Hereditary macrothrombocytopathia, nephritis and deafness

Zebrafish as a model for podocyte research

Podocytes, specialized postmitotic cells, are central players in various kidney-related diseases. Zebrafish have become a valuable model system for studying podocyte biology because they are genetically easy to manipulate, transparent, and their glomerular structure is similar to that of mammals. This review provides an overview of the knowledge of podocyte biology in zebrafish larvae, with particular focus on their essential contribution to understanding the mechanisms that underlie kidney diseases as well as supporting drug development. In addition, special attention is given to advances in live-imaging techniques allowing the observation of dynamic processes, including podocyte motility, podocyte process behavior, and glomerulus maturation. The review further addresses the functional aspects of podocytes in zebrafish larvae. This includes topics such as glomerular filtration, ultrastructural analyses, and evaluation of podocyte response to nephrotoxic insults. Studies presented in this context have provided important insights into the maintenance and resistance of the glomerular filtration barrier in zebrafish larvae and explored the potential transferability of these findings to mammals such as mice, rats, and most importantly, humans. The recent ability to identify potential therapeutic targets represents a promising new way to identify drugs that could effectively treat podocyte-associated glomerulopathies in humans. In summary, this review gives an overview about the importance of zebrafish as a model for podocyte-related disease and targeted drug development. It also highlights the key role of advanced imaging techniques in transparent zebrafish larvae, improving our understanding of glomerular diseases and the significant potential for translation of these findings to humans.

A De Novo Mutation in MYH9 in a Child With Severe and Prolonged Macrothrombocytopenia

Congenital macrothrombocytopenia is a diverse group of hereditary disorders caused by mutations in the MYH9 gene, which encodes the nonmuscle myosin heavy chain-A, an important motor protein in hemopoietic cells. Thus, the term MYH9-related disease has been proposed, but the clinicopathologic basis of MYH9 mutations has been poorly investigated. Here, we report a sporadic case of Epstein syndrome, an MYH9 disorder, in a 4-year-old Chinese boy who presented with macrothrombocytopenia. He had no family history of thrombocytopenia, hearing loss, or renal failure. A de novo heterozygous MYH9 mutation, c.287C>T; p. (Ser96Leu), was found in this patient. Genotype-phenotype analysis of all reported mutations suggested a domain-specific relationship between the location of the MYH9 mutation and the penetrance of the nonhematologic characteristics of MYH9-related disorders. Our study highlights the importance of suspecting MYH9-related disease even in cases of chronic macrothrombocytopenia without a family history or extrahematologic symptoms.

Successful cochlear implantation in a patient with Epstein syndrome during long-term follow-up

Epstein syndrome is a rare disease characterized by macrothrombocytopenia, nephritis and progressive sensorineural hearing loss (SNHL). This syndrome is presently recognized as an autosomal dominant disease caused by mutations of non-muscle myosin heavy chain 9 (MYH9). Little information is available about the progress of SNHL, the efficacy of cochlear implants (CI) or the perioperative management of thrombocytopenia in patients with Epstein syndrome. We herein report a case of a patient with Epstein syndrome with the MYH9:c.2105G>A:p.R702H variant who underwent cochlear implantation after 27 years of follow-up for her progressive SNHL. The deterioration rates of hearing were 3.48 dB/year on the right ear and 2.46 dB/year on the left ear. The patient derived benefits from CI and had a speech recognition test result (for sentences) of 93% at 6-months postoperatively. Thrombocytopenia was successfully managed without any bleeding complications by using eltrombopag, an oral thrombopoietic agent, making transfusion of platelets unnecessary. The accurate diagnosis of Epstein syndrome was made only after long-term follow-up as the thrombocytopenia was initially diagnosed as idiopathic thrombocytopenic purpura. This case report highlights the perioperative management of thrombocytopenia, the progress of SNHL and the potential pitfalls of diagnosis.

The use of pan-retinal photocoagulation to treat recurrent vitreous haemorrhage with neovascularisation in the context of Epstein syndrome: an MYH9-related disorder

A female patient presented with stable chronic thrombocytopaenia with large platelets, sensorineuronal deafness and renal impairment. Her treatment was refractory to intravenous immunoglobulins (IVIG) and steroids for a putative diagnosis of immune thrombocytopaenic purpura (ITP). She underwent genetic testing which revealed a MYH9 mutation in-keeping with a diagnosis of Epstein Syndrome. Subsequently to this she developed globally constricted fields on Goldmann visual field testing. MRI pituitary was unremarkable but she was diagnosed with a pituitary microprolactinoma secondary to raised prolactin in the blood responsive to carbegoline therapy. She subsequently developed retinal haemorrhages and recurrent vitreous haemorrhages due to neovascularisation. Fluorescein angiography revealed the extent of the neovascularisation and microvascular ischaemia. She underwent pan-retinal photocoagulation (PRP) to treat the ischaemic stimulus which resulted in regression of the new vessels and cessation of vitreous haemorrhages. There are no previous reported cases of microvascular retinal disease in the literature in the context of Epstein Syndrome, and this is the first report of successful treatment with PRP.

MYH9-related disorders display heterogeneous kidney involvement and outcome

Background

MYH9-related diseases (MYH9-RD) are autosomal dominant disorders caused by mutations of the MYH9 gene encoding the non-muscle myosin heavy chain IIA. They are characterized by congenital thrombocytopenia, giant platelets and leucocyte inclusions. Hearing impairment, pre-senile cataract and nephropathy can also occur. We aimed to evaluate renal involvement and outcome in MYH9-RD patients followed-up by nephrologists.

Methods

We conducted a retrospective multicentre observational study of 13 patients among 9 families with MYH9 mutation diagnosed by genetic testing and immunofluorescence assay referred to nephrologists.

Results

At initial referral, median age was 30 (range 14–76) years. Median estimated glomerular filtration rate was 66 mL/min/1.73 m² (0–141) and two patients had already end-stage renal disease (ESRD). Renal presentation associated proteinuria (n = 12), haematuria (n = 6) and hypertension (n = 6). Three patients developed a rapid onset ESRD whereas five others had a relatively stable kidney function over a 3-year median follow-up (1–34). Extra-renal features varied widely, with hearing impairment in six patients, cataract in two and mild liver dysfunction in seven. Thrombocytopenia existed at referral in 11 patients. Time to diagnosis varied from 0 to 29 years (median 3 years). Initial diagnoses such as idiopathic thrombocytopenic purpura (n = 4) and focal segmental glomerulosclerosis (n = 1) led to corticosteroid administration (n = 4), intravenous immunoglobulins (n = 3), cyclophosphamide (n = 1) and splenectomy (n = 1).

Conclusions

Renal involvement and outcome in MYH9-RD are heterogeneous. The diagnosis is often delayed and misdiagnoses can lead to unnecessary treatments. MYH9-RD should be considered in any patient with glomerular involvement associated with a low or slightly decreased platelet count and/or hearing loss and liver dysfunction.

Basement Membrane Defects in Genetic Kidney Diseases

The glomerular basement membrane (GBM) is a specialized structure with a significant role in maintaining the glomerular filtration barrier. This GBM is formed from the fusion of two basement membranes during development and its function in the filtration barrier is achieved by key extracellular matrix components including type IV collagen, laminins, nidogens, and heparan sulfate proteoglycans. The characteristics of specific matrix isoforms such as laminin-521 (α5β2γ1) and the α3α4α5 chain of type IV collagen are essential for the formation of a mature GBM and the restricted tissue distribution of these isoforms makes the GBM a unique structure. Detailed investigation of the GBM has been driven by the identification of inherited abnormalities in matrix proteins and the need to understand pathogenic mechanisms causing severe glomerular disease. A well-described hereditary GBM disease is Alport syndrome, associated with a progressive glomerular disease, hearing loss, and lens defects due to mutations in the genes COL4A3, COL4A4, or COL4A5. Other proteins associated with inherited diseases of the GBM include laminin β2 in Pierson syndrome and LMX1B in nail patella syndrome. The knowledge of these genetic mutations associated with GBM defects has enhanced our understanding of cell-matrix signaling pathways affected in glomerular disease. This review will address current knowledge of GBM-associated abnormalities and related signaling pathways, as well as discussing the advances toward disease-targeted therapies for patients with glomerular disease.

Septin 7 reduces nonmuscle myosin IIA activity in the SNAP23 complex and hinders GLUT4 storage vesicle docking and fusion

Glomerular epithelial cells, podocytes, are insulin responsive and can develop insulin resistance. Here, we demonstrate that the small GTPase septin 7 forms a complex with nonmuscle myosin heavy chain IIA (NMHC-IIA; encoded by MYH9), a component of the nonmuscle myosin IIA (NM-IIA) hexameric complex. We observed that knockdown of NMHC-IIA decreases insulin-stimulated glucose uptake into podocytes. Both septin 7 and NM-IIA associate with SNAP23, a SNARE protein involved in GLUT4 storage vesicle (GSV) docking and fusion with the plasma membrane. We observed that insulin decreases the level of septin 7 and increases the activity of NM-IIA in the SNAP23 complex, as visualized by increased phosphorylation of myosin regulatory light chain. Also knockdown of septin 7 increases the activity of NM-IIA in the complex. The activity of NM-IIA is increased in diabetic rat glomeruli and cultured human podocytes exposed to macroalbuminuric sera from patients with type 1 diabetes. Collectively, the data suggest that the activity of NM-IIA in the SNAP23 complex plays a key role in insulin-stimulated glucose uptake into podocytes. Furthermore, we observed that septin 7 reduces the activity of NM-IIA in the SNAP23 complex and thereby hinders GSV docking and fusion with the plasma membrane.

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Septin 7, nonmuscle myosin heavy chain IIA (NMHC-IIA) and SNAP23 form a complex.

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Knockdown of septin 7 increases NM-IIA activity in the SNAP23 complex.

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Insulin decreases septin 7 level and increases NM-IIA activity in the SNAP23 complex.

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Septin 7 hinders GSV docking/fusion by reducing NM-IIA activity in the SNAP23 complex.

Pitfalls in the Diagnosis of Immune Thrombocytopenic Purpura in Children: 4 Case Reports

Acute idiopathic thrombocytopenic purpura is the most common cause of thrombocytopenia in childhood, and diagnosis of idiopathic thrombocytopenic purpura is made clinically based on the exclusion of other causes of thrombocytopenia. Patients with diverse causes of thrombocytopenia are sometimes erroneously diagnosed as having idiopathic thrombocytopenic purpura. However, for the prevention of misdiagnoses, careful inspection of peripheral blood smear is of utmost importance. This report presents 4 cases presumed as acute idiopathic thrombocytopenic purpura that were finally identified as pseudothrombocytopenia, inherited macrothrombocytopenia (MHY9 disorders) possibly Epstein syndrome, Bernard-Soulier syndrome, and drug-induced thrombocytopenia. They draw attention to the importance of platelet morphology to exclude inherited macrothrombocytopenia and history to exclude drug-induced thrombocytopenia. Better diagnostic approaches would be possible by the awareness of these relatively rare causes of isolated thrombocytopenia.

Preoperative use of platelets in a 6-year-old with acute appendicitis and a myosin heavy chain 9-related disorder: a case report and review of literature

BACKGROUND

Mutations of nonmuscle myosin heavy chain 9 (MYH9) gene are an autosomal dominant cause of inherited thrombocytopenia in children. MYH9 spectrum disorders include May-Hegglin anomaly and Sebastian, Fechtner, and Epstein syndromes. Patients with these disorders often present with macroplatelets and thrombocytopenia and have a mild bleeding tendency; extrahematologic manifestations (nephropathy, deafness, and cataracts) correlate with specific mutations. No definitive guidelines exist for preoperative prophylactic platelet (PLT) transfusion in these patients.

STUDY DESIGN AND METHODS

This was a case study and literature review.

RESULTS

A 6-year-old male presented with appendicitis. Review of records revealed that he and his siblings had thrombocytopenia; polymerase chain reaction amplification with DNA sequence analysis showed a variation in the MYH9 gene previously reported as a known cause of MYH9-related disorders. Involvement of other organ systems was not found on initial work-up. The patient's PLT count on admission was 20 × 10(9) /L. The degree of thrombocytopenia prompted transfusion of apheresis PLTs and he had good response (73 × 10(9) /L). After infusion he developed hives, rash, itching, and nausea, which resolved after administration of epinephrine and hydrocortisone. Transfusion reaction work-up was negative and symptoms were interpreted as an allergic reaction. The appendectomy was uneventful.

CONCLUSION

This patient's PLT count was within guidelines to warrant transfusion; however, some patients with MYH9 mutations have counts above the transfusion threshold. To the authors' knowledge, there are no set guidelines for preoperative prophylaxis in a patient with an MYH9 deficiency. The management of the bleeding diathesis in these patients, especially in the setting of invasive procedures, is uncertain.

Sporadic Epstein syndrome with macrothrombocytopenia, sensorineural hearing loss and renal failure

We report here a sporadic case of Epstein syndrome, one of the MYH9 disorders. A Japanese boy was first noted to have thrombocytopenia at 3 years of age. Blood smear showed giant platelets but no Döhle-like bodies in the neutrophils. He had no family history of thrombocytopenia, hearing loss, and/or renal failure. Thrombocytopenia took a chronic course and platelet count fluctuated in the range 18 000-46 000/μL, not responding to i.v. immunoglobulin or prednisolone treatment. The patient had episodes of gross nasal bleeding at 7 and 18 years of age. Mild hearing loss was suspected at 6, and proteinuria was first noted at 14 years of age. At the development of renal failure at 24 years of age, he was identified to have de novo R702H MYH9 mutation. This case illustrates the importance of suspecting MYH9 disorder even in cases of chronic macrothrombocytopenia without family history.

A case of cochlear implantation in a patient with Epstein syndrome

Epstein syndrome is a rare autosomal dominant platelet disorder characterized by thrombocytopenia, giant platelets, and sensorineural hearing loss. It is included among four overlapping syndromes, the others being May-Hegglin anomaly, Fechtner syndrome, and Sebastian syndrome. It is now established that all four disorders are caused by mutations in the MYH9 gene. We report the case of a patient with Epstein syndrome in whom bilateral profound hearing impairment developed and cochlear implantation was carried out. A cochlear implant was successfully used with a speech discrimination score of 100% on a Japanese sentence recognition test. This report offers the second description of the performance of a cochlear implant in a patient with Epstein syndrome. This case study may offer hope for patients and their family members with this kind of mutation.

How benign is hematuria? Using genetics to predict prognosis

Hematuria is a common presenting feature of glomerular disease and is sometimes associated with kidney failure later in life. Where isolated microscopic hematuria occurs in children and young adults, an underlying monogenic disorder, such as Alport syndrome or thin basement membrane nephropathy, is frequently responsible. In this review, these and other diseases, which often present with isolated microscopic hematuria, including hereditary angiopathy, nephropathy, aneurysms, and muscle cramps (HANAC) syndrome, IgA nephropathy, and CFHR5 nephropathy, are discussed together with the associated molecular pathology, clinical features, and prognosis. Genetic testing for these conditions used in clinical practice can provide important diagnostic and prognostic information that is relevant to the patient and their family, particularly when kidney transplantation is considered.

Familial cases with MYH9 disorders caused by MYH9 S96L mutation

We report familial cases with MYH9 disorders: a 1-year-old Japanese boy who presented only with macrothrombocytopenia, and his 33-year-old father who had been diagnosed with refractory chronic idiopathic thrombocytopenic purpura, and suffered from hearing loss and chronic renal failure. Peripheral blood smears revealed giant platelets but no Döhle body-like cytoplasmic inclusion bodies in neutrophils. Heterozygous MYH9 S96L mutations were found in the patient and his father, resulting in the diagnosis of a familial case with MYH9 disorders. The possibility of MYH9 disorders including Epstein syndrome should be assessed in cases of thrombocytopenia through the careful examination of hematological features.

Cell-matrix adhesion of podocytes in physiology and disease

Cell-matrix adhesion is crucial for maintaining the mechanical integrity of epithelial tissues. Podocytes--a key component of the glomerular filtration barrier--are exposed to permanent transcapillary filtration pressure and must therefore adhere tightly to the underlying glomerular basement membrane (GBM). The major cell-matrix adhesion receptor in podocytes is the integrin α3β1, which connects laminin 521 in the GBM through various adaptor proteins to the intracellular actin cytoskeleton. Other cell-matrix adhesion receptors expressed by podocytes include the integrins α2β1 and αvβ3, α-dystroglycan, syndecan-4 and type XVII collagen. Mutations in genes encoding any of the components critical for podocyte adhesion cause glomerular disease. This Review highlights recent advances in our understanding of the cell biology and genetics of podocyte adhesion with special emphasis on glomerular disease.

Non-muscle myosin IIA is required for the development of the zebrafish glomerulus

Mutations in the MYH9 gene, coding for the non-muscle myosin heavy chain IIA (NMHC-IIA), are responsible for syndromes characterized by macrothrombocytopenia associated with deafness, cataracts, and severe glomerular disease. Electron microscopy of renal biopsies from these patients found glomerular abnormalities characterized by alterations in mesangial cells, podocytes, and thickening of the glomerular basement membrane. Knockout of NMHC-IIA in mice is lethal, and therefore little is known about the glomerular-related functions of Myh9. Here, we use zebrafish as a model to study the role and function of zNMHC-IIA in the glomerulus. Knockdown of zNMHC-IIA resulted in malformation of the glomerular capillary tuft characterized by few and dilated capillaries of the pronephros. In zNMHC-IIA morphants, endothelial cells failed to develop fenestrations, mesangial cells were absent or reduced, and the glomerular basement membrane appeared nonuniformly thickened. Knockdown of zNMHC-IIA did not impair the formation of podocyte foot processes or slit diaphragms; however, podocyte processes were less uniform in these morphants compared to controls. In vivo clearance of fluorescent dextran indicated that the glomerular barrier function was not compromised by zNMHC-IIA knockdown; however, glomerular filtration was significantly reduced. Thus, our results demonstrate an important role of zNMHC-IIA for the proper formation and function of the glomerulus in zebrafish.

Plasma from a case of recurrent idiopathic FSGS perturbs non-muscle myosin IIA (MYH9 protein) in human podocytes

The MYH9 gene encodes a non-muscle myosin IIA heavy chain (NMMHC-IIA) expressed in podocytes. Heterozygous MYH9 mutations cause a set of overlapping syndromes characterized by variable degrees of deafness, morphologic abnormalities of platelets and focal segmental glomerulosclerosis (FSGS) with progressive renal dysfunction. Similar glomerular lesions are seen in a variety of nephropathies, including an idiopathic form of FSGS in children which recurs in renal allografts, implying a circulating factor that affects glomerular podocyte biology. It is unknown whether NMMHC-IIA is perturbed in the idiopathic form of FSGS. We describe a pediatric patient with typical idiopathic FSGS, in whom proteinuria recurred within hours of deceased donor renal transplantation but who responded to plasmapheresis. We demonstrate in vitro that plasmapheresis effluent from our patient rapidly decreased cultured podocyte levels of the phosphorylated myosin light chain (MLC) that mediates NMMHC-IIA binding to actin and induced dispersion of NMMHC-IIA from its usual position along actin stress fibers. FSGS plasma also caused dispersion of slit diaphragm proteins (nephrin and podocin) and vinculin-positive focal adhesion complexes. Our observations suggest that the putative circulating factor in idiopathic FSGS disrupts normal NMMHC-IIA function in podocytes and might contribute to the pathogenesis of recurrent FSGS in other children.

Glomerular pathology in autosomal dominant MYH9 spectrum disorders: what are the clues telling us about disease mechanism?

Genetic variation in MYH9, encoding non-muscle heavy chain IIA, has been recognized for over a decade as the cause of an autosomal dominant syndrome characterized by macrothrombocytopenia, neutrophil inclusions, and glomerular pathology. More recently, genetic variation in the MYH9 region on chromosome 22 has been associated with chronic kidney disease in African-descent individuals. A better understanding of the disease mechanisms responsible for glomerular injury in autosomal dominant MYH9 syndromes will lead to fuller appreciation of the role of this gene in glomerular biology.

MYH-9 Related Platelet Disorders: Strategies for Management and Diagnosis

MYH-9 related platelet disorders belong to the group of inherited giant platelet disorders. The MYH-9 gene encodes the non-muscular myosin heavy chain IIA (NMMHCIIA), a cytoskeletal contractile protein. Several mutations in the MYH-9 gene lead to macrothrombocytopenia, and cytoplasmic inclusion bodies within leukocytes, while the number of megakaryocytes in the bone marrow is normal. Four overlapping syndromes, known as May-Hegglin anomaly, Epstein syndrome, Fechtner syndrome and Sebastian platelet syndrome, describe different clinical manifestations of MYH9 gene mutations. Macrothrombocytopenia is present in all affected individuals, whereas only some develop additional clinical manifestations such as renal failure, hearing loss and presenile cataracts. The bleeding tendency is usually moderate, with menorrhagia and easy bruising being most frequent. The biggest risk for the individual is inappropriate treatment due to misdiagnosis of chronic autoimmune thrombocytopenia. More than 30 mutations within the 40 exons of the MYH-9 gene leading to macrothrombocytopenia have been identified, of which the upstream mutations up to amino acid ~1400 are more likely associated with syndromic manifestations than the downstream mutations. Diagnosis is based on identification of the granulocyte inclusion bodies using blood smears and immunofluorescence and is finally confirmed by identifying the mutation. Treatment is supportive and should be aimed to prevent iron deficiency anemia. Beside renal failure, the biggest risk for patients affected by a MYH-9 disorder are the adverse effects resulting form treatment based on the misdiagnosis of immune thrombocytopenia.

Patients with Epstein-Fechtner syndromes owing to MYH9 R702 mutations develop progressive proteinuric renal disease

Recent linkage analyses of nondiabetic African-American patients with focal segmental glomerulosclerosis (FSGS) have identified MYH9, encoding nonmuscle myosin heavy chain IIA (NMMHC-IIA), as a gene having a critical role in this disease. Abnormalities of the MYH9 locus also underlie rare autosomal dominant diseases such as May-Hegglin anomaly, and Sebastian, Epstein (EPS), and Fechtner (FTNS) syndromes that are characterized by macrothrombocytopenia and cytoplasmic inclusion bodies in granulocytes. Among these diseases, patients with EPS or FTNS develop progressive nephritis and hearing disability. We analyzed clinical features and pathophysiological findings of nine EPS-FTNS patients with MYH9 mutations at the R702 codon hot spot. Most developed proteinuria and/or hematuria in early infancy and had a rapid progression of renal impairment during adolescence. Renal histopathological findings in one patient showed changes compatible with FSGS. The intensity of immunostaining for NMMHC-IIA in podocytes was decreased in this patient compared with control patients. Thus, MYH9 R702 mutations display a strict genotype-phenotype correlation, and lead to the rapid deterioration of podocyte structure. Our results highlight the critical role of NMMHC-IIA in the development of FSGS.

Epstein syndrome presenting as renal failure in young patients

Two young Chinese patients presented with renal failure and thrombocytopenia. Further investigations showed the presence of large platelets and high-frequency sensorineural hearing deficit. Genetic studies confirmed mutations in the gene encoding the myosin heavy chain (MYH-9), and Epstein Syndrome was diagnosed. One patient underwent deceased-donor kidney transplantation with satisfactory graft function. Epstein Syndrome is a rare genetic disorder with autosomal dominant inheritance. Clinicians should be aware of this entity when a young patient presents with renal failure and thrombocytopenia.

CKD in MYH9-related disorders

MYH9-related disorders are rare causes of chronic kidney disease (CKD) presenting as chronic glomerulonephritis and derive from mutations of the MYH9 gene, which encodes for the nonmuscle myosin heavy chain IIA. These disorders are autosomal dominant and include May-Hegglin anomaly and Sebastian, Fechtner, and Epstein syndromes. Diagnosis of these disorders is made first in early childhood because of the characteristic peripheral-blood smear findings of thrombocytopenia, giant platelets, and variably detected basophilic cytoplasmic inclusion bodies in leukocytes. CKD typically develops later in adulthood and may progress to end-stage renal disease. MYH9-related disorders may be associated with deafness and cataract; hence, Alport syndrome becomes important in the differential diagnosis. However, the autosomal dominance pattern of inheritance and characteristic peripheral-blood smear findings in the former help differentiate the two conditions. New evidence suggests that MYH9 gene alterations also are associated with a greater risk of focal segmental glomerulosclerosis and hypertensive nephrosclerosis in African Americans. The purpose of this review is to focus on the known, but rarely recognized association of MYH9-related disorders with CKD and highlight the recent discoveries related to the MYH9 gene that may explain the reason for a high CKD burden in African Americans.

Regulation of platelet biogenesis: insights from the May-Hegglin anomaly and other MYH9-related disorders

Megakaryocyte (MK) maturation culminates in release of blood platelets through proplatelet extensions. MKs presumably delay elaborating proplatelets until synthesis of platelet constituents is complete. Recent insights from investigation of a classic human congenital macrothrombocytopenia, the May-Hegglin anomaly, and related MYH9-associated disorders shed new light on underlying mechanisms. The findings reviewed in this article implicate myosin IIA, the non-muscle myosin heavy chain product of the MYH9 gene, in restraining proplatelet formation until MKs achieve terminal maturity. Loss of myosin IIA function, through dominant inhibitory mutations in humans, targeted gene disruption in mice, or manipulation of cultured MKs, seems to accelerate proplatelet formation. The resulting process is inefficient and produces platelets that vary widely in size, shape and content. Several lines of evidence suggest that the Rho-ROCK-myosin light chain pathway restrains proplatelet formation through myosin IIA. These findings illustrate that mammalian thrombopoiesis is complex and subject to both positive and negative regulation.

Abnormal megakaryocyte morphology and proplatelet formation in mice with megakaryocyte-restricted MYH9 inactivation

Mutations in the MYH9 gene encoding nonmuscle myosin IIA lead to macrothrombocytopenia as observed in MYH9-related disorders. We used mice with megakaryocyte-restricted MYH9 inactivation to explore the role of myosin in thrombopoiesis. In situ, bone marrow MYH9Delta megakaryocytes were irregularly shaped, appearing leaky with poorly defined limits. The demarcation membranes were abnormally organized and poorly developed, pointing to an insufficient reservoir for the future formation of platelets. The cytoskeletal-rich peripheral zone was lacking due to the absence of the myosin filament network that normally surrounds the granular zone in wild-type cells. In vitro studies of cultured cells showed that MYH9Delta megakaryocytes were unable to form stress fibers upon adhesion to collagen, suggesting that the leaky shape results from defects in internal tension and anchorage to the extracellular environment. Surprisingly, the proportion of cells extending proplatelets was increased in MYH9Delta megakaryocytes and the proplatelet buds were larger. Overall, this study provides evidence for a role of myosin in different steps of megakaryocyte development through its participation in the maintenance of cell shape, formation and organization of the demarcation membranes and the peripheral zone, anchorage to the extracellular matrix, and proplatelet formation.

Historical hematology: May-Hegglin anomaly

May-Hegglin anomaly is a rare autosomal dominant platelet disorder characterized by thrombocytopenia, giant platelets, and unique leukocyte inclusion bodies. This disorder was first described by May, a German physician, in 1909, and was subsequently described by a Swiss physician, Hegglin, in 1945. The pathogenesis of the disorder had been unknown until recently, when mutations in the gene encoding for nonmuscle myosin heavy chain IIA (MYH9) were identified. Unique cytoplasmic inclusion bodies are aggregates of nonmuscle myosin heavy chain IIA, and are only present in granulocytes. It is not yet known why inclusion bodies are not present in platelets, monocytes, and lymphocytes, or how giant platelets are formed. Interestingly, MYH9 is also found to be responsible for several related disorders with macrothrombocytopenia and leukocytes inclusion, including Sebastian, Fechtner, and Epstein syndromes, which feature deafness, nephritis, and/or cataract. Current interest is centered upon the mechanisms by which a single mutation causes a variety of phenotypes.

[Alport syndrome or progressive hereditary nephritis with hearing loss]

Alport syndrome is an inherited disorder characterized by progressive hematuric nephritis with structural defects of the glomerular basement membrane, and sensorineural deafness. Ocular abnormalities are frequently associated. The incidence is approximatively 1/5000. The renal disease is severe in male patients and should be responsible for 2% of end-stage renal failure. Alport syndrome is heterogeneous at the clinical and genetic levels. It occurs as a consequence of structural abnormalities in type IV collagen, the major constituent of basement membranes. Six genetically distinct chains of type IV collagen have been identified. Mutations in the COL4A5 gene located at Xq22, and encoding the alpha 5(IV) chain are responsible for X-linked Alport syndrome whereas COL4A3 or COL4A4 located "head to head" on chromosome 2 are involved in the rarer autosomal forms of the disease.

Haematological characteristics of MYH9 disorders due to MYH9 R702 mutations

OBJECTIVE

MYH9 disorders are characterised by giant platelets, thrombocytopenia, and Döhle body-like cytoplasmic granulocyte inclusion bodies that result from mutations in MYH9, the gene for non-muscle myosin heavy chain-IIA (NMMHC-IIA). MYH9 R702 mutations are highly associated with Alport manifestations and result in Epstein syndrome. The aim of our study was to determine the haematological characteristics of MYH9 disorders as a result of R702 mutations to aid in making a proper diagnosis.

PATIENTS AND METHODS

Platelet size of patients with MYH9 disorders was determined as platelet diameter by microscopic observation of 200 platelets on stained peripheral blood smears. Double in situ hybridisation using a biotinylated oligo(dT) probe and immunofluorescence analysis of neutrophil NMMHC-IIA was performed on peripheral blood smears.

RESULTS

Patients carrying R702 mutations had significantly larger platelets than those with other MYH9 mutations. Although granulocyte inclusion bodies were mostly invisible on stained blood smears, immunofluorescence analysis for NMMHC-IIA showed an abnormal type II localisation in all neutrophils. We first showed that poly(A)+ RNA coincided with accumulated NMMHC-IIA at inclusion bodies in patients with MYH9 disorders. However, no condensation of poly(A)+ RNA at inclusion bodies was observed in patients with R702 mutations.

CONCLUSION

Our study shows that R702 mutations result in especially large platelets and inclusion bodies being faint and mostly invisible on conventionally stained blood smears. We further demonstrated that poly(A)+ RNA content but not NMMHC-IIA accumulation is responsible for the morphological appearance/stainability of inclusion bodies on stained blood smears and the amount of poly(A)+ RNA is decreased in those with R702 mutations.

Identification of inherited macrothrombocytopenias based on mean platelet volume among patients diagnosed with idiopathic thrombocytopenia

Inherited macrothrombocytopenia is a rare illness that is often misdiagnosed as idiopathic thrombocytopenia (ITP), a more widespread acquired disease. Automated blood cell counters in routine clinical use usually miss giant platelets and underestimate mean platelet volume (MPV). Incorrect diagnoses might expose patients to a risk of unnecessary treatment. The ADVIA 120 hematology counter efficiently detects large platelets based on two-dimensional laser light scatter. The present study measures and re-evaluates MPV using the ADVIA 120 in 112 patients who had initially been diagnosed with ITP. We identified 11 unrelated patients as having probable macrothrombocytopenia (average MPV of 19.2+/-3.8 fL; normal range 7.8-10.2). Functional, phenotypical and DNA analyses confirmed that three of these patients had Bernard-Soulier syndrome and one had MYH9-related disease, both of which are the most common forms of inherited macrothrombocytopenia. We stress that a conventional automated hematology analyzer had overlooked giant platelets in these patients, and that all of them had received high-dose steroid therapy and/or splenectomy before this study according to a diagnosis of ITP. Thus, checking MPV using the ADVIA 120 in thrombocytopenic patients is a useful method of correctly diagnosing inherited macrothrombocytopenia, and thus avoiding patient exposure to unnecessary and sometimes toxic treatment.

Le syndrome MYH9 : à propos d'une nouvelle observation et de la mise en évidence d'une nouvelle mutation du gène MYH9

INTRODUCTION

Familial macrothrombocytopenias are a group of rare autosomal dominant platelet disorders including many syndromes in particular the May-Hegglin anomaly. They are characterized by thrombocytopenia with giant platelets and in some cases neutrophilic inclusions in peripheral blood granulocytes. Recently these different clinical entities have been demonstrated to be linked to mutations in the same gene, MYH9.

CASE REPORT

We report in a young African woman presenting as a May-Hegglin anomaly a new mutation of the MYH9 gene. In regard of this case we present a brief review of the MYH9 syndrome.

CONCLUSION

The MYH9 syndrome includes now several clinical entities who share some common clinical and biological characteristics such as a thrombocytopenia with giant platelets, presence or absence of other manifestations including Dohle like bodies, nephritis, sensorineural hearing loss, cataract. We report a new case in which a new mutation of the MYH9 gene was evidenced.

Congenital macrothrombocytopenias

Congenital macrothrombocytopenias comprise a heterogeneous group of rare disorders, characterized by abnormal giant platelets, thrombocytopenia and bleeding tendency with variable severity. Many of these disorders share common clinical and laboratory features, making accurate diagnosis difficult and patients are often misdiagnosed with and treated for idiopathic thrombocytopenic purpura. Recent progress in the elucidation of underlying defects and further developments of specific diagnostic techniques for several congenital macrothrombocytopenias have renewed our approach to the classification and the diagnosis of the disease. This review summarizes the current knowledge on the clinical and laboratory features of common congenital macrothrombocytopenias and discusses how that knowledge aids in making a proper diagnosis.