Gray Platelet Syndrome

A number sign (#) is used with this entry because gray platelet syndrome (GPS) is caused by homozygous or compound heterozygous mutation in the NBEAL2 gene (614169) on chromosome 3p21.

Description

The gray platelet syndrome (GPS) is a rare inherited disorder characterized by mild to moderate bleeding tendency, moderate thrombocytopenia, and a marked decrease or absence of platelet alpha-granules and of the proteins contained in alpha-granules. The platelets are enlarged, but not giant, and have a gray appearance on light microscopy of Wright-stained peripheral blood smears due to decreased granules. Many patients with gray platelet syndrome develop a stable myelofibrosis (summary by Nurden and Nurden, 2007).

Cases suggesting autosomal dominant and autosomal recessive inheritance have been described, indicating that GPS is probably a genetically heterogeneous disorder with more than one molecular cause.

Clinical Features

Raccuglia (1971) provided the first report of gray platelet syndrome, which they described as a qualitative defect in platelets (Gerrard et al., 1980). The boy had petechiae and ecchymoses as a newborn, and a bruising tendency throughout childhood. The platelet count was mildly decreased compared to normal, but increased after splenectomy at age 9 years. Peripheral blood platelets were relatively large, vacuolated, and almost devoid of cytoplasmic granulation. By electron microscopy of this patient's platelets, White (1979) showed that they contained normal numbers of mitochondria, dense bodies, peroxisomes, and lysosomes, but specifically lacked alpha-granules.

Gerrard et al. (1980) reported a girl with gray platelet syndrome who also had Goldenhar syndrome (164210), a probable coincidence. She developed a recurrent petechial rash and bruising tendency at about 8 months of age associated with a low-normal platelet count. Ultrastructural analysis of the platelets from the girl and the patient reported by Raccuglia (1971) showed decreased alpha-granules (less than 15% of controls) as well as deficiencies of alpha-granule proteins, including beta-thromboglobulin (see PPBP, 121010), platelet-derived growth factor (PDGFB; 190040), fibrinogen (FGA; 134820), platelet factor-4 (PF4; 173460), and thrombospondin (THBS1; 188060). Functional studies showed a deficiency in platelet aggregation in response to collagen and thrombin, but normal response to arachidonic acid and ristocetin. Alpha-granules were distinct from dense granules and lysosomes. Gerrard et al. (1980) postulated a defect in the assembly or binding of certain proteins into the alpha-granules.

Berrebi et al. (1988) reported a 68-year-old man of Ashkenazi Jewish origin with an 18-year history of thrombocytopenia and mild splenomegaly. Bleeding episodes consisted of ecchymoses and postsurgical bleeding. Peripheral blood smear showed large agranular gray platelets, and electron microscopy showed absence of alpha-granules. Bone marrow biopsy showed normal numbers of small megakaryocytes. Platelet-derived thrombospondin and fibronectin (FN1; 135600) were absent. A 4-year follow-up showed no changes and no evidence of a myeloproliferative disorder, suggesting that patients with GPS may have long survival.

Lutz et al. (1992) estimated that 40 cases of GPS had been reported. They reported a 4-year-old boy who presented with frequent ecchymoses. The bleeding time was long, and platelets on blood smears appeared gray. Stimulated platelets did not release factor-4 or beta-thromboglobulin.

Jantunen et al. (1994) reported a patient with GPS and splenomegaly, indicating extramedullary hematopoiesis, who developed myelofibrosis. He had normalization of the platelet count after splenectomy, but the bleeding tendency continued. The myelofibrosis remained nonprogressive after more than 15 years of follow-up.

Alkhairy (1995) described the gray platelet syndrome in a teenager with menorrhagia and thrombocytopenia from menarche at 11 years. She had a history of easy bruising and epistaxis since childhood. Three of her 5 sibs, all male, had a bleeding tendency. All 3 bled for 24 hours after hospital-based circumcision and required multiple ligations to achieve hemostasis. All 4 affected sibs displayed typical morphology of gray platelet syndrome. The parents were first cousins of Palestinian origin with normal hematologic parameters. This family raised the possibility of a recessive form of the disorder, although gonadal mosaicism for a dominant gene defect could not be excluded.

Gunay-Aygun et al. (2010) reported 21 patients with GPS from 14 families. The diagnosis was based on electron microscopic evidence of the lack of alpha-granules in platelets. Peripheral smears showed typical large, pale gray platelets, with normal granulocytes. Most patients had bleeding symptoms from early childhood, with a mean age of onset of 2.6 years (range, birth to 6 years). Of 19 patients, bleeding severity was variable: 7 (37%) had mild, 4 (21%) moderate, and 8 (42%) severe bleeding. Of the 8 patients with severe bleeding, 7 were females in whom menorrhagia and resultant severe anemia required repeated blood transfusions, resulting in death in 2. Two males had no bleeding symptoms. Five patients had mild symptoms in the form of bruising and epistaxis. Most patients required supplemental iron, perioperative and peripartum DDVAP, or platelet transfusions. Thrombocytopenia was variable but increased with age, and 7 of 8 patients showed variable degrees of myelofibrosis on bone marrow biopsy, which also progressed with age. Platelet aggregation responses to most agents were normal. Twelve of 13 patients studied showed markedly increased serum vitamin B12.

Clinical Variability

Drouin et al. (2001) described 3 sibs from normal parents with classic platelet abnormalities of GPS. In addition to gray platelets on blood smear, there were also gray polymorphonuclear neutrophils with decreased or abnormally distributed components of secretory compartments. Secondary granules were also decreased in number as assayed by immunoelectron microscopy, thus confirming that the secretory compartments in neutrophils were also deficient in this family.

Nurden et al. (2004) reported a 55-year-old woman, born of consanguineous parents, with a lifelong bleeding disorder manifest as epistaxis, bleeding at menarche and during delivery, and hemorrhage during surgery. She had an enlarged spleen and had also developed a deep vein thrombosis. Laboratory studies showed thrombocytopenia and enlarged gray platelets. She had 2 unaffected sons, indicating an autosomal recessive condition. Functional studies showed normal agglutination with arachidonic acid and ristocetin, but no aggregation with various forms of collagen. Flow cytometry, immunostaining, and Western blot analysis showed a deficiency of platelet glycoprotein VI (GP6; 605546). Residual GP6 protein appeared normal, and there were no mutations in the GP6 gene, suggesting normal synthesis. Expression of the Fc receptor gamma-chain (FCER1G; 147139) was decreased, but downstream signaling appeared normal. Nurden et al. (2004) suggested that GPS is a heterogeneous syndrome and that GP6 deficiency may represent a specific subgroup of the disorder. In the patient reported by Nurden et al. (2004), Nurden et al. (2008) also found decreased levels of platelet membrane TLT1 (TREML1; 609714), which is located in the membrane of alpha-granules, and decreased levels of P-selectin (SELP; 173610). The authors postulated that excess activity of metalloproteases may have caused the decrease in select proteins. In contrast, an unrelated patient with GPS who had relatively normal collagen-induced platelet aggregation, had normal levels of GP6, TLT1, and P-selectin, suggesting biochemical, phenotypic, and molecular heterogeneity in GPS. The second patient was from the Manouche Gypsy tribe in France, and the disorder was consistent with autosomal dominant inheritance.

Biochemical Features

Nurden et al. (1982) studied a brother and sister with GPS. Detailed analysis of platelets showed decreased concentrations of alpha-granule proteins.

The Weibel-Palade bodies (WPB) of endothelial cells, like the alpha-granules of megakaryocytes and platelets, contain von Willebrand factor (VWF; 613160) and P-selectin (SELP; 173610). Gebrane-Younes et al. (1993) undertook an ultrastructural study of endothelial cells in the dermis capillary network by transmission electron microscopy in 2 patients with the gray platelet syndrome. They demonstrated normal WPB containing both VWF and P-selectin with a normal intracellular distribution in endothelial cells. The authors concluded that these proteins are normally synthesized, but improperly targeted to the alpha-granules. The fundamental targeting defect in the gray platelet syndrome appeared to be specific to the megakaryocytic cell line.

Inheritance

In Japan, Mori et al. (1984) found 24 affected patients in a single family. There was at least 1 instance of male-to-male transmission, consistent with autosomal dominant inheritance. Mori et al. (1984) suggested that the families reported by Crowell and Eisner (1972) and Chesney et al. (1974) may have the same disorder. Inheritance was autosomal dominant in both families.

Alkhairy (1995) reported the gray platelet syndrome in 4 sibs from a consanguineous Palestinian Arab family, suggesting possible autosomal recessive inheritance.

Gunay-Aygun et al. (2010) reported 14 families with GPS, including 11 with clear autosomal recessive inheritance, as evidenced by consanguinity or multiple affected sibs with unaffected parents. The families had various backgrounds, including Bedouin, Turkish, Mennonite, French, German, Somali, African American, and mixed Northern and Southern European.

Mapping

By genomewide linkage analysis of 6 families with autosomal recessive GPS, followed by fine mapping in 12 families, Gunay-Aygun et al. (2010) refined the GPS locus to a 9.4-Mb region on chromosome 3p22.1-3p21.1 (chr3:42,663,630 to 52,036,954, NCBI36) with a combined lod score of 11.3. Exome sequencing of 1,423 (69%) of the 2,075 exons in the candidate region did not identify any pathogenic variants, and the remaining 652 exons could not be amplified or sequenced despite several attempts.

Molecular Genetics

Simultaneously and independently, Gunay-Aygun et al. (2011), Albers et al. (2011), and Kahr et al. (2011) identified biallelic mutations in the NBEAL2 gene in patients with gray platelet syndrome. Using exome sequencing of the 3p21 locus, Gunay-Aygun et al. (2011) identified homozygous or compound heterozygous mutations in the NBEAL2 gene (see, e.g., 614169.0001-614169.0003) in 15 unrelated patients with the disorder who had been linked to the 3p21 locus (Gunay-Aygun et al., 2010). There were 5 missense, 3 nonsense, 4 frameshift, and 3 splice site mutations. The mutations were not found in 629 control genomes or 100 control individuals. There were no apparent genotype/phenotype correlations. Albers et al. (2011) also used exome sequencing of the 3p21 locus to identify homozygous or compound heterozygous NBEAL2 mutations (see, e.g., 614169.0003-614169.0005) in affected members of 4 families with GPS. The mutations occurred throughout the gene. Kahr et al. (2011) used platelet-derived mRNA to identify the NBEAL2 gene as mutant in a patient with GPS. Sequencing of the gene identified biallelic mutations (see, e.g., 614169.0006 and 614169.0007) in affected individuals of 3 families with the disorder. One family was Pakistani, 1 was Native American, and a third was Native American/Mexican. All the authors noted that NBEAL2 belongs to a family of proteins involved in membrane dynamics and intracellular vesicle trafficking. One such protein, LYST (606897), is mutant in Chediak-Higashi syndrome (CHS; 214500), which is characterized by defects in platelet granules and other lysosome-related organelles. The findings indicated that NBEAL2 is critical for the development of platelet alpha-granules.

Animal Model

Albers et al. (2011) found that knockout of zebrafish Nbeal2 via morpholino oligonucleotides resulted in spontaneous tail bleeding in 41% of embryos. Morpholino knockdown resulted in complete abrogation of thrombocytes, the zebrafish equivalent of platelets, whereas mature erythrocytes were not affected.