Inherited thrombocytopenias: the beat goes on

In this issue of Blood, Bottega et al document mutations in ACTN1, which encodes the cytoskeletal protein α-actinin 1, in 10 of 239 consecutive probands with an inherited thrombocytopenia—making ACTN1 an important cause of familial thrombocytopenia.¹ 

Way back in 1909, Richard May, and in 1945, Robert Hegglin independently described a clinical entity characterized by inclusions in white cells, large platelets, and thrombocytopenia, an association that was subsequently named the May-Hegglin anomaly.²  However, it was only at the turn of the century that the genetic basis of this entity was established as a mutation in MYH9, the gene encoding for the non–muscle myosin heavy chain IIA.²  This amalgamated several syndromes with eponyms (Sebastian, Fechtner, Epstein) under 1 rubric of MYH9-related disorders. However, the genetic and molecular basis has been largely unknown in most patients with inherited thrombocytopenias.

In the past 2 decades, there has been remarkable progress in our understanding of inherited thrombocytopenias. Several novel mutations have been elucidated using technologies such as whole exome sequencing. These include mutations in NBEAL2 in patients with gray platelet syndrome, RBM8A in thrombocytopenia with absent radii, HOXA11 in congenital thrombocytopenia with radioulnar synostosis, and ANKYD26, GFI1B, and PRKCAG.^3-6  To date, about 2 dozen genes have been linked to inherited thrombocytopenias. To this rapidly moving field, the present article by Bottega et al¹  adds evidence that ACTN1 mutations are an important mechanism.

In 2013, Kunishima et al⁷  reported 6 Japanese pedigrees with an autosomal dominant macrothrombocytopenia, mild bleeding symptoms, and mutations in ACTN1, the gene encoding α-actinin 1 and a member of the actin-cross-linking protein superfamily that participates in cytoskeletal organization. α-Actinins exist as antiparallel dimers and have 2 functional domains—an N-terminal actin-binding domain and a C-terminal calmodulin-like domain. Platelets and megakaryocytes express mainly α-actinin 1. These authors found ACTN1 mutations in 6 of 13 pedigrees with autosomal dominant macrothrombocytopenia, all in the ACTN1 functional domains. Moreover, they showed that expression of the ACTN1 variants in Chinese hamster ovary cells or in mouse fetal liver–derived megakaryocytes caused disruption of the normal actin-based cytoskeleton structure; in mouse megakaryocytes, there was a reduction in proplatelet tips, consistent with impaired platelet production. Of note, ACTN1 mutations accounted for 5.5% of their patients with autosomal dominant macrothrombocytopenia and were the fourth most common cause.

Also in 2013, Gueguen et al⁸  reported a missense mutation in ACTN1 in a large French kindred of 55 members, of whom 26 had autosomal dominant macrothrombocytopenia. The mutation in the actin-binding domain of α-actinin 1 segregated with macrothrombocytopenia and studies in COS-7 cells corroborated a disruption of the actin cytoskeleton by the mutation. Interestingly, in the French pedigree and in 5 of 6 Japanese pedigrees,⁷  the ACTN1 mutations were cytosine guanine dinucleotide mutations; the cytosine guanine dinucleotide site is a common mutational hot spot in the human genome.

The current article by Bottega et al¹  strengthens the association of ACTN1 mutations with inherited thrombocytopenias. The authors had previously identified a causative mutation in 111 of 239 consecutive probands with inherited thrombocytopenias. Here they report missense mutations in ACTN1 in 10 of the remaining 128 index cases (4.2% of total cohort) through studies using whole exome sequencing in 7 subjects and Sanger sequencing in 121. Eight were novel variants affecting highly conserved amino acid residues in ACTN1. These patients had mild thrombocytopenia (mean platelet count, 103 × 10⁹ ± 26 × 10⁹/L in 31 subjects) with increased mean platelet volume (12.6 ± 1.7 fL; 50 controls: 10.1 ± 1.4 fL) and mild to no bleeding manifestations, although 4 of 18 subjects undergoing surgery had increased blood loss. Of note, 3 individuals from one family developed leukemia; the association of ACTN1 mutations with myeloid malignancies needs to be clarified. The platelet aggregation responses were normal in the patients. These observations were complemented by studies in fibroblasts showing altered actin cytoskeleton by the ACTN1 mutations. Overall, these 3 studies^1,7,8  provide convincing evidence to link ACTN1 mutations with inherited thrombocytopenias and reflect on the relative high prevalence in such patients.

A better understanding of inherited thrombocytopenias is important in clinical practice, beyond predisposition to bleeding, which is highly variable and often mild. First, many of these patients are first recognized in their adulthood,⁵  and in the absence of a family history of thrombocytopenia, there is the risk of misdiagnosis as immune thrombocytopenia and resulting unnecessary therapy. Second, some gene mutations have prognostic implications, such as the association with myeloid malignancies with RUNX1 and ANKRD26 mutations or worsening renal function or hearing loss with MYH9 mutations. Not all patients with MYH9 mutations demonstrate inclusions in neutrophils on the routine peripheral smear, and additional studies are needed. Last, there may be therapeutic implications; for example, the role of eltrombopag in patients with MYH9 mutations.⁵ 

From a mechanistic perspective, inherited thrombocytopenias involve mutations in genes with diverse functions, although for many the specific function in megakaryocytes/platelets remains unclear. The genes include genes coding for transcription factors (RUNX1, FLI1, GATA1, GFI1B, HOXA11), thrombopoietin receptor (MPL), cytoskeletal proteins and related signaling (MYH9, TUBB1, ACTN1, FLNA, WASP), surface membrane glycoproteins and related signaling (GPIBA, GPIBB, GPIX, ITGA2B, ITGB3), proteins involved in vesicle transport (NBEAL2), exon-junction complex involved in RNA processing (RBM8A), and cyclic adenosine monophosphate–dependent protein kinase (PRKACG).^3-6  Mutations in the ANKRD26 5′ untranslated region result in loss of RUNX1 and FLI1 binding and in ANKRD26 silencing.⁹  Thus, different genes likely affect distinct mechanisms in megakaryocytes and platelets.

With new insights come new questions. The currently identified mutations explain the genetic basis in about half of patients, even at specialized centers. What do the others have? What are the unrecognized clinical implications (such as predisposition to malignancies) of specific mutations? What measures can be used to raise platelet counts on a temporary basis besides platelet transfusions?

Patients with inherited thrombocytopenias constitute an untapped reservoir of information into the basic biology of the cells involved. Studies are needed to identify the impact of the mutations on specific processes in megakaryocytes and platelets. In some entities, there is strong evidence for associated platelet dysfunction, as in patients with RUNX1 mutations.¹⁰ 

Recent advances in the area of inherited thrombocytopenias are impressive. The available state-of-the-art technologies will continue unraveling new causes and yield insights into megakaryocyte and platelet biology. The beat goes on—and the tempo is high.