Syndromic Thrombocytopenia Research Articles

STRENGTHENING THE EVIDENCE: RAP1B GENE ASSOCIATION WITH SYNDROMIC THROMBOCYTOPENIA

STRENGTHENING THE EVIDENCE: RAP1B GENE ASSOCIATION WITH SYNDROMIC THROMBOCYTOPENIA

Somatic RAP1B gain-of-function variant underlies isolated thrombocytopenia and immunodeficiency.

The ubiquitously expressed small GTPase Ras-related protein 1B (RAP1B) acts as a molecular switch that regulates cell signaling, cytoskeletal remodeling, and cell trafficking and activates integrins in platelets and lymphocytes. The residue G12 in the P-loop is required for the RAP1B-GTPase conformational switch. Heterozygous germline RAP1B variants have been described in patients with syndromic thrombocytopenia. However, the causality and pathophysiological impact remained unexplored. We report a boy with neonatal thrombocytopenia, combined immunodeficiency, neutropenia, and monocytopenia caused by a heterozygous de novo single nucleotide substitution, c.35G>A (p.G12E) in RAP1B. We demonstrate that G12E and the previously described G12V and G60R were gain-of-function variants that increased RAP1B activation, talin recruitment, and integrin activation, thereby modifying late responses such as platelet activation, T cell proliferation, and migration. We show that in our patient, G12E was a somatic variant whose allele frequency decreased over time in the peripheral immune compartment, but remained stable in bone marrow cells, suggesting a differential effect in distinct cell populations. Allogeneic hematopoietic stem cell transplantation fully restored the patient's hemato-immunological phenotype. Our findings define monoallelic RAP1B gain-of-function variants as a cause for constitutive immunodeficiency and thrombocytopenia. The phenotypic spectrum ranged from isolated hematological manifestations in our patient with somatic mosaicism to complex syndromic features in patients with reported germline RAP1B variants.

Adding to the evidence of gene-disease association of RAP1B and syndromic thrombocytopenia.

Syndromic constitutive thrombocytopenia encompasses a heterogeneous group of disorders characterised by quantitative and qualitative defects of platelets while featuring other malformations. Recently, heterozygous, de novo variants in RAP1B were reported in three cases of syndromic thrombocytopenia. Here, we report two additional, unrelated individuals identified retrospectively in our data repository with heterozygous variants in RAP1B: NM_001010942.2(RAP1B):c.35G>A, p.(Gly12Glu) (de novo) and NM_001010942.2(RAP1B):c.178G>A, p.(Gly60Arg). Both individuals had thrombocytopenia, as well as congenital malformations, and neurological, behavioural, and dysmorphic features, in line with previous reports. Our data supports the causal role of monoallelic RAP1B variants that disrupt RAP1B GTPase activity in syndromic congenital thrombocytopenia.

OC 65.5 Platelet-Specific Gene Dysregulation and Intron Retention in a Patient with Syndromic Thrombocytopenia

OC 65.5 Platelet-Specific Gene Dysregulation and Intron Retention in a Patient with Syndromic Thrombocytopenia

Frameshift mutation S368fs in the gene encoding cytoskeletal β-actin leads to ACTB-associated syndromic thrombocytopenia by impairing actin dynamics

Heterozygous dominant mutations in the ubiquitously produced cytoskeletal β–actin isoform lead to a broad range of human disease phenotypes, which are currently classified as three distinct clinical entities termed Baraitser-Winter–Cerebrofrontofacial syndrome (BWCFF), ACTB–associated pleiotropic malformation syndrome with intellectual disability (ACTB–PMSID), and ACTB–associated syndromic thrombocytopenia (ACTB–AST). The latter two are distinguishable from BWCFF by the presence of milder craniofacial features and less pronounced developmental abnormalities, or the absence of craniofacial features in combination with a characteristic thrombocytopenia with platelet anisotropy. Production and correct function of β–actin is required for multiple essential processes in all types of cells. Directed cell migration, cytokinesis and morphogenesis are amongst the functions that are supported by β–actin. Here we report the recombinant production and biochemical characterization of the ACTB–AST mutant p.S368fs, resulting in an altered sequence in the C–terminal region of β–actin that includes a replacement of the last 8 residues and an elongation of the molecule by 4 residues. The mutation affects a region important for actin polymerization and actin–profilin interaction. Accordingly, we measured markedly reduced rates of nucleation and polymerization during spontaneous actin assembly and lower affinity of p.S368fs for human profilin–1. The reduced affinity is also reflected in the lower propensity of profilin–1 to extend the nucleation phase of p.S368fs. While localized in close proximity to actin–cofilin and actin–myosin interfaces, we determined only minor effects of the mutation on the interaction of mutant filaments with cofilin and myosin family members. However, allosteric effects on sites distant from the mutation manifest themselves in a 7.9 °C reduction in thermal denaturation temperature, a 2–fold increase in the observed IC50 for DNase–I, and changes in nucleotide exchange kinetics. Our results support a disease mechanism involving impaired actin dynamics and function through disruption of actin–profilin interactions and further exacerbated by allosteric perturbations.

EP190: A third patient with RAP1B-related syndromic thrombocytopenia and novel clinical findings

RAP1B is a member of the RAS family of small GTP binding proteins (GTPases), activated in many signal transduction pathways regulating numerous cellular processes including proliferation, adhesion, and migration. RAP1B-related syndromic thrombocytopenia is an ultra-rare genetic disorder characterized by hematologic abnormalities, neurodevelopmental delays, and congenital birth defects. Initial report of a germline RAP1B variant described a patient with a Kabuki syndrome-like phenotype due to RAP1B loss-of-function.

De novo missense variants in the RAP1B gene identified in two patients with syndromic thrombocytopenia.

We present two independent cases of syndromic thrombocytopenia with multiple malformations, microcephaly, learning difficulties, dysmorphism and other features. Exome sequencing identified two novel de novo heterozygous variants in these patients, c.35G>T p.(Gly12Val) and c.178G>C p.(Gly60Arg), in the RAP1B gene (NM_001010942.2). These variants have not been described previously as germline variants, however functional studies in literature strongly suggest a clinical implication of these two activating hot spot positions. We hypothesize that pathogenic missense variants in the RAP1B gene cause congenital syndromic thrombocytopenia with a spectrum of associated malformations and dysmorphism, possibly through a gain of function mechanism.

Familial leukemia due to germline RUNX1 mutations: lessons learned from two decades of research and unsolved problems

The RUNX1 gene is a critical transcription factor for the generation and maintenance of hematopoietic stem cells. RUNX1 is also one of the most frequently mutated gene in sporadic leukemias. Heterozygous loss-of-function mutations of the RUNX1 gene in the germline cause a rare autosomal dominant disorder called familial platelet disorder with propensity to acute myelogenous leukemia (FPD/AML). Besides the preexisting platelet disorder in FPD/AML patients, AML also develops in 20-60% of affected individuals. Since its discovery by the Gilliland group in 1999, RUNX1 mutation in the germline has been extensively investigated in the field. The past two decades of research have taught us three important lessons: 1) patients with FPD/AML display atypical symptoms and they have a widened clinical spectrum of FPD, such as eczema and syndromic thrombocytopenia, 2) the elucidation of variant of uncertain significance (VUS) of RUNX1 have revealed their role in epigenetic functions and involvement in the Fanconi anemia DNA repair pathway, and 3) non-coding mutations of RUNX1 also causes FPD/AML. In three distinct familial cases, an enhancer for RUNX1, eR1, was either lost or disconnected from the promoter through genetic deletion or chromosomal translocation abnormalities. This experience, with congenital mutations of RUNX1, will be very useful for future research for a series of other leukemia-causing germline mutations that have been recently identified.

Variants in exons 5 and 6 of ACTB cause syndromic thrombocytopenia

Germline mutations in the ubiquitously expressed ACTB, which encodes β-cytoplasmic actin (CYA), are almost exclusively associated with Baraitser-Winter Cerebrofrontofacial syndrome (BWCFF). Here, we report six patients with previously undescribed heterozygous variants clustered in the 3′-coding region of ACTB. Patients present with clinical features distinct from BWCFF, including mild developmental disability, microcephaly, and thrombocytopenia with platelet anisotropy. Using patient-derived fibroblasts, we demonstrate cohort specific changes to β-CYA filament populations, which include the enhanced recruitment of thrombocytopenia-associated actin binding proteins (ABPs). These perturbed interactions are supported by in silico modeling and are validated in disease-relevant thrombocytes. Co-examination of actin and microtubule cytoskeleton constituents in patient-derived megakaryocytes and thrombocytes indicates that these β-CYA mutations inhibit the final stages of platelet maturation by compromising microtubule organization. Our results define an ACTB-associated clinical syndrome with a distinct genotype-phenotype correlation and delineate molecular mechanisms underlying thrombocytopenia in this patient cohort.

Practice patterns in the diagnosis of inherited platelet disorders within a single institution.

: The diagnosis of inherited platelet disorders (IPDs) is challenging with variable diagnostic practices existing between institutions. To determine patterns and utility of diagnostic testing practices for IPDs within a single institution, a retrospective cohort study was performed. Records of 50 patients (50% women), median age 32 years (1 day to 81 years) were analyzed. In total, 28 (53%) had a positive International Society of Thrombosis and Hemostasis Bleeding Assessment Tool score. Test-ordering patterns were highly variable. All patients had platelet morphology analysis by light microscopy. In total, 42 (84%) underwent light transmission aggregometry, 43 (86%) platelet function analyzer, 37 (74%) platelet electron microscopy, 25 (50%) flow cytometry, and 15 (30%) genetic testing. Platelet function analyzer and light transmission aggregometry were always used as first-order tests, followed by platelet transmission electron microscopy and flow cytometry (81 and 84%, respectively). Genetic testing was obtained up front in five cases (33% of orders), mostly in patients with syndromic thrombocytopenia or in the setting of a known genetic disorder. Test-ordering practices did not adhere to published algorithms. Even within a single institution, great heterogeneity exists in the testing approach to IPDs. Although, a large proportion of cases were studied with platelet transmission electron microscopy and flow cytometry, standard platelet assays established the diagnosis in a great majority. Standardization of testing practices, first beginning at the institutional level is a much needed step forward.

Syndromic mental retardation with thrombocytopenia due to 21q22.11q22.12 deletion: Report of three patients

During the last few years, an increasing number of microdeletion/microduplication syndromes have been delineated. This rapid evolution is mainly due to the availability of microarray technology as a routine diagnostic tool. Microdeletions of the 21q22.11q22.12 region encompassing the RUNX1 gene have been reported in nine patients presenting with syndromic thrombocytopenia and mental retardation. RUNX1 gene is responsible for an autosomal dominant platelet disorder with predisposition to acute myelogenous leukemia. We report on three novel patients with an overlapping "de novo" interstitial deletion involving the band 21q22 characterized by array-CGH. All our patients presented with severe developmental delay, dysmorphic features, behavioral problems, and thrombocytopenia. Comparing the clinical features of our patients with the overlapping ones already reported two potential phenotypes related to 21q22 microdeletion including RUNX1 were highlighted: thrombocytopenia with +/- mild dysmorphic features and syndromic thrombocytopenia with growth and developmental delay.

Myelodisplastic Syndrome with Trysomy 21 IN A Patient with Constitutional Submicroscopic 21Q22 Deletion.

Abstract 4831 IntroductionPrevious studies showed that chromosomal and genomic aberrations leading to activation of oncogenes or haploisufficiency of tumor suppressor genes are well-known pathogenic mechanisms in cancer. Additional copies of chromosme 21 are frequently found in Myelodisplastic Syndrome (MDS) and in Acute Myeloid Leukemia (AML); the presence of these chromosomal abnormalities and the high incidence of acute leukemias in subjects with constitutional trisomy 21, suggest that genes on chromosome 21, including RUNX1/AML1, play a particular role in leukemogenesis and hematopoiesis. We describe a patient with syndromic trombocytopenia ( average platelet count= 70000/mm3), psychomothor delay, microcephaly and low stature, that developed a progressive anemia and became transfusion-dependent at seventeen years of age. Materials and methodsCytogenetic analysis was performed on bone marrow cells and on peripheral blood lymphocytes, with standard techniques and evaluated with Giemsa-trypsin-Giemsa banding according to International System for Human Cytogenetic Nomenclature (ISCN 2005). Fluorescent In Situ Hybridization (FISH) experiments was performed on bone marrow samples with LSI AML1/ETO Dual Color, Dual Fusion Translocation and, at the same time, the High-resolution oligo array-CGH (Agilent Human Genome CGH Microarray 44B) was performed on the DNA of the patient. ResultsThe bone marrow cells showed marked dysplastic morphology and the following abnormal karyotype: 46,XX[14]/47,XX,+21[6]; the peripheral blood karyotype was normal. The High-resolution oligo array-CGH demonstrated a constitutional de novo microdeletion of one chromosome 21. The interstitial deletion was found to be approximately 4,4Mb (Megabases), extending from 32,29 Mb to 36,51 Mb on band 21q22.11-12, involving MRAP, IFNAR2, IFNGRR2, KCNE2, KCNE1 and RUNX1 genes. The FISH performed on bone marrow cells, revealed two orange signals representing normal copies of ETO and one green signal for AML1 in 60% interphase cells and two orange signals and two green signals in the remaining 40% cells. The first pattern of signals, for AML1, is related to cells with karyotype 46,XX, while the second pattern of signals is related to cells with karyotype 47,XX,+21. These results indicate that in the myelodisplastic clone the third chromosome 21 are not deleted on band 21q22. ConclusionThree cases were recently published of syndromic thrombocytopenia with 21q22 constitutional deletion, including RUNX1, and variable degree of dysmorphic features and mental delay. One of the three patients developed LMA at the age of six years. Our results further support the fundamental role, in the pathogenetic mechanism of syndromic trombocytopenia and MDS/AML, of the numerical abnormalities of chromosome 21 associated with submicroscopic rearrangement of RUNX1 and other dosage-sensitive unknown genes on chromosome 21. DisclosuresNo relevant conflicts of interest to declare.

Syndromic thrombocytopenia and predisposition to acute myelogenous leukemia caused by constitutional microdeletions on chromosome 21q

AbstractSeveral lines of evidence support the presence of dosage-sensitive genes on chromosome 21 that regulate leukemogenesis and hematopoiesis. We report a detailed clinical and molecular characterization of 3 patients with chronic thrombocytopenia caused by distinct constitutional microdeletions involving chromosomal region 21q22.12. The patients exhibited growth restriction, dysmorphic features, and developmental delays. One patient developed acute myelogenous leukemia (AML) at 6 years of age. All 3 deletions included the RUNX1, CLIC6, DSCR, and KCNE1 genes. Our data provide additional support for the role of RUNX1 haploinsufficiency in megakaryopoiesis and predisposition to AML. The leukemic clone had trisomy 21 resulting from duplication of chromosome 21 containing the RUNX1 deletion. This shows that genes other than RUNX1 must also play a role in AML associated with trisomy 21. We recommend that children with syndromic thrombocytopenia have clinical array-comparative genomic hybridization analysis and appropriate cytogenetic studies to facilitate our ability to provide a definitive diagnosis.