Abstract Number: OC 31.4
Meeting: ISTH 2021 Congress
Theme: Platelet Disorders, von Willebrand Disease and Thrombotic Microangiopathies » Inherited Thrombocytopenias
Background: Inherited macrothrombocytopenias (IMTP) are rare and heterogeneous disorders caused by defects in several genes involved in megakaryopoiesis. To date, only two unrelated families have been diagnosed with thrombocytopenia due to genetic variants in TPM4 (TPM4-RT), the gene encoding for tropomyosin-4 (TPM4).
Aims: To characterize the third unrelated family with a novel nonsense TPM4 variant identified by whole-exome sequencing (WES).
Methods: WES was performed in a family with lifelong dominant IMTP (Table 1). Bleeding score (BS) was recorded by ISTH-BAT. Platelet phenotyping included platelet count (P), blood smear, aggregometry (LTA), flow cytometry (FC), and electron microscopy (EM). Cytoskeleton proteins were analyzed in resting/spreading conditions (fibrinogen) by immunofluorescence (IF).
Results: Patient clinical characteristics are shown in table 1. The index case was a 40 years-old female with lifelong MCT (P=111×109/L, MVP=14.7 fL) and mild bleeding tendency (BS=6). She showed normal expression of platelet glycoproteins and granules secretion, but mildly impaired aggregation with arachidonic acid. WES revealed a novel nonsense variant (c.322C>T; p.Gln108*) in TPM4 (NM_003290.3), classified as pathogenic variant (Table 1). This variant affects domain1 in TPM4. IF assays showed altered TPM4 localization in platelets, while other cytoskeleton proteins (actin, actinin, β-tubulin, and non-muscle myosin IIA) were not affected. Platelet spreading and filipodia/lamellipodia production was altered after platelet-stimulation with TRAP-6 and CRP (Figure 1).
| Patient | Age | Platelets (x109/L) | MPV (fL) | BS | TPM4 variant (p.Gln108*) |
| II-1 | 60 | 145 | 12.7 | 2 | Heterozygous |
| II-2 | 64 | 121 | 13.5 | 2 | Heterozygous |
| II-3 | – | – | – | – | N/A died |
| III-1 | 34 | 140 | 11.8 | 4 | Heterozygous |
| III-2 | 40 | 111 | 14.7 | 6 | Heterozygous |
| III-3 | 36 | 180 | 10.9 | 2 | WT |
| III-4 | 33 | 124 | 13.4 | 0 | Heterozygous |
Table 1: Clinical characteristics of the family members.
Figure 1. Representative picture of platelet spreading in fibrinogen, in control and patient´ platelets untreated or stimualted with TRAP-6 50µM stimulation.
Conclusions: This new family with TPM4-RT consolidates this rare form of IMTP and supports including TPM4 in Tier1 gene for inherited thrombocytopenias. TPM4 localization was altered in spreading platelets. Our IF data suggested that TPM4 plays a crucial role for a correct cytoskeletal remodeling.
Funding: ISCIII (PI17/01966, PI 17/01311, PI20/00926), GRS (GRS2061A/19, GRS2135/A/2020), Fundación Séneca (19873/GERM/15), Fundación Mutua Madrileña (AP172142019), Premio López Borrasca (2019), Grupo Trabajo Patología Hemorrágica-SETH (2020).
To cite this abstract in AMA style:
Marín-Quílez A, Fernández-Infante C, Manrique Gonzalo MÁ, Llorente-González C, Millán-Salanova M, Santos-Mínguez S, Miguel-García C, Rodríguez-Iglesias I, López-Cadenas F, González T, Palma-Barqueros V, Benito R, Díez-Campelo M, González-Porras JR, Hernández-Rivas JM, Vicente-Manzanares M, Rivera J, Bastida JM. Characterization of a New Family with Lifelong Macrothrombocytopenia Caused by a Novel Nonsense Variant in TPM4 [abstract]. Res Pract Thromb Haemost. 2021; 5 (Suppl 2). https://abstracts.isth.org/abstract/characterization-of-a-new-family-with-lifelong-macrothrombocytopenia-caused-by-a-novel-nonsense-variant-in-tpm4/. Accessed November 29, 2021.« Back to ISTH 2021 Congress
ISTH Congress Abstracts - https://abstracts.isth.org/abstract/characterization-of-a-new-family-with-lifelong-macrothrombocytopenia-caused-by-a-novel-nonsense-variant-in-tpm4/