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X-linked thrombocytopenia (THC1/XLT)

Written2019-08Jyoti Kumar
Stanford University, Department of Pathology Stanford, CA; kumarj@stanford.edu
Abstract X-linked thrombocytopenia (XLT) is a rare bleeding disorder characterized by isolated thrombocytopenia with small-sized platelets. XLT is a milder clinical variant of Wiskott-Aldrich syndrome (WAS), which is characterized by the clinical triad of eczema, susceptibility to infection and thrombocytopenia, while patients with XLT only present with thrombocytopenia. XLT is caused by a mutation in the WAS gene.

Keyword X-linked thrombocytopenia, THC1, XLT, WAS gene, thrombocytopenia

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Identity

Atlas_Id 10170
Inheritance X-linked recessive with estimated incidence between 1 and 10 per million males worldwide. This condition is rarer among females.

Clinics

Phenotype and clinics The disease presents as a bleeding disorder with easy bruising, mucosal bleeding, such as nosebleeds, and mild to severe anemia. It is less common to have eczema or immunological disturbances as compared to WAS.
Thrombocytopenia associated with small platelet volume is a consistent finding. In most patients the mean platelet volume is half that of normal control subjects. Life-threatening bleeding, including severe oral bleeding, gastrointestinal bleeding, and intracranial hemorrhage, has been reported.
Autoimmune diseases are rare, but have also been reported to occur.
Additionally, there is a milder form of the disorder called intermittent thrombocytopenia where patients have normal platelet production with intermittent episodes of thrombocytopenia.
Differential diagnosis Wiskott-Aldrich syndrome: X-linked thrombocytopenia (XLT) is recognized as a variant of WAS. Both are caused by mutations in WAS. Males suffering from WAS or XLT have a congenital thrombocytopenia from moderate to very severe with small platelets. However, patients with WAS also have a severe immune dysregulation with susceptibility to infections, eczema, or autoimmune phenomena, and are at risk of lymphoproliferative disorders. If the WAS protein is truncated or not expressed, patients are likely to suffer from the severe WAS phenotype. Instead, XLT is typically associated with missense mutations and patients often show expression, even at reduced levels, of normal-sized WAS protein (Balduini 2012).
GATA1-related thrombocytopenias: Germline mutations in the X-linked GATA1 cause two rare GATA1-related thrombocytopenias: dyserythropoietic anemia with thrombocytopenia and X-linked thrombocytopenia with beta-thalassemia (XLTT). In contrast to XLT, both are characterized by dyserythropoiesis and dysmegakaryopoiesis with variable degrees of anemia and macrothrombocytopenia (large platelets) whereas in XLT, platelets are small. Moreover, in XLTT, there is an unbalanced α:β globin chain synthesis manifesting as mild β-thalassemia (Balduini 2012). Thus, GATA1-related thrombocytopenias should be suspected in males with congenital thrombocytopenia, abnormal red cell morphology and a hypercellular bone marrow.
Neoplastic risk In one study, malignancy developed at a median age of 34 years, half of which were hematolymphoid in origin (Albert et al. 2010). Overall, the incidence of malignancies in patients of the XLT phenotype is increased, but lower than that in classic WAS.
Treatment Treatment options are controversial. XLT is typically treated conservatively and do not require standard prophylactic interventions. Patients with declining immune function may benefit from IVIG. Allogeneic hematopoietic stem cell transplantation (HSCT) may be considered on a case by case basis (Albert et al. 2010).
Prognosis XLT patients generally have a benign disease course and excellent long-term survival, and life expectancy is not as significantly affected. However, severe disease-related events such as infections, bleeding, autoimmune diseases, and malignancies can result (Albert et al. 2010).

Genes involved and Proteins

Gene NameWAS
Alias IMD2, THC, thrombocytopenia 1 (X-linked), Wiskott-Aldrich syndrome (eczema-thrombocytopenia), WASP, WASPA, SCNX, THC1
Location Xp11.23
Note Mutations of the WAS gene result in three distinct phenotypes: Wiskott-Aldrich Syndrome (WAS), X-linked thrombocytopenia (XLT), and X-linked neutropenia (XLN).
Protein
Description 502 amino acids; 54 kDa; consists of an N-terminal Ena-VASP homology domain 1 (EVH1), a basic domain, a GTPase binding domain (GBD), polyproline domain and the C-terminal domain comprising of a cluster of verprolin homology (V), central (C) and acidic regions (A) (the VCA domain).
Expression Consitutively expressed in all haematopoietic stem-cell-derived lineages, except in mature red blood cells. WASL (Neural WASP, N-WASP) and WASP family verprolin homologous protein 1 WASF1, WASF2 and (WAVE1, WAVE2 and WAVE3) are more widely expressed.
Localisation Located in the cytoplasmic compartment with highest density along the cell membrane.
Function WAS protein acts as an adaptor to bring together downstream mediators that facilitate ACTR2/ ACTR3 (Arp2/3)-mediated actin polymerization. A lack of WAS results in cytoskeletal defects that compromise multiple aspects of normal cellular activity including proliferation, phagocytosis, immune synapse formation, adhesion and directed migration.
Mutations
Note The XLT phenotype is generally those with missense or splice-site mutations that allow expression of normal-sized mutated protein, often in reduced quantity. Three of the six hotspot mutations (168C>T; 290 C>N/291 G>N and IVS6+5g>a) have been found consistently in WASP-positive patients.
In one study that evaluated 81 affected members form 75 unrelated families, all of the patients with XLT (except one) had missense mutations with 75% having detectable WASp in the peripheral blood, and the majority of patients with classic WAS had null mutations or splice site mutations without expression of WASp. These findings suggest there is a potential association of genotype and phenotype (Liu 2015). Thus, perhaps the clinical phenotype and long-term outcome of patients with WAS are potentially influenced by the effect of these WAS mutations. Patients with missense mutations that allow expression of mutated WASp and those with splice anomalies, which result in generation of multiple products, including normal WASp, present with the attenuated XLT phenotype and show better prognosis (Liu 2015).
Germinal More than 295 unique mutations have been identified.
158 unique WASP gene mutations had been identified in a cohort of 270 unrelated WAS/XLT families. The most common are missense mutations, followed by splice-site mutations, short deletions, and nonsense mutations. Insertions, complex mutations, and large deletions are less frequent. Most deletions and insertions involve fewer than 10 nucleotides and result in frame shifting and early termination of transcription.
Amino acid substitutions are typically located in exons 1-4. Splice-site mutations occur predominantly in the downstream half of the WASP gene (introns 6-12).
Mutations affecting invariant splice sites may result in multiple splicing products, which often include small amounts of normal WASP cDNA.
Six mutational hotspots, defined as occurring in > 2,5% of the population, have been identified. Three of these hotspots represent point mutations within the coding regions, whereas the other three involve splice sites.
Somatic Patients with WAS and XLT have variable clinical findings even within families. This variability could be also associated with somatic mutations that are commonly detected in lymphocytes of WAS patients. Many of these somatic events restore the expression of WASp conferring a selective proliferation and differentiation advantage to affected cells. However, it still unknown whether revertant clones are associated with clinical improvement (Balduini 2012).

Bibliography

X-linked thrombocytopenia (XLT) due to WAS mutations: clinical characteristics, long-term outcome, and treatment options
Albert MH, Bittner TC, Nonoyama S, Notarangelo LD, Burns S, Imai K, Espanol T, Fasth A, Pellier I, Strauss G, Morio T, Gathmann B, Noordzij JG, Fillat C, Hoenig M, Nathrath M, Meindl A, Pagel P, Wintergerst U, Fischer A, Thrasher AJ, Belohradsky BH, Ochs HD
Blood 2010 Apr 22;115(16):3231-8
PMID 20173115
 
Genetics of familial forms of thrombocytopenia
Balduini CL, Savoia A
Hum Genet 2012 Dec;131(12):1821-32
PMID 22886561
 
Clinical course of patients with WASP gene mutations
Imai K, Morio T, Zhu Y, Jin Y, Itoh S, Kajiwara M, Yata J, Mizutani S, Ochs HD, Nonoyama S
Blood 2004 Jan 15;103(2):456-64
PMID 12969986
 
Mutations of the Wiskott-Aldrich Syndrome Protein (WASP): hotspots, effect on transcription, and translation and phenotype/genotype correlation
Jin Y, Mazza C, Christie JR, Giliani S, Fiorini M, Mella P, Gandellini F, Stewart DM, Zhu Q, Nelson DL, Notarangelo LD, Ochs HD
Blood 2004 Dec 15;104(13):4010-9
PMID 15284122
 
Wiskott-Aldrich syndrome/X-linked thrombocytopenia in China: Clinical characteristic and genotype-phenotype correlation
Liu DW, Zhang ZY, Zhao Q, Jiang LP, Liu W, Tu WW, Song WX, Zhao XD
Pediatr Blood Cancer 2015 Sep;62(9):1601-8
PMID 25931402
 
X-linked thrombocytopenia and Wiskott-Aldrich syndrome are allelic diseases with mutations in the WASP gene
Villa A, Notarangelo L, Macchi P, Mantuano E, Cavagni G, Brugnoni D, Strina D, Patrosso MC, Ramenghi U, Sacco MG, et al
Nat Genet 1995 Apr;9(4):414-7
PMID 7795648
 

Citation

This paper should be referenced as such :
Kumar J
X-linked thrombocytopenia (THC1/XLT);
Atlas Genet Cytogenet Oncol Haematol. in press
On line version : http://AtlasGeneticsOncology.org/Tumors/XlinkThrombocytopenID10170.html


External links

OMIM313900
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