ACTN1 mutations cause congenital macrothrombocytopenia

Comparative genetic analysis of blood and semen samples in sperm donors from Hunan, China

OBJECTIVES

At present, most genetic tests or carrier screening are performed with blood samples, and the known carrier rate of disease-causing variants is also derived from blood. For semen donors, what is really passed on to offspring is the pathogenic variant in their sperm. This study aimed to determine whether pathogenic variants identified in the sperm of young semen donors are also present in their blood, and whether matching results for blood are consistent with results for sperm.

METHODS

We included 40 paired sperm and blood samples from 40 qualified semen donors at the Hunan Province Human Sperm Bank of China. All samples underwent exome sequencing (ES) analysis, and the pathogenicity was assessed according to the American College of Medical Genetics (ACMG) guidelines. Scoring for sperm donation matching, which was based on gene scoring and variant scoring, was also used to assess the consistency of sperm and blood genetic test results.

RESULTS

A total of 108 pathogenic (P)/likely pathogenic (LP) variants in 82 genes were identified. The highest carrier had 7 variants, and there was also one donor did not carry any P/LP variant. On average, each donor carried 2.7 P/LP variants. Among all the P/LP variants, missense mutation was the dominant type and most of them were located in exonic regions. Chromosome 1 harboured the largest number of variants and no pathogenic copy number variants (CNV) was identified in semen donors. The P/LP variant of all the 40 semen donors was consistent by comparing sperm and blood. Except for one case that was slightly different, the rest simulated matching results for blood were all consistent with results for sperm.

CONCLUSIONS

It is reasonable to choose either blood or sperm for genetic screening in semen donors.

Implementation and clinical utility of multigene panels for bleeding, platelet, and thrombotic disorders

High-throughput sequencing, with its capacity to simultaneously sequence large volumes of genomic data, has evolved from a research-focused technology to a clinical tool. This review outlines key steps in the development of a clinical hemostasis and thrombosis genetics service leveraging a multigene panel. We discuss its value across inherited bleeding, platelet, and thrombotic disorders (BPTDs) in the context of published studies utilizing multigene panels for these conditions. Benefits of sequencing include establishing a diagnosis through the simultaneous assessment of multiple candidate genes, exclusion of genocopies, and predictions of phenotype to deliver targeted therapy. The presence of concomitant variants may modify phenotype; however, predictions on disease course from oligogenic modifiers are not yet used to guide patient care or counseling. Limitations in the widespread roll-out of multigene panels for clinical diagnosis of BPTDs exist. These challenges relate to detection of structural variants, variable diagnostic hit rates, and management of incidental findings. Variants of uncertain significance also frustrate diagnostic yield. Family segregation studies, in vitro characterization, and protein modeling aid interpretation of variant pathogenicity. Although multigene panels offer substantial opportunities to improve BPTD diagnostics, their implementation should be guided by appropriate expertise and in conjunction with clinical research to ensure safe and ethical care.

Dedicator of cytokinesis 2 regulates cytoskeletal actin dynamics and is essential for platelet biogenesis and functions

AIMS

Dedicator of cytokinesis 2 (DOCK2), a member of the DOCK family of guanine nucleotide exchange factors that specifically act on the Rho GTPases including Rac and Cdc42, plays pivotal roles in the regulation of leukocyte homeostasis. However, its functions in platelets remain unknown.

METHODS AND RESULTS

Using mice with genetic deficiency of DOCK2 (Dock2-/-), we showed that Dock2-/- mice exhibited a macrothrombocytopenic phenotype characterized as decreased platelet count and enlarged platelet size by transmission electron microscopy. Dock2-/- megakaryocytes had reduced polyploidization determined by propidium iodide staining and defective proplatelet formation by confocal microscopy. DOCK2 deficiency led to enriched F-actin level in resting platelets but defective F-actin assembly in activated platelets by phalloidin staining, and mechanistically, attenuated activity of Rac1, unchanged Cdc42 but enhanced RhoA measured by immunoprecipitation of GTP-bound proteins. Immunoblotting analysis showed that Dock2-/- platelets had reduced immunoreceptor tyrosine-based activation motif signaling downstream of impaired clustering of GPVI receptors determined by stochastic optical reconstruction microscopy. Further, DOCK2 deficiency resulted in reduced density and branches of fibrin fibres in the clots in vitro and diminished platelet aggregation in a microfluidic chamber ex vivo. Dock2-/- platelets exhibited impaired incorporation into a growing thrombus in cremaster arterioles following allogeneic transfusion into a WT recipient and defective heterotypic interactions with neutrophils in cremaster venules as reflected by decreased platelet-neutrophil aggregate formation in vitro under stirring condition. In addition, myeloid deficiency of DOCK2 caused prolonged tail bleeding times. Finally, pharmacological inhibition of DOCK2 using a small-molecular inhibitor CPYPP suppressed actin dynamics leading to impaired responses to GPVI activation and defects in platelet spreading, clot retraction, and thrombus formation.

CONCLUSION

DOCK2 plays critical roles in the regulation of platelet biogenesis and functions by controlling Rac1 activity and cytoskeletal actin dynamics and may be a novel target for the treatment of thrombotic and thrombo-inflammatory diseases.

α-Actinin-1 deficiency in megakaryocytes causes low platelet count, platelet dysfunction, and mitochondrial impairment

ABSTRACT

Cytoskeletal remodeling and mitochondrial bioenergetics play important roles in thrombocytopoiesis and platelet function. Recently, α-actinin-1 mutations have been reported in patients with congenital macrothrombocytopenia. However, the role and underlying mechanism of α-actinin-1 in thrombocytopoiesis and platelet function remain elusive. Using megakaryocyte (MK)-specific α-actinin-1 knockout (KO; PF4-Actn1-/-) mice, we demonstrated that PF4-Actn1-/- mice exhibited reduced platelet counts. The decreased platelet number in PF4-Actn1-/- mice was due to defects in thrombocytopoiesis. Hematoxylin and eosin staining and flow cytometry revealed a decrease in the number of MKs in the bone marrow of PF4-Actn1-/- mice. The absence of α-actinin-1 increased the proportion of 2 N-4 N MKs and decreased the proportion of 8 N-32 N MKs. Colony-forming unit-MK colony formation, the ratio of proplatelet formation-bearing MKs, and MK migration in response to stromal cell-derived factor-1 signaling were inhibited in PF4-Actn1-/- mice. Platelet spreading, clot retraction, aggregation, integrin αIIbβ3 activation, and CD62P exposure in response to various agonists were decreased in PF4-Actn1-/- platelets. Notably, PF4-Actn1-/- platelets inhibited calcium mobilization, reactive oxygen species (ROS) generation, and actin polymerization in response to collagen and thrombin. Furthermore, the PF4-Actn1-/- mice exhibited impaired hemostasis and thrombosis. Mechanistically, proteomic analysis of low-ploidy (2-4 N) and high-ploidy (≥8 N) PF4-Actn1-/- MKs revealed that α-actinin-1 deletion reduced platelet activation and mitochondrial function. PF4-Actn1-/- platelets and Actn1 KO 293T cells exhibited reduced mitochondrial membrane potential, mitochondrial ROS generation, mitochondrial calcium mobilization, and mitochondrial bioenergetics. Overall, in this study, we report that mice with α-actinin-1 deficiency in MKs exhibit low platelet count and impaired platelet function, thrombosis, and mitochondrial bioenergetics.

Structural and functional insights into α-actinin isoforms and their implications in cardiovascular disease

α-actinin (ACTN) is a pivotal member of the actin-binding protein family, crucial for the anchoring and organization of actin filaments within the cytoskeleton. Four isoforms of α-actinin exist: two non-muscle isoforms (ACTN1 and ACTN4) primarily associated with actin stress fibers and focal adhesions, and two muscle-specific isoforms (ACTN2 and ACTN3) localized to the Z-disk of the striated muscle. Although these isoforms share structural similarities, they exhibit distinct functional characteristics that reflect their specialized roles in various tissues. Genetic variants in α-actinin isoforms have been implicated in a range of pathologies, including cardiomyopathies, thrombocytopenia, and non-cardiovascular diseases, such as nephropathy. However, the precise impact of these genetic variants on the α-actinin structure and their contribution to disease pathogenesis remains poorly understood. This review provides a comprehensive overview of the structural and functional attributes of the four α-actinin isoforms, emphasizing their roles in actin crosslinking and sarcomere stabilization. Furthermore, we present detailed structural modeling of select ACTN1 and ACTN2 variants to elucidate mechanisms underlying disease pathogenesis, with a particular focus on macrothrombocytopenia and hypertrophic cardiomyopathy. By advancing our understanding of α-actinin's role in both normal cellular function and disease states, this review lays the groundwork for future research and the development of targeted therapeutic interventions.

Looking Under the Hood at the Cytoskeletal Engine of Platelet Production

Blood platelets are anucleate cells essential for normal blood hemostasis. To maintain a normal platelet count of 150 000 to 400 000 per μL of blood, 1011 platelets must be released each day from precursor cells called megakaryocytes. In this review, we aim to provide an overview of platelet production and evaluate the proposed mechanisms of platelet generation. We will discuss novel cytoskeletal mechanisms of platelet production, including microtubule and actin-based systems. We present new evidence that supports a cytoplasmic trigger for platelet production, discuss centrosome clustering as a new mechanism to trigger proplatelet production, and review new data supporting the bone marrow as the major location of platelet production.

Application of platelet transcriptomics for assessing treatment effectiveness and predicting long-term platelet counts recovery in aplastic anemia

BACKGROUND

Aplastic anemia (AA) is a bone marrow failure disease for which the means of assessing and predicting the therapeutic effectiveness are still relatively limited. Thrombocytopenia is often the earliest and most severe symptom in patients newly diagnosed with AA. While clinical consideration is usually given to the quantitative changes in platelets during treatment, there is little focus on the resolution of the molecular characteristics of platelets in AA.

OBJECTIVES

To investigate the changes in platelet molecular characteristics throughout the treatment process of AA, and to explore the use of transcriptomics for monitoring and predicting treatment outcomes.

METHODS

We comprehensively analyzed platelet transcriptomic changes in patients with AA at initial diagnosis and different stages of treatment effectiveness using bulk transcriptome sequencing.

RESULTS

Genes associated with cell proliferation, erythroid function, and amino acid transport were elevated in newly diagnosed AA. Conversely, genes linked to histones, thrombopoiesis, mitochondrial energy metabolism, and signaling pathways were significantly downregulated. Additionally, 60.6% of the differentially expressed genes were substantially restored following complete remission. Furthermore, through the examination of longitudinal samples, we identified recovery ascending genes that could serve as viable biomarkers for assessing treatment effectiveness in AA. Besides, we observed that higher expression levels of recovery ascending genes may predict superior long-term platelet counts recovery 6 months in advance in patients with partial response.

CONCLUSION

The platelet transcriptome undergoes profound changes and can serve as a potential indicator for assessing treatment effectiveness and predicting long-term platelet counts recovery in AA.

Comprehensive genetic analysis for identification of monogenic disorders and selection of appropriate treatments in pediatric patients with persistent thrombocytopenia

Persistent thrombocytopenia is caused by various diseases, including immune thrombocytopenia, inherited thrombocytopenia, and inherited bone marrow failure syndromes. Considering the large number of genes responsible for inherited disorders, comprehensive genetic analysis is required to diagnose monogenic disorders. In this study, we enrolled 53 pediatric patients with persistent thrombocytopenia exhibiting visually small or normal-sized platelets. We performed whole-exome sequencing, including 56 genes responsible for inherited thrombocytopenia, and evaluated clinical parameters according to disease type. Among 53 patients, 12 patients (22.6%) were diagnosed with monogenic disorders. Nine patients had a family history of thrombocytopenia. Pathogenic or novel variants of genes responsible for inherited thrombocytopenia were identified in three and six patients, respectively. The variants in genes for inherited thrombocytopenia with large or giant platelets were unexpectedly identified in six patients. Pathogenic variants in genes for inherited bone marrow failure syndromes with systemic features were identified in three patients with atypical symptoms. Since the definitive diagnostic methods for immune thrombocytopenia are limited, and a substantial number of patients with inherited thrombocytopenia are at a high risk of developing malignancies, comprehensive genetic analysis is indispensable for selecting appropriate therapies, avoidance of unnecessary treatments for immune thrombocytopenia, and long-term follow-up of patients with inherited thrombocytopenia.

Gaining Insights into Inherited Bleeding Disorders of Complex Etiology in Pediatric Patients: Whole-Exome Sequencing as First-Line Investigation Tool

INTRODUCTION

 Investigation of the molecular basis of inherited bleeding disorders (IBD) is mostly performed with gene panel sequencing. However, the continuous discovery of new related genes underlies the limitation of this approach. This study aimed to identify genetic variants responsible for IBD in pediatric patients using whole-exome sequencing (WES), and to provide a detailed description and reclassification of candidate variants.

MATERIAL AND METHODS

 WES was performed for 18 pediatric patients, and variants were filtered using a first-line list of 290 genes. Variant prioritization was discussed in a multidisciplinary team based on genotype-phenotype correlation, and segregation studies were performed with available family members.

RESULTS

 The study identified 22 candidate variants in 17 out of 18 patients (94%). Eleven patients had complete genotype-phenotype correlation, resulting in a diagnostic yield of 61%, 5 (28%) were classified as partially solved, and 2 (11%) remained unsolved. Variants were identified in platelet (ACTN1, ANKRD26, CYCS, GATA1, GFI1B, ITGA2, NBEAL2, RUNX1, SRC, TUBB1), bleeding (APOLD1), and coagulation (F7, F8, F11, VWF) genes. Notably, 9 out of 22 (41%) variants were previously unreported. Variant pathogenicity was assessed according to the American College of Medical Genetics and Genomics guidelines and reclassification of three variants based on family segregation evidence, resulting in the identification of 10 pathogenic or likely pathogenic variants, 6 variants of uncertain significance, and 6 benign or likely benign variants.

CONCLUSION

 This study demonstrated the high potential of WES in identifying rare molecular defects causing IBD in pediatric patients, improving their management, prognosis, and treatment, particularly for patients at risk of malignancy and/or bleeding due to invasive procedures.

ACTN1-related thrombocytopenia: Homozygosity for an ACTN1 variant results in a more severe phenotype

ACTN1-related thrombocytopenia is a rare disorder caused by heterozygous variants in the ACTN1 gene characterized by macrothrombocytopenia and mild bleeding tendency. We describe for the first time two patients affected with ACTN1-RT caused by a homozygous variant in ACTN1 (c.982G>A) with mild heart valve defects unexplained by any other genetic variants investigated by WES. Within the reported family, the homozygous sisters have moderate thrombocytopenia and marked platelet macrocytosis with giant platelets, revealing a more severe haematological phenotype compared to their heterozygous relatives and highlighting a significant effect of allelic burden on platelet size. Moreover, we hypothesize that some ACTN1 variants, especially when present in the homozygous state, may also contribute to the cardiac abnormalities.

Trans-ancestry epigenome-wide association meta-analysis of DNA methylation with lifetime cannabis use

Cannabis is widely used worldwide, yet its links to health outcomes are not fully understood. DNA methylation can serve as a mediator to link environmental exposures to health outcomes. We conducted an epigenome-wide association study (EWAS) of peripheral blood-based DNA methylation and lifetime cannabis use (ever vs. never) in a meta-analysis including 9436 participants (7795 European and 1641 African ancestry) from seven cohorts. Accounting for effects of cigarette smoking, our trans-ancestry EWAS meta-analysis revealed four CpG sites significantly associated with lifetime cannabis use at a false discovery rate of 0.05 ( p < 5.85 × 10 - 7 ) : cg22572071 near gene ADGRF1, cg15280358 in ADAM12, cg00813162 in ACTN1, and cg01101459 near LINC01132. Additionally, our EWAS analysis in participants who never smoked cigarettes identified another epigenome-wide significant CpG site, cg14237301 annotated to APOBR. We used a leave-one-out approach to evaluate methylation scores constructed as a weighted sum of the significant CpGs. The best model can explain 3.79% of the variance in lifetime cannabis use. These findings unravel the DNA methylation changes associated with lifetime cannabis use that are independent of cigarette smoking and may serve as a starting point for further research on the mechanisms through which cannabis exposure impacts health outcomes.

Exome sequencing in 116 patients with inherited thrombocytopenia that remained of unknown origin after systematic phenotype-driven diagnostic workup

Inherited thrombocytopenias (IT) are genetic diseases characterized by low platelet count, sometimes associated with congenital defects or a predisposition to develop additional conditions. Next-generation sequencing has substantially improved our knowledge of IT, with more than 40 genes identified so far, but obtaining a molecular diagnosis remains a challenge especially for patients with non-syndromic forms, having no clinical or functional phenotypes that raise suspicion about specific genes. We performed exome sequencing (ES) in a cohort of 116 IT patients (89 families), still undiagnosed after a previously validated phenotype-driven diagnostic algorithm including a targeted analysis of suspected genes. ES achieved a diagnostic yield of 36%, with a gain of 16% over the diagnostic algorithm. This can be explained by genetic heterogeneity and unspecific genotype-phenotype relationships that make the simultaneous analysis of all the genes, enabled by ES, the most reasonable strategy. Furthermore, ES disentangled situations that had been puzzling because of atypical inheritance, sex-related effects or false negative laboratory results. Finally, ES-based copy number variant analysis disclosed an unexpectedly high prevalence of RUNX1 deletions, predisposing to hematologic malignancies. Our findings demonstrate that ES, including copy number variant analysis, can substantially contribute to the diagnosis of IT and can solve diagnostic problems that would otherwise remain a challenge.

Excessive serine from the bone marrow microenvironment impairs megakaryopoiesis and thrombopoiesis in Multiple Myeloma

Thrombocytopenia is a major complication in a subset of patients with multiple myeloma (MM). However, little is known about its development and significance during MM. Here, we show thrombocytopenia is linked to poor prognosis in MM. In addition, we identify serine, which is released from MM cells into the bone marrow microenvironment, as a key metabolic factor that suppresses megakaryopoiesis and thrombopoiesis. The impact of excessive serine on thrombocytopenia is mainly mediated through the suppression of megakaryocyte (MK) differentiation. Extrinsic serine is transported into MKs through SLC38A1 and downregulates SVIL via SAM-mediated tri-methylation of H3K9, ultimately leading to the impairment of megakaryopoiesis. Inhibition of serine utilization or treatment with TPO enhances megakaryopoiesis and thrombopoiesis and suppresses MM progression. Together, we identify serine as a key metabolic regulator of thrombocytopenia, unveil molecular mechanisms governing MM progression, and provide potential therapeutic strategies for treating MM patients by targeting thrombocytopenia.

Endometriosis of the Cervix: A Rare Clinical Case with the Possibility of Comparing the Eutopic and Ectopic Endometrium at the Cellular Level

Endometriosis of the cervix is a rare form of genital endometriosis, which is characterized by the appearance of tissue on the vaginal part of the cervix, similar to the tissue of the mucous membrane of the uterine cavity. We describe a clinical case in which we compared the content of cytoskeletal proteins, H3 histone modifications and DNA methylation (total and 5-hydroxymethylcytosine content) in the eutopic endometrium and in tissue from endometriosis foci on the cervix. The patient had elevated levels of estradiol, interleukin-1β and interleukin-8. At the cellular level, the content of tubulin and the marker of stable microtubules were reduced in the ectopic endometrium (by 45% and 37%, p < 0.05, respectively), but the alpha-actinin-1 content was increased (by 75%, p < 0.05) with an increase in the expression of its gene. At the same time, the total level of DNA methylation in the endometriotic focus was reduced by more than 2 times with the accumulation of the intermediate product 5-hydroxymethylcytosine (the content increased by more than 3 times), probably due to an increase in the content of tet methylcytosine dioxygenase 1 (more than 4 times).

Genetic Polymorphisms Associated with Prothrombin Time and Activated Partial Thromboplastin Time in Chinese Healthy Population

(1) Background: The purpose of this study was to evaluate the effect of gene polymorphisms on prothrombin time (PT) and activated partial thromboplastin time (APTT) in a healthy Chinese population. (2) Methods: A total of 403 healthy volunteers from a series of novel oral anticoagulants (NOACs) bioequivalence trials in China were included. Coagulation tests for PT and APTT were performed in the central lab at Peking University First Hospital. Whole-exome sequencing (WES) and genome-wide association analysis were performed. (3) Results: In the correlation analysis of PT, 105 SNPs from 84 genes reached the genome-wide significance threshold (p < 1 × 10−5). Zinc Finger Protein 594 (ZNF594) rs184838268 (p = 4.50 × 10−19) was most significantly related to PT, and Actinin Alpha 1 (ACTN1) was found to interact most with other candidate genes. Significant associations with previously reported candidate genes Aurora Kinase B (AURKB), Complement C5(C5), Clock Circadian Regulator (CLOCK), and Histone Deacetylase 9(HDAC9) were detected in our dataset (p < 1 × 10−5). PiggyBac Transposable Element Derived 2(PGBD2) rs75935520 (p = 4.49 × 10−6), Bromodomain Adjacent To Zinc Finger Domain 2A(BAZ2A) rs199970765 (p = 5.69 × 10−6) and Protogenin (PRTG) rs80064850 (p = 8.69 × 10−6) were significantly correlated with APTT (p < 1 × 10−5). The heritability values of PT and APTT were 0.83 and 0.64, respectively; (4) Conclusion: The PT and APTT of healthy populations are affected by genetic polymorphisms. ZNF594 and ACTN1 variants could be novel genetic markers of PT, while PRTG polymorphisms might be associated with APTT levels. The findings could be attributed to ethnic differences, and need further investigation.

Identification of key biomarkers in Angelman syndrome by a multi-cohort analysis

The Angelman Syndrome (AS) is an extreme neurodevelopmental disorder without effective treatments. While most patients with this disease can be diagnosed by genetic testing, there are still a handful of patients have an unrecognized genetic cause for their illness. Thus, novel approaches to clinical diagnosis and treatment are urgently needed. The aim of this study was to identify and characterize differentially expressed genes involved in AS and built potential diagnostic panel for AS by NGS sequencing. A multi-cohort analysis framework was used to analyze stem cell-derived neurons from AS patients in GSE160747 dataset. We identified three differentially expressed genes (ACTN1, ADAMTS2, SLC30A8) differentiates AS patients from controls. Moreover, we validated the expression patterns of these genes in GSE146640, GSE120225. Receiver operating characteristic (ROC) curves analysis demonstrated that these genes could function as potential diagnostic biomarkers [AUC = 1 (95% CI 1–1)]. This study may provide new approach for diagnosing patients with AS and helping to develop novel therapies in treating AS patients.

Genetics of inherited thrombocytopenias

The inherited thrombocytopenia syndromes are a group of disorders characterized primarily by quantitative defects in platelet number, though with a variety demonstrating qualitative defects and/or extrahematopoietic findings. Through collaborative international efforts applying next-generation sequencing approaches, the list of genetic syndromes that cause thrombocytopenia has expanded significantly in recent years, now with over 40 genes implicated. In this review, we focus on what is known about the genetic etiology of inherited thrombocytopenia syndromes and how the field has worked to validate new genetic discoveries. We highlight the important role for the clinician in identifying a germline genetic diagnosis and strategies for identifying novel causes through research-based endeavors.

Don't you forget about me(gakaryocytes)

Platelets (small, anucleate cell fragments) derive from large precursor cells, megakaryocytes (MKs), that reside in the bone marrow. MKs emerge from hematopoietic stem cells in a complex differentiation process that involves cytoplasmic maturation, including the formation of the demarcation membrane system, and polyploidization. The main function of MKs is the generation of platelets, which predominantly occurs through the release of long, microtubule-rich proplatelets into vessel sinusoids. However, the idea of a 1-dimensional role of MKs as platelet precursors is currently being questioned because of advances in high-resolution microscopy and single-cell omics. On the one hand, recent findings suggest that proplatelet formation from bone marrow-derived MKs is not the only mechanism of platelet production, but that it may also occur through budding of the plasma membrane and in distant organs such as lung or liver. On the other hand, novel evidence suggests that MKs not only maintain physiological platelet levels but further contribute to bone marrow homeostasis through the release of extracellular vesicles or cytokines, such as transforming growth factor β1 or platelet factor 4. The notion of multitasking MKs was reinforced in recent studies by using single-cell RNA sequencing approaches on MKs derived from adult and fetal bone marrow and lungs, leading to the identification of different MK subsets that appeared to exhibit immunomodulatory or secretory roles. In the following article, novel insights into the mechanisms leading to proplatelet formation in vitro and in vivo will be reviewed and the hypothesis of MKs as immunoregulatory cells will be critically discussed.

CRLF3 plays a key role in the final stage of platelet genesis and is a potential therapeutic target for thrombocythemia

The process of platelet production has so far been understood to be a 2-stage process: megakaryocyte maturation from hematopoietic stem cells followed by proplatelet formation, with each phase regulating the peripheral blood platelet count. Proplatelet formation releases into the bloodstream beads-on-a-string preplatelets, which undergo fission into mature platelets. For the first time, we show that preplatelet maturation is a third, tightly regulated, critical process akin to cytokinesis that regulates platelet count. We show that deficiency in cytokine receptor-like factor 3 (CRLF3) in mice leads to an isolated and sustained 25% to 48% reduction in the platelet count without any effect on other blood cell lineages. We show that Crlf3-/- preplatelets have increased microtubule stability, possibly because of increased microtubule glutamylation via the interaction of CRLF3 with key members of the Hippo pathway. Using a mouse model of JAK2 V617F essential thrombocythemia, we show that a lack of CRLF3 leads to long-term lineage-specific normalization of the platelet count. We thereby postulate that targeting CRLF3 has therapeutic potential for treatment of thrombocythemia.

Screening and diagnosis of inherited platelet disorders

Inherited platelet disorders are important conditions that often manifest with bleeding. These disorders have heterogeneous underlying pathologies. Some are syndromic disorders with non-blood phenotypic features, and others are associated with an increased predisposition to developing myelodysplasia and leukemia. Platelet disorders can present with thrombocytopenia, defects in platelet function, or both. As the underlying pathogenesis of inherited thrombocytopenias and platelet function disorders are quite diverse, their evaluation requires a thorough clinical assessment and specialized diagnostic tests, that often challenge diagnostic laboratories. At present, many of the commonly encountered, non-syndromic platelet disorders do not have a defined molecular cause. Nonetheless, significant progress has been made over the past few decades to improve the diagnostic evaluation of inherited platelet disorders, from the assessment of the bleeding history to improved standardization of light transmission aggregometry, which remains a "gold standard" test of platelet function. Some platelet disorder test findings are highly predictive of a bleeding disorder and some show association to symptoms of prolonged bleeding, surgical bleeding, and wound healing problems. Multiple assays can be required to diagnose common and rare platelet disorders, each requiring control of preanalytical, analytical, and post-analytical variables. The laboratory investigations of platelet disorders include evaluations of platelet counts, size, and morphology by light microscopy; assessments for aggregation defects; tests for dense granule deficiency; analyses of granule constituents and their release; platelet protein analysis by immunofluorescent staining or flow cytometry; tests of platelet procoagulant function; evaluations of platelet ultrastructure; high-throughput sequencing and other molecular diagnostic tests. The focus of this article is to review current methods for the diagnostic assessment of platelet function, with a focus on contemporary, best diagnostic laboratory practices, and relationships between clinical and laboratory findings.

Mutations in the most divergent α-tubulin isotype, α8-tubulin, cause defective platelet biogenesis

BACKGROUND

In the panel of genes commonly associated with inherited macrothrombocytopenia, an important fraction encodes key cytoskeletal proteins such as tubulin isotypes, the building blocks of microtubules. Macrothrombocytopenia-causing mutations have been identified in the TUBB1 and TUBA4A genes, emphasizing their importance in the formation of platelets and their marginal band, a unique microtubule ring-like structure that supports the platelet typical disc-shaped morphology. This raised the hypothesis that other tubulin isotypes normally expressed in platelets could play a similar role in their formation.

OBJECTIVES

To assess whether tubulin isotype genes other than TUBA4A and TUBB1 could be implicated in inherited macrothrombocytopenia.

METHODS

We used high throughput sequencing to screen a cohort of 448 French blood donors with mild thrombocytopenia for mutations in a panel of selected genes known or suspected to be involved in platelet biogenesis.

RESULTS

We identified six distinct novel mutations in TUBA8, which encodes the most-divergent α-tubulin, as the causative determinant of macrothrombocytopenia and platelet marginal band defects. Functionally, all TUBA8 mutations were found to fully or partially inhibit the incorporation of the mutated α8-tubulin in the microtubule network.

CONCLUSION

This study provides strong support for a key role of multiple tubulin genes in platelet biogenesis by discovering variants in a tubulin gene that was previously not known to be important for platelets.

Essential role of zyxin in platelet biogenesis and glycoprotein Ib-IX surface expression

Platelets are generated from the cytoplasm of megakaryocytes (MKs) via actin cytoskeleton reorganization. Zyxin is a focal adhesion protein and wildly expressed in eukaryotes to regulate actin remodeling. Zyxin is upregulated during megakaryocytic differentiation; however, the role of zyxin in thrombopoiesis is unknown. Here we show that zyxin ablation results in profound macrothrombocytopenia. Platelet lifespan and thrombopoietin level were comparable between wild-type and zyxin-deficient mice, but MK maturation, demarcation membrane system formation, and proplatelet generation were obviously impaired in the absence of zyxin. Differential proteomic analysis of proteins associated with macrothrombocytopenia revealed that glycoprotein (GP) Ib-IX was significantly reduced in zyxin-deficient platelets. Moreover, GPIb-IX surface level was decreased in zyxin-deficient MKs. Knockdown of zyxin in a human megakaryocytic cell line resulted in GPIbα degradation by lysosomes leading to the reduction of GPIb-IX surface level. We further found that zyxin was colocalized with vasodilator-stimulated phosphoprotein (VASP), and loss of zyxin caused diffuse distribution of VASP and actin cytoskeleton disorganization in both platelets and MKs. Reconstitution of zyxin with VASP binding site in zyxin-deficient hematopoietic progenitor cell-derived MKs restored GPIb-IX surface expression and proplatelet generation. Taken together, our findings identify zyxin as a regulator of platelet biogenesis and GPIb-IX surface expression through VASP-mediated cytoskeleton reorganization, suggesting possible pathogenesis of macrothrombocytopenia.

Advances in understanding the pathogenesis of hereditary macrothrombocytopenia

Low platelet count, or thrombocytopenia, is a common haematological abnormality, with a wide differential diagnosis, which may represent a clinically significant underlying pathology. Macrothrombocytopenia, the presence of large platelets in combination with thrombocytopenia, can be acquired or hereditary and indicative of a complex disorder. In this review, we discuss the interpretation of platelet count and volume measured by automated haematology analysers and highlight some important technical considerations relevant to the analysis of blood samples with macrothrombocytopenia. We review how large cohorts, such as the UK Biobank and INTERVAL studies, have enabled an accurate description of the distribution and co-variation of platelet parameters in adult populations. We discuss how genome-wide association studies have identified hundreds of genetic associations with platelet count and mean platelet volume, which in aggregate can explain large fractions of phenotypic variance, consistent with a complex genetic architecture and polygenic inheritance. Finally, we describe the large genetic diagnostic and discovery programmes, which, simultaneously to genome-wide association studies, have expanded the repertoire of genes and variants associated with extreme platelet phenotypes. These have advanced our understanding of the pathogenesis of hereditary macrothrombocytopenia and support a future clinical diagnostic strategy that utilises genotype alongside clinical and laboratory phenotype data.

The Importance of Alpha-Actinin Proteins in Platelet Formation and Function, and Their Causative Role in Congenital Macrothrombocytopenia

The actin cytoskeleton plays a central role in platelet formation and function. Alpha-actinins (actinins) are actin filament crosslinking proteins that are prominently expressed in platelets and have been studied in relation to their role in platelet activation since the 1970s. However, within the past decade, several groups have described mutations in ACTN1/actinin-1 that cause congenital macrothrombocytopenia (CMTP)-accounting for approximately 5% of all cases of this condition. These findings are suggestive of potentially novel functions for actinins in platelet formation from megakaryocytes in the bone marrow and/or platelet maturation in circulation. Here, we review some recent insights into the well-known functions of actinins in platelet activation before considering possible roles for actinins in platelet formation that could explain their association with CMTP. We describe what is known about the consequences of CMTP-linked mutations on actinin-1 function at a molecular and cellular level and speculate how these changes might lead to the alterations in platelet count and morphology observed in CMTP patients. Finally, we outline some unanswered questions in this area and how they might be addressed in future studies.

Thymosin β4 is essential for thrombus formation by controlling the G-actin/F-actin equilibrium in platelets

Coordinated rearrangements of the actin cytoskeleton are pivotal for platelet biogenesis from megakaryocytes but also orchestrate key functions of peripheral platelets in hemostasis and thrombosis, such as granule release, the formation of filopodia and lamellipodia, or clot retraction. Along with profilin (Pfn) 1, thymosin β4 (encoded by Tmsb4x) is one of the two main G-actin-sequestering proteins within cells of higher eukaryotes, and its intracellular concentration is particularly high in cells that rapidly respond to external signals by increased motility, such as platelets. Here, we analyzed constitutive Tmsb4x knockout (KO) mice to investigate the functional role of the protein in platelet production and function. Thymosin β4 deficiency resulted in a macrothrombocytopenia with only mildly increased platelet volume and an unaltered platelet life span. Megakaryocyte numbers in the bone marrow and spleen were unaltered, however, Tmsb4x KO megakaryocytes showed defective proplatelet formation in vitro and in vivo. Thymosin β4-deficient platelets displayed markedly decreased G-actin levels and concomitantly increased F-actin levels resulting in accelerated spreading on fibrinogen and clot retraction. Moreover, Tmsb4x KO platelets showed activation defects and an impaired immunoreceptor tyrosine-based activation motif (ITAM) signaling downstream of the activating collagen receptor glycoprotein VI. These defects translated into impaired aggregate formation under flow, protection from occlusive arterial thrombus formation in vivo and increased tail bleeding times. In summary, these findings point to a critical role of thymosin β4 for actin dynamics during platelet biogenesis, platelet activation downstream of glycoprotein VI and thrombus stability.

Actinin BioID reveals sarcomere crosstalk with oxidative metabolism through interactions with IGF2BP2

Actinins are strain-sensing actin cross-linkers that are ubiquitously expressed and harbor mutations in human diseases. We utilize CRISPR, pluripotent stem cells, and BioID to study actinin interactomes in human cardiomyocytes. We identify 324 actinin proximity partners, including those that are dependent on sarcomere assembly. We confirm 19 known interactors and identify a network of RNA-binding proteins, including those with RNA localization functions. In vivo and biochemical interaction studies support that IGF2BP2 localizes electron transport chain transcripts to actinin neighborhoods through interactions between its K homology (KH) domain and actinin’s rod domain. We combine alanine scanning mutagenesis and metabolic assays to disrupt and functionally interrogate actinin-IGF2BP2 interactions, which reveal an essential role in metabolic responses to pathological sarcomere activation using a hypertrophic cardiomyopathy model. This study expands our functional knowledge of actinin, uncovers sarcomere interaction partners, and reveals sarcomere crosstalk with IGF2BP2 for metabolic adaptation relevant to human disease.

Ladha et al. combine BioID, human cardiomyocytes, and CRISPR-Cas9 to interrogate the actinin interactome. This reveals 324 actinin proximity partners, including RNA-binding proteins that bind transcripts encoding ETC functional components. Among these RNA-binding proteins, IGF2BP2 directly binds actinin, and actinin-IGF2BP2 interactions regulate ETC transcript localization and metabolic adaptation to sarcomere function.

Megakaryocyte Cytoskeletal Proteins in Platelet Biogenesis and Diseases

Thrombopoiesis governs the formation of blood platelets in bone marrow by converting megakaryocytes into long, branched proplatelets on which individual platelets are assembled. The megakaryocyte cytoskeleton responds to multiple microenvironmental cues, including chemical and mechanical stimuli, sustaining the platelet shedding. During the megakaryocyte's life cycle, cytoskeletal networks organize cell shape and content, connect them physically and biochemically to the bone marrow vascular niche, and enable the release of platelets into the bloodstream. While the basic building blocks of the cytoskeleton have been studied extensively, new sets of cytoskeleton regulators have emerged as critical components of the dynamic protein network that supports platelet production. Understanding how the interaction of individual molecules of the cytoskeleton governs megakaryocyte behavior is essential to improve knowledge of platelet biogenesis and develop new therapeutic strategies for inherited thrombocytopenias caused by alterations in the cytoskeletal genes.

Actin/microtubule crosstalk during platelet biogenesis in mice is critically regulated by Twinfilin1 and Cofilin1

Rearrangements of the microtubule (MT) and actin cytoskeleton are pivotal for platelet biogenesis. Hence, defects in actin- or MT-regulatory proteins are associated with platelet disorders in humans and mice. Previous studies in mice revealed that loss of the actin-depolymerizing factor homology (ADF-H) protein Cofilin1 (Cof1) in megakaryocytes (MKs) results in a moderate macrothrombocytopenia but normal MK numbers, whereas deficiency in another ADF-H protein, Twinfilin1 (Twf1), does not affect platelet production or function. However, recent studies in yeast have indicated a critical synergism between Twf1 and Cof1 in the regulation of actin dynamics. We therefore investigated platelet biogenesis and function in mice lacking both Twf1 and Cof1 in the MK lineage. In contrast to single deficiency in either protein, Twf1/Cof1 double deficiency (DKO) resulted in a severe macrothrombocytopenia and dramatically increased MK numbers in bone marrow and spleen. DKO MKs exhibited defective proplatelet formation in vitro and in vivo as well as impaired spreading and altered assembly of podosome-like structures on collagen and fibrinogen in vitro. These defects were associated with aberrant F-actin accumulation and, remarkably, the formation of hyperstable MT, which appears to be caused by dysregulation of the actin- and MT-binding proteins mDia1 and adenomatous polyposis coli. Surprisingly, the mild functional defects described for Cof1-deficient platelets were only slightly aggravated in DKO platelets suggesting that both proteins are largely dispensable for platelet function in the peripheral blood. In summary, these findings reveal critical redundant functions of Cof1 and Twf1 in ensuring balanced actin/microtubule crosstalk during thrombopoiesis in mice and possibly humans.

RhoA/Cdc42 signaling drives cytoplasmic maturation but not endomitosis in megakaryocytes

Megakaryocytes (MKs), the precursors of blood platelets, are large, polyploid cells residing mainly in the bone marrow. We have previously shown that balanced signaling of the Rho GTPases RhoA and Cdc42 is critical for correct MK localization at bone marrow sinusoids in vivo. Using conditional RhoA/Cdc42 double-knockout (DKO) mice, we reveal here that RhoA/Cdc42 signaling is dispensable for the process of polyploidization in MKs but essential for cytoplasmic MK maturation. Proplatelet formation is virtually abrogated in the absence of RhoA/Cdc42 and leads to severe macrothrombocytopenia in DKO animals. The MK maturation defect is associated with downregulation of myosin light chain 2 (MLC2) and β1-tubulin, as well as an upregulation of LIM kinase 1 and cofilin-1 at both the mRNA and protein level and can be linked to impaired MKL1/SRF signaling. Our findings demonstrate that MK endomitosis and cytoplasmic maturation are separately regulated processes, and the latter is critically controlled by RhoA/Cdc42.

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.

Inherited Platelet Disorders: An Updated Overview

Platelets play a major role in hemostasis as ppwell as in many other physiological and pathological processes. Accordingly, production of about 10¹¹ platelet per day as well as appropriate survival and functions are life essential events. Inherited platelet disorders (IPDs), affecting either platelet count or platelet functions, comprise a heterogenous group of about sixty rare diseases caused by molecular anomalies in many culprit genes. Their clinical relevance is highly variable according to the specific disease and even within the same type, ranging from almost negligible to life-threatening. Mucocutaneous bleeding diathesis (epistaxis, gum bleeding, purpura, menorrhagia), but also multisystemic disorders and/or malignancy comprise the clinical spectrum of IPDs. The early and accurate diagnosis of IPDs and a close patient medical follow-up is of great importance. A genotype-phenotype relationship in many IPDs makes a molecular diagnosis especially relevant to proper clinical management. Genetic diagnosis of IPDs has been greatly facilitated by the introduction of high throughput sequencing (HTS) techniques into mainstream investigation practice in these diseases. However, there are still unsolved ethical concerns on general genetic investigations. Patients should be informed and comprehend the potential implications of their genetic analysis. Unlike the progress in diagnosis, there have been no major advances in the clinical management of IPDs. Educational and preventive measures, few hemostatic drugs, platelet transfusions, thrombopoietin receptor agonists, and in life-threatening IPDs, allogeneic hematopoietic stem cell transplantation are therapeutic possibilities. Gene therapy may be a future option. Regular follow-up by a specialized hematology service with multidisciplinary support especially for syndromic IPDs is mandatory.

Role of Rho-GTPases in megakaryopoiesis

Megakaryocytes (MKs) are the bone marrow (BM) cells that generate blood platelets by a process that requires: i) polyploidization responsible for the increased MK size and ii) cytoplasmic organization leading to extension of long pseudopods, called proplatelets, through the endothelial barrier to allow platelet release into blood. Low level of localized RHOA activation prevents actomyosin accumulation at the cleavage furrow and participates in MK polyploidization. In the platelet production, RHOA and CDC42 play opposite, but complementary roles. RHOA inhibits both proplatelet formation and MK exit from BM, whereas CDC42 drives the development of the demarcation membranes and MK migration in BM. Moreover, the RhoA or Cdc42 MK specific knock-out in mice and the genetic alterations in their down-stream effectors in human induce a thrombocytopenia demonstrating their key roles in platelet production. A better knowledge of Rho-GTPase signalling is thus necessary to develop therapies for diseases associated with platelet production defects.

Abbreviations: AKT: Protein Kinase BARHGEF2: Rho/Rac Guanine Nucleotide Exchange Factor 2ARP2/3: Actin related protein 2/3BM: Bone marrowCDC42: Cell division control protein 42 homologCFU-MK: Colony-forming-unit megakaryocyteCIP4: Cdc42-interacting protein 4mDIA: DiaphanousDIAPH1; Protein diaphanous homolog 1ECT2: Epithelial Cell Transforming Sequence 2FLNA: Filamin AGAP: GTPase-activating proteins or GTPase-accelerating proteinsGDI: GDP Dissociation InhibitorGEF: Guanine nucleotide exchange factorHDAC: Histone deacetylaseLIMK: LIM KinaseMAL: Megakaryoblastic leukaemiaMARCKS: Myristoylated alanine-rich C-kinase substrateMKL: Megakaryoblastic leukaemiaMLC: Myosin light chainMRTF: Myocardin Related Transcription FactorOTT: One-Twenty Two ProteinPACSIN2: Protein Kinase C And Casein Kinase Substrate In Neurons 2PAK: P21-Activated KinasePDK: Pyruvate Dehydrogenase kinasePI3K: Phosphoinositide 3-kinasePKC: Protein kinase CPTPRJ: Protein tyrosine phosphatase receptor type JRAC: Ras-related C3 botulinum toxin substrate 1RBM15: RNA Binding Motif Protein 15RHO: Ras homologousROCK: Rho-associated protein kinaseSCAR: Suppressor of cAMP receptorSRF: Serum response factorSRC: SarcTAZ: Transcriptional coactivator with PDZ motifTUBB1: Tubulin β1VEGF: Vascular endothelial growth factorWAS: Wiskott Aldrich syndromeWASP: Wiskott Aldrich syndrome proteinWAVE: WASP-family verprolin-homologous proteinWIP: WASP-interacting proteinYAP: Yes-associated protein

Megakaryocyte migration defects due to nonmuscle myosin IIA mutations underlie thrombocytopenia in MYH9-related disease

Megakaryocytes (MKs), the precursor cells for platelets, migrate from the endosteal niche of the bone marrow (BM) toward the vasculature, extending proplatelets into sinusoids, where circulating blood progressively fragments them into platelets. Nonmuscle myosin IIA (NMIIA) heavy chain gene (MYH9) mutations cause macrothrombocytopenia characterized by fewer platelets with larger sizes leading to clotting disorders termed myosin-9-related disorders (MYH9-RDs). MYH9-RD patient MKs have proplatelets with thicker and fewer branches that produce fewer and larger proplatelets, which is phenocopied in mouse Myh9-RD models. Defective proplatelet formation is considered to be the principal mechanism underlying the macrothrombocytopenia phenotype. However, MYH9-RD patient MKs may have other defects, as NMII interactions with actin filaments regulate physiological processes such as chemotaxis, cell migration, and adhesion. How MYH9-RD mutations affect MK migration and adhesion in BM or NMIIA activity and assembly prior to proplatelet production remain unanswered. NMIIA is the only NMII isoform expressed in mature MKs, permitting exploration of these questions without complicating effects of other NMII isoforms. Using mouse models of MYH9-RD (NMIIAR702C+/-GFP+/-, NMIIAD1424N+/-, and NMIIAE1841K+/-) and in vitro assays, we investigated MK distribution in BM, chemotaxis toward stromal-derived factor 1, NMIIA activity, and bipolar filament assembly. Results indicate that different MYH9-RD mutations suppressed MK migration in the BM without compromising bipolar filament formation but led to divergent adhesion phenotypes and NMIIA contractile activities depending on the mutation. We conclude that MYH9-RD mutations impair MK chemotaxis by multiple mechanisms to disrupt migration toward the vasculature, impairing proplatelet release and causing macrothrombocytopenia.

Learning the Ropes of Platelet Count Regulation: Inherited Thrombocytopenias

Inherited thrombocytopenias (IT) are a group of hereditary disorders characterized by a reduced platelet count sometimes associated with abnormal platelet function, which can lead to bleeding but also to syndromic manifestations and predispositions to other disorders. Currently at least 41 disorders caused by mutations in 42 different genes have been described. The pathogenic mechanisms of many forms of IT have been identified as well as the gene variants implicated in megakaryocyte maturation or platelet formation and clearance, while for several of them the pathogenic mechanism is still unknown. A range of therapeutic approaches are now available to improve survival and quality of life of patients with IT; it is thus important to recognize an IT and establish a precise diagnosis. ITs may be difficult to diagnose and an initial accurate clinical evaluation is mandatory. A combination of clinical and traditional laboratory approaches together with advanced sequencing techniques provide the highest rate of diagnostic success. Despite advancement in the diagnosis of IT, around 50% of patients still do not receive a diagnosis, therefore further research in the field of ITs is warranted to further improve patient care.

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.

Case Report: Exome Sequencing Identified a Novel Frameshift Mutation of α-Actin 1 in a Chinese Family With Macrothrombocytopenia and Mild Bleeding

Inherited macrothrombocytopenia (IMTP) is a rare disorder characterized by a reduced platelet count and abnormally large platelets. The main clinical symptom of IMTP is mild bleeding in some patients. At present, more than 30 genes have been identified in patients with syndromic and non-syndromic IMTP. In this study, a 3-year-old boy and his mother who presented with mild epistaxis and/or gingival bleeding were diagnosed as having IMTP. Wen then selected whole sequencing to explore the genetic lesion of the patients. After data filtering and mutation validation, a novel frameshift mutation (NM_001130004: c.398_399insTGCG, p.F134AfsX60) of α-actin 1 (ACTN1) was identified in the proband and his mother but absent in other unaffected individuals. Previous studies have proven that mutations in ACTN1 may lead to IMTP with mild to absent bleeding phenotype. The novel mutation, resulting in a truncated protein in exon 4 of the ACTN1 gene, was absent in the public database, such as 1000G and genomAD. Further Western blot revealed that the expression of α-actin 1 in the proband was decreased overtly, which indicated that the novel frameshift mutation may induce non-sense-mediated mRNA decay. In summary, this study not only broadened the variants spectrum of ACTN1 gene, which may contribute to the genetic counseling of IMTP, but also confirmed the diagnosis of IMTP, which may help the management and prognosis for the family members.

Inherited thrombocytopenias: an updated guide for clinicians

The great advances in the knowledge of inherited thrombocytopenias (ITs) made since the turn of the century have significantly changed our view of these conditions. To date, ITs encompass 45 disorders with different degrees of complexity of the clinical picture and very wide variability in the prognosis. They include forms characterized by thrombocytopenia alone, forms that present with other congenital defects, and conditions that predispose to acquire additional diseases over the course of life. In this review, we recapitulate the clinical features of ITs with emphasis on the forms predisposing to additional diseases. We then discuss the key issues for a rational approach to the diagnosis of ITs in clinical practice. Finally, we aim to provide an updated and comprehensive guide to the treatment of ITs, including the management of hemostatic challenges, the treatment of severe forms, and the approach to the manifestations that add to thrombocytopenia.

Plaquettes sanguines de culture : état de l’art

Les plaquettes sanguines sont des éléments anucléés du sang. D’un diamètre de 2 à 3 μm, ce sont les plus petits éléments figurés du sang. Alors que leur rôle principal est d’arrêter ou prévenir les saignements, elles sont également impliquées dans d’autres fonctions, comme l’immunité, l’inflammation ou la progression tumorale. L’essor des biotechnologies et les connaissances acquises sur les mécanismes qui régulent la biogénèse des plaquettes permettent aujourd’hui d’envisager la production de plaquettes de culture. Dès lors, ce type de produit pourrait avoir sa place pour relever un certain nombre de défis transfusionnels comme l’allo-immunisation ou les états réfractaires. Cependant les rendements de culture restent faibles et de nombreux obstacles doivent encore être franchis avant d’envisager une application en transfusion. Cet article recense les arguments qui motivent la production de plaquettes de culture à visée transfusionnelle et récapitule les principales avancées dans le domaine tout en soulignant ses limites.

Recent advances in inherited platelet disorders

PURPOSE OF REVIEW

The increasing use of high throughput sequencing and genomic analysis has facilitated the discovery of new causes of inherited platelet disorders. Studies of these disorders and their respective mouse models have been central to understanding their biology, and also in revealing new aspects of platelet function and production. This review covers recent contributions to the identification of genes, proteins and variants associated with inherited platelet defects, and highlights how these studies have provided insights into platelet development and function.

RECENT FINDINGS

Novel genes recently implicated in human platelet dysfunction include the galactose metabolism enzyme UDP-galactose-4-epimerase in macrothrombocytopenia, and erythropoietin-producing hepatoma-amplified sequence receptor transmembrane tyrosine kinase EPHB2 in a severe bleeding disorder with deficiencies in platelet agonist response and granule secretion. Recent studies of disease-associated variants established or clarified roles in platelet function and/or production for the membrane receptor G6b-B, the FYN-binding protein FYB1/ADAP, the RAS guanyl-releasing protein RASGRP2/CalDAG-GEFI and the receptor-like protein tyrosine phosphatase PTPRJ/CD148. Studies of genes associated with platelet disorders advanced understanding of the cellular roles of neurobeachin-like 2, as well as several genes influenced by the transcription regulator RUNT-related transcription factor 1 (RUNX1), including NOTCH4.

SUMMARY

The molecular bases of many hereditary platelet disorders have been elucidated by the application of recent advances in cell imaging and manipulation, genomics and protein function analysis. These techniques have also aided the detection of new disorders, and enabled studies of disease-associated genes and variants to enhance understanding of platelet development and function.

Pathogenic and likely pathogenic variants in at least five genes account for approximately 3% of mild isolated nonsyndromic thrombocytopenia

BACKGROUND

Thrombocytopenia has a variety of different etiologies, both acquired and hereditary. Inherited thrombocytopenia may be associated with other symptoms (syndromic forms) or may be strictly isolated. To date, only about half of all the familial forms of thrombocytopenia have been accounted for in terms of well-defined genetic abnormalities. However, data are limited on the nature and frequency of the underlying causative genetic variants in individuals with mild isolated nonsyndromic thrombocytopenia.

STUDY DESIGN AND METHODS

Thirteen known or candidate genes for isolated thrombocytopenia were included in a gene panel analysis in which targeted next-generation sequencing was performed on 448 French blood donors with mild isolated nonsyndromic thrombocytopenia.

RESULTS

A total of 68 rare variants, including missense, splice site, frameshift, nonsense, and in-frame variants (all heterozygous) were identified in 11 of the 13 genes screened. Twenty-nine percent (N = 20) of the variants detected were absent from both the French Exome Project and gnomAD exome databases. Using stringent criteria and an unbiased approach, we classified seven predicted loss-of-function variants (three in ITGA2B and four in TUBB1) and four missense variants (one in GP1BA, two in ITGB3 and one in ACTN1) as being pathogenic or likely pathogenic. Altogether, they were found in 13 members (approx. 3%) of our studied cohort.

CONCLUSION

We present the results of gene panel sequencing of known and candidate thrombocytopenia genes in mild isolated nonsyndromic thrombocytopenia. Pathogenic and likely pathogenic variants in five known thrombocytopenia genes were identified, accounting for approximately 3% of individuals with the condition.

Absence of bleeding upon dual antiplatelet therapy in a patient with a immune GPVI deficiency

Acquired deficiencies in platelet glycoprotein VI are rare and have not been found associated with other defects. Here we report the case of a 64-year old male patient presenting an immune GPVI deficiency associated to a mutation in the alpha-actinin gene and who has been treated with dual anti platelet therapy without bleeding.Introduction: Glycoprotein (GP) VI, a pluripotent receptor interacting with collagen and fibrin(ogen) is responsible for thrombus formation, growth and stability (1-4). It is co-expressed with the Fc receptor γ (FcRγ) chain (5). GPVI is not critical for haemostasis since subjects with a GPVI deficiency usually present low or even no bleeding tendency (6, 7). Acquired GPVI deficiency due to antibody-induced GPVI depletion is the most frequent finding. At least 10 patients have been described with an acquired GPVI deficiency, most often associated to immune thrombocytopenia, moderate bleeding and impaired collagen-induced platelet aggregation (7). Several mechanisms leading to the GPVI deficiency are proposed including antibody-triggered GPVI internalization and/or shedding of the extracellular domain (8, 9). We report the case of a patient presenting an acquired GPVI deficiency different from those previously described: (i) he is male whereas all previous cases were female, (ii) he is heterozygous for a mutation in α (alpha)-actinin-1 gene and (iii) he was treated with dual antiplatelet therapy with no haemorrhagic manifestation.

Phosphorylation of ACTN4 Leads to Podocyte Vulnerability and Proteinuric Glomerulosclerosis

BACKGROUND

Genetic mutations in α-actinin-4 (ACTN4)-an important actin crosslinking cytoskeletal protein that provides structural support for kidney podocytes-have been linked to proteinuric glomerulosclerosis in humans. However, the effect of post-translational modifications of ACTN4 on podocyte integrity and kidney function is not known.

METHODS

Using mass spectrometry, we found that ACTN4 is phosphorylated at serine (S) 159 in human podocytes. We used phosphomimetic and nonphosphorylatable ACTN4 to comprehensively study the effects of this phosphorylation in vitro and in vivo. We conducted x-ray crystallography, F-actin binding and bundling assays, and immunofluorescence staining to evaluate F-actin alignment. Microfluidic organ-on-a-chip technology was used to assess for detachment of podocytes simultaneously exposed to fluid flow and cyclic strain. We then used CRISPR/Cas9 to generate mouse models and assessed for renal injury by measuring albuminuria and examining kidney histology. We also performed targeted mass spectrometry to determine whether high extracellular glucose or TGF-β levels increase phosphorylation of ACTN4.

RESULTS

Compared with the wild type ACTN4, phosphomimetic ACTN4 demonstrated increased binding and bundling activity with F-actin in vitro. Phosphomimetic Actn4 mouse podocytes exhibited more spatially correlated F-actin alignment and a higher rate of detachment under mechanical stress. Phosphomimetic Actn4 mice developed proteinuria and glomerulosclerosis after subtotal nephrectomy. Moreover, we found that exposure to high extracellular glucose or TGF-β stimulates phosphorylation of ACTN4 at S159 in podocytes.

CONCLUSIONS

These findings suggest that increased phosphorylation of ACTN4 at S159 leads to biochemical, cellular, and renal pathology that is similar to pathology resulting from human disease-causing mutations in ACTN4. ACTN4 may mediate podocyte injury as a consequence of both genetic mutations and signaling events that modulate phosphorylation.

Myelodysplastic syndromes in a pediatric patient with Cri du Chat syndrome with a ring chromosome 5

Few hematological complications have previously been reported in association with Cri du Chat syndrome (CdCS). A case of myelodysplastic syndromes (MDS) in a pediatric patient with CdCS is herein presented. A 17-year-old female with CdCS caused by ring chromosome 5 was admitted to the hospital for investigation of a 1-month history of anemia. Based on the morphological findings of bone marrow, the patient was diagnosed with refractory cytopenia with multilineage dysplasia. The risk group was classified as intermediate-1 in the International Prognostic Scoring System (IPSS), and low in the revised IPSS. Assessment by microarray comparative genomic hybridization (CGH) identified the breakpoints of ring chromosome 5 as 46,XX,r(5)(p14.3q35.3). This revealed that the 5q terminal deletion did not include the common deleted region of MDS with del(5q). Treatment with azacitidine was initiated to control disease progression and improve quality of life. At baseline, the patient had a mean transfusion requirement of 3 units/month, which decreased to 2 units/month after six cycles of azacitidine and to 1 unit/month after 10 cycles of azacitidine. Cytopenia observed in the presented case seemed irrelevant to ring chromosome 5 which is the causative cytogenetic abnormality of CdCS, and further analyses may be needed to clarify the pathogenesis.

Inherited thrombocytopenias: history, advances and perspectives

Over the last 100 years the role of platelets in hemostatic events and their production by megakaryocytes have gradually been defined. Progressively, thrombocytopenia was recognized as a cause of bleeding, first through an acquired immune disorder; then, since 1948, when Bernard-Soulier syndrome was first described, inherited thrombocytopenia became a fascinating example of Mendelian disease. The platelet count is often severely decreased and platelet size variable; associated platelet function defects frequently aggravate bleeding. Macrothrombocytopenia with variable proportions of enlarged platelets is common. The number of circulating platelets will depend on platelet production, consumption and lifespan. The bulk of macrothrombocytopenias arise from defects in megakaryopoiesis with causal variants in transcription factor genes giving rise to altered stem cell differentiation and changes in early megakaryocyte development and maturation. Genes encoding surface receptors, cytoskeletal and signaling proteins also feature prominently and Sanger sequencing associated with careful phenotyping has allowed their early classification. It quickly became apparent that many inherited thrombocytopenias are syndromic while others are linked to an increased risk of hematologic malignancies. In the last decade, the application of next-generation sequencing, including whole exome sequencing, and the use of gene platforms for rapid testing have greatly accelerated the discovery of causal genes and extended the list of variants in more common disorders. Genes linked to an increased platelet turnover and apoptosis have also been identified. The current challenges are now to use next-generation sequencing in first-step screening and to define bleeding risk and treatment better.

Diagnosis of Inherited Platelet Disorders on a Blood Smear

Inherited platelet disorders (IPDs) are rare diseases featured by low platelet count and defective platelet function. Patients have variable bleeding diathesis and sometimes additional features that can be congenital or acquired. Identification of an IPD is desirable to avoid misdiagnosis of immune thrombocytopenia and the use of improper treatments. Diagnostic tools include platelet function studies and genetic testing. The latter can be challenging as the correlation of its outcomes with phenotype is not easy. The immune-morphological evaluation of blood smears (by light- and immunofluorescence microscopy) represents a reliable method to phenotype subjects with suspected IPD. It is relatively cheap, not excessively time-consuming and applicable to shipped samples. In some forms, it can provide a diagnosis by itself, as for MYH9-RD, or in addition to other first-line tests as aggregometry or flow cytometry. In regard to genetic testing, it can guide specific sequencing. Since only minimal amounts of blood are needed for the preparation of blood smears, it can be used to characterize thrombocytopenia in pediatric patients and even newborns further. In principle, it is based on visualizing alterations in the distribution of proteins, which result from specific genetic mutations by using monoclonal antibodies. It can be applied to identify deficiencies in membrane proteins, disturbed distribution of cytoskeletal proteins, and alpha as well as delta granules. On the other hand, mutations associated with impaired signal transduction are difficult to identify by immunofluorescence of blood smears. This review summarizes technical aspects and the main diagnostic patterns achievable by this method.

Novel ACTN1 variants in cases of thrombocytopenia

The ACTN1 gene has been implicated in inherited macrothrombocytopenia. To decipher the spectrum of variants and phenotype of ACTN1‐related thrombocytopenia, we sequenced the ACTN1 gene in 272 cases of unexplained chronic or familial thrombocytopenia. We identified 15 rare, monoallelic, nonsynonymous and likely pathogenic ACTN1 variants in 20 index cases from 20 unrelated families. Thirty‐one family members exhibited thrombocytopenia. Targeted sequencing was carried out on 12 affected relatives, which confirmed presence of the variant. Twenty‐eight of 32 cases with monoallelic ACTN1 variants had mild to no bleeding complications. Eleven cases harbored 11 different unreported ACTN1 variants that were monoallelic and likely pathogenic. Nine variants were located in the α‐actinin‐1 (ACTN1) rod domain and were predicted to hinder dimer formation. These variants displayed a smaller increase in platelet size compared with variants located outside the rod domain. In vitro expression of the new ACTN1 variants induced actin network disorganization and led to increased thickness of actin fibers. These findings expand the repertoire of ACTN1 variants associated with thrombocytopenia and highlight the high frequency of ACTN1‐related thrombocytopenia cases. The rod domain, like other ACTN1 functional domains, may be mutated resulting in actin disorganization in vitro and thrombocytopenia with normal platelet size in most cases.

Coactosin-like 1 integrates signaling critical for shear-dependent thrombus formation in mouse platelets

Platelet aggregate formation is a multistep process involving receptor-mediated, as well as biomechanical, signaling cascades, which are highly dependent on actin dynamics. We have previously shown that actin depolymerizing factor (ADF)/n-cofilin and Twinfilin 2a, members of the ADF homology (ADF-H) protein family, have distinct roles in platelet formation and function. Coactosin-like 1 (Cotl1) is another ADF-H protein that binds actin and was also shown to enhance biosynthesis of pro-inflammatory leukotrienes (LT) in granulocytes. Here, we generated mice lacking Cotl1 in the megakaryocyte lineage (Cotl1^{-/- )} to investigate its role in platelet production and function. Absence of Cotl1 had no impact on platelet counts, platelet activation or cytoskeletal reorganization under static conditions in vitro In contrast, Cotl1 deficiency markedly affected platelet aggregate formation on collagen and adhesion to immobilized von Willebrand factor at high shear rates in vitro, pointing to an impaired function of the platelet mechanoreceptor glycoprotein (GP) Ib. Furthermore, Cotl1 ^-/-platelets exhibited increased deformability at high shear rates, indicating that the GPIb defect may be linked to altered biomechanical properties of the deficient cells. In addition, we found that Cotl1 deficiency markedly affected platelet LT biosynthesis. Strikingly, exogenous LT addition restored defective aggregate formation of Cotl1^-/- platelets at high shear in vitro, indicating a critical role of platelet-derived LT in thrombus formation. In vivo, Cotl1 deficiency translated into prolonged tail bleeding times and protection from occlusive arterial thrombus formation. Together, our results show that Cotl1 in platelets is an integrator of biomechanical and LT signaling in hemostasis and thrombosis.