The beta 1 tubulin R307H single nucleotide polymorphism is associated with treatment failures in immune thrombocytopenia (ITP)

A circle of life: platelet and megakaryocyte cytoskeleton dynamics in health and disease

Platelets are blood cells derived from megakaryocytes that play a central role in regulating haemostasis and vascular integrity. The microtubule cytoskeleton of megakaryocytes undergoes a critical dynamic reorganization during cycles of endomitosis and platelet biogenesis. Quiescent platelets have a discoid shape maintained by a marginal band composed of microtubule bundles, which undergoes remarkable remodelling during platelet activation, driving shape change and platelet function. Disrupting or enhancing this process can cause platelet dysfunction such as bleeding disorders or thrombosis. However, little is known about the molecular mechanisms underlying the reorganization of the cytoskeleton in the platelet lineage. Recent studies indicate that the emergence of a unique platelet tubulin code and specific pathogenic tubulin mutations cause platelet defects and bleeding disorders. Frequently, these mutations exhibit dominant negative effects, offering valuable insights into both platelet disease mechanisms and the functioning of tubulins. This review will highlight our current understanding of the role of the microtubule cytoskeleton in the life and death of platelets, along with its relevance to platelet disorders.

Quantitative and structural changes of blood platelet cytoskeleton proteins in multiple sclerosis (MS)

Epidemiological studies show that cardiovascular events related to platelet hyperactivity remain the leading causes of death among multiple sclerosis (MS) patients. Quantitative or structural changes of platelet cytoskeleton alter their morphology and function. Here, we demonstrated, for the first time, the structural changes in MS platelets that may be related to their hyperactivity. MS platelets were found to form large aggregates compared to control platelets. In contrast to the control, the images of overactivated, irregularly shaped MS platelets show changes in the cytoskeleton architecture, fragmented microtubule rings. Furthermore, MS platelets have long and numerous pseudopodia rich in actin filaments. We showed that MS platelets and megakaryocytes, overexpress β1-tubulin and β-actin mRNAs and proteins and have altered post-translational modification patterns. Moreover, we identified two previously undisclosed mutations in the gene encoding β1-tubulin in MS. We propose that the demonstrated structural changes of platelet cytoskeleton enhance their ability to adhere, aggregate, and degranulate fueling the risk of adverse cardiovascular events in MS.

Evaluation of the hematological inflammatory parameters in the patients with immune thrombocytopenic purpura: A case-control study

Background and Aims

Inflammation is one of the immune thrombocytopenic purpura (ITP)'s aggravating elements due to inflammatory cells' function. This study aims to identify and evaluate hematological inflammatory parameters, including neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and hemoglobin-to-platelet ratio (HPR), in patients with ITP compared to the control group.

Methods

We retrospectively analyzed the profile of 190 ITP patients from August 2019 to January 2021 at Imam Reza Hospital of Mashhad, Iran, along with 100 healthy individuals who had no ITP-related clinical or laboratory symptoms. Immune cell counts, NLR, PLR, and HPR were calculated using the complete blood count at the time of diagnosis and after the treatment. The results were analyzed through MedCalc, SPSS software, and the receiver operating characteristic curve.

Results

The result showed that white blood cell (WBC) and neutrophil counts were higher in ITP patients (WBC: p: 0.001, neutrophil: p: 0.001), and conversely, platelet and lymphocyte counts were higher in the control group compared to ITP patients (platelets: p: 0.001, lymphocytes: p: 0.001). The indices analysis between the two groups revealed that NLR was significantly increased in ITP patients (p: 0.001), but PLR was significantly reduced in ITP patients (with the mean platelet count of 23.44 ± 35.26 × 109/L) compared to the control group (with the mean platelet count of 234.04 ± 55.88 × 109/L). The HPR index also significantly increased in ITP patients (p: 0.001).

Conclusion

An increase in NLR, PLR, and a decrease in HPR can be considered a valuable diagnostic algorithm in patients with ITP.

Expanding the genetic spectrum of TUBB1-related thrombocytopenia

β1-Tubulin plays a major role in proplatelet formation and platelet shape maintenance, and pathogenic variants in TUBB1 lead to thrombocytopenia and platelet anisocytosis (TUBB1-RT). To date, the reported number of pedigrees with TUBB1-RT and of rare TUBB1 variants with experimental demonstration of pathogenicity is limited. Here, we report 9 unrelated families presenting with thrombocytopenia carrying 6 β1-tubulin variants, p.Cys12LeufsTer12, p.Thr107Pro, p.Gln423*, p.Arg359Trp, p.Gly109Glu, and p.Gly269Asp, the last of which novel. Segregation studies showed incomplete penetrance of these variants for platelet traits. Indeed, most carriers showed macrothrombocytopenia, some only increased platelet size, and a minority had no abnormalities. Moreover, only homozygous carriers of the p.Gly109Glu variant displayed macrothrombocytopenia, highlighting the importance of allele burden in the phenotypic expression of TUBB1-RT. The p.Arg359Trp, p.Gly269Asp, and p.Gly109Glu variants deranged β1-tubulin incorporation into the microtubular marginal ring in platelets but had a negligible effect on platelet activation, secretion, or spreading, suggesting that β1-tubulin is dispensable for these processes. Transfection of TUBB1 missense variants in CHO cells altered β1-tubulin incorporation into the microtubular network. In addition, TUBB1 variants markedly impaired proplatelet formation from peripheral blood CD34+ cell-derived megakaryocytes. Our study, using in vitro modeling, molecular characterization, and clinical investigations provides a deeper insight into the pathogenicity of rare TUBB1 variants. These novel data expand the genetic spectrum of TUBB1-RT and highlight a remarkable heterogeneity in its clinical presentation, indicating that allelic burden or combination with other genetic or environmental factors modulate the phenotypic impact of rare TUBB1 variants.

Identification of novel TUBB1 variants in patients with macrothrombocytopenia

Background/aim

Macrothrombocytopenia is an autosomal-dominant disorder characterized by increased platelet size and a decreased number of circulating platelets. The membrane skeleton and the link between actin filaments of the skeleton and microtubules, which consist of alpha and beta tubulin [including the tubulin beta-1 chain (TUBB1)] heterodimers, are important for normal platelet morphology, and defects in these systems are associated with macrothrombocytopenia.

Materials and methods

In this study, we sequenced the exons of the TUBB1 gene using DNA isolated from the peripheral blood samples of healthy controls (n = 47) and patients with macrothrombocytopenia (n = 37) from Turkey. The TUBB1 expression levels in fractioned blood samples from patients and healthy controls were analyzed by RT-qPCR and Western blot. Microtubule organization of the platelets in the peripheral blood smears of patients, and in mutant TUBB1-transfected HeLa cells, were analyzed by immunofluorescence staining.

Results

A new TUBB1 c.803G>T (p.T178T) variant was detected in all of the control and patient samples. Importantly, we found 3 new heterozygous TUBB1 variants predicting amino acid substitutions: G146R (in 1 patient), E123Q (in 1 patient), and T274M (in 4 patients); the latter variant was associated with milder thrombocytopenia in cancer patients treated with paclitaxel. Ectopic expression of TUBB1 T274M/R307H variant in HeLa cells resulted in irregular microtubule organization.

Conclusion

Further clinical and functional studies of the newly identified TUBB1 variants may offer important insights into their pathogenicity in macrothrombocytopenia.

Identification of a pathogenic TUBB1 variant in a Chinese family with congenital macrothrombocytopenia through whole genome sequencing

Congenital macrothrombocytopenia is a genetically heterogeneous group of rare disorders. We herein report a large Chinese family presented with phenotypic variability involving thrombocytopenia and/or giant platelets. Whole genome sequencing (WGS) of the proband and one of his affected brothers identified a potentially pathogenic c.952 C > T heterozygous variant in the TUBB1 gene. This p.R318W β1-tubulin variant was also identified in three additional siblings and five members of the next generation. These findings were consistent with an autosomal dominant inheritance with incomplete penetrance. Moreover, impaired platelet agglutination in response to ristocetin was detected in the patient's brother. Half of the family members harboring the p.R318W mutation displayed significantly decreased external release of p-selectin by stimulated platelets. The p.R318W β1-tubulin mutation was identified for the first time in a Chinese family with congenital macrothrombocytopenia using WGS as an unbiased sequencing approach. Affected individuals within the family demonstrated impaired platelet aggregation and/or release functions.

A Glanzmann thrombasthenia family associated with a TUBB1-related macrothrombocytopenia

BACKGROUND

Macrothrombocytopenia (MTP) is a rare but enigmatic complication of Glanzmann thrombasthenia (GT), an inherited bleeding disorder caused by the absence of platelet aggregation due to deficiencies of the αIIbβ3 integrin.

OBJECTIVES

We report a family with type I GT and a prolonged bleeding time but unusually associated with congenital mild thrombocytopenia and platelet size heterogeneity with giant forms.

METHODS AND RESULTS

Sanger sequencing of DNA from the propositus identified 2 heterozygous ITGB3 gene mutations: p.P189S and p.C210S both of which prevent αIIbβ3 expression and are causative of GT but without explaining the presence of enlarged platelets. High-throughput screening led to the detection of a predicted disease-causing heterozygous mutation in the TUBB1 gene: p.G146R, encoding β1-tubulin, a component of the platelet cytoskeleton and a gene where mutations are a known cause of MTP.

CONCLUSIONS

Family screening confirmed that this rare phenotype results from oligogenic inheritance while suggesting that the GT phenotype dominates clinically.

Association between gene polymorphisms and clinical features in idiopathic thrombocytopenic purpura patients

: Immune thrombocytopenic purpura (ITP) is an autoimmune disease in which increased platelet destruction and thrombocytopenia are diagnostic features. In fact, the exact pathogenesis of this disease is still unknown, but genetic changes can be a potential factor in the development of ITP. In this study, the relationship between polymorphisms with platelet destruction has been studied, which leads to decreased platelet count. Relevant literature was identified by a PubMed search (2000-2016) of English language papers using the terms 'ITP', 'polymorphism,' and 'immune system'. The majority of genetic changes (polymorphisms) occur in immune system genes, including interferon (IFN)-γ gene. These changes lead to the dysfunction of immune system and production of pathogenic antibodies against platelet surface glycoproteins such as glycoprotein IIb/IIIa, which eventually result in the destruction of platelets and increasing disease severity. In addition, IFN-γ as well as factors and cytokines involved in megakaryopoiesis, including stem cell factor and interleukin-3 (IL-3), leads to the differentiation of megakaryocytes and platelet release. Considering the fact that IFN-γ is a factor of inflammation and thrombocytopenia, coexistence of this cytokine with thrombopoietin, stem cell factor, and IL-3 results in megakaryocytes differentiation and platelet production, which can be effective to reduce disease severity and increase the platelet counts.

Bone marrow niche in immune thrombocytopenia: a focus on megakaryopoiesis

Immune thrombocytopenia (ITP) is an autoimmune disorder characterized by increased bleeding tendency and thrombocytopenia. In fact, the precise pathogenesis of this disease is still not clear. Megakaryopoiesis involves complete differentiation of megakaryocyte (MK) progenitors to functional platelets. This complex process occurs in specific bone marrow (BM) niches composed of several hematopoietic and non-hematopoietic cell types, soluble factors, and extracellular matrix proteins. These specialized microenvironments sustain MK maturation and localization to sinusoids as well as platelet release into circulation. However, MKs in ITP patients show impaired maturation and signs of degradation. Intrinsic defects in MKs and their extrinsic environment have been implicated in altered megakaryopoiesis in this disease. In particular, aberrant expression of miRNAs directing MK proliferation, differentiation, and platelet production; defective MK apoptosis; and reduced proliferation and differentiation rate of the MSC compartment observed in these patients may account for BM defects in ITP. Furthermore, insufficient production of thrombopoietin is another likely reason for ITP development. Therefore, identifying the signaling pathways and transcription factors influencing the interaction between MKs and BM niche in ITP patients will contribute to increased platelet production in order to prevent incomplete MK maturation and destruction as well as BM fibrosis and apoptosis in ITP. In this review, we will examine the interaction and role of BM niches in orchestrating megakaryopoiesis in ITP patients and discuss how these factors can be exploited to improve the quality of patient treatment and prognosis.

Update on the inherited platelet disorders

PURPOSE OF REVIEW

The inherited platelet disorders have witnessed a surge in our understanding of molecular mechanisms of disease in the past few years due in large to part to the introduction of next-generation sequencing for discovery of novel genes. The purpose of this review is to update the reader on the novel discoveries with regard to the inherited platelet disorders, with a particular focus on describing the novel disorders described most recently.

RECENT FINDINGS

The description of novel mechanisms of disease including mutations in PRKACG, in a family with severe macrothrombocytopenia, RUNX1 and FLI1 mutations in patients with inherited mild platelet function disorders and CalDAG-GEFI resulting in a severe platelet bleeding phenotype show that there is still much to be learned from studying families and molecular sequencing of patients with well phenotyped platelet disorders.

SUMMARY

The implications for clinical practice of the continually growing list of genes described in small numbers of families makes whole exome/genome tempting as an option for evaluation of patients, but use outside of the research setting still needs to be done with extreme caution as interpretation of variants is likely to require additional studies.

The European Hematology Association Roadmap for European Hematology Research: a consensus document

The European Hematology Association (EHA) Roadmap for European Hematology Research highlights major achievements in diagnosis and treatment of blood disorders and identifies the greatest unmet clinical and scientific needs in those areas to enable better funded, more focused European hematology research. Initiated by the EHA, around 300 experts contributed to the consensus document, which will help European policy makers, research funders, research organizations, researchers, and patient groups make better informed decisions on hematology research. It also aims to raise public awareness of the burden of blood disorders on European society, which purely in economic terms is estimated at €23 billion per year, a level of cost that is not matched in current European hematology research funding. In recent decades, hematology research has improved our fundamental understanding of the biology of blood disorders, and has improved diagnostics and treatments, sometimes in revolutionary ways. This progress highlights the potential of focused basic research programs such as this EHA Roadmap.The EHA Roadmap identifies nine 'sections' in hematology: normal hematopoiesis, malignant lymphoid and myeloid diseases, anemias and related diseases, platelet disorders, blood coagulation and hemostatic disorders, transfusion medicine, infections in hematology, and hematopoietic stem cell transplantation. These sections span 60 smaller groups of diseases or disorders.The EHA Roadmap identifies priorities and needs across the field of hematology, including those to develop targeted therapies based on genomic profiling and chemical biology, to eradicate minimal residual malignant disease, and to develop cellular immunotherapies, combination treatments, gene therapies, hematopoietic stem cell treatments, and treatments that are better tolerated by elderly patients.

β-1 tubulin R307H SNP alters microtubule dynamics and affects severity of a hereditary thrombocytopenia

BACKGROUND

Single nucleotide polymorphisms (SNPs) in platelet-associated genes partly explain inherent variability in platelet counts. Patients with monoallelic Bernard Soulier syndrome due to the Bolzano mutation (GPIBA A156V) have variable platelet counts despite a common mutation for unknown reasons.

OBJECTIVES

We investigated the effect of the most common SNP (R307H) in the hematopoietic-specific tubulin isotype β-1 in these Bernard Soulier patients and potential microtubule-based mechanisms of worsened thrombocytopenia.

PATIENTS/METHODS

Ninety-four monoallelic Bolzano mutation patients were evaluated for the R307H β-1 SNP and had platelet counts measured by three methods; the Q43P SNP was also evaluated. To investigate possible mechanisms underlying this association, we used molecular modeling of β-1 tubulin with and without the R307H SNP. We transfected SNP or non-SNP β-1 tubulin into MCF-7 and CMK cell lines and measured microtubule regrowth after nocodazole-induced depolymerization.

RESULTS

We found that patients with at least one R307H SNP allele had significantly worse thrombocytopenia; manual platelet counting revealed a median platelet count of 124 in non-SNP patients and 76 in SNP patients (both ×10(9)  L(-1) ; P < 0.01). The Q43P SNP had no significant association with platelet count. Molecular modeling suggested a structural relationship between the R307H SNP and microtubule stability via alterations in the M-loop of β tubulin; in vitro microtubule recovery assays revealed that cells transfected with R307H SNP β-1 had significantly impaired microtubule recovery.

CONCLUSIONS

Our data show that the R307H SNP is significantly associated with the degree of thrombocytopenia in congenital and acquired platelet disorders, and may affect platelets by altering microtubule behavior.

Rituximab and three dexamethasone cycles provide responses similar to splenectomy in women and those with immune thrombocytopenia of less than two years duration

Adults with newly diagnosed or persistent immunothrombocytopenia frequently relapse upon tapering steroids; adults and children with chronic disease have an even lower likelihood of lasting response. In adults with newly-diagnosed immunothrombocytopenia, two studies showed that dexamethasone 40 mg/day × four days and 4 rituximab infusions were superior to dexamethasone alone. Studies have also shown three cycles of dexamethasone are better than one and patients with persistent/chronic immunothrombocytopenia respond less well to either dexamethasone or rituximab. Therefore, 375 mg/m(2) × 4 rituximab was combined with three 4-day cycles of 28 mg/m(2) (max. 40 mg) dexamethasone at 2-week intervals and explored in 67 ITP patients. Best long-term response was assessed as complete (platelet count ≥ 100 × 10(9)/L) or partial (50-99 × 10(9)/L). Only 5 patients had not been previously treated. Fifty achieved complete (n=43, 64%) or partial (n=7, 10%) responses. Thirty-five of 50 responders maintained treatment-free platelet counts over 50 × 10(9)/L at a median 17 months (range 4-67) projecting 44% event-free survival. Duration of immunothrombocytopenia less than 24 months, achieving complete responses, and being female were associated with better long-term response (P<0.01). Adverse events were generally mild-moderate, but 3 patients developed serum sickness and 2 colitis; there were no sequelae. Dexamethasone could be difficult to tolerate. Fourteen patients became hypogammaglobulinemic and half had increased frequency of minor infections; 9 of 12 evaluable patients recovered their IgG levels. Rituximab combined with three cycles of dexamethasone provides apparently better results to reported findings with rituximab alone, dexamethasone alone, or the combination with one cycle of dexamethasone. The results suggest medical cure may be achievable in immunothrombocytopenia, especially in women and in patients within two years of diagnosis. (clinicaltrials.gov identifier:02050581).