RUNX1 familial platelet disorder

RUNX1 familial platelet disorder with associated myeloid malignancy is an autosomal dominant constitutional thrombocytopenia caused by heterozygous germline loss-of-function variants in RUNX1, in which platelets are normal in size but functionally defective and the lifetime risk of a myeloid or lymphoid malignancy is approximately 35% to 45%

RUNX1 encodes a core-binding transcription factor that is required for definitive haematopoiesis and for megakaryocyte maturation, so haploinsufficiency produces both a quantitative and a qualitative platelet defect and establishes a pre-leukaemic state that can acquire secondary somatic mutations

Affected individuals usually present with easy bruising, epistaxis, menorrhagia or excessive bleeding after dental work, and the diagnosis is established by germline RUNX1 testing rather than by platelet count alone because many carriers have only mild thrombocytopenia

Terminology and classification

• The disorder is recorded under several names, including RUNX1 familial platelet disorder (RUNX1-FPD), familial platelet disorder with associated myeloid malignancies (FPDMM) and the older FPD/AML, all of which describe the same germline RUNX1 entity
• It belongs to the inherited platelet disorders and, more specifically, to the hereditary thrombocytopenias with normal platelet volume, and it is distinguished from the acquired immune thrombocytopenias by its lifelong course, family history and cancer predisposition
• The fifth edition of the World Health Organization classification of haematolymphoid tumours (2022) and the International Consensus Classification (2022) both recognise myeloid neoplasms with germline predisposition as a distinct diagnostic group, and RUNX1 is one of the defining genes of that group
• ICD-10 assigns D69.1 to qualitative platelet defects and ICD-11 places the condition at 3B62.3, while Orphanet uses ORPHA:71290 and MONDO uses MONDO:0011071 for the same entity
• Additional cross-references used in interoperability work are OMIM 601399, MeSH C563324, UMLS C1832388 and GARD 10352, and OMIM 601399 refers to the familial platelet disorder phenotype rather than to the RUNX1 gene entry
• The main differential within the inherited thrombocytopenias is Bernard-Soulier syndrome, which has large platelets, and MYH9-related disease, GATA1-related disease and Wiskott-Aldrich syndrome, none of which carries the same leukaemia predisposition

History

• In 1999 Song and colleagues linked a family with thrombocytopenia and acute myelogenous leukaemia to heterozygous loss-of-function changes in CBFA2, the gene now called RUNX1, on chromosome 21q22 and named the disorder familial platelet disorder with predisposition to AML
• RUNX1 had already been recognised as the AML1 partner of the t(8
• Mouse models with reduced Runx1 dosage confirmed that the platelet and megakaryocyte abnormalities are cell-autonomous and that a second somatic hit is required before overt malignancy develops, which established the two-hit model still used in counselling
• The 2016 revision of the World Health Organization classification introduced myeloid neoplasms with germline predisposition as a provisional category, and the 2022 fifth edition converted germline RUNX1 predisposition into a fully recognised diagnostic qualifier
• An international consensus classification published in 2022 aligned the haematological and molecular criteria with the WHO scheme and emphasised that germline testing must be performed on a non-haematopoietic tissue when a RUNX1 variant is suspected
• The National Institutes of Health RUNX1 natural history study has assembled more than 200 carriers since its inception and in 2026 reported that 31 of 213 participants had developed a haematological malignancy, providing the largest prospective risk estimates available
• The earlier assumption that bleeding severity simply tracks the platelet count was revised after aggregometry and granule studies showed a disproportionate functional defect, which changed pre-operative planning from count-based thresholds to platelet function assessment

Epidemiology

• No population-based prevalence has been established because most carriers are identified through family studies, and Orphanet classifies the disorder as rare with an unknown prevalence rather than publishing a point estimate
• The National Institutes of Health cohort of 213 carriers is the largest systematically phenotyped series and yields a lifetime haematological malignancy risk of 42% with a 95% confidence interval of 26% to 55%, slightly above the 35% to 45% range quoted in earlier reviews
• The median age at malignancy diagnosis in that cohort was 36 years with a range from 1 to 74 years, so the risk begins in childhood and does not remit in older adulthood, which argues against discharging adult carriers from surveillance
• Paediatric carriers have a cumulative malignancy incidence of 7.3% by age 18 years, which is 57.7-fold higher than the general paediatric population and 3.4-fold higher than children with Fanconi anaemia by age 5 years
• Thrombocytopenia is mild to moderate, with a mean platelet count of 131 ± 49 ×10^9/L in paediatric carriers versus 314 ± 55 ×10^9/L in unaffected relatives, and mean platelet volume remains normal, which is a useful discriminator from macrothrombocytopenias
• Abnormal bleeding is reported in 32% of paediatric carriers and 42% of adults, and mucosal bleeding such as epistaxis and menorrhagia predominates over deep bleeding, reflecting the platelet-functional rather than coagulation nature of the defect
• The sex distribution is equal and no founder population or ethnic predilection has been described, in contrast to SOST-related sclerosing bone disease or the Afrikaner founder effect seen in sclerosteosis
• Surveillance depends on family registries, the RUNX1db variant database and the RUNX1 Research Program rather than on notifiable-disease reporting, because no jurisdiction mandates registration of germline predisposition syndromes

Pathophysiology

• Most pathogenic variants are truncating or missense changes in the Runt homology domain that abolish DNA binding or reduce protein output, and large deletions of RUNX1 produce the same phenotype through haploinsufficiency rather than through a dominant-negative mechanism
• RUNX1 dimerises with CBFB to regulate enhancers of megakaryocyte genes such as PF4, ITGA2B and GP1BA, and reduced RUNX1 activity impairs proplatelet formation, alpha-granule content and agonist-induced aggregation
• Haploinsufficiency alone is insufficient for leukaemia, and progression requires cooperating somatic lesions such as additional RUNX1 mutations, ASXL1, SRSF2, PHF6 or RAS pathway changes that expand a pre-leukaemic clone
• Clonal haematopoiesis is detectable in a substantial fraction of carriers before blood counts change, and the clone size and mutation profile are being evaluated as biomarkers that could trigger earlier bone marrow assessment
• The platelet defect includes storage-pool and signal-transduction abnormalities that are not corrected by platelet transfusion alone, which explains why bleeding can exceed what the count predicts during surgery or dental extraction
• RUNX1 also regulates T-cell and epithelial differentiation, and this pleiotropy is thought to underlie the eczema, psoriasis and autoimmune or atopic conditions reported in carriers independently of their haematological status

Clinical features

• The cardinal bleeding phenotype is easy bruising and prolonged bleeding after minor trauma, dental extraction or tonsillectomy, and women frequently report menorrhagia and postpartum haemorrhage that is out of proportion to the platelet count
• Physical examination is usually normal apart from ecchymoses, and splenomegaly, lymphadenopathy or hepatomegaly should raise concern for an evolving haematological malignancy rather than being attributed to the platelet disorder
• Acute myeloid leukaemia and myelodysplastic syndrome account for most malignancies, but acute lymphoblastic leukaemia, chronic myelomonocytic leukaemia, hairy-cell leukaemia and non-Hodgkin lymphoma have all been reported in carriers
• Children who develop malignancy differ from adults in having a higher proportion of lymphoid disease and more chromosomal changes but fewer cooperating point mutations, so paediatric surveillance cannot simply extrapolate from adult algorithms
• Skin involvement includes eczema and psoriasis, and carriers also report asthma, allergic rhinitis, arthritis, autoimmune thyroid disease and gastrointestinal symptoms more often than their unaffected relatives
• Warning signs of transformation are a falling platelet count, new anaemia or neutropenia, macrocytosis, circulating blasts, dysplastic neutrophils and unexplained constitutional symptoms such as fever, weight loss or night sweats

Diagnosis

• The diagnosis is suspected when a lifelong bleeding tendency coexists with mild thrombocytopenia, normal platelet size and a family history of leukaemia, and it should also be considered in any young adult with myelodysplasia and a personal or family bleeding history
• Initial laboratory work includes a complete blood count with mean platelet volume, peripheral smear review, platelet aggregometry and flow cytometric granule studies, which together document the qualitative defect that a count alone misses
• Molecular confirmation requires RUNX1 sequencing plus deletion and duplication analysis by MLPA or chromosomal microarray, and germline status must be confirmed on cultured skin fibroblasts or saliva when blood or marrow shows the variant
• Baseline bone marrow aspiration and biopsy are recommended at diagnosis once clinically acceptable, followed by annual marrow examination and quarterly blood counts, because morphological or cytogenetic evolution can precede overt leukaemia by months
• Cascade testing of first-degree relatives is recommended after a germline variant is found, and predictive testing of asymptomatic children is offered when surveillance or transplant planning would change, with genetic counselling before testing
• The differential includes immune thrombocytopenia, Bernard-Soulier syndrome, MYH9-related disease, GATA1-related thrombocytopenia, Wiskott-Aldrich syndrome and other germline predisposition syndromes such as CEBPA, DDX41, GATA2 and ETV6, each separated by platelet size, syndromic features and the specific gene
• Variants of uncertain significance are common because most RUNX1 changes are private, and the Myeloid Malignancy Variant Curation Expert Panel framework plus functional transactivation and localisation assays are used to reclassify them

Management

• Care is coordinated by a haematologist with genetics input, and every carrier should receive a written surveillance plan, a medical alert card and education about bleeding precautions before elective procedures
• Aspirin, non-steroidal anti-inflammatory drugs and other antiplatelet agents are avoided unless clearly indicated, and desmopressin, tranexamic acid or epsilon aminocaproic acid are used to cover dental work, surgery and trauma
• Platelet transfusion is reserved for severe bleeding or procedures with a high bleeding risk, and HLA-matched or cross-matched products are preferred for repeatedly transfused patients to limit alloimmunisation
• Allogeneic haematopoietic stem cell transplantation is the only curative option and is considered for myelodysplasia, acute leukaemia or early morphological evidence of transformation, with timing individualised according to donor availability and comorbidity
• When acute myeloid leukaemia develops, induction and consolidation follow European LeukemiaNet recommendations and the WHO or International Consensus Classification, and allogeneic transplant is offered in first remission because germline predisposition increases relapse risk
• Surveillance consists of clinical review every 6 to 12 months for constitutional symptoms and organomegaly, complete blood counts every 3 months and annual bone marrow assessment, with earlier investigation for any unexplained cytopenia or clonal change
• Genetic counselling explains the 50% autosomal dominant transmission risk, the incomplete and age-dependent penetrance of malignancy, the availability of prenatal and preimplantation testing and the importance of informing relatives before they undergo surgery or donate marrow
• Overall survival is determined mainly by whether and when a haematological malignancy develops, and carriers who remain free of malignancy have a near-normal life expectancy but require lifelong surveillance because the risk persists into old age

Research and controversies

• A 2026 functional pipeline combining transactivation reporter assays, electrophoretic mobility shift assays and subcellular localisation reclassified 17 of 26 RUNX1 variants of uncertain significance, and it showed that current expert-panel thresholds exclude some functionally impaired alleles
• Paediatric data published in 2026 showed that childhood malignancies in RUNX1-FPDMM are more often lymphoid and more often associated with chromosomal changes than adult malignancies, challenging the assumption that the disorder is purely a myeloid predisposition syndrome
• Whether all carriers should undergo baseline bone marrow biopsy, and how often marrow should be repeated, remains contested because procedure risk and patient burden must be balanced against the modest yield of detecting early dysplasia in asymptomatic people
• No RUNX1-directed therapy exists, and research focuses on epigenetic modulators, differentiation agents and immune-based strategies that might eliminate the pre-leukaemic clone before a second hit produces overt disease
• The RUNX1 Research Program, the RUNX1db variant database and international natural-history collaboration are the main sources of longitudinal data, and harmonising variant classification and surveillance protocols across registries is regarded as the priority for improving outcomes