Glanzmann thrombasthenia (GT) is an inherited hemorrhagic disorder characterised by impaired platelet functions, manifested clinically as spontaneous bleeding. It is usually inherited in an autosomal recessive manner. Platelet dysfunction in patients with GT is caused by quantitative and/or qualitative deficiencies in αIIbβ3, which result from mutations in the genes encoding αIIbβ3. These genetic alterations lead to platelet dysfunction characterised by impaired fibrinogen binding capacity upon agonist stimulation, defective aggregation and spreading. While classical GT typically exhibits normal platelet counts and morphology, very rare mutations in ITGA2B (encoding αIIb) and/or ITGB3 (encoding β3) cause macrothrombocytopenia or increased platelet anisotropy (heterogeneity of platelet size and morphology). This type of mutation mainly localises in the membrane-proximal region of αIIbβ3 and is inherited in an autosomal dominant manner. This particular type of disorder is called ITGA2B/ITGB3-related macrothrombocytopenia and has been considered a subset of congenital macrothrombocytopenia. Current research suggests that gain-of-function mutations in ITGA2B or ITGB3 underlie the pathogenesis of most ITGA2B/ITGB3-related macrothrombocytopenia and mechanistically distinguish it from classical GT. However, recent reports have documented non-activating ITGB3 mutations that also cause macrothrombocytopenia, presenting a profound challenge to the mechanistic understanding of ITGA2B/ITGB3-related macrothrombocytopenia. This review summarises the reported cases of gain-of-function mutations in ITGA2B and ITGB3 associated with ITGA2B/ITGB3-related macrothrombocytopenia hitherto and discusses the potential molecular pathways contributing to the unique phenotypes in ITGA2B/ITGB3-related macrothrombocytopenia.

Ankyrin repeat domain 26 (ANKRD26)-related thrombocytopenia (ANKRD26-RT) is an autosomal dominant disorder caused by variants in the 5'-untranslated region of the ANKRD26 gene, leading to its overexpression and impaired megakaryocyte maturation. We identified a novel ANKRD26 variant (c.-129_-112del) and investigated its transcriptional impact, platelet function, and haematopoietic effects. We analysed ANKRD26 transcription in megakaryocytes derived from 21 ANKRD26-RT patients using quantitative reverse transcription droplet digital polymerase chain reaction (PCR). Furthermore, platelet function, megakaryocyte morphology and RNA sequencing were assessed. The novel variant exhibited significantly higher ANKRD26 transcription levels than previously reported variants (p < 0.001), but did not show any characteristic clinical phenotypes. Megakaryocytes with the novel variant showed reduced proplatelet formation and lower nuclear ploidy. RNA sequencing revealed upregulation of cell proliferation genes, including dedicator of cytokinesis 2 (DOCK2), cluster of differentiation (CD38) and interleukin-1 receptor accessory protein (IL1RAP). No platelet-related or megakaryocyte-related genes other than ANKRD26 were detected in these variants. The new variant disrupted the runt-related transcription factor 1 (RUNX1)-binding site, leading to ANKRD26 overexpression and possible haematopoietic abnormalities, though it did not directly correlate with clinical severity. However, genes linked to cell proliferation (CD38, DOCK2 and IL1RAP) showed increased expression. Our results suggested a potential link to leucocytosis, although further research is required to confirm this.

Primary atopic disorders (PAD) are monogenic disorders caused by pathogenic gene variants encoding proteins that are key for the maintenance of a healthy skin barrier and a well-functioning immune system. Physicians face the challenge to find single, extremely rare PAD patients/families among the millions of individuals with common allergic diseases. We describe case scenarios with signature PAD. We review the literature and deduct specific clinical red flags for PAD detection. They include a positive family history and/or signs of pathological susceptibility to infections, immunodysregulation, or syndromic disease. Results of conventional laboratory and most immunological lab studies are not sufficient to make a definitive diagnosis of PAD. In the past, multistep narrowing of differential diagnoses by various immunological and other laboratory tests led to testing of single genes or gene panel analyses, which was a time-consuming and often unsuccessful approach. The implementation of whole-genomic analyses in the routine diagnostics has led to a paradigm shift. Upfront genome-wide analysis by whole genome sequencing (WGS) will shorten the time to diagnosis, save patients from unnecessary investigations, and reduce morbidity and mortality. We propose a rational, clinical landmark-based approach for deciding which cases pass the filter for carrying out early WGS. WGS result interpretation requires a great deal of caution regarding the causal relationship of variants in PAD phenotypes and absence of proof by adequate functional tests. In case of negative WGS results, a re-iteration attitude with re-analyses of the data (using the latest data base annotation)) may eventually lead to PAD diagnosis. PAD, like many other rare genetic diseases, will only be successfully managed, if physicians from different clinical specialties and geneticists interact regularly in multidisciplinary conferences.

Familial platelet disorder with associated myeloid malignancies (FPDMM) is an autosomal dominant disease caused by heterozygous germline mutations in RUNX1. It is characterized by thrombocytopenia, platelet dysfunction, and a predisposition to hematological malignancies. Although FPDMM is a precursor for diseases involving abnormal DNA methylation, the DNA methylation status in FPDMM remains unknown, largely due to a lack of animal models and challenges in obtaining patient-derived samples. Here, using genome editing techniques, we established two lines of human induced pluripotent stem cells (iPSCs) with different FPDMM-mimicking heterozygous RUNX1 mutations. These iPSCs showed defective differentiation of hematopoietic progenitor cells (HPCs) and megakaryocytes (Mks), consistent with FPDMM. The FPDMM-mimicking HPCs showed DNA methylation patterns distinct from those of wild-type HPCs, with hypermethylated regions showing the enrichment of ETS transcription factor (TF) motifs. We found that the expression of FLI1, an ETS family member, was significantly downregulated in FPDMM-mimicking HPCs with a RUNX1 transactivation domain (TAD) mutation. We demonstrated that FLI1 promoted binding-site-directed DNA demethylation, and that overexpression of FLI1 restored their megakaryocytic differentiation efficiency and hypermethylation status. These findings suggest that FLI1 plays a crucial role in regulating DNA methylation and correcting defective megakaryocytic differentiation in FPDMM-mimicking HPCs with a RUNX1 TAD mutation.

LCP1 encodes L-plastin, an actin-bundling protein primarily expressed in hematopoietic cells. In mouse and fish models, LCP1 deficiency has been shown to result in hematologic and immune defects. This study aimed to determine the nature of a human inborn error of immunity resulting from a novel genetic variant of LCP1. We performed genetic, protein, and cellular analysis of PBMCs from a kindred with apparent autosomal dominant immune deficiency. We identified a candidate causal mutation in LCP1, which we evaluated by engineering the orthologous mutation in mice and Jurkat cells. A splice-site variant in LCP1 segregated with lymphopenia, neutropenia, and thrombocytopenia. The splicing defect resulted in at least 2 aberrant transcripts, producing an in-frame deletion of 24 nucleotides, and a frameshift deletion of exon 8. Cellular analysis of the kindred revealed a proportionate reduction of T and B cells and a mild expansion of transitional B cells. Similarly, mice carrying the orthologous genetic variant exhibited the same in-frame aberrant transcript, reduced expression Lcp1 and gene dose-dependent leukopenia, mild thrombocytopenia, and lymphopenia, with a significant reduction of T-cell populations. Functional analysis revealed that LCP1c740-1G>A confers a defect in platelet development and function with aberrant spreading on collagen. Immunologic analysis revealed defective actin organization in T cells, reduced migration of PBMCs from patients, splenocytes from mutant mice, and a mutant Jurkat cell line in response to CXCL12; impaired germinal center B-cell expansion after immunization; and reduced cytokinesis during T cell proliferation. We describe a unique human hematopoietic defect affecting neutrophils, lymphocytes, and platelets arising from partial LCP1 deficiency.