDGFRA fusion gene was not identified. In the absence of ‘B’ and ‘C’ findings, we diagnosed an indolent form of SM. For 2 years after the diagnosis, the absolute eosinophil count progressively increased. Bone marrow evaluation showed myeloid hyperplasia and the FIP1L1-PDGFRA fusion gene was detected. Thus, the diagnosis of myeloid neoplasm with PDGFRA rearrangement was established. The patient was treated with imatinib 100 mg daily and rapidly obtained a complete molecular remission. The clinical, biological, and therapeutic aspects of SM might be challenging, especially when another associated hematological disease is diagnosed. Little is known about the underlying molecular and immunological mechanisms that can promote one entity prevailing over the other one. Currently, the preferred concept of SM pathogenesis is a multimutated neoplasm in which KIT mutations represent a “phenotype modifier” toward SM. Our patient showed an evolution from KIT mutated indolent SM to a myeloid neoplasm with PDGFRA rearrangement; when the eosinophilic component expanded, a regression of the MC counterpart was observed. In conclusion, extensive clinical monitoring associated with molecular testing is essential to better define these rare diseases and consequently their prognosis and treatment.

Anti-CD117 immunotherapy to eliminate hematopoietic and leukemia stem cells

Precise replacement of diseased or dysfunctional organs is the goal of regenerative medicine and has appeared to be a distant goal for a long time. In the field of hematopoietic stem cell transplantation, this goal is now becoming tangible as gene-editing technologies and novel conditioning agents are entering the clinical arena. Targeted immunologic depletion of hematopoietic stem cells (HSCs), which are at the very root of the hematopoietic system, will enable more selective and potentially more effective hematopoietic stem cell transplantation in patients with hematological diseases. In contrast to current conditioning regimes based on ionizing radiation and chemotherapy, immunologic conditioning will spare mature hematopoietic cells and cause substantially less inflammation and unspecific collateral damage to other organs. Biological agents that target the stem cell antigen CD117 are the frontrunners for this purpose and have exhibited preclinical activity in depletion of healthy HSCs. The value of anti-CD117 antibodies as conditioning agents is currently being evaluated in early clinical trials. Whereas mild, antibody-based immunologic conditioning concepts might be appropriate for benign hematological disorders in which incomplete replacement of diseased cells is sufficient, higher efficacy will be required for treatment and elimination of hematologic stem cell malignancies such as acute myeloid leukemia and myelodysplastic syndrome. Antibody-drug conjugates, bispecific T-cell engaging and activating antibodies (TEAs), or chimeric antigen receptor (CAR) T cells might offer increased efficacy compared with naked antibodies and yet higher tolerability and safety compared with current genotoxic conditioning approaches. Here, we summarize the current state regarding immunologic conditioning concepts for the treatment of HSC disorders and outline potential future developments.

Clinical Impact of Inherited and Acquired Genetic Variants in Mastocytosis

Mastocytosis is a rare and complex disease characterized by expansion of clonal mast cells (MC) in skin and/or various internal organ systems. Involvement of internal organs leads to the diagnosis of systemic mastocytosis (SM). The WHO classification divides SM into indolent SM, smoldering SM and advanced SM variants, including SM with an associated hematologic neoplasm, aggressive SM, and MC leukemia. Historically, genetic analysis of individuals with pure cutaneous mastocytosis (CM) and SM have focused primarily on cohort studies of inherited single nucleotide variants and acquired pathogenic variants. The most prevalent pathogenic variant (mutation) in patients with SM is KIT p.D816V, which is detectable in most adult patients. Other somatic mutations have also been identified-especially in advanced SM-in TET2, SRSF2, ASXL1, RUNX1, CBL and JAK2, and shown to impact clinical and cellular phenotypes. Although only small patient cohorts have been analyzed, disease associations have also been identified in several germline variants within genes encoding certain cytokines or their receptors (IL13, IL6, IL6R, IL31, IL4R) and toll-like receptors. More recently, an increased prevalence of hereditary alpha-tryptasemia (HαT) caused by increased TPSAB1 copy number encoding alpha-tryptase has been described in patients with SM. Whereas HαT is found in 3-6% of general Western populations, it is identified in up to 17% of patients with SM. In the current manuscript we review the prevalence, functional role and clinical impact of various germline and somatic genetic variants in patients with mastocytosis.

A shift of dynamic equilibrium between the KIT active and inactive states causes drug resistance

Tyrosine phosphorylation, a highly regulated post-translational modification, is carried out by the enzyme tyrosine kinase (TK). TKs are important mediators in signaling cascades, facilitating diverse biological processes in response to stimuli. TKs may acquire mutations leading to malignancy and are viable targets for anti-cancer drugs. Mast/stem cell growth factor receptor KIT is a TK involved in cell differentiation, whose dysregulation leads to various types of cancer, including gastrointestinal stromal tumors, leukemia, and melanoma. KIT can be targeted by a range of inhibitors that predominantly bind to the inactive state of the enzyme. A mutation Y823D in the activation loop of KIT is known to be responsible for the loss of sensitivity to some drugs in metastatic tumors. We used all-atom molecular dynamics simulations to study the impact of Y823D on the KIT conformation and dynamics and compared it to the effect of phosphorylation of Y823. We simulated in total 6.4 μs of wild-type, mutant and phosphorylated KIT in the active- and inactive-state conformations. We found that Y823D affects the protein dynamics differently: in the active state, the mutation increases the protein stability, whereas in the inactive state it induces local destabilization, thus shifting the dynamic equilibrium towards the active state, altering the communication between distant regulatory regions. The observed dynamics of the Y823D mutant is similar to the dynamics of KIT phosphorylated at position Y823, thus we hypothesize that this mutation mimics a constitutively active kinase, which is not responsive to inhibitors that bind its inactive conformation.

Recent Advances in the Molecular Biology of Systemic Mastocytosis: Implications for Diagnosis, Prognosis, and Therapy

In recent years, molecular characterization and management of patients with systemic mastocytosis (SM) have greatly benefited from the application of advanced technologies. Highly sensitive and accurate assays for KIT D816V mutation detection and quantification have allowed the switch to non-invasive peripheral blood testing for patient screening; allele burden has prognostic implications and may be used to monitor therapeutic efficacy. Progress in genetic profiling of KIT, together with the use of next-generation sequencing panels for the characterization of associated gene mutations, have allowed the stratification of patients into three subgroups differing in terms of pathogenesis and prognosis: i) patients with mast cell-restricted KIT D816V; ii) patients with multilineage KIT D816V-involvement; iii) patients with "multi-mutated disease". Thanks to these findings, new prognostic scoring systems combining clinical and molecular data have been developed. Finally, non-genetic SETD2 histone methyltransferase loss of function has recently been identified in advanced SM. Assessment of SETD2 protein levels and activity might provide prognostic information and has opened new research avenues exploring alternative targeted therapeutic strategies. This review discusses how progress in recent years has rapidly complemented previous knowledge improving the molecular characterization of SM, and how this has the potential to impact on patient diagnosis and management.

Systemic Mastocytosis: Following the Tyrosine Kinase Inhibition Roadmap

Systemic mastocytosis is a rare and heterogeneous disease characterized by mast cell proliferation and activation. KIT is a transmembrane tyrosine kinase which plays a key role in mast cell growth, differentiation and survival. After interaction with its ligand, the stem cell factor, KIT dimerizes activating downstream pathways involving multiple tyrosine kinases (PI3K, JAK/STAT, RAS/ERK). Activating mutations in KIT are detected in most cases of systemic mastocytosis, being the most common KIT D816V. Therefore, since the emergence of tyrosine kinase inhibitors, KIT inhibition has been an attractive approach when facing mastocytosis treatment. Initial reports showed that only the rare KIT D816V negative cases were responsive to tyrosine kinase inhibitors. However, the development of new tyrosine kinase inhibitors such as midostaurin or avapritinib with activity against mast cells carrying the D816V KIT mutation, has changed the landscape of this disease.

The First 3D Model of the Full-Length KIT Cytoplasmic Domain Reveals a New Look for an Old Receptor

Receptor tyrosine kinases (RTKs) are key regulators of normal cellular processes and have a critical role in the development and progression of many diseases. RTK ligand-induced stimulation leads to activation of the cytoplasmic kinase domain that controls the intracellular signalling. Although the kinase domain of RTKs has been extensively studied using X-ray analysis, the kinase insert domain (KID) and the C-terminal are partially or fully missing in all reported structures. We communicate the first structural model of the full-length RTK KIT cytoplasmic domain, a crucial target for cancer therapy. This model was achieved by integration of ab initio KID and C-terminal probe models into an X-ray structure, and by their further exploration through molecular dynamics (MD) simulation. An extended (2-µs) MD simulation of the proper model provided insight into the structure and conformational dynamics of the full-length cytoplasmic domain of KIT, which can be exploited in the description of the KIT transduction processes.

Rational Drug Design Approach of Receptor Tyrosine Kinase Type III Inhibitors

Rational drug design is accomplished through the complementary use of structural biology and computational biology of biological macromolecules involved in disease pathology. Most of the known theoretical approaches for drug design are based on knowledge of the biological targets to which the drug binds. This approach can be used to design drug molecules that restore the balance of the signaling pathway by inhibiting or stimulating biological targets by molecular modeling procedures as well as by molecular dynamics simulations. Type III receptor tyrosine kinase affects most of the fundamental cellular processes including cell cycle, cell migration, cell metabolism, and survival, as well as cell proliferation and differentiation. Many inhibitors of successful rational drug design show that some computational techniques can be combined to achieve synergistic effects.

The Spectrum of Aggressive Mastocytosis: A Workshop Report and Literature Review

Most cases of mastocytosis are indolent, usually cutaneous mastocytosis or indolent systemic mastocytosis (SM). Aggressive mast cell (MC) diseases are very rare and often fatal. They can develop de novo or due to progression of indolent forms and can present in different ways; either as MC sarcoma or as advanced SM which includes aggressive SM, MC leukemia, and SM with an associated hematological neoplasm. This review will describe these different aggressive forms of mastocytosis, illustrated by cases submitted to the workshop of the 18th Meeting of the European Association for Haematopathology, Basel 2016, organized by the European Bone Marrow Working Group. In addition, the diagnostic criteria for identifying myelomastocytic leukemia, an aggressive myeloid neoplasm with partial MC differentiation that falls short of the criteria for SM, and disease progression in patients with established mastocytosis are discussed.

Tyrosine Kinase Inhibition in Mastocytosis: KIT and Beyond KIT

Mastocytosis is a group of rare disorders characterized by abnormal accumulation of mast cells in one or several organs. Mastocytosis can be seen at any age; but, in adults, the disease is usually systemic and chronic. Patients with indolent systemic mastocytosis (SM) are usually treated symptomatically, but cytoreductive treatments are needed in more advanced SM. In most patients with SM, an activating KIT D816V mutation is found. Thus, patients with advanced SM benefit from treatment with KIT-targeting tyrosine kinase inhibitors. However, none of these drugs are curative; new targeted drugs or combinations are still needed to improve patients' outcome.

"Infostery" analysis of short molecular dynamics simulations identifies highly sensitive residues and predicts deleterious mutations

Characterizing a protein mutational landscape is a very challenging problem in Biology. Many disease-associated mutations do not seem to produce any effect on the global shape nor motions of the protein. Here, we use relatively short all-atom biomolecular simulations to predict mutational outcomes and we quantitatively assess the predictions on several hundreds of mutants. We perform simulations of the wild type and 175 mutants of PSD95’s third PDZ domain in complex with its cognate ligand. By recording residue displacements correlations and interactions, we identify “communication pathways” and quantify them to predict the severity of the mutations. Moreover, we show that by exploiting simulations of the wild type, one can detect 80% of the positions highly sensitive to mutations with a precision of 89%. Importantly, our analysis describes the role of these positions in the inter-residue communication and dynamical architecture of the complex. We assess our approach on three different systems using data from deep mutational scanning experiments and high-throughput exome sequencing. We refer to our analysis as “infostery”, from “info” - information - and “steric” - arrangement of residues in space. We provide a fully automated tool, COMMA2 (www.lcqb.upmc.fr/COMMA2), that can be used to guide medicinal research by selecting important positions/mutations.

Distinct Tie2 tyrosine phosphorylation sites dictate phenotypic switching in endothelial progenitor cells

Angiogenesis is a regulated process involving the proliferation, migration, and remodeling of different cell types particularly mature endothelial and their progenitor cells, nominated as endothelial progenitor cells (EPCs). Tie2/Tek is a tyrosine kinase receptor expressed by endothelial cells that induces signal transduction pathways involved in endothelial biology. To address the potential importance of the various tyrosine residues of Tie2 in EPC development, we generated a series of Tie2 tyrosine mutated (Y1106F, Y1100F, and Y1111F) EPCs and then assess the biological features of these cells. Clonogenic, tubulogenic, proliferative, migratory, and functional properties of these cells were analyzed. Next, GFP-positive EPCs containing Tie2 tyrosine mutations were systemically transplanted into sublethaly irradiated mice to analyze the potency of these cells for marrow reconstitution. We found that mutation in the Tie2 tyrosine 1106 residue directed EPCs toward a mature endothelial phenotype, which was associated with augmented tubulogenic and migratory properties, and increased phosphorylation of the active site (tyrosine 992) as well as increased vascular perfusion in the in vivo Matrigel plug assay. Moreover, transplantation of 1106 Tie2 mutant EPCs failed to reconstitute the bone marrow after myeloablation, whereas transplantation of EPCs with the 1100 or 1111 Tie2 tyrosine mutation resulted in bone marrow engraftment, leading to improved survival of recipient mice. Our findings demonstrate that the tyrosine 1106 residue in Tie2 plays a key role to maintain the stemness features of EPCs.

KIT Suppresses BRAFV600E-Mutant Melanoma by Attenuating Oncogenic RAS/MAPK Signaling

The receptor tyrosine kinase KIT promotes survival and migration of melanocytes during development, and excessive KIT activity hyperactivates the RAS/MAPK pathway and can drive formation of melanomas, most notably of rare melanomas that occur on volar and mucosal surfaces of the skin. The much larger fraction of melanomas that occur on sun-exposed skin is driven primarily by BRAF- or NRAS-activating mutations, but these melanomas exhibit a surprising loss of KIT expression, which raises the question of whether loss of KIT in these tumors facilitates tumorigenesis. To address this question, we introduced a kit(lf) mutation into a strain of Tg(mitfa:BRAF^V600E); p53(lf) melanoma-prone zebrafish. Melanoma onset was accelerated in kit(lf); Tg(mitfa:BRAF^V600E); p53(lf) fish. Tumors from kit(lf) animals were more invasive and had higher RAS/MAPK pathway activation. KIT knockdown also increased RAS/MAPK pathway activation in a BRAF^V600E-mutant human melanoma cell line. We found that pathway stimulation upstream of BRAF^V600E could paradoxically reduce signaling downstream of BRAF^V600E, and wild-type BRAF was necessary for this effect, suggesting that its activation can dampen oncogenic BRAF^V600E signaling. In vivo, expression of wild-type BRAF delayed melanoma onset, but only in a kit-dependent manner. Together, these results suggest that KIT can activate signaling through wild-type RAF proteins, thus interfering with oncogenic BRAF^V600E-driven melanoma formation. Cancer Res; 77(21); 5820-30. ©2017 AACR.

Co-occurrence of FLT3-TKD and NPM1 mutations defines a highly favorable prognostic AML group

Although FLT3 internal tandem duplication (ITD) mutations in acute myeloid leukemia (AML) confer an adverse prognosis, co-occurrence with a nucleophosphomin (NPM1) mutation partially improves response and survival outcomes. In contrast, simultaneous NPM1 and FLT3 tyrosine kinase domain (TKD) mutations were reported to improve response over that of an isolated NPM1 mutation in one as yet unverified report. To validate this, we explored the impact of the co-occurrence of FLT3-TKD and NPM1 mutations on clinical outcomes. Study populations included 21 patients (8%) with FLT3-TKD⁺NPM1⁺ mutated, 18 patients (7%) with FLT3-TKD-only-mutated, 117 patients (44%) with NPM1-only-mutated, and 107 patients (41%) with FLT3-ITD⁺NPM1-mutated AML. Compared with NPM1⁺ -only-mutated AML, FLT3-TKD/NPM1 double mutation status was associated with a significantly superior relapse-free survival (median, not reached vs 18.3 months; P = .03) and a trend toward improved overall survival (OS). The presence of FLT3-TKD/NPM1 double mutation status was an independent positive predictor in multivariable analysis. Allogeneic stem cell transplant did not improve outcomes in the FLT3-TKD/NPM1 cohort. Consistent with historical data, the co-mutation status defined a highly favorable prognostic group characterized by high response rates and prolonged disease-free and OS. These study findings substantiate previous data describing this intriguing paradoxical cooperative effect. Our results emphasize the need for elucidating the mechanistic links between FLT3-TKD and NPM1 in future molecular and murine model studies.

Insight on Mutation-Induced Resistance from Molecular Dynamics Simulations of the Native and Mutated CSF-1R and KIT

The receptors tyrosine kinases (RTKs) for the colony stimulating factor-1, CSF-1R, and for the stem cell factor, SCFR or KIT, are important mediators of signal transduction. The abnormal function of these receptors, promoted by gain-of-function mutations, leads to their constitutive activation, associated with cancer or other proliferative diseases. A secondary effect of the mutations is the alteration of receptors' sensitivity to tyrosine kinase inhibitors, compromising effectiveness of these molecules in clinical treatment. In particular, the mutation V560G in KIT increases its sensitivity to Imatinib, while the D816V in KIT, and D802V in CSF-1R, triggers resistance to the drug. We analyzed the Imatinib binding affinity to the native and mutated KIT (mutations V560G, S628N and D816V) and CSF-1R (mutation D802V) by using molecular dynamics simulations and energy calculations of Imatinib•target complexes. Further, we evaluated the sensitivity of the studied KIT receptors to Imatinib by measuring the inhibition of KIT phosphorylation. Our study showed that (i) the binding free energy of Imatinib to the targets is highly correlated with their experimentally measured sensitivity; (ii) the electrostatic interactions are a decisive factor affecting the binding energy; (iii) the most deleterious impact to the Imatinib sensitivity is promoted by D802V (CSF-1R) and D816V (KIT) mutations; (iv) the role of the juxtamembrane region, JMR, in the imatinib binding is accessory. These findings contribute to a better description of the mutation-induced effects alternating the targets sensitivity to Imatinib.

Imatinib in systemic mastocytosis: a phase IV clinical trial in patients lacking exon 17 KIT mutations and review of the literature

Resistance to imatinib has been recurrently reported in systemic mastocytosis (SM) carrying exon 17 KIT mutations. We evaluated the efficacy and safety of imatinib therapy in 10 adult SM patients lacking exon 17 KIT mutations, 9 of which fulfilled criteria for well-differentiated SM (WDSM). The World Health Organization 2008 disease categories among WDSM patients were mast cell (MC) leukemia (n = 3), indolent SM (n = 3) and cutaneous mastocytosis (n = 3); the remainder case had SM associated with a clonal haematological non-MC disease. Patients were given imatinib for 12 months -400 or 300 mg daily depending on the presence vs. absence of > 30% bone marrow (BM) MCs and/or signs of advanced disease-. Absence of exon 17 KIT mutations was confirmed in highly-purified BM MCs by peptide nucleic acid-mediated PCR, while mutations involving other exons were investigated by direct sequencing of purified BM MC DNA. Complete response (CR) was defined as resolution of BM MC infiltration, skin lesions, organomegalies and MC-mediator release-associated symptoms, plus normalization of serum tryptase. Criteria for partial response (PR) included ≥ 50% reduction in BM MC infiltration and improvement of skin lesions and/or organomegalies. Treatment was well-tolerated with an overall response rate of 50%, including early and sustained CR in four patients, three of whom had extracellular mutations of KIT, and PR in one case. This later patient and all non-responders (n = 5) showed wild-type KIT. These results together with previous data from the literature support the relevance of the KIT mutational status in selecting SM patients who are candidates for imatinib therapy.

Oncogenic KIT mutations in different exons lead to specific changes in melanocyte phospho-proteome

Mutations in the proto-oncogene c-KIT (KIT) are found in several cancers, and the site of these mutations differs markedly between cancer types. We used site directed mutagenesis to induce KIT(559), KIT(642) and KIT(816) mutations in primary human melanocytes (PHM) and we investigated the impact of each mutation on KIT function. We studied canonical KIT-signaling pathways by immunoblotting, and we used stable isotope labeling by amino acids in cell culture (SILAC) and kinase prediction models to identify kinases differently activated in respective mutants. We validated our results with the analysis of phosphorylation levels of selected substrates for each kinase. We concluded that CK1 ε and δ are more active in cell clones harboring KIT(559) and KIT(642) mutations, whereas PAK4 is more active in clones with KIT(816) mutation. Our findings might help to develop further therapeutic options for tumors with specific KIT mutations in different domains.

BIOLOGICAL SIGNIFICANCE

Different types of cancers harbor mutations in the oncogene KIT. The use of small molecules inhibitors directly targeting KIT had a limited success in the treatment of patients with KIT mutant cancers. Our study describes specific phospho-proteome changes due to different KIT mutations, and provides targets of further therapeutic options.

Mastocytosis: a mutated KIT receptor induced myeloproliferative disorder

Although more than 90% systemic mastocytosis (SM) patients express gain of function mutations in the KIT receptor, recent next generation sequencing has revealed the presence of several additional genetic and epigenetic mutations in a subset of these patients, which confer poor prognosis and inferior overall survival. A clear understanding of how genetic and epigenetic mutations cooperate in regulating the tremendous heterogeneity observed in these patients will be essential for designing effective treatment strategies for this complex disease. In this review, we describe the clinical heterogeneity observed in patients with mastocytosis, the nature of relatively novel mutations identified in these patients, therapeutic strategies to target molecules downstream from activating KIT receptor and finally we speculate on potential novel strategies to interfere with the function of not only the oncogenic KIT receptor but also epigenetic mutations seen in these patients.

Discovery of Dual Inhibitors for Wild Type and D816V Mutant of c-KIT Kinase through Virtual and Biochemical Screening of Natural Products

Although stem cell factor receptor (c-KIT) kinase is responsible for various malignant human cancers, the presence of constitutively active gain-of-function mutants has made it difficult to discover new anticancer agents using c-KIT as the target protein. To identify the common inhibitors of wild-type c-KIT and the most abundant gain-of-function mutant (D816V), the virtual screening of natural products was performed for the two target proteins in parallel with the scoring function improved by implementing a sophisticated solvation free energy term. As a result, four common inhibitors of natural origin are found with biochemical potencies ranging from low micromolar to submicromolar levels. The results of extensive docking simulations show that although the natural-product inhibitors establish weaker hydrophobic interactions with the D816V mutant than with the wild type, they exhibit a little higher inhibitory activity for the former than the latter by strengthening the hydrogen-bond interactions to a sufficient extent. Of the four natural-product inhibitors, (Z)-6-hydroxy-2-(4-methoxybenzylidene)benzofuran-3(2H)-one (3) is anticipated to serve as a new molecular core for the structure-activity relationship studies to optimize the biochemical potencies because it exhibits good inhibitory activity against both the wild type and D816V mutant despite its low molecular weight (268.3 amu).

Dissecting protein architecture with communication blocks and communicating segment pairs

BACKGROUND

Proteins adapt to environmental conditions by changing their shape and motions. Characterising protein conformational dynamics is increasingly recognised as necessary to understand how proteins function. Given a conformational ensemble, computational tools are needed to extract in a systematic way pertinent and comprehensive biological information.

RESULTS

Here, we present a method, Communication Mapping (COMMA), to decipher the dynamical architecture of a protein. The method first extracts residue-based dynamic properties from all-atom molecular dynamics simulations. Then, it integrates them in a graph theoretic framework, where it identifies groups of residues or protein regions that mediate short- and long-range communication. COMMA introduces original concepts to contrast the different roles played by these regions, namely communication blocks and communicating segment pairs, and evaluates the connections and communication strengths between them. We show the utility and capabilities of COMMA by applying it to three archetypal proteins, namely protein A, the tyrosine kinase KIT and the tumour suppressor p53.

CONCLUSION

Our method permits to compare in a direct way the dynamical behaviour either of proteins with different characteristics or of the same protein in different conditions. It is useful to identify residues playing a key role in protein allosteric regulation and to explain the effects of deleterious mutations in a mechanistic way. COMMA is a fully automated tool with broad applicability. It is freely available to the community at www.lcqb.upmc.fr/COMMA .

How Intrinsic Molecular Dynamics Control Intramolecular Communication in Signal Transducers and Activators of Transcription Factor STAT5

Signal Transducer and Activator of Transcription STAT5 is a key mediator of cell proliferation, differentiation and survival. While STAT5 activity is tightly regulated in normal cells, its constitutive activation directly contributes to oncogenesis and is associated with a broad range of hematological and solid tumor cancers. Therefore the development of compounds able to modulate pathogenic activation of this protein is a very challenging endeavor. A crucial step of drug design is the understanding of the protein conformational features and the definition of putative binding site(s) for such modulators. Currently, there is no structural data available for human STAT5 and our study is the first footprint towards the description of structure and dynamics of this protein. We investigated structural and dynamical features of the two STAT5 isoforms, STAT5a and STAT5b, taken into account their phosphorylation status. The study was based on the exploration of molecular dynamics simulations by different analytical methods. Despite the overall folding similarity of STAT5 proteins, the MD conformations display specific structural and dynamical features for each protein, indicating first, sequence-encoded structural properties and second, phosphorylation-induced effects which contribute to local and long-distance structural rearrangements interpreted as allosteric event. Further examination of the dynamical coupling between distant sites provides evidence for alternative profiles of the communication pathways inside and between the STAT5 domains. These results add a new insight to the understanding of the crucial role of intrinsic molecular dynamics in mediating intramolecular signaling in STAT5. Two pockets, localized in close proximity to the phosphotyrosine-binding site and adjacent to the channel for communication pathways across STAT5, may constitute valid targets to develop inhibitors able to modulate the function-related communication properties of this signaling protein.

Oncogenic KIT-induced aggressive systemic mastocytosis requires SHP2/PTPN11 phosphatase for disease progression in mice

Acquired mutations in KIT are driver mutations in systemic mastocytosis (SM). Here, we tested the role of SHP2/PTPN11 phosphatase in oncogenic KIT signaling using an aggressive SM mouse model. Stable knock-down (KD) of SHP2 led to impaired growth, colony formation, and increased rates of apoptosis in P815 cells. This correlated with defects in signaling to ERK/Bim, Btk, Lyn, and Stat5 pathways in P815-KD cells compared to non-targeting (NT). Retro-orbital injections of P815 NT cells in syngeneic DBA/2 mice resulted in rapid development of aggressive SM within 13-16 days characterized by splenomegaly, extramedullary hematopoiesis, and multifocal liver tumors. In contrast, mice injected with P815 SHP2 KD cells showed less disease burden, including normal spleen weight and cellularity, and significant reductions in mastocytoma cells in spleen, bone marrow, peripheral blood and liver compared to NT controls. Treatment of human mast cell leukemia HMC-1 cells or P815 cells with SHP2 inhibitor II-B08, resulted in reduced colony formation and cell viability. Combining II-B08 with multi-kinase inhibitor Dasatinib showed enhanced efficacy than either inhibitor alone in blocking cell growth pathways and cell viability. Taken together, these results identify SHP2 as a key effector of oncogenic KIT and a therapeutic target in aggressive SM.

Genome-Wide Association Study of Golden Retrievers Identifies Germ-Line Risk Factors Predisposing to Mast Cell Tumours

Canine mast cell tumours (CMCT) are one of the most common skin tumours in dogs with a major impact on canine health. Certain breeds have a higher risk of developing mast cell tumours, suggesting that underlying predisposing germ-line genetic factors play a role in the development of this disease. The genetic risk factors are largely unknown, although somatic mutations in the oncogene C-KIT have been detected in a proportion of CMCT, making CMCT a comparative model for mastocytosis in humans where C-KIT mutations are frequent. We have performed a genome wide association study in golden retrievers from two continents and identified separate regions in the genome associated with risk of CMCT in the two populations. Sequence capture of associated regions and subsequent fine mapping in a larger cohort of dogs identified a SNP associated with development of CMCT in the GNAI2 gene (p = 2.2x10-16), introducing an alternative splice form of this gene resulting in a truncated protein. In addition, disease associated haplotypes harbouring the hyaluronidase genes HYAL1, HYAL2 and HYAL3 on cfa20 and HYAL4, SPAM1 and HYALP1 on cfa14 were identified as separate risk factors in European and US golden retrievers, respectively, suggesting that turnover of hyaluronan plays an important role in the development of CMCT.

Structure-based de novo design and identification of D816V mutant-selective c-KIT inhibitors

To identify potent and selective inhibitors of D816V, the most common gain-of-function c-KIT mutant, we carried out structure-based de novo design using 7-azaindole as the core and the scoring function improved by implementing an accurate solvation free energy term. This approach led to the identification of new c-KIT inhibitors specific for the D816V mutant. The 3-(3,4-dimethoxyphenyl)-7-azaindole scaffold was optimized and represents a lead structure for the design of the potent and specific inhibitors of the D816V mutant. The results of molecular dynamics simulations indicate that hydrogen bonding interactions between the 7-azadindole moiety and the backbone groups of Cys673 are the most significant determinant for the potency and selectivity of c-KIT inhibitors.

Systemic Mastocytosis: Clinical Update and Future Directions

Systemic mastocytosis (SM) is defined as the accumulation of abnormal mast cells (MC) in 1 or more extracutaneous tissues. Symptoms are due to either MC activation or organ infiltration and vary depending on disease subtype. More benign forms of SM, such as indolent SM, result in a life expectancy similar to the general population, while more aggressive subtypes, such as MC leukemia (MCL), have a median survival measured on the order of months. Treatment of indolent SM is directed at controlling the symptoms associated with MC activation. In advanced forms, such as aggressive SM and MCL, agents targeting MC proliferation such as KIT tyrosine kinase inhibitors, cladribine, and thalidomide may be provided. Newer agents based on preclinical rationale are also being actively investigated. However, the only potentially curative therapy for aggressive SM/MCL remains hematopoietic stem cell transplantation. Given that SM is a relatively rare disease, clinicians are often underprepared to evaluate, diagnose, and effectively treat this clinically heterogeneous condition. Here we seek to familiarize clinicians with this orphan disease and review current and future treatment approaches.

KIT mutation analysis in mast cell neoplasms: recommendations of the European Competence Network on Mastocytosis

Although acquired mutations in KIT are commonly detected in various categories of mastocytosis, the methodologies applied to detect and quantify the mutant type and allele burden in various cells and tissues are poorly defined. We here propose a consensus on methodologies used to detect KIT mutations in patients with mastocytosis at diagnosis and during follow-up with sufficient precision and sensitivity in daily practice. In addition, we provide recommendations for sampling and storage of diagnostic material as well as a robust diagnostic algorithm. Using highly sensitive assays, KIT D816V can be detected in peripheral blood leukocytes from most patients with systemic mastocytosis (SM) that is a major step forward in screening and SM diagnosis. In addition, the KIT D816V allele burden can be followed quantitatively during the natural course or during therapy. Our recommendations should greatly facilitate diagnostic and follow-up investigations in SM in daily practice as well as in clinical trials. In addition, the new tools and algorithms proposed should lead to a more effective screen, early diagnosis of SM and help to avoid unnecessary referrals.

Reduced DNA methylation and hydroxymethylation in patients with systemic mastocytosis

OBJECTIVE

As disruption of epigenetic control is a frequent event in solid tumors and leukemia, we investigated changes in DNA methylation (5mC) and hydroxymethylation (5hmC) in patients with systemic mastocytosis (SM), a rare myeloproliferative disease with a wide spectrum of severity, characterized by the accumulation of mast cells in various organs.

METHODS

We measured overall genomic levels of 5hmC and 5mC in patients with SM by dot blot, as well as by quantitative immunofluorescence in samples of cutaneous mastocytosis.

RESULTS

Overall 5hmC levels were reduced in all patients with SM, but to a greater extent in the presence of higher D816V mutational load in the KIT oncogene, which affects prognosis and therapeutic options in these patients. Loss of 5hmC was likely due to systemic effects of SM as it did not correlate with overall mast cell burden in these patients, nor it was due to inactivating mutations of TET2 or reduced TET2 expression.

CONCLUSIONS

The correlation between SM diagnosis and significantly low 5hmC levels suggests that reduction of 5hmC represents a systemic effect of SM that may be useful for patient stratification and that measurements of 5hmC levels may serve as a better prognostic marker than TET2 mutations.

An integrated molecular dynamics, principal component analysis and residue interaction network approach reveals the impact of M184V mutation on HIV reverse transcriptase resistance to lamivudine

The emergence of different drug resistant strains of HIV-1 reverse transcriptase (HIV RT) remains of prime interest in relation to viral pathogenesis as well as drug development. Amongst those mutations, M184V was found to cause a complete loss of ligand fitness. In this study, we report the first account of the molecular impact of M184V mutation on HIV RT resistance to 3TC (lamivudine) using an integrated computational approach. This involved molecular dynamics simulation, binding free energy analysis, principle component analysis (PCA) and residue interaction networks (RINs). Results clearly confirmed that M184V mutation leads to steric conflict between 3TC and the beta branched side chain of valine, decreases the ligand (3TC) binding affinity by ∼7 kcal mol(-1) when compared to the wild type, changes the overall conformational landscape of the protein and distorts the native enzyme residue-residue interaction network. The comprehensive molecular insight gained from this study should be of great importance in understanding drug resistance against HIV RT as well as assisting in the design of novel reverse transcriptase inhibitors with high ligand efficacy on resistant strains.

Expression of Kit and Etv1 in restricted brain regions supports a brain-cell progenitor as an origin for cranial germinomas

Mismigrating germ-cell progenitors have historically been accepted as the cell of origin for central nervous system (CNS) germinomas. However, an alternative hypothesis suggests that CNS germinomas arise from a brain-cell progenitor. Germinomas often acquire Kit signaling pathway mutations, and there is evidence for an oncogenic relationship between KIT and the ETV1 transcription factor. KIT appears to be necessary to stabilize ETV1, and ETV1 then activates oncogenesis-associated genes. ETV1 expression is not increased by KIT, so ETV1 already needs to be expressed in order for KIT to have an oncogenic function. Therefore, if brain-cell progenitors are the cell of origin for germinomas, those cells would already need to coexpress ETV1 and KIT. We examined Kit and Etv1 in situ hybridization data from the Allen Brain Atlas, for mouse brain tissue at various stages of development. Both Kit and Etv1 were expressed in the regions where germinomas most commonly arise, and in the medulla oblongata. All human cases of germinomas correlated to the regions where ETV1 and KIT are coexpressed. We therefore postulate that germinomas in the brain share a similar mechanism with other KIT-driven cancers, which supports the hypothesis that germinomas arise from a brain-cell progenitor.

Single H5N1 influenza A neuraminidase mutation develops resistance to oseltamivir due to distorted conformational and drug binding landscape: multiple molecular dynamics analyses

Clinical studies showed that a single mutation, I117V, develops severe resistance to oseltamivir, the first orally active influenza A neuraminidase inhibitor, in highly pathogenic H5N1 influenza A viruses.

Hotspot mutations in KIT receptor differentially modulate its allosterically coupled conformational dynamics: impact on activation and drug sensitivity

Receptor tyrosine kinase KIT controls many signal transduction pathways and represents a typical allosterically regulated protein. The mutation-induced deregulation of KIT activity impairs cellular physiological functions and causes serious human diseases. The impact of hotspots mutations (D816H/Y/N/V and V560G/D) localized in crucial regulatory segments, the juxtamembrane region (JMR) and the activation (A-) loop, on KIT internal dynamics was systematically studied by molecular dynamics simulations. The mutational outcomes predicted in silico were correlated with in vitro and in vivo activation rates and drug sensitivities of KIT mutants. The allosteric regulation of KIT in the native and mutated forms is described in terms of communication between the two remote segments, JMR and A-loop. A strong correlation between the communication profile and the structural and dynamical features of KIT in the native and mutated forms was established. Our results provide new insight on the determinants of receptor KIT constitutive activation by mutations and resistance of KIT mutants to inhibitors. Depiction of an intra-molecular component of the communication network constitutes a first step towards an integrated description of vast communication pathways established by KIT in physiopathological contexts.

Receptor tyrosine kinase KIT plays a crucial role in the regulation of cell signaling. This allosterically controlled activity may be affected by gain-of-function mutations that promote the development of several cancers. Identification of the molecular basis of KIT constitutive activation and allosteric regulation has inspired computational study of KIT hotspot mutations. In the present contribution, we investigated the mutation-induced effects on KIT conformational dynamics and intra-protein communication conditionally on the mutation location and the nature of the substituting amino acid. Our data elucidate that all studied mutations stabilize an inactive non-autoinhibited state of KIT over the inactive auto-inhibited state prevalent for the native protein. This shift in the protein conformational landscape promotes KIT constitutive activation. Our in silico analysis established correlations between the structural and dynamical effects induced by oncogenic mutations and the mutants auto-activation rates and drug sensitivities measured in vitro and in vivo. Particularly, the A-loop mutations stabilize the drug-resistant forms, while the JMR mutations may facilitate inhibitors binding to the active site. Cross-correlations established between local and long-range structural and dynamical effects demonstrate the allosteric character of the gain-of-function mutations mode of action.

Allosteric pathway identification through network analysis: from molecular dynamics simulations to interactive 2D and 3D graphs

Allostery is a universal phenomenon that couples the information induced by a local perturbation (effector) in a protein to spatially distant regulated sites. Such an event can be described in terms of a large scale transmission of information (communication) through a dynamic coupling between structurally rigid (minimally frustrated) and plastic (locally frustrated) clusters of residues. To elaborate a rational description of allosteric coupling, we propose an original approach - MOdular NETwork Analysis (MONETA) - based on the analysis of inter-residue dynamical correlations to localize the propagation of both structural and dynamical effects of a perturbation throughout a protein structure. MONETA uses inter-residue cross-correlations and commute times computed from molecular dynamics simulations and a topological description of a protein to build a modular network representation composed of clusters of residues (dynamic segments) linked together by chains of residues (communication pathways). MONETA provides a brand new direct and simple visualization of protein allosteric communication. A GEPHI module implemented in the MONETA package allows the generation of 2D graphs of the communication network. An interactive PyMOL plugin permits drawing of the communication pathways between chosen protein fragments or residues on a 3D representation. MONETA is a powerful tool for on-the-fly display of communication networks in proteins. We applied MONETA for the analysis of communication pathways (i) between the main regulatory fragments of receptors tyrosine kinases (RTKs), KIT and CSF-1R, in the native and mutated states and (ii) in proteins STAT5 (STAT5a and STAT5b) in the phosphorylated and the unphosphorylated forms. The description of the physical support for allosteric coupling by MONETA allowed a comparison of the mechanisms of (a) constitutive activation induced by equivalent mutations in two RTKs and (b) allosteric regulation in the activated and non-activated STAT5 proteins. Our theoretical prediction based on results obtained with MONETA was validated for KIT by in vitro experiments. MONETA is a versatile analytical and visualization tool entirely devoted to the understanding of the functioning/malfunctioning of allosteric regulation in proteins - a crucial basis to guide the discovery of next-generation allosteric drugs.

Differential effects of CSF-1R D802V and KIT D816V homologous mutations on receptor tertiary structure and allosteric communication

The colony stimulating factor-1 receptor (CSF-1R) and the stem cell factor receptor KIT, type III receptor tyrosine kinases (RTKs), are important mediators of signal transduction. The normal functions of these receptors can be compromised by gain-of-function mutations associated with different physiopatological impacts. Whereas KIT D816V/H mutation is a well-characterized oncogenic event and principal cause of systemic mastocytosis, the homologous CSF-1R D802V has not been identified in human cancers. The KIT D816V oncogenic mutation triggers resistance to the RTK inhibitor Imatinib used as first line treatment against chronic myeloid leukemia and gastrointestinal tumors. CSF-1R is also sensitive to Imatinib and this sensitivity is altered by mutation D802V. Previous in silico characterization of the D816V mutation in KIT evidenced that the mutation caused a structure reorganization of the juxtamembrane region (JMR) and facilitated its departure from the kinase domain (KD). In this study, we showed that the equivalent CSF-1R D802V mutation does not promote such structural effects on the JMR despite of a reduction on some key H-bonds interactions controlling the JMR binding to the KD. In addition, this mutation disrupts the allosteric communication between two essential regulatory fragments of the receptors, the JMR and the A-loop. Nevertheless, the mutation-induced shift towards an active conformation observed in KIT D816V is not observed in CSF-1R D802V. The distinct impact of equivalent mutation in two homologous RTKs could be associated with the sequence difference between both receptors in the native form, particularly in the JMR region. A local mutation-induced perturbation on the A-loop structure observed in both receptors indicates the stabilization of an inactive non-inhibited form, which Imatinib cannot bind.

Harnessing allostery: a novel approach to drug discovery

Allostery is the most direct and efficient way for regulation of biological macromolecule function, ranging from the control of metabolic mechanisms to signal transduction pathways. Allosteric modulators target to allosteric sites, offering distinct advantages compared to orthosteric ligands that target to active sites, such as greater specificity, reduced side effects, and lower toxicity. Allosteric modulators have therefore drawn increasing attention as potential therapeutic drugs in the design and development of new drugs. In recent years, advancements in our understanding of the fundamental principles underlying allostery, coupled with the exploitation of powerful techniques and methods in the field of allostery, provide unprecedented opportunities to discover allosteric proteins, detect and characterize allosteric sites, design and develop novel efficient allosteric drugs, and recapitulate the universal features of allosteric proteins and allosteric modulators. In the present review, we summarize the recent advances in the repertoire of allostery, with a particular focus on the aforementioned allosteric compounds.

Functional deregulation of KIT: link to mast cell proliferative diseases and other neoplasms

In this review, the authors discuss common gain-of-function mutations in the stem cell factor receptor KIT found in mast cell proliferation disorders and summarize the current understanding of the molecular mechanisms by which these transforming mutations may affect KIT structure and function leading to altered downstream signaling and cellular transformation. Drugs targeting KIT have shown mixed success in the treatment of mastocytosis and other hyperproliferative diseases. A brief overview of the most common KIT inhibitors currently used, the reasons for the varied clinical results of such inhibitors and a discussion of potential new strategies are provided.