Bruton's tyrosine kinase: cell biology, sequence conservation, mutation spectrum, siRNA modifications, and expression profiling

Cell-penetrating peptide-conjugated, splice-switching oligonucleotides mitigate the phenotype in BTK/Tec double deficient X-linked agammaglobulinemia model

Splice-switching oligonucleotides (SSOs) have been developed as a treatment for various disorders, including Duchenne muscular dystrophy and spinal muscular atrophy. Here, the activity of several different SSOs was investigated as potential treatments for B lymphocyte disorders with a focus on X-linked agammaglobulinemia (XLA), caused by defects in the gene encoding Bruton's tyrosine kinase (BTK). In this study, the activity of locked nucleic acid (LNA), tricyclo-DNA (tcDNA), phosphoryl guanidine oligonucleotides (PGO) and phosphorodiamidate morpholino oligomers (PMO) were compared, targeting the pseudoexon region of BTK pre-mRNA. We further investigated the effect of conjugating cell-penetrating peptides, including Pip6a, to the SSOs. The effect was measured as splice-switching in vitro as well as in a further developed, bacterial artificial chromosome transgenic mouse model of XLA. Therapy in the form of intravenous infusions 2 times a week during 3 weeks of PMO oligomers conjugated to Pip6a was sufficient to partly restore the in vivo B lineage phenotype. SSOs treatment also provides a unique opportunity to get insights into a restoration process, when B lymphocytes of different maturation stages are simultaneously splice-corrected.

Wash-free fluorescent tools based on organic molecules: Design principles and biomedical applications

Fluorescence-assisted tools based on organic molecules have been extensively applied to interrogate complex biological processes in a non-invasive manner with good sensitivity, high resolution, and rich contrast. However, the signal-to-noise ratio is an essential factor to be reckoned with during collecting images for high fidelity. In view of this, the wash-free strategy is proven as a promising and important approach to improve the signal-to-noise ratio, thus a thorough introduction is presented in the current review about wash-free fluorescent tools based on organic molecules. Firstly, generalization and summarization of the principles for designing wash-free molecular fluorescent tools (WFTs) are made. Subsequently, to make the thought of molecule design more legible, a wash-free strategy is highlighted in recent studies from four diverse but tightly binding aspects: (1) special chemical structures, (2) molecular interactions, (3) bio-orthogonal reactions, (4) abiotic reactions. Meanwhile, biomedical applications including bioimaging, biodetection, and therapy, are ready to be accompanied by. Finally, the prospects for WFTs are elaborated and discussed. This review is a timely conclusion about wash-free strategy in the fluorescence-guided biomedical applications, which may bring WFTs to the forefront and accelerate their extensive applications in biology and medicine.

Bruton tyrosine kinase promotes wound healing after myocardial infarction by inhibiting the transcription of u-PA

BACKGROUNDS

Bruton tyrosine kinase (BTK), which is highly expressed in immune cells, plays a critical role in regulating the function of macrophages. A growing body of evidence has demonstrated that the accumulation of macrophages in cardiac tissue after myocardial infarction (MI) significantly affects wound healing and ventricular remodeling during the early phase of repair after MI. However, the role of BTK in cardiac repair post-MI, especially in macrophage-mediated repair, remains unclear.

METHODS

MI was induced by permanent left anterior descending (LAD) artery ligation in wild-type (WT) mice and macrophage-specific BTK-knockout (BTKMAC-KO) mice. Expression of BTK and phosphorylated BTK were assessed by western blotting. Then, RNA sequencing and ChIP-qPCR assay were performed to explore potential BTK targets and transcriptional regulatory sites.

RESULTS

BTK, which was mainly expressed in macrophages, was upregulated in mice after MI. Compared with WT mice, BTKMAC-KO mice had significantly greater mortality due to heart rupture, reduced wall thickness and severe impairment of left ventricular (LV) function after MI. In addition, increased matrix metalloproteinase-9 (MMP-9) expression and decreased α-SMA and collagen expression were observed in BTKMAC-KO mice after MI. Further experiments revealed that BTK deficiency in macrophages reduces the expression of VEGF and impairs angiogenesis after MI. By RNA sequencing, we found that Nf-kB family genes, as well as the urokinase-type plasminogen activator (uPA), were significantly upregulated in BTK-deficient macrophages. By ChIP-qPCR analysis, we confirmed that uPA was transcriptionally activated by the Nf-kB p65 subunit. Finally, the application of plasminogen activator inhibitor-1 (PAI-1), an uPA inhibitor, markedly protected against cardiac rupture, lowered the mortality rate, and improved cardiac function by increasing collagen deposition and promoting tissue healing in BTKMAC-KO mice after MI.

CONCLUSIONS

The present study identifies PAI-1 as a novel cardioprotective agent for cardiac repair post-MI that increases collagen deposition and promotes tissue healing. A therapeutic strategy targeting BTK may be a promising treatment for cardiac repair post-MI.

Reinvent Aliphatic Arsenicals as Reversible Covalent Warheads toward Targeted Kinase Inhibition and Non-acute Promyelocytic Leukemia Cancer Treatment

The success of arsenic in acute promyelocytic leukemia (APL) treatment is hardly transferred to non-APL cancers, mainly due to the low selectivity and weak binding affinity of traditional arsenicals to oncoproteins critical for cancer survival. We present herein the reinvention of aliphatic trivalent arsenicals (As) as reversible covalent warheads of As-based targeting inhibitors toward Bruton's tyrosine kinase (BTK). The effects of As warheads' valency, thiol protection, methylation, spacer length, and size on inhibitors' activity were studied. We found that, in contrast to the bulky and rigid aromatic As warhead, the flexible aliphatic As warheads were well compatible with the well-optimized guiding group to achieve nanomolar inhibition against BTK. The optimized As inhibitors effectively blocked the BTK-mediated oncogenic signaling pathway, leading to elevated antiproliferative activities toward lymphoma cells and xenograft tumor. Our study provides a promising strategy enabling rational design of new aliphatic arsenic-based reversible covalent inhibitors toward non-APL cancer treatment.

Protein kinases: drug targets for immunological disorders

Protein kinases play a major role in cellular activation processes, including signal transduction by diverse immunoreceptors. Given their roles in cell growth and death and in the production of inflammatory mediators, targeting kinases has proven to be an effective treatment strategy, initially as anticancer therapies, but shortly thereafter in immune-mediated diseases. Herein, we provide an overview of the status of small molecule inhibitors specifically generated to target protein kinases relevant to immune cell function, with an emphasis on those approved for the treatment of immune-mediated diseases. The development of inhibitors of Janus kinases that target cytokine receptor signalling has been a particularly active area, with Janus kinase inhibitors being approved for the treatment of multiple autoimmune and allergic diseases as well as COVID-19. In addition, TEC family kinase inhibitors (including Bruton's tyrosine kinase inhibitors) targeting antigen receptor signalling have been approved for haematological malignancies and graft versus host disease. This experience provides multiple important lessons regarding the importance (or not) of selectivity and the limits to which genetic information informs efficacy and safety. Many new agents are being generated, along with new approaches for targeting kinases.

Readministration of high-dose adeno-associated virus gene therapy vectors enabled by ImmTOR nanoparticles combined with B cell-targeted agents

Tolerogenic ImmTOR nanoparticles encapsulating rapamycin have been demonstrated to mitigate immunogenicity of adeno-associated virus (AAV) gene therapy vectors, enhance levels of transgene expression, and enable redosing of AAV at moderate vector doses of 2 to 5E12 vg/kg. However, recent clinical trials have often pushed AAV vector doses 10-fold to 50-fold higher, with serious adverse events observed at the upper range. Here, we assessed combination therapy of ImmTOR with B cell-targeting drugs for the ability to increase the efficiency of redosing at high vector doses. The combination of ImmTOR with a monoclonal antibody against B cell activation factor (aBAFF) exhibited strong synergy leading to more than a 5-fold to 10-fold reduction of splenic mature B cells and plasmablasts while increasing the fraction of pre-/pro-B cells. In addition, this combination dramatically reduced anti-AAV IgM and IgG antibodies, thus enabling four successive AAV administrations at doses up to 5E12 vg/kg and at least two AAV doses at 5E13 vg/kg, with the transgene expression level in the latter case being equal to that observed in control animals receiving a single vector dose of 1E14 vg/kg. Similar synergistic effects were seen with a combination of ImmTOR and a Bruton's tyrosine kinase inhibitor, ibrutinib. These results suggest that ImmTOR could be combined with B cell-targeting agents to enable repeated vector administrations as a potential strategy to avoid toxicities associated with vector doses above 1E14 vg/kg.

Phosphoinositides and intracellular calcium signaling: novel insights into phosphoinositides and calcium coupling as negative regulators of cellular signaling

Intracellular calcium (Ca2+) and phosphoinositides (PIPs) are crucial for regulating cellular activities such as metabolism and cell survival. Cells maintain precise intracellular Ca2+ and PIP levels via the actions of a complex system of Ca2+ channels, transporters, Ca2+ ATPases, and signaling effectors, including specific lipid kinases, phosphatases, and phospholipases. Recent research has shed light on the complex interplay between Ca2+ and PIP signaling, suggesting that elevated intracellular Ca2+ levels negatively regulate PIP signaling by inhibiting the membrane localization of PIP-binding proteins carrying specific domains, such as the pleckstrin homology (PH) and Ca2+-independent C2 domains. This dysregulation is often associated with cancer and metabolic diseases. PIPs recruit various proteins with PH domains to the plasma membrane in response to growth hormones, which activate signaling pathways regulating metabolism, cell survival, and growth. However, abnormal PIP signaling in cancer cells triggers consistent membrane localization and activation of PIP-binding proteins. In the context of obesity, an excessive intracellular Ca2+ level prevents the membrane localization of the PIP-binding proteins AKT, IRS1, and PLCδ via Ca2+-PIPs, contributing to insulin resistance and other metabolic diseases. Furthermore, an excessive intracellular Ca2+ level can cause functional defects in subcellular organelles such as the endoplasmic reticulum (ER), lysosomes, and mitochondria, causing metabolic diseases. This review explores how intracellular Ca2+ overload negatively regulates the membrane localization of PIP-binding proteins.

BTKbase, Bruton Tyrosine Kinase Variant Database in X-Linked Agammaglobulinemia: Looking Back and Ahead

BTKbase is an international database for disease-causing variants in Bruton tyrosine kinase (BTK) leading to X-linked agammaglobulinemia (XLA), a rare primary immunodeficiency of antibody production. BTKbase was established in 1994 as one of the first publicly available variation databases. The number of cases has more than doubled since the last update; it now contains information for 2310 DNA variants in 2291 individuals. 1025 of the DNA variants are unique. The human genome contains more than 500 protein kinases, among which BTK has the largest number of unique disease-causing variants. The current version of BTKbase has numerous novel features: the database has been reformatted, it has moved to LOVD database management system, it has been internally harmonized, etc. Systematics and standardization have been increased, including Variation Ontology annotations for variation types. There are some regions with lower than expected variation frequency and some hotspots for variations. BTKbase contains, in addition to variant descriptions at DNA, RNA and protein levels, also laboratory parameters and clinical features for many patients. BTKbase has served clinical and research communities in the diagnosis of XLA cases and provides general insight into effects of variations, especially in signalling pathways. Amino acid substitutions and their effects were investigated, predicted, and visualized at 3D level in the protein domains. BTKbase is freely available.

Bruton's Tyrosine Kinase Inhibitors: The Next Frontier of B-Cell-Targeted Therapies for Cancer, Autoimmune Disorders, and Multiple Sclerosis

Bruton's tyrosine kinase (BTK) is an important protein belonging to the tyrosine kinase family that plays a key role in the intracellular signaling and proliferation, migration, and survival of normal and malignant B-lymphocytes and myeloid cells. Understanding the role of BTK in the B-cell signaling pathway has led to the development of BTK inhibitors (BTKi) as effective therapies for malignancies of myeloid origin and exploration as a promising therapeutic option for other cancers. Given its central function in B-cell receptor signaling, inhibition of BTK is an attractive approach for the treatment of a wide variety of autoimmune diseases that involve aberrant B-cell function including systemic lupus erythematosus (SLE), rheumatoid arthritis (RA), and multiple sclerosis (MS). Here, we review the role of BTK in different cell signaling pathways, the development of BTKi in B-cell malignancies, and their emerging role in the treatment of MS and other autoimmune disorders.

Discovery of a potent BTK and IKZF1/3 triple degrader through reversible covalent BTK PROTAC development

Building on our previous work on ibrutinib-based reversible covalent Bruton's tyrosine kinase (BTK) PROTACs, we explored a different irreversible BTK inhibitor poseltinib as the BTK binder for PROTAC development. Different from ibrutinib, converting the irreversible cysteine reacting acrylamide group of poseltinib to a reversible covalent cyano-acrylamide group dramatically decreases the binding affinity to BTK by over 700 folds. Interestingly, one of the reversible covalent BTK PROTACs based on poseltinib with a rigid linker, dubbed as PS-RC-1, is highly potent (IC50 = ~10 nM) in Mino cells but not in other mantle cell lymphoma (MCL) cell lines, such as Jeko-1 and Rec-R cells. We showed that PS-RC-1 potently induces degradation of IKZF1 and IKZF3 but not BTK or GSPT1, accounting for its toxicity in Mino cells. We further decreased the molecular size of PS-RC-1 by shrinking the BTK binding moiety and developed PS-2 as a potent BTK and IKZF1/3 triple degrader with high specificity.

Ibrutinib in the Treatment of Solid Tumors: Current State of Knowledge and Future Directions

Bruton's Tyrosine Kinase (BTK) is considered crucial in the activation and survival of both physiological and malignant B-cells. In recent years, ibrutinib, an oral BTK inhibitor, became a breakthrough therapy for hematological malignancies, such as chronic lymphocytic. However, ibrutinib's feasibility might not end there. Several other kinases with established involvement with solid malignancies (i.e., EGFR, HER2) have been found to be inhibited by this agent. Recent discoveries indicate that BTK is a potential anti-solid tumor therapy target. Consequently, ibrutinib, a BTK-inhibitor, has been studied as a therapeutic option in solid malignancies. While most preclinical studies indicate ibrutinib to be an effective therapeutic option in some specific indications, such as NSCLC and breast cancer, clinical trials contradict these observations. Nevertheless, while ibrutinib failed as a monotherapy, it might become an interesting part of a multidrug regime: not only has a synergism between ibrutinib and other compounds, such as trametinib or dactolisib, been observed in vitro, but this BTK inhibitor has also been established as a radio- and chemosensitizer. This review aims to describe the milestones in translating BTK inhibitors to solid tumors in order to understand the future potential of this agent better.

Retrospective study of 98 patients with X-linked agammaglobulinemia complicated with arthritis

OBJECTIVE

We preformed this retrospective study of clinical manifestation, imaging feature, and mutations to describe joint involvement in X-linked agammaglobulinemia (XLA) patients, aimed to provide recommendation for physicians.

METHODS

A total number of 98 XLA patients who have been diagnosed between January 2000 and February 2020 were enrolled and grouped based on whether they developed arthritis and analyzed for the clinical, imaging, and gene mutation data using the t test or the Mann-Whitney test.

RESULTS

Forty-five out of 98 patients (45.9%) had joint involvement, 40.8% had symptom prior to the diagnosis of XLA, and 54.1% had no articular symptom. Patients with joint involvement had a higher median diagnostic age of XLA and initial IgG level than patients without it, while their intravenous immunoglobulin was lower (p < 0.05). Knee, hip, and ankle were the most frequent joint, and oligoarthritis (≦ 4 joints) was more common than polyarthritis (88.9% vs 11.1%). Red and tenderness were the most frequent clinical symptoms (80%) with 24.4% reporting limited activity and 8.9% reporting deformity. Imaging data collected from 32 patients indicated that joint effusion (53.3%), synovitis (15.5%), and swollen soft tissue (15.5%) were the most common feature. Seventeen patients were treated by antibiotics plus intravenous immunoglobulin (IVIG) with an effective rate of 70.6%, and 28 patients only received IVIG with an effective rate of 67.9%. In comparison to patients without arthritis who have higher frequency nonsense and frameshift mutation, patients with arthritis had a higher incidence of missense mutation (p < 0.05).

CONCLUSION

High prevalence of arthritis among X-linked agammaglobulinemia patients and subsequent progression through IVIG replacement therapy highlight the importance of timely diagnosis and better management of these patients. Our finding indicated a potential correlation between genotype and phenotype, and further research on the mechanism of arthritis in XLA patients could increase physicians' awareness and improve patients' prognosis. Key Points • This study described the feature of arthritis in XLA patients and indicated a potential correlation between this complication and genotype.

Bruton's tyrosine kinase inhibition in the treatment of preclinical models and multiple sclerosis

Significant progress has been made in understanding the immunopathogenesis of multiple sclerosis (MS) over recent years. Successful clinical trials with CD20-depleting monoclonal antibodies have corroborated the fundamental role of B cells in the pathogenesis of MS and reinforced the notion that cells of the B cell lineage are an attractive treatment target. Therapeutic inhibition of Bruton's tyrosine kinase (BTK), an enzyme involved in B cell and myeloid cell activation and function, is regarded as a next-generation approach that aims to attenuate both errant innate and adaptive immune functions. Moreover, brain-penetrant BTK inhibitors may impact compartmentalized inflammation and neurodegeneration within the central nervous system by targeting brain-resident B cells and microglia, respectively. Preclinical studies in animal models of MS corroborated an impact of BTK inhibition on meningeal inflammation and cortical demyelination. Notably, BTK inhibition attenuated the antigen-presenting capacity of B cells and the generation of encephalitogenic T cells. Evobrutinib, a selective oral BTK inhibitor, has been tested recently in a phase 2 study of patients with relapsing-remitting MS. The study met the primary endpoint of a significantly reduced cumulative number of Gadolinium-enhancing lesions under treatment with evobrutinib compared to placebo treatment. Thus, the results of ongoing phase 2 and 3 studies with evobrutinib, fenobrutinib, and tolebrutinib in relapsing-remitting and progressive MS are eagerly awaited. This review article introduces the physiological role of BTK, summarizes the pre-clinical and trial evidence, and addresses the potential beneficial effects of BTK inhibition in MS.

Identification of 22 novel BTK gene variants in B cell deficiency with hypogammaglobulinemia

X-linked agammaglobulinemia (XLA) is an inborn error of immunity caused by pathogenic variants in the BTK gene, resulting in impaired B cell differentiation and maturation. Over 900 variants have already been described in this gene, however, new pathogenic variants continue to be identified. In this report, we describe 22 novel variants in BTK, associated with B cell deficiency with hypo- or agammaglobulinemia in male patients or in asymptomatic female carriers. Genetic data was correlated with BTK protein expression by flow cytometry, and clinical and family history to obtain a comprehensive assessment of the clinico-pathologic significance of these new variants in the BTK gene. For one novel missense variant, p.Cys502Tyr, site-directed mutagenesis was performed to determine the impact of the sequence change on protein expression and stability. Genetic data should be correlated with protein and/or clinical and immunological data, whenever possible, to determine the clinical significance of the gene sequence alteration.

Emerging small-molecule inhibitors of the Bruton's tyrosine kinase (BTK): Current development

Therapy based on Bruton's tyrosine kinase (BTK) inhibitors one of the major treatment options currently recommended for lymphoma patients. The first generation of BTK inhibitor, Ibrutinib, achieved remarkable progress in the treatment of B-cell malignancies, but still has problems with drug-resistance or off-target induced serious side effects. Therefore, numerous new BTK inhibitors were developed to address this unmet medical need. In parallel, the effect of BTK inhibitors against immune-related diseases has been evaluated in clinical trials. This review summarizes recent progress in the research and development of BTK inhibitors, with a focus on structural characteristics and structure-activity relationships. The structure-refinement process of representative pharmacophores as well as their effects on binding affinity, biological activity and pharmacokinetics profiles were analyzed. The advantages and disadvantages of reversible/irreversible BTK inhibitors and their potential implications were discussed to provide a reference for the rational design and development of novel potent BTK inhibitors.

Bruton's Tyrosine Kinase Inhibitors Ibrutinib and Acalabrutinib Counteract Anthracycline Resistance in Cancer Cells Expressing AKR1C3

Simple Summary

The enzyme aldo-keto reductase 1C3 (AKR1C3) is present in several cancers, in which it is capable of actively metabolising different chemotherapy drugs and decreasing their cytotoxic effects. Therefore, the combination with specific inhibitors of AKR1C3 might prevent drug metabolism and increase its efficacy. We investigated the ability of Bruton’s tyrosine kinase inhibitors ibrutinib and acalabrutinib to block the AKR1C3 mediated inactivation of the anthracycline daunorubicin. Experimentation with recombinant AKR1C3 and different cancer cells expressing this enzyme outlined BTK-inhibitors as potential partners to synergise daunorubicin cytotoxicity in vitro. This evidence could be useful to improve the clinical outcome of anthracycline-based chemotherapies.

Abstract

Over the last few years, aldo-keto reductase family 1 member C3 (AKR1C3) has been associated with the emergence of multidrug resistance (MDR), thereby hindering chemotherapy against cancer. In particular, impaired efficacy of the gold standards of induction therapy in acute myeloid leukaemia (AML) has been correlated with AKR1C3 expression, as this enzyme metabolises several drugs including anthracyclines. Therefore, the development of selective AKR1C3 inhibitors may help to overcome chemoresistance in clinical practice. In this regard, we demonstrated that Bruton’s tyrosine kinase (BTK) inhibitors ibrutinib and acalabrutinib efficiently prevented daunorubicin (Dau) inactivation mediated by AKR1C3 in both its recombinant form as well as during its overexpression in cancer cells. This revealed a synergistic effect of BTK inhibitors on Dau cytotoxicity in cancer cells expressing AKR1C3 both exogenously and endogenously, thus reverting anthracycline resistance in vitro. These findings suggest that BTK inhibitors have a novel off-target action, which can be exploited against leukaemia through combination regimens with standard chemotherapeutics like anthracyclines.

Clinical characteristics and prenatal diagnosis for 22 families in Henan Province of China with X-linked agammaglobulinemia (XLA) related to Bruton's tyrosine kinase (BTK) gene mutations

BACKGROUND

X-linked agammaglobulinaemia (XLA) is a rare immunodeficiency disease for which recurrent severe infection is the major clinical symptom. BTK is the main causative gene, with X chromosome recessive inheritance. However, the mutations reported to date do not fully explain the disorder.

METHODS

We detected the percentage of CD19+ B cells and serum immunoglobulin (IgG, IgA, and IgM) levels by flow cytometry and rate scatter immunoturbidimetry, and investigated the BTK mutation profile in 22 XLA patients using Sanger sequencing and real-time PCR .

RESULTS

We evaluated the clinical symptoms of 22 XLA patients and investigated genetic mutations present, identifying six novel mutations in the BTK gene: 2 missense mutations (c.23G > T and c.112 T > C), 2 frameshift mutations (c.522_523insC and c.1060delA), 1 large deletion (deletion of exon 2 to 5), and 1 splice-site mutation (c.1631 + 2 T > C). Prenatal diagnoses were performed in six families (F10, F11, F15, F18, F20 and F21), with the following results: the male fetus in Family 10 (F10) did not carry the c.922_923delGA mutation; the male fetus in Family 15 (F15) did not carry the c.1631 + 1G > T splicing mutation; the female fetus in Family 20 (F20) did not carry the c.1931 T > C mutation; the female fetus in Family 21 (F21) did not carry the large deletion mutation. Hence, these four fetuses are not likely to develop XLA. Male fetuses with c.1060delA and c.1684C > T mutations were identified in Family 11 and Family 18, respectively. The pregnant woman in F18 chose to terminate the pregnancy, whereas the pregnant woman in F11 chose to continue the pregnancy.

CONCLUSION

We confirmed the diagnosis of 22 XLA patients from 22 unrelated families and detected six new pathogenic mutations. Prenatal diagnosis was performed in six families. Early genetic diagnosis and routine lifelong immunoglobulin replacement therapy can prevent and treat infections in XLA children, saving their lives.

Nanoparticle-Mediated Therapeutic Application for Modulation of Lysosomal Ion Channels and Functions

Applications of nanoparticles in various fields have been addressed. Nanomaterials serve as carriers for transporting conventional drugs or proteins through lysosomes to various cellular targets. The basic function of lysosomes is to trigger degradation of proteins and lipids. Understanding of lysosomal functions is essential for enhancing the efficacy of nanoparticles-mediated therapy and reducing the malfunctions of cellular metabolism. The lysosomal function is modulated by the movement of ions through various ion channels. Thus, in this review, we have focused on the recruited ion channels for lysosomal function, to understand the lysosomal modulation through the nanoparticles and its applications. In the future, lysosomal channels-based targets will expand the therapeutic application of nanoparticles-associated drugs.

Structural mechanism for Bruton's tyrosine kinase activation at the cell membrane

Bruton’s tyrosine kinase (Btk) activation on the cell membrane is critical for B cell proliferation and development, and Btk inhibition is a promising treatment for several hematologic cancers and autoimmune diseases. Here, we examine Btk activation using the results of long-timescale molecular dynamics simulations. In our simulations, Btk lipid-binding modules dimerized on the membrane in a single predominant conformation. We observed that the phospholipid PIP₃—in addition to its expected role of recruiting Btk to the membrane—allosterically mediated dimer formation and stability by binding at two novel sites. Our results provide strong evidence that PIP₃-mediated dimerization of Btk at the cell membrane is a critical step in Btk activation and suggest a potential approach to allosteric Btk inhibitor development.

Bruton’s tyrosine kinase (Btk) is critical for B cell proliferation and activation, and the development of Btk inhibitors is a vigorously pursued strategy for the treatment of various B cell malignancies. A detailed mechanistic understanding of Btk activation has, however, been lacking. Here, inspired by a previous suggestion that Btk activation might depend on dimerization of its lipid-binding PH–TH module on the cell membrane, we performed long-timescale molecular dynamics simulations of membrane-bound PH–TH modules and observed that they dimerized into a single predominant conformation. We found that the phospholipid PIP₃ stabilized the dimer allosterically by binding at multiple sites, and that the effects of PH–TH mutations on dimer stability were consistent with their known effects on Btk activity. Taken together, our simulation results strongly suggest that PIP₃-mediated dimerization of Btk at the cell membrane is a critical step in Btk activation.

Ibrutinib in the management of Waldenstrom macroglobulinemia

Bruton tyrosine kinase plays a critical role in hastening cell proliferation. Bruton tyrosine kinase inhibitors are a class of immunotheraputic agents that disrupt this signaling pathway. Ibrutinib, a novel Bruton tyrosine kinase inhibitor approved by the Food and Drug Administration (FDA) for the treatment of Waldenstrom macroglobulinemia in patients who have failed treatment with other agents, has emerged as an important therapeutic agent in the management of Waldenstrom macroglobulinemia and other plasma cell dyscrasias. Ibrutinib has shown to increase progression free survival and improve overall mortality. We present a review of ibrutinib, beginning with an overview of the Bruton tyrosine kinase pathway and clinically relevant gene mutations impacting treatment and prognosis for patients with Waldenstrom macroglobulinemia, followed by evidence supporting therapeutic indications for ibrutinib, and detailing its safety and efficacy evidence, current clinical guidelines, adverse effects and their management, and finally challenges of drug resistance. We also present findings on newly developed Bruton tyrosine kinase inhibitors in the therapeutic pipeline to provide readers insight into this rapidly evolving corner of oncology pharmacy practice.

Molecular characterization and function analysis of grouper (Epinephelus coioides) Bruton's tyrosine kinase BTK

Bruton's tyrosine kinase (BTK) is a Tec-family tyrosine kinase and plays a crucial role in B cell antigen receptor (BCR) signal pathway. Mutations in humans and mice BTK gene results in X-linked agammaglobulinemia (XLA) and X-linked immunodeficiency (XLD), respectively. To study the function of BTK in teleost, we cloned a BTK gene from orange-spotted grouper. Homology analysis showed that the grouper BTK (EcBTK) had a high amino acid identity with other vertebrates (63%-92%) and shared the highest amino acid identity with ballan wrasse Labrus bergylta BTK. EcBTK comprises a Bruton's tyrosine kinase pleckstrin homology (PH) domain, a Tec homology (TH) domain, a Src homology 3 (SH3) domain, a Src homology 2 (SH2) domain and a Protein Kinases, catalytic (PKc) domain. Tissue distribution analysis showed that EcBTK was mainly expressed in immune organs. EcBTK was uniform distributed throughout the cytoplasm of transfected HEK293T cells and overexpression of EcBTK slightly down-regulates NF-κB activity. Ibrutinib treatment can reduce the phosphorylation level of grouper's BTK. In groupers infected with Cryptocaryon irritans, up-regulation of EcBTK were not seen in the early stage of infected skin and gill until days 14-21. The phosphorylation level of grouper BTK was significantly increased in infected skin and gill.

The hidden potential of lysosomal ion channels: A new era of oncogenes

Lysosomes serve as the control centre for cellular clearance. These membrane-bound organelles receive biomolecules destined for degradation from intracellular and extracellular pathways; thus, facilitating the production of energy and shaping the fate of the cell. At the base of their functionality are the lysosomal ion channels which mediate the function of the lysosome through the modulation of ion influx and efflux. Ion channels form pores in the membrane of lysosomes and allow the passage of ions, a seemingly simple task which harbours the potential of overthrowing the cell's stability. Considered the master regulators of ion homeostasis, these integral membrane proteins enable the proper operation of the lysosome. Defects in the structure or function of these ion channels lead to the development of lysosomal storage diseases, neurodegenerative diseases and cancer. Although more than 50 years have passed since their discovery, lysosomes are not yet fully understood, with their ion channels being even less well characterized. However, significant improvements have been made in the development of drugs targeted against these ion channels as a means of combating diseases. In this review, we will examine how Ca²⁺, K⁺, Na⁺ and Cl^- ion channels affect the function of the lysosome, their involvement in hereditary and spontaneous diseases, and current ion channel-based therapies.

Acquired thrombotic thrombocytopenic purpura

The von Willebrand factor (VWF)-cleaving metalloprotease, ADAMTS13 (a disintegrin and metalloprotease with thrombospondin type 1 motifs-13) is the only known target of the dysregulated immune response in acquired TTP. Autoantibodies to ADAMTS13 either neutralize its activity or accelerate its clearance, thereby causing a severe deficiency of ADAMTS13 in plasma. As a consequence, size regulation of VWF is impaired and the persistence of ultra-large VWF (ULVWF) multimers facilitates micro vascular platelet aggregation causing microangiopathic haemolytic anaemia and ischaemic organ damage. Autoimmune TTP although a rare disease with an annual incidence of 1.72 cases has a mortality rate of 20% even with adequate therapy.

We describe the mechanisms involved in ADAMTS13 autoimmunity with a focus on the role of B- and T-cells in the pathogenesis of this disorder. We discuss the potential translation of recent experimental findings into future therapeutic concepts for the treatment of acquired TTP.

Delayed diagnosis in X-linked agammaglobulinemia and its relationship to the occurrence of mutations in BTK non-kinase domains

BACKGROUND

X-linked agammaglobulinemia (XLA) is characterized by the absence of immunoglobulin and B cells. Patients suffer from recurrent bacterial infections from early childhood, and require lifelong immunoglobulin replacement therapy. Mutations in BTK (Bruton's Tyrosine Kinase) are associated with this phenotype. Some patients that present XLA do not show typical clinical symptoms, resulting in delayed diagnosis due to the lack of a severe phenotype. This study presents a report of five XLA patients from four different families and attempts to determine a relationship between delayed diagnosis and the occurrence of BTK mutations.

METHODS

Samples from patients with antibody deficiency were analyzed to determine BTK expression, immunophenotyping and mutation analysis. Clinical and laboratory data was analyzed and presented for each patient.

RESULTS

Most patients presented here showed atypical clinical and laboratory data for XLA, including normal IgM, IgG, or IgA levels. Most patients expressed detectable BTK protein. Sequencing of BTK showed that these patients harbored missense mutations in the pleckstrin homology and Src-homology-2 domains. When it was compared to public databases, BTK sequencing exhibited a new change, along with three other previously reported changes.

CONCLUSIONS

Delayed diagnosis and atypical manifestations in XLA might be related to mutation type and BTK expression.

Inherent X-Linked Genetic Variability and Cellular Mosaicism Unique to Females Contribute to Sex-Related Differences in the Innate Immune Response

Females have a longer lifespan and better general health than males. Considerable number of studies also demonstrated that, after trauma and sepsis, females present better outcomes as compared to males indicating sex-related differences in the innate immune response. The current notion is that differences in the immuno-modulatory effects of sex hormones are the underlying causative mechanism. However, the field remains controversial and the exclusive role of sex hormones has been challenged. Here, we propose that polymorphic X-linked immune competent genes, which are abundant in the population are important players in sex-based immuno-modulation and play a key role in causing sex-related outcome differences following trauma or sepsis. We describe the differences in X chromosome (ChrX) regulation between males and females and its consequences in the context of common X-linked polymorphisms at the individual as well as population level. We also discuss the potential pathophysiological and immune-modulatory aspects of ChrX cellular mosaicism, which is unique to females and how this may contribute to sex-biased immune-modulation. The potential confounding effects of ChrX skewing of cell progenitors at the bone marrow is also presented together with aspects of acute trauma-induced de novo ChrX skewing at the periphery. In support of the hypothesis, novel observations indicating ChrX skewing in a female trauma cohort as well as case studies depicting the temporal relationship between trauma-induced cellular skewing and the clinical course are also described. Finally, we list and discuss a selected set of polymorphic X-linked genes, which are frequent in the population and have key regulatory or metabolic functions in the innate immune response and, therefore, are primary candidates for mediating sex-biased immune responses. We conclude that sex-related differences in a variety of disease processes including the innate inflammatory response to injury and infection may be related to the abundance of X-linked polymorphic immune-competent genes, differences in ChrX regulation, and inheritance patterns between the sexes and the presence of X-linked cellular mosaicism, which is unique to females.

Increased intracellular Ca2+ concentrations prevent membrane localization of PH domains through the formation of Ca2+-phosphoinositides

Insulin resistance is a metabolic disorder in which target cells fail to respond to physiological levels of circulating insulin, leading to hyperinsulinemia and glucose intolerance. The molecular mechanism underlying insulin resistance is still largely unknown. Here, we found that intracellular Ca²⁺ overloading in obesity attenuates insulin-stimulated phosphorylation of protein kinase B and its downstream signaling by preventing membrane localization of various pleckstrin homology (PH) domains. When at high intracellular levels, Ca²⁺ binds tightly with phosphoinositides to yield Ca²⁺-phosphoinositides (PIPs), abrogating the membrane targeting of PH domains and disrupting insulin signaling. Thus, we identified a previously unknown physiological function of intracellular Ca²⁺ as a critical negative regulator of insulin signaling, especially through the formation of Ca²⁺-PIPs.

Insulin resistance, a key etiological factor in metabolic syndrome, is closely linked to ectopic lipid accumulation and increased intracellular Ca²⁺ concentrations in muscle and liver. However, the mechanism by which dysregulated intracellular Ca²⁺ homeostasis causes insulin resistance remains elusive. Here, we show that increased intracellular Ca²⁺ acts as a negative regulator of insulin signaling. Chronic intracellular Ca²⁺ overload in hepatocytes during obesity and hyperlipidemia attenuates the phosphorylation of protein kinase B (Akt) and its key downstream signaling molecules by inhibiting membrane localization of pleckstrin homology (PH) domains. Pharmacological approaches showed that elevated intracellular Ca²⁺ inhibits insulin-stimulated Akt phosphorylation and abrogates membrane localization of various PH domain proteins such as phospholipase Cδ and insulin receptor substrate 1, suggesting a common mechanism inhibiting the membrane targeting of PH domains. PH domain-lipid overlay assays confirmed that Ca²⁺ abolishes the binding of various PH domains to phosphoinositides (PIPs) with two adjacent phosphate groups, such as PI(3,4)P₂, PI(4,5)P₂, and PI(3,4,5)P₃. Finally, thermodynamic analysis of the binding interaction showed that Ca²⁺-mediated inhibition of targeting PH domains to the membrane resulted from the tight binding of Ca²⁺ rather than PH domains to PIPs forming Ca²⁺-PIPs. Thus, Ca²⁺-PIPs prevent the recognition of PIPs by PH domains, potentially due to electrostatic repulsion between positively charged side chains in PH domains and the Ca²⁺-PIPs. Our findings provide a mechanistic link between intracellular Ca²⁺ dysregulation and Akt inactivation in insulin resistance.

BTK suppresses myeloma cellular senescence through activating AKT/P27/Rb signaling

We previously explored the role of BTK in maintaining multiple myeloma stem cells (MMSCs) self-renewal and drug-resistance. Here we investigated the elevation of BTK suppressing MM cellular senescence, a state of irreversible cellular growth arrest. We firstly discovered that an increased expression of BTK in MM samples compared to normal controls by immunohistochemistry (IHC), and significant chromosomal gain in primary samples. In addition, BTK high-expressing MM patients are associated with poor outcome in both Total Therapy 2 (TT2) and TT3 cohorts. Knockdown BTK expression by shRNA induced MM cellular senescence using β-galactosidase (SA-b-gal) staining, cell growth arrest by cell cycle staining and decreased clonogenicity while forcing BTK expression in MM cells abrogated these characteristics. We also validated this feature in mouse embryonic fibroblast cells (MEFs), which showed that elevated BTK expression was resistant to MEF senescence after serial cultivation in vitro. Further mechanism study revealed that BTK activated AKT signaling leading to down-regulation of P27 expression and hindered RB activity while AKT inhibitor, LY294002, overcame BTK-overexpression induced cellular senescence resistance. Eventually we demonstrated that BTK inhibitor, CGI-1746, induced MM cellular senescence, colony reduction and tumorigenecity inhibition in vivo. Summarily, we designate a novel mechanism of BTK in mediating MM growth, and BTK inhibitor is of great potential in vivo and in vitro suggesting BTK is a promising therapeutic target for MM.

ANKRD54 preferentially selects Bruton's Tyrosine Kinase (BTK) from a Human Src-Homology 3 (SH3) domain library

Bruton's Tyrosine Kinase (BTK) is a cytoplasmic protein tyrosine kinase with a fundamental role in B-lymphocyte development and activation. The nucleocytoplasmic shuttling of BTK is specifically modulated by the Ankyrin Repeat Domain 54 (ANKRD54) protein and the interaction is known to be exclusively SH3-dependent. To identify the spectrum of the ANKRD54 SH3-interactome, we applied phage-display screening of a library containing all the 296 human SH3 domains. The BTK-SH3 domain was the prime interactor. Quantitative western blotting analysis demonstrated the accuracy of the screening procedure. Revealing the spectrum and specificity of ANKRD54-interactome is a critical step toward functional analysis in cells and tissues.

From identification of the BTK kinase to effective management of leukemia

BTK is a cytoplasmic protein-tyrosine kinase, whose corresponding gene was isolated in the early 1990s. BTK was initially identified by positional cloning of the gene causing X-linked agammaglobulinemia and independently in a search for new kinases. Given the phenotype of affected patients, namely lack of B-lymphocytes and plasma cells with the ensuing inability to mount humoral immune responses, BTK inhibitors were anticipated to have beneficial effects on antibody-mediated pathologies, such as autoimmunity. In contrast to, for example, the SRC-family of cytoplasmic kinases, there was no obvious way in which structural alterations would yield constitutively active forms of BTK, and such mutations were also not found in leukemias or lymphomas. In 2007, the first efficient inhibitor, ibrutinib, was reported and soon became approved both in the United States and in Europe for the treatment of three B-cell malignancies, mantle cell lymphoma, chronic lymphocytic leukemia and Waldenström's macroglobulinemia. Over the past few years, additional inhibitors have been developed, with acalabrutinib being more selective, and recently demonstrating fewer clinical adverse effects. The antitumor mechanism is also not related to mutations in BTK. Instead tumor residency in lymphoid organs is inhibited, making these drugs highly versatile. BTK is one of the only 10 human kinases that carry a cysteine in the adenosine triphosphate-binding cleft. As this allows for covalent, irreversible inhibitor binding, it provides these compounds with a highly advantageous character. This quality may be crucial and bodes well for the future of BTK-modifying medicines, which have been estimated to reach annual multi-billion dollar sales in the future.

Bruton's Tyrosine Kinase Small Molecule Inhibitors Induce a Distinct Pancreatic Toxicity in Rats

Bruton's tyrosine kinase (BTK) is a member of the Tec family of cytoplasmic tyrosine kinases involved in B-cell and myeloid cell signaling. Small molecule inhibitors of BTK are being investigated for treatment of several hematologic cancers and autoimmune diseases. GDC-0853 ((S)-2-(3'-(hydroxymethyl)-1-methyl-5-((5-(2-methyl-4-(oxetan-3-yl)piperazin-1-yl)pyridin-2-yl)amino)-6-oxo-1,6-dihydro-[3,4'-bipyridin]-2'-yl)-7,7-dimethyl-3,4,7,8-tetrahydro-2H-cyclopenta[4,5]pyrrolo[1,2-a]pyrazin-1(6H)-one) is a selective and reversible oral small-molecule BTK inhibitor in development for the treatment of rheumatoid arthritis and systemic lupus erythematosus. In Sprague-Dawley (SD) rats, administration of GDC-0853 and other structurally diverse BTK inhibitors for 7 days or longer caused pancreatic lesions consisting of multifocal islet-centered hemorrhage, inflammation, fibrosis, and pigment-laden macrophages with adjacent lobular exocrine acinar cell atrophy, degeneration, and inflammation. Similar findings were not observed in mice or dogs at much higher exposures. Hemorrhage in the peri-islet vasculature emerged between four and seven daily doses of GDC-0853 and was histologically similar to spontaneously occurring changes in aging SD rats. This suggests that GDC-0853 could exacerbate a background finding in younger animals. Glucose homeostasis was dysregulated following a glucose challenge; however, this occurred only after 28 days of administration and was not directly associated with onset or severity of pancreatic lesions. There were no changes in other common serum biomarkers assessing endocrine and exocrine pancreatic function. Additionally, these lesions were not readily detectable via Doppler ultrasound, computed tomography, or magnetic resonance imaging. Our results indicate that pancreatic lesions in rats are likely a class effect of BTK inhibitors, which may exacerbate an islet-centered pathology that is unlikely to be relevant to humans.

Clinical characteristics and genetic profiles of 174 patients with X-linked agammaglobulinemia: Report from Shanghai, China (2000-2015)

X-linked agammaglobulinemia (XLA) is a humoral primary immunodeficiency. XLA patients typically present with very low numbers of peripheral B cells and a profound deficiency of all immunoglobulin isotypes. Most XLA patients carry mutations in Bruton tyrosine kinase (BTK) gene.The genetic background and clinical features of 174 Chinese patients with XLA were investigated. The relationship between specific BTK gene mutations and severity of clinical manifestations was also examined. Mutations were graded from mild to severe based on structural and functional prediction through bioinformatics analysis.One hundred twenty-seven mutations were identified in 142 patients from 124 families, including 45 novel mutations and 82 recurrent mutations that were distributed over the entire BTK gene sequence. Variation in phenotypes was observed, and there was a tendency of association between genotype and age of disease onset.This report constitutes the largest group of patients with BTK mutations in China. A genotype-phenotype correlation was observed in this study. Early diagnosis of congenital agammaglobulinemia should be based on clinical symptoms, family history, and molecular analysis of the BTK gene.

Substitution scanning identifies a novel, catalytically active ibrutinib-resistant BTK cysteine 481 to threonine (C481T) variant

Irreversible Bruton tyrosine kinase (BTK) inhibitors, ibrutinib and acalabrutinib have demonstrated remarkable clinical responses in multiple B-cell malignancies. Acquired resistance has been identified in a sub-population of patients in which mutations affecting BTK predominantly substitute cysteine 481 in the kinase domain for catalytically active serine, thereby ablating covalent binding of inhibitors. Activating substitutions in the BTK substrate phospholipase Cγ2 (PLCγ2) instead confers resistance independent of BTK. Herein, we generated all six possible amino acid substitutions due to single nucleotide alterations for the cysteine 481 codon, in addition to threonine, requiring two nucleotide substitutions, and performed functional analysis. Replacement by arginine, phenylalanine, tryptophan or tyrosine completely inactivated the catalytic activity, whereas substitution with glycine caused severe impairment. BTK with threonine replacement was catalytically active, similar to substitution with serine. We identify three potential ibrutinib resistance scenarios for cysteine 481 replacement: (1) Serine, being catalytically active and therefore predominating among patients. (2) Threonine, also being catalytically active, but predicted to be scarce, because two nucleotide changes are needed. (3) As BTK variants replaced with other residues are catalytically inactive, they presumably need compensatory mutations, therefore being very scarce. Glycine and tryptophan variants were not yet reported but likely also provide resistance.

Clinical and mutational features of X-linked agammaglobulinemia in Mexico

X-linked agammaglobulinemia (XLA) is caused by BTK mutations, patients typically show <2% of peripheral B cells and reduced levels of all immunoglobulins; they suffer from recurrent infections of bacterial origin; however, viral infections, autoimmune-like diseases, and an increased risk of developing gastric cancer are also reported. In this work, we report the BTK mutations and clinical features of 12 patients diagnosed with XLA. Furthermore, a clinical revision is also presented for an additional cohort of previously reported patients with XLA. Four novel mutations were identified, one of these located in the previously reported mutation refractory SH3 domain. Clinical data support previous reports accounting for frequent respiratory, gastrointestinal tract infections and other symptoms such as the occurrence of reactive arthritis in 19.2% of the patients. An equal proportion of patients developed septic arthritis; missense mutations and mutations in SH1, SH2 and PH domains predominated in patients who developed arthritis.

Analysis of the Effects of the Bruton's tyrosine kinase (Btk) Inhibitor Ibrutinib on Monocyte Fcγ Receptor (FcγR) Function

The irreversible Bruton's tyrosine kinase (Btk) inhibitor ibrutinib has shown efficacy against B-cell tumors such as chronic lymphocytic leukemia and B-cell non-Hodgkin lymphoma. Fcγ receptors (FcγR) on immune cells such as macrophages play an important role in tumor-specific antibody-mediated immune responses, but many such responses involve Btk. Here we tested the effects of ibrutinib on FcγR-mediated activities in monocytes. We found that ibrutinib did not affect monocyte FcγR-mediated phagocytosis, even at concentrations higher than those achieved physiologically, but suppressed FcγR-mediated cytokine production. We confirmed these findings in macrophages from Xid mice in which Btk signaling is defective. Because calcium flux is a major event downstream of Btk, we tested whether it was involved in phagocytosis. The results showed that blocking intracellular calcium flux decreased FcγR-mediated cytokine production but not phagocytosis. To verify this, we measured activation of the GTPase Rac, which is responsible for actin polymerization. Results showed that ibrutinib did not inhibit Rac activation, nor did the calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl ester). We next asked whether the effect of ibrutinib on monocyte FcγR-mediated cytokine production could be rescued by IFNγ priming because NK cells produce IFNγ in response to antibody therapy. Pretreatment of monocytes with IFNγ abrogated the effects of ibrutinib on FcγR-mediated cytokine production, suggesting that IFNγ priming could overcome this Btk inhibition. Furthermore, in monocyte-natural killer cell co-cultures, ibrutinib did not inhibit FcγR-mediated cytokine production despite doing so in single cultures. These results suggest that combining ibrutinib with monoclonal antibody therapy could enhance chronic lymphocytic leukemia cell killing without affecting macrophage effector function.

Targets for Ibrutinib Beyond B Cell Malignancies

Ibrutinib (Imbruvica^™) is an irreversible, potent inhibitor of Bruton's tyrosine kinase (BTK). Over the last few years, ibrutinib has developed from a promising drug candidate to being approved by FDA for the treatment of three B cell malignancies, a truly remarkable feat. Few, if any medicines are monospecific and ibrutinib is no exception; already during ibrutinib's initial characterization, it was found that it could bind also to other kinases. In this review, we discuss the implications of such interactions, which go beyond the selective effect on BTK in B cell malignancies. In certain cases, the outcome of ibrutinib treatment likely results from the combined inhibition of BTK and other kinases, causing additive or synergistic, effects. Conversely, there are also examples when the clinical outcome seems unrelated to inhibition of BTK. Thus, more specifically, adverse effects such as enhanced bleeding or arrhythmias could potentially be explained by different interactions. We also predict that during long‐term treatment bone homoeostasis might be affected due to the inhibition of osteoclasts. Moreover, the binding of ibrutinib to molecular targets other than BTK or effects on cells other than B cell‐derived malignancies could be beneficial and result in new indications for clinical applications.

BTK mutations selectively regulate BTK expression and upregulate monocyte XBP1 mRNA in XLA patients

Mutations in the Bruton agammaglobulinemia tyrosine kinase (BTK) gene are responsible for X-linked agammaglobulinemia (XLA). Unfolded or misfolded proteins can trigger stress pathways in the endoplasmic reticulum (ER), known as unfolded protein response (UPR). The aim was to clarify the involvement of UPR in XLA pathophysiology. By reverse transcription-quantitative PCR, we evaluated the expression of BTK and 12 UPR-related genes in eight patients. Moreover, we assessed the BTK protein expression and pattern in the patients' monocytes by flow cytometry and fluorescence immunocytochemistry. We found a reduced BTK expression in patients with stop codon mutations (P < 0.02). However, missense mutations did not affect BTK expression. Flow cytometry showed a reduction of BTK in patients which was corroborated by an absent or nonfunctional protein synthesis revealed by immunocytochemistry. In contrast with the other UPR-related genes, X-box binding protein 1 (XBP1) was markedly upregulated in the patients (P < 0.01), suggesting Toll-like receptor (TLR) activation since BTK directly interacts with TLRs as a negative regulator and XBP1 can be activated in direct response to TLR ligation. Different BTK mutations can be identified by the BTK expression. Inasmuch as UPR-related genes were downregulated or unaltered in patients, we speculate the involvement of the TLRs-XBP1 axis in the XLA pathophysiology. Such data could be the basis for further studies of this novel pathomechanism concerning XLA.

Splice-correction strategies for treatment of X-linked agammaglobulinemia

X-linked agammaglobulinemia (XLA) is a primary immunodeficiency disease caused by mutations in the gene coding for Bruton’s tyrosine kinase (BTK). Deficiency of BTK leads to a developmental block in B cell differentiation; hence, the patients essentially lack antibody-producing plasma cells and are susceptible to various infections. A substantial portion of the mutations in BTK results in splicing defects, consequently preventing the formation of protein-coding mRNA. Antisense oligonucleotides (ASOs) are therapeutic compounds that have the ability to modulate pre-mRNA splicing and alter gene expression. The potential of ASOs has been exploited for a few severe diseases, both in pre-clinical and clinical studies. Recently, advances have also been made in using ASOs as a personalized therapy for XLA. Splice-correction of BTK has been shown to be feasible for different mutations in vitro, and a recent proof-of-concept study demonstrated the feasibility of correcting splicing and restoring BTK both ex vivo and in vivo in a humanized bacterial artificial chromosome (BAC)-transgenic mouse model. This review summarizes the advances in splice correction, as a personalized medicine for XLA, and outlines the promises and challenges of using this technology as a curative long-term treatment option.

Characterization of all possible single-nucleotide change caused amino acid substitutions in the kinase domain of Bruton tyrosine kinase

Knowledge about features distinguishing deleterious and neutral variations is crucial for interpretation of novel variants. Bruton tyrosine kinase (BTK) contains the highest number of unique disease-causing variations among the human protein kinases, still it is just 10% of all the possible single-nucleotide substitution-caused amino acid variations (SNAVs). In the BTK kinase domain (BTK-KD) can appear altogether 1,495 SNAVs. We investigated them all with bioinformatic and protein structure analysis methods. Most disease-causing variations affect conserved and buried residues disturbing protein stability. Minority of exposed residues is conserved, but strongly tied to pathogenicity. Sixty-seven percent of variations are predicted to be harmful. In 39% of the residues, all the variants are likely harmful, whereas in 10% of sites, all the substitutions are tolerated. Results indicate the importance of the entire kinase domain, involvement in numerous interactions, and intricate functional regulation by conformational change. These results can be extended to other protein kinases and organisms.

Role of Bruton's tyrosine kinase inhibitors in HIV-1-infected cells

Many cellular cofactors have been documented to be critical for various stages of viral replication. Using high-throughput proteomic assays, we have previously identified Bruton's tyrosine kinase (BTK) as a host protein that was uniquely upregulated in the plasma membrane of human immunodeficiency virus (HIV-1)-infected T cells. Here, we have further characterized the BTK expression in HIV-1 infection and show that this cellular factor is specifically expressed in infected myeloid cells. Significant upregulation of the phosphorylated form of BTK was observed in infected cells. Using size exclusion chromatography, we found BTK to be virtually absent in the uninfected U937 cells; however, new BTK protein complexes were identified and distributed in both high molecular weight (∼600 kDa) and a small molecular weight complex (∼60-120 kDa) in the infected U1 cells. BTK levels were highest in cells either chronically expressing virus or induced/infected myeloid cells and that BTK translocated to the membrane following induction of the infected cells. BTK knockdown in HIV-1-infected cells using small interfering RNA (siRNA) resulted in selective death of infected, but not uninfected, cells. Using BTK-specific antibody and small-molecule inhibitors including LFM-A13 and a FDA-approved compound, ibrutinib (PCI-32765), we have found that HIV-1-infected cells are sensitive to apoptotic cell death and result in a decrease in virus production. Overall, our data suggests that HIV-1-infected cells are sensitive to treatments targeting BTK expressed in infected cells.

Bruton's TK inhibitors: structural insights and evolution of clinical candidates

Bruton's TK (BTK) is a promising biological target for therapeutic intervention of several diseases including inflammatory diseases and cancer/B cell malignancies. Numerous research groups are actively engaged in investigating the functions of BTK, and discovering potent and selective BTK inhibitors as drug candidates. Revealed by x-ray crystal structures with ligands of diverse chemical structures, the ability of BTK kinase domain to adopt various inactive conformations offers unique opportunities to identify highly potent and exquisitely selective inhibitors. Both reversible and covalent inhibitor approaches have yielded candidates demonstrating safety profiles and efficacies in multiple preclinical models of autoimmunity and oncology. Two BTK inhibitors have entered human clinical trials for oncology indications. Ibrutinib won the US FDA approval in November 2013 to become the first-in-class BTK inhibitor for treating mantle cell lymphoma. This encouraging outcome and the other on-going human studies could ultimately expand the utility of BTK inhibitors to broader autoimmune disease areas.

Autoinhibition of Bruton's tyrosine kinase (Btk) and activation by soluble inositol hexakisphosphate

Bruton's tyrosine kinase (Btk), a Tec-family tyrosine kinase, is essential for B-cell function. We present crystallographic and biochemical analyses of Btk, which together reveal molecular details of its autoinhibition and activation. Autoinhibited Btk adopts a compact conformation like that of inactive c-Src and c-Abl. A lipid-binding PH-TH module, unique to Tec kinases, acts in conjunction with the SH2 and SH3 domains to stabilize the inactive conformation. In addition to the expected activation of Btk by membranes containing phosphatidylinositol triphosphate (PIP3), we found that inositol hexakisphosphate (IP6), a soluble signaling molecule found in both animal and plant cells, also activates Btk. This activation is a consequence of a transient PH-TH dimerization induced by IP6, which promotes transphosphorylation of the kinase domains. Sequence comparisons with other Tec-family kinases suggest that activation by IP6 is unique to Btk.

Mortality and morbidity in patients with X-linked agammaglobulinaemia

BACKGROUND

X-linked agammaglobulinaemia (XLA) is a genetic disorder characterised by a defect in the generation of mature B cells, lack of antibodies production, and susceptibility to recurrent bacterial infections. Understanding of the risk factors responsible for morbidity and mortality in these patients can help in a better management of this disorder. However, there is a lack of specific studies in the literature regarding the morbidity and mortality of XLA patients. This study is designed to evaluate morbidities and mortality and survival rates in Iranian patients with XLA diagnosis during the past 20 years.

METHODS

We have registered the clinical data of the XLA patients and followed them up until 2010. At the time of diagnosis, a four-page questionnaire including complete medical information was filled out for all patients. Follow-up information was gathered either by reviewing the patients' hospital records or regularly visiting the patients.

RESULTS

Among 41 patients, 26.8% died during the follow up period. All of the complications before the initiation of treatment such as pneumonia, otitis media and diarrhoea were reduced after immunoglobulin replacement, except sinusitis and conjunctivitis. There were significant associations between some immunological and clinical characteristics such as lymphocyte subsets, consanguinity marriage and mortality.

CONCLUSION

Despite recent advances in the treatment of XLA, these patients still suffer from severe complications. Associations between poor prognosis and clinical and some immunological characteristics of the patients may help physicians to select poor prognoses patients at higher risk of mortality to develop prevention strategies for them.