Identification of six novel MYH9 mutations and genotype-phenotype relationships in autosomal dominant macrothrombocytopenia with leukocyte inclusions

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.

An inherited macrothrombocytopenic disorder with abnormal large granules

Inherited giant platelet syndromes are a heterogeneous group of rare bleeding disorders. In the current study, a patient was reported with prolonged bleeding time, thrombocytopenia and giant platelets. Both the patient's red and white cell counts were normal without morphological abnormalities. The electron microscopy of platelets showed abnormal large electron-dense granules in the cytoplasm with exocytosis being easily observed. Her fathers platelets had the same abnormalities. The expression of glycoprotein (GP)Ib, GPIIb and GPIIIa on the surface of the patient's platelets was normal, and her platelet aggregation in response to ADP and ristocetin was also normal. Immunogold probes combined to monoclonal antibodies against GPIIIa, P-selectin or CD63 could not identify any relationship between the aberrant structures and alpha granules or lysosomes of platelet. Serotonin level was normal in the platelets of the patient, indicating the electron opaque granules are not dense bodies. The morphological abnormalities of the platelets from the patient are clearly distinguishable from other hereditary giant platelet disorders. We propose that the abnormal large granules from the patient's platelets probably represent a novel inherited thrombocytopenic disorder.

Identification of three in-frame deletion mutations in MYH9 disorders suggesting an important hot spot for small rearrangements in MYH9 exon 24

MYH9 disorders include hereditary macrothrombocytopenias with leukocyte inclusion bodies. Among more than 200 genetically confirmed families, the vast majority of cases exhibit single point mutations including substitutions and deletions of the COOH-terminus in the protein-coding sequence of MYH9. Only four in-frame deletions have been reported to date. In the current study, we describe three in-frame deletions including p.E1084del, p.E1066_A1072del and p.G1055_Q1068del, all of which are localized to exon 24. Interestingly, these three deletions were found to induce the diverse clinical manifestations on the non-hematological symptoms, while they equally demonstrated type I staining of inclusion bodies. As a result of these findings, we suggest that exon 24 represents a potential 'hot spot' for unequal homologous recombination, which may generate in-frame deletions in the coiled-coil rod of non-muscle myosin heavy chain-IIA. The exact length and position of these deletions may also determine the severity of the non-hematological manifestations, however does not appear to affect the morphology of the leukocyte inclusion bodies. These findings further our current understanding of the molecular pathogenesis underlying MYH9 disorders.

Clinical and molecular genetic analysis of a family with macrothrombocytopenia and early onset sensorineural hearing loss

A kindred with inherited macrothrombocytopenia (MTCP) and sensorineural hearing loss (SNHL) from Ghent, Belgium was identified. Currently, joint expression of MTCP and hearing loss are linked to mutations within MYH9 only. Thus, we tested the hypothesis that a mutation within MYH9 is responsible for the autosomal dominant inheritance of MTCP and hearing loss in the Ghent family. A mutation screen of MYH9 coding region including its intron-exon junctions, as well as common hearing loss genes GJB2, GJB3, and GJB6, was performed. However, no pathogenic sequence alteration was identified. Patients' leukocytes were determined to be normal for NMMHC-A distribution via immunofluorescence analysis and free of Döhle body-like inclusions, identified as aggregates of mutant NMHC-IIA in MYH9 disorders. Also, western blot analysis with anti-NMHC-IIA antibody identified a single 220 kDa immunoreactive band with normal expression level of NMHC-IIA within the platelets and leukocytes of the affected family members. The immunoblot analysis eliminates the possibility of a large deletion within MYH9 that can escape detection by direct sequencing. Collectively, these results suggest that molecular genetic etiology of the Ghent family disorder may be due to as yet unidentified gene whose mutation(s) yields a phenocopy of the MYH9-related disease.

Identification of the first in cis mutations in MYH9 disorder

Here, we report the first in cis mutations in exon 1 of the MYH9 gene in a patient with MYH9 disorder. The patient was a 5-yr-old girl with macrothrombocytopenia and conspicuous cytoplasmic inclusion bodies in neutrophils. Immunofluorescence analysis of neutrophil non-muscle myosin heavy chain-II A (NMMHC-IIA) indicated several cytoplasmic spots of NMMHC-IIA aggregates that were circular to oval in shape (type II pattern). Mutational analysis showed two mutations, c.99G > T and c.103C > G, which would result in p.W33C and p.P35A, respectively, in exon 1 of the MYH9 gene. In addition, concurrent mutations were present on the same chromosome. Inclusion bodies are usually faint or mostly invisible in MYH9 disorders with a mutation in exon 1. In this case, double mutations might have caused the large myosin protein aggregation and accumulation. Although not observed in this patient, the development of Alport manifestations should be monitored by careful follow-up.

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.

Identification and characterization of the first large deletion of the MYH9 gene associated with MYH9 disorders

MYH9 disorders are autosomal dominant macrothrombocytopenias with leukocyte inclusion bodies. Single point mutations in the protein-coding sequence of the MYH9 gene are the most common cause. So far no large gene deletion/insertion and splicing defects have been reported. Conventional DNA sequencing of each MYH9-coding exon showed no abnormalities in a patient. Reverse transcription- polymerase chain reaction (PCR) amplification and sequencing of neutrophil mRNA identified an inframe deletion of exon 25. Further long-range PCR amplification of genomic DNA revealed a deletion of 1220 nucleotides including entire exon 25. Immunoblot analysis showed a small, abnormal protein in neutrophils but not in platelets. This is the first report of a large deletion of the MYH9 gene leading to the development of MYH9 disorders.

Differential expression of wild-type and mutant NMMHC-IIA polypeptides in blood cells suggests cell-specific regulation mechanisms in MYH9 disorders

MYH9 disorders such as May-Hegglin anomaly are characterized by macrothrombocytopenia and cytoplasmic granulocyte inclusion bodies that result from mutations in MYH9, the gene for nonmuscle myosin heavy chain-IIA (NMMHC-IIA). We examined the expression of mutant NMMHC-IIA polypeptide in peripheral blood cells from patients with MYH9 5770delG and 5818delG mutations. A specific antibody to mutant NMMHC-IIA (NT629) was raised against the abnormal carboxyl-terminal residues generated by 5818delG. NT629 reacted to recombinant 5818delG NMMHC-IIA but not to wild-type NMMHC-IIA, and did not recognize any cellular components of normal peripheral blood cells. Immunofluorescence and immunoblotting revealed that mutant NMMHC-IIA was present and sequestrated only in inclusion bodies within neutrophils, diffusely distributed throughout lymphocyte cytoplasm, sparsely localized on a diffuse cytoplasmic background in monocytes, and uniformly distributed at diminished levels only in large platelets. Mutant NMMHC-IIA did not translocate to lamellipodia in surface activated platelets. Wild-type NMMHC-IIA was homogeneously distributed among megakaryocytes derived from the peripheral blood CD34(+) cells of patients, but coarse mutant NMMHC-IIA was heterogeneously scattered without abnormal aggregates in the cytoplasm. We show the differential expression of mutant NMMHC-IIA and postulate that cell-specific regulation mechanisms function in MYH9 disorders.

Perioperative management of MYH9 hereditary macrothrombocytopenia (Fechtner syndrome)

OBJECTIVE

Hereditary thrombocytopenias characterized by mutations in the gene for non-muscle myosin heavy chain IIA (NMMHC-IIA) are known as MYH9-related hereditary macrothrombocytopenia, and include the May-Hegglin anomaly, Sebastian platelet syndrome, Fechtner syndrome, and Epstein syndrome. Despite the presence of thrombocytopenia, these patients often have only mild or non-bleeding phenotypes. A major risk for these patients can be inappropriate treatment with long-term corticosteroids or splenectomy for misdiagnosed chronic autoimmune thrombocytopenia, as well as inadequate peri- and postoperative management.

METHODS

Using the case of a 44-yr-old male with Fechtner syndrome (macrothrombocytopenia, leukocyte inclusions, sensorineural deafness, glomerulonephritis) who underwent neurosurgery for an intracerebral arteriovenous malformation, we describe current methods to diagnose hereditary MYH9-related macrothombocytopenia by analysis of the blood smear, immunofluorescence staining of the NMMHC-IIA in leucocytes, and by MYH9-gene sequencing.

RESULTS

Clusters of NMMHC-IIA in granulocytes and a R1165C mutation in the MYH9-gene in two macrothrombocytopenic family members confirmed the diagnosis of a MYH9-related disease. The patient had no bleeding diathesis by history or physical examination. Thus no perioperative prohemostatic pharmacologic therapies or transfusions were given, with only minimal bleeding observed. Postoperative antithrombotic thromboprophylaxis was not given because of anticipated enhanced risk for bleeding. However, the patient developed symptomatic pulmonary embolism on postoperative day 6, which was successfully managed with 8 months of anticoagulation.

CONCLUSION

MYH9-related hereditary macrothrombocytopenia does not necessarily protect against postoperative venous thromboembolism, and affected patients who do not evince bleeding diathesis should be considered for routine postoperative pharmacologic thromboprophylaxis.

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.

Acute myeloid leukemia in a child with hereditary thrombocytopenia

A child with a known diagnosis of an autosomal dominant macrothrombocytopenia, Fechtner Syndrome, developed acute myeloid leukemia (AML). Recently the disease gene for the inherited macrothrombocytopenias has been identified as MYH9, encoding for non-muscle myosin heavy chain-A. MYH9 has never been associated with the development of acute leukemia, but MYH11 is disrupted in the M4 eosinophilia sub-type of AML (inv16). The patients leukemic blasts did carry the common t(8;21) which yields an AML1-ETO fusion protein that inhibits AML-1. Despite his thrombocytopenia, the patient successfully completed intensive bone marrow cytoreduction without significant bleeding complications and is now in remission for over 3 years.

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.

Analysis of clinical manifestations, mutant gene and encoded protein in two Chinese MYH9-related disease families

BACKGROUND

MYH9-related disease is a rare autosomal dominant disorder characterized by the triad of giant platelet, thrombocytopenia and inclusion bodies in neutrophil. In recent years, much progress has been made in the investigation of its clinical feature and pathogenesis.

METHODS

Clinical manifestations were analyzed in two Chinese MYH9-related disease families. Polymerase chain reaction (PCR), DNA sequencing and CpoI restrictive endonuclease map analysis were used to identify spot mutation in nonmuscle myosin heavy chain 9 (MYH9) gene. Indirect immunofluence combined propidium iodine (PI) nuclei count-staining technology was applied to probe nonmuscle myosin heavy chain IIA (NMMHC-A) in MYH9-related disease neutrophils and platelets. Western blot was undergone to examine the expression of NMMHC-A in MYH9-related disease patients.

RESULTS

All of the patients manifested with the typical triad, mild to moderate bleeding tendency were their common clinical feature, some patients were accompanied by renal lesion. G5521A mutation in MYH9 gene was identified in both families. Spindle-like inclusions with yellow fluorescence in MYH9-related disease neutrophils were clearly revealed by indirect immunofluence combined PI nuclei count-staining technology, which matched very well with the inclusions, detected by Wright-Giemsa's stain. An upregulation of NMMHC-A in MYH9-related disease neutrophils was observed by Western blotting analysis.

CONCLUSION

Mutation of MYH9 gene exists in cases of Chinese MYH9-related disease. In the two families, the point mutation was located in exon 38(G5521A), and the transference rule of the MYH9 gene mutation is corresponding with clinical phenotype distribution. Indirect immunofluorescence combining with PI nuclei staining technology is sensitive and more specific than Wright-Giemsa's staining in detecting MYH9-related disease inclusions, with which we might easily distinguish MYH9-related disease inclusions from infection-associated inclusions. The expression of the NMMHC-A in MYH9-related disease neutrophils was upregulated than normal control.

Absence of hearing loss in a mouse model for DFNA17 and MYH9-related disease: the use of public gene-targeted ES cell resources

Multiple mouse embryonic stem (ES) cell banks expand the capability to characterize functions of genes implicated in human disease and to develop mouse models for the further understanding of disease pathology. Genetic diseases that result in hearing loss can provide insight into causative molecular mechanisms for deafness. We utilized BayGenomics, the public mouse ES cell bank, to identify gene-trapped ES cell lines associated with hearing loss. We identified two gene-trapped ES cell lines specific for the non-muscle myosin heavy chain class IIA or myosin heavy chain IX (Myh9). Inherited mutations in the Myh9 gene have been linked to non-syndromic hereditary hearing impairment DFNA17 as well as 'MYH9-related disease' characterized by macrothrombocytopenia, leukocyte inclusions, and in some patients deafness. Mutant Myh9 mice were derived from one of these ES cell lines that underwent germline transmission for in-depth otological examination. No homozygous mice however were identified at birth, consistent with recently published data describing the embryonic lethality of homozygous mutations in Myh9. We provide evidence that adult heterozygous Myh9 mouse inner ears contain half wild-type levels of Myh9 mRNA. Hearing loss however was not observed in heterozygous Myh9 mice in contrast to human Myh9-related diseases. Aged heterozygous Myh9 mice also did not show signs of cochleosaccular degeneration common in DFNA17. Although inheritance of Myh9 mutations in humans is dominant, we conclude that heterozygous loss of Myh9 is not critical to hearing function in mice by itself.

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.

Genotype-phenotype correlation in MYH9-related thrombocytopenia

Mutation of the non-muscle myosin heavy chain type II-A results in MYH9-related hereditary macrothrombocytopenia (HMTC), including four autosomal dominant platelet disorders: May-Hegglin anomaly (MHA), Sebastian (SBS), Fechtner (FS) and Epstein (EPS) syndrome. Denaturing high-performance liquid chromatography (DHPLC) was optimised for rapid screening of the seven exons harbouring all but one of the previously reported mutations of MYH9. Individuals from 13 families with phenotypes suggestive of MYH9-related HMTC were screened for mutations by DHPLC followed by direct sequencing of samples with aberrant column retention time. Mutations were identified in all 13 families. Six distinct missense heterozygous mutations were found in 10 families, including six families with MHA or SBS (E1841K, D1424N), three families with FS (R702H, R1165C, and D1424Y), and one family with EPS (S96L). A truncating mutation (R1933X) was found in three MHA families. A review of all published mutations suggests that mutation in the C-terminal coiled coil region or truncation of the tailpiece is associated with haematological-only phenotype, while mutation of the head ATPase domain frequently is associated with nephropathy and/or hearing loss. Mutations of other regions have intermediate expression of non-haematological characteristics. Further study is required to confirm these associations and understand the molecular basis for this genotype-phenotype relationship.

Pathogenetic mechanisms of hematological abnormalities of patients with MYH9 mutations

Mutations of MYH9, the gene for non-muscle myosin heavy chain IIA (NMMHC-IIA), cause a complex clinical phenotype characterized by macrothrombocytopenia and granulocyte inclusion bodies, often associated with deafness, cataracts and/or glomerulonephritis. The pathogenetic mechanisms of these defects are either completely unknown or controversial. In particular, it is a matter of debate whether haploinsufficiency or a dominant-negative effect of mutant allele is responsible for hematological abnormalities. We investigated 11 patients from six pedigrees with different MYH9 mutations. We evaluated NMMHC-IIA levels in platelets and granulocytes isolated from peripheral blood and in megakaryocytes (Mks) cultured from circulating progenitors. NMMHC-IIA distribution in Mks and granulocytes was also assessed. We demonstrated that all the investigated patients had a 50% reduction of NMMHC-IIA expression in platelets and that a similar defect was present also in Mks. In subjects with R1933X and E1945X mutations, the whole NMMHC-IIA of platelets and Mks was wild-type. No NMMHC-IIA inclusions were observed at any time of Mk maturation. In granulocytes, the extent of NMMHC-IIA reduction in patients with respect to control cells was significantly greater than that measured in platelets and Mks, and we found that wild-type protein was sequestered within most of the NMMHC-IIA inclusions. Altogether these results indicate that haploinsufficiency of NMMHC-IIA in megakaryocytic lineage is the mechanism of macrothrombocytopenia consequent to MYH9 mutations, whereas in granulocytes a dominant-negative effect of mutant allele is involved in the formation of inclusion bodies. The finding that the same mutations act through different mechanisms in different cells is surprising and requires further investigation.

Altered cytoskeleton organization in platelets from patients with MYH9-related disease

MYH9-related disease (MYH9-RD) is an autosomal dominant disorder deriving from mutations in the MYH9 gene encoding for the heavy chain of non-muscle myosin IIA, and characterized by thrombocytopenia and giant platelets. Isoform IIA of myosin is the only one expressed in platelets, but the possibility that MYH9 mutations affect the organization of contractile structures in these blood elements has never been investigated. In this work we have analyzed the composition and the agonist-induced reorganization of the platelet cytoskeleton from seven MYH9-RD patients belonging to four different families. We found that an increased amount of myosin was constitutively associated with actin in the cytoskeleton of resting MYH9-RD platelets. Upon platelet stimulation, an impaired increase in the total cytoskeletal proteins was observed. Moreover, selected membrane glycoproteins, tyrosine kinases, and small GTPases failed to interact with the cytoskeleton in agonist-stimulated MYH9-RD platelets. These results demonstrate for the first time that mutations of MYH9 result in an alteration of the composition and agonist-induced reorganization of the platelet cytoskeleton. We suggest that these abnormalities may represent the biochemical basis for the previously reported functional alterations of MYH9-RD platelets, and for the abnormal platelet formation from megakaryocytes, resulting in thrombocytopenia and giant platelets.

First description of somatic mosaicism in MYH9 disorders

MYH9 disorders are characterized by giant platelets, thrombocytopenia, and Dohle body-like cytoplasmic granulocyte inclusion bodies that result from mutations in MYH9, which encodes non-muscle myosin heavy chain-A (NMMHCA). These disorders are known to be transmitted in an autosomal dominant manner, although about 20% of cases are considered to be sporadic. We report here the first case of a MYH9 disorder because of somatic mosaicism. The patient was the father of a male with typical May-Hegglin anomaly. The father had normal platelet counts, however, both normal-sized and giant platelets were observed on his peripheral blood smears. In addition, 14% of neutrophils contained inclusion bodies and the rest showed a normal morphology. Quantitative fluorescent polymerase chain reaction analysis showed that only 6% of DNA from peripheral blood leucocytes harboured the mutation. The mosaicism was demonstrated at a similar rate in different tissues, buccal mucosa cells and hair bulb cells, implying that the mutation had occurred before gastrulation. Mosaicism might account for some de novo mutations in MYH9 disorders.

Targeted disruption of mouse ortholog of the human MYH9 responsible for macrothrombocytopenia with different organ involvement: hematological, nephrological, and otological studies of heterozygous KO mice

Among three different isoforms of non-muscle myosin heavy chains (NMMHCs), only NMMHCA is associated with inherited human disease, called MYH9 disorders, characterized by macrothrombocytopenia and characteristic granulocyte inclusions. Here targeted gene disruption was performed to understand fundamental as well as pathological role of the gene for NMMHCA, MYH9. Heterozygous intercrosses yielded no homozygous animals among 552 births, suggesting that MYH9 expression is required for embryonic development. In contrast, MYH9+/- mice were viable and fertile without gross anatomical, hematological, and nephrological abnormalities. Immunofluorescence analysis also showed the normal cytoplasmic distribution of NMMHCA. We further measured the auditory brainstem response and found two of six MYH9+/- mice had hearing losses, whereas the remaining four were comparable to wild-type mice. Such observation may parallel the diverse expression of Alport's manifestations of human individuals with MYH9 disorders and suggest the limited requirement of the gene for maintenance and function of specific organs.

Detection of unique neutrophil non-muscle myosin heavy chain-A localization by immunofluorescence analysis in MYH9 disorder presented with macrothrombocytopenia without leukocyte inclusions and deafness

MYH9 disorders are autosomal-dominant macrothrombocytopenias with leukocyte inclusions caused by mutations in the MYH9 gene, which encodes the non-muscle myosin heavy chain-A (NMMHCA). We report a patient with an MYH9 disorder who presented with macrothrombocytopenia without leukocyte inclusions and severe bilateral sensory deafness. Conventional May-Grunwald-Giemsa staining failed to detect granulocyte cytoplasmic inclusions, whereas immunofluorescence analysis clearly demonstrated abnormal neutrophil NMMHCA localization. Genetic analyses revealed a novel heterozygous 18 base deletion in MYH9, leading to a six-amino acid in-frame deletion (N76_S81del) in NMMHCA. These results further support the usefulness of immunofluorescence analysis in differential diagnosis of MYH9 disorders.

A Unique Immunofluorescence Method Promotes Accurate Diagnosis in MYH9 Disorders: A Case Report

The identification of a mutation in the MYH9 gene in hereditary macrothrombocytopenia has established a distinct entity proposed as "MYH9 disorders," which previously have often been misdiagnosed as chronic immune thrombocytopenic purpura. The authors describe clinical and laboratory characterization of a family with the disorder demonstrating giant platelets, thrombocytopenia, and leukocyte inclusion bodies. The authors emphasize the efficacy of a unique immunofluorescence method for the nonmuscle myosin heavy chain A in the diagnosis, because it is more sensitive than May-Grünwald-Giemsa staining and more practical than electron microscopy or direct sequencing. MYH9 disorders may be much more common than previously realized if accurately diagnosed.

Rod mutations associated with MYH9-related disorders disrupt nonmuscle myosin-IIA assembly

MYH9-related disorders are autosomal dominant syndromes, variably affecting platelet formation, hearing, and kidney function, and result from mutations in the human nonmuscle myosin-IIA heavy chain gene. To understand the mechanisms by which mutations in the rod region disrupt nonmuscle myosin-IIA function, we examined the in vitro behavior of 4 common mutant forms of the rod (R1165C, D1424N, E1841K, and R1933Stop) compared with wild type. We used negative-stain electron microscopy to analyze paracrystal morphology, a model system for the assembly of individual myosin-II molecules into bipolar filaments. Wild-type tail fragments formed ordered paracrystal arrays, whereas mutants formed aberrant aggregates. In mixing experiments, the mutants act dominantly to interfere with the proper assembly of wild type. Using circular dichroism, we find that 2 mutants affect the alpha-helical coiled-coil structure of individual molecules, and 2 mutants disrupt the lateral associations among individual molecules necessary to form higher-order assemblies, helping explain the dominant effects of these mutants. These results demonstrate that the most common mutations in MYH9, lesions in the rod, cause defects in nonmuscle myosin-IIA assembly. Further, the application of these methods to biochemically characterize rod mutations could be extended to other myosins responsible for disease.

Correlation between the clinical phenotype of MYH9 -related disease and tissue distribution of class II nonmuscle myosin heavy chains

Nonmuscle myosin heavy chain II-A is responsible for MYH9-related disease, which is characterized by macrothrombocytopenia, granulocyte inclusions, deafness, cataracts, and renal failure. Since another two highly conserved nonmuscle myosins, II-B and II-C, are known, an analysis of their tissue distribution is fundamental for the understanding of their biological roles. In mouse, we found that all forms are ubiquitously expressed. However, megakaryocytic and granulocytic lineages express only II-A, suggesting that congenital features, macrothrombocytopenia, and leukocyte inclusions correlate with its exclusive presence. In kidney, eye, and ear, where clinical manifestations have a late onset, as well as in other tissues apparently not affected in patients, II-A and at least one of the other two isoforms are expressed, suggesting that II-B and II-C can partially compensate for each other. We hypothesize that cells expressing only II-A manifest the congenital defects, while tissues expressing additional myosin II isoforms show either late onset of abnormalities or no pathological sign.

Inherited platelet disorders

Inherited platelet disorders are important causes of bleeding that can quantitatively and qualitatively alter platelets, impairing their function. The purpose of this review is to summarize current knowledge on the different types of inherited platelet disorders, their clinical and laboratory features, molecular genetic causes, and the therapies used in clinical practice to manage these conditions.

Asp1424Asn MYH9 mutation results in an unstable protein responsible for the phenotypes in May-Hegglin anomaly/Fechtner syndrome

May-Hegglin anomaly (MHA), Fechtner syndrome (FTNS), Sebastian syndrome (SBS), and Epstein syndrome (EPS) are a group of rare, autosomal dominant disorders characterized by thrombocytopenia, giant platelets, and Döhle-like inclusion bodies, together with variable manifestations of Alport-like symptoms that include high-tone sensorineural deafness, cataracts, and nephritis. These disorders result from mutations in the MYH9 gene, which encodes for the nonmuscle myosin heavy chain A protein (also known as NMMHC-A). To date 20 different mutations have been characterized for this gene, but no clear phenotype-genotype correlation has been established, and very little is known regarding the molecular pathogenesis of this group of diseases. Here, we describe 2 new families with MHA/FTNS phenotypes that have been characterized in terms of their mutations, protein localization in megakaryocytes, protein expression, and mRNA stability. Our findings suggest that, at least for the Asp1424Asn mutation in the MYH9 gene, the phenotypes result from a highly unstable protein. No abnormalities in protein localization or mRNA stability were observed. We hypothesize that haploinsufficiency of the MYH9 results in a failure to properly reorganize the cytoskeleton in megakaryocytes as required for efficient platelet production.

MYH9-related disease: May-Hegglin anomaly, Sebastian syndrome, Fechtner syndrome, and Epstein syndrome are not distinct entities but represent a variable expression of a single illness

May-Hegglin anomaly, Sebastian syndrome, Fechtner syndrome, and Epstein syndrome are autosomal dominant macrothrombocytopenias distinguished by different combinations of clinical and laboratory signs, such as sensorineural hearing loss, cataract, nephritis, and polymorphonuclear Döhle-like bodies. Mutations in the MYH9 gene encoding for the nonmuscle myosin heavy chain IIA (NMMHC-IIA) have been identified in all these syndromes. To understand the role of the MYH9 mutations, we report the molecular defects in 12 new cases, which together with our previous works represent a cohort of 19 families. Since no genotype-phenotype correlation was established, we performed an accurate clinical and biochemical re-evaluation of patients. In addition to macrothrombocytopenia, an abnormal distribution of NMMHC-IIA within leukocytes was observed in all individuals, including those without Döhle-like bodies. Selective, high-tone hearing deficiency and cataract was diagnosed in 83% and 23%, respectively, of patients initially referred as having May-Hegglin anomaly or Sebastian syndrome. Kidney abnormalities, such as hematuria and proteinuria, affected not only patients referred as Fechtner syndrome and Epstein syndrome but also those referred as May-Hegglin anomaly and Sebastian syndrome. These findings allowed us to conclude that May-Hegglin anomaly, Sebastian syndrome, Fechtner syndrome, and Epstein syndrome are not distinct entities but rather a single disorder with a continuous clinical spectrum varying from mild macrothrombocytopenia with leukocyte inclusions to a severe form complicated by hearing loss, cataracts, and renal failure. For this new nosologic entity, we propose the term "MHY9-related disease," which better interprets the recent knowledge in this field and identifies all patients at risk of developing renal, hearing, or visual defects.

Macrothrombocytopenia and progressive deafness is due to a mutation in MYH9

BACKGROUND

In 1992, a family with hereditary macrothrombocytopenia and progressive sensorineural hearing impairment without renal dysfunction was described. Recently, mutations in MYH9, a nonmuscle myosin heavy chain, have been found in several forms of hereditary macrothrombocytopenia.

HYPOTHESIS

The hereditary macrothrombocytopenia and hearing loss in the previously reported family is due to a mutation in MYH9 gene.

METHODS

Genomic DNA was extracted from the affected proband. Mutation screening of all MYH9 coding exons was carried out using denaturing high-performance liquid chromatography. Abnormal results were followed by direct sequencing of the exon and comparison of the sequence with the normal MYH9 sequence.

RESULTS

The results of denaturing high-performance liquid chromatography suggested a potential sequence alteration in exon 30 of MYH9. Direct sequence analysis of this exon in the affected individual identified a G to A single base pair transition at nucleotide 4270 altering codon 1424. This mutations leads to an amino acid change from aspartate (D) to asparagine (N) in the highly conserved coiled-coil domain.

CONCLUSIONS

A single base pair transition in MYH9, resulting in an amino acid substitution D1424N, is responsible for macrothrombocytopenia and hearing loss in the kindred under study. The presence of hearing impairment and the absence of renal symptoms, as reported in other families with the same mutation MYH9, further highlights the role of genetic background in expression and modification of the affected phenotype.

Immunofluorescence analysis of neutrophil nonmuscle myosin heavy chain-A in MYH9 disorders: association of subcellular localization with MYH9 mutations

The autosomal dominant macrothrombocytopenia with leukocyte inclusions, May-Hegglin anomaly, Sebastian syndrome, and Fechtner syndrome, are rare human disorders characterized by a triad of giant platelets, thrombocytopenia, and characteristic Döhle body-like cytoplasmic inclusions in granulocytes. Epstein syndrome is another autosomal dominant macrothrombocytopenia associated with Alport syndrome but without leukocyte inclusions. These disorders are caused by mutations in the same gene, the MYH9, which encodes the nonmuscle myosin heavy chain-A (NMMHCA). The term, MYH9 disorders, has been proposed, but the clinicopathologic basis of MYH9 mutations has been poorly investigated. In this study, a total of 24 cases with MYH9 disorders and suspected cases were subjected to immunofluorescence analysis by a polyclonal antibody against human platelet NMMHCA. Abnormal subcellular localization of NMMHCA was observed in every neutrophil from individuals with MYH9 mutations. Comparison with May-Grünwald-Giemsa staining revealed that the NMMHCA always coexisted with the neutrophil inclusion bodies, suggesting that NMMHCA is associated with such bodies. In three cases, neutrophil inclusions were not detected on conventional May-Grünwald-Giemsa-stained blood smears but immunofluorescence analysis revealed the abnormal NMMHCA localization. In contrast, cases with Epstein syndrome and the isolated macrothrombocytopenia with normal NMMHCA localization had no MYH9 mutations. An antibody that recognizes the C-terminal 12 mer peptides showed similar immunoreactivity from the patients heterozygous for truncated mutations that abolished the C-terminal epitope, suggesting that normal NMMHCA dimerizes with abnormal NMMHCA to form inclusion bodies. We further propose that the localization pattern can be classified into three groups according to the number, size, and shape of the fluorescence-labeled NMMHCA granule. Immunofluorescence analysis of neutrophil NMMHCA is useful as a screening test for the clear hematopathologic classification of MYH9 disorders.

Mutations in human nonmuscle myosin IIA found in patients with May-Hegglin anomaly and Fechtner syndrome result in impaired enzymatic function

A family of autosomal-dominant diseases including May-Hegglin anomaly, Fechtner syndrome, Sebastian syndrome, Alport syndrome, and Epstein syndrome are commonly characterized by giant platelets and thrombocytopenia. In addition, there may be leukocyte inclusions, deafness, cataracts, and nephritis, depending on the syndrome. Mutations in the human nonmuscle myosin IIA heavy chain gene (MYH9) have been linked to these diseases. Two of the recently described mutations, N93K and R702C, are conserved in smooth and nonmuscle myosins from vertebrates and lie in the head domain of myosin. Interestingly, the two mutations lie within close proximity in the three-dimensional structure of myosin. These two mutations were engineered into a heavy meromyosin-like recombinant fragment of nonmuscle myosin IIA, which was expressed in baculovirus along with the appropriate light chains. The R702C mutant displays 25% of the maximal MgATPase activity of wild type heavy meromyosin and moves actin filaments at half the wild type rate. The effects of the N93K mutation are more dramatic. This heavy meromyosin has only 4% of the maximal MgATPase activity of wild type and does not translocate actin filaments in an in vitro motility assay. Biochemical characterization of the mutant is consistent with this mutant being unable to fully adopt the "on" conformation.