Myeloid neoplasms with germ line RUNX1 mutation
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Familial platelet disorder with propensity to myeloid malignancies (FPD/AML) is an autosomal dominant disorder characterized by quantitative and/or qualitative platelet defects with a tendency to develop a variety of hematological malignancies. Heterozygous germ line mutations in the RUNX1 gene are responsible genetic events for FPD/AML. Notably, about half of individuals in the family with germ line mutations in RUNX1 develop overt hematological malignancies. The latency is also relatively long as an average age at diagnosis is more than 30 years. Similar to what is observed in sporadic hematological malignancies, acquired additional genetic events cooperate with inherited RUNX1 mutations to progress the overt malignant phase. Reflecting recent increased awareness of hematological malignancies with germ line mutations, FPD/AML was added in the revised WHO 2016 classification. In this review, we provide an update on FPD/AML with recent clinical and experimental findings.
KeywordsRUNX1 FPD/AML FPD/MM Germ line mutation
RUNX1 is a well-known critical transcription factor for embryogenesis and definitive hematopoiesis [1, 2, 3, 4, 5]. Somatic mutation in the RUNX1 gene is one of the most frequent mutations identified in the patients with a variety of myeloid malignancies, such as myelodysplastic syndromes (MDS), myeloproliferative neoplasms (MPN), and acute myeloid leukemia (AML) [6, 7]. In most of cases, those RUNX1 mutations are considered as “subclonal mutation” . On the other hand, germ line mutations in RUNX1 gene cause familial platelet disorder with a propensity to myeloid malignancy (FPD/AML), an autosomal dominant disorder which typically presents with quantitative/qualitative platelet defect with a tendency to develop primarily myeloid malignancies, such as MDS and AML . In this case, heterozygous inherited RUNX1 mutations play a fundamental role in the pathogenesis of FPD/AML. However, these inherited RUNX1 mutations are not enough to develop the leukemic stage. Additional “subclonal” mutations are required to develop myeloid malignancies.
Since a certain number of cases have begun to be recognized as germ line mutation-driven myeloid diseases, “myeloid neoplasms with germ line predisposition” has recently been added as a discrete section of the WHO 2016 classification (“WHO blue book”) . FPD/AML is described in this section and its awareness by clinicians is increasing. In this perspective, we describe an overview of FPD/AML pathogenesis and its recent findings.
Role of RUNX1 in hematopoiesis and hematological malignancies
The t(8;21) is frequently identified in patients with AML . RUNX1 was originally cloned from the sample obtained from an AML patient with t(8;21) . The RUNX1 gene was located at the breakpoint on chromosome 21 . The critical roles of RUNX1 in a variety of physiological processes have been uncovered primarily using transgenic mice. Runx1 deficiency is embryonically lethal because of hemorrhage in the central nervous system and a lack of definitive hematopoiesis . Conditional deletion of Runx1 gene in adult mice causes aberrant megakaryocytopoiesis, thrombocytopenia, lymphocytopenia, and expansion of hematopoietic stem cells (HSCs)/myeloid progenitors [4, 5]. Recently, it has been shown that Runx1 deficiency causes a defect of ribosomal biogenesis (RiBi) .
Given that RUNX1 plays pivotal roles in the regulation of physiological hematopoiesis, the dysregulation of RUNX1 leads to defective hematopoiesis. Indeed, RUNX1 mutations have frequently been identified in patients with MDS and AML [6, 7]. Furthermore, a higher frequency of RUNX1 mutations (30–50%) has been reported in therapy-related and radiation-associated MDS and AML [13, 14, 15]. In general, RUNX1 mutations are considered to result in the loss of RUNX1 functions. On the other hand, recent several reports indicate that wild-type RUNX1 is also important for growth and survival of leukemia cells .
Familial platelet disorder (FPD) with propensity to myeloid malignancies
Identification of RUNX1 mutations as a causative genetic event in FPD/AML
In 1996, using the samples from the pedigree described by Dowton et al., the link between FPD/AML and genes on human chromosome 21q22.1-22.2 has been reported . In 1999, Song et al. identified heterozygous germ line RUNX1 mutations as the causal abnormality in FPD/AML . RUNX1 genes encode a DNA-binding subunit that contains highly conserved Runt-homology domain (RHD) for sequence-specific DNA binding. The RUNX1 mutations identified in the individuals of FPD/AML pedigrees are generally located in the RHD, leading to disruption of DNA binding ability. Although the incidence is less common, the mutations in the C-terminal region have been reported . In this case, the mutation generates a frameshift and premature chain termination, resulting in the lack of transactivation domain . Intragenic deletions, which are not identified by traditional sequence analysis, have also been reported in a few families [9, 26, 27]. In FPD/AML, most of RUNX1 mutations cause haploinsufficiency , while some mutations are considered to function in a dominant negative manner [28, 29]. It is suggested that the mutations which have dominant negative effects are associated with a high frequency of leukemia progression compared to the mutations which act via haploinsufficiency .
Mechanism of thrombocytopenia
The molecular mechanisms of how RUNX1 haploinsufficiency causes abnormalities of megakaryopoiesis and platelets remain to be elucidated. Recently, it has been shown that wild-type RUNX1 negatively regulates the expression of non-muscle myosin IIB heavy chain 10 (MYH10), resulting in the megakaryocyte maturation . Thus, haploinsufficiency of RUNX1 might constitutively activate MYH10 expression and affect megakaryocyte polyploidization . RUNX1 also directly regulates the expression of myosin light chain 9 (MYL9) in megakaryocytes. Thus, downregulation of MYL9 due to RUNX1 haploinsufficiency could affect abnormal platelet production and function . Platelet storage pool deficiency and low levels of MPL receptor expression have also been reported [25, 32].
Additional genetic events for mutant clone expansion and leukemia progression
CBL mutations have been found in various myeloid malignancies, such as MDS, AML, and chronic myelomonocytic leukemia (CMML) [33, 34, 35]. CBL mutations have also been identified in the patients with FPD/AML. Shiba et al. reported that CBL mutation and 11q-acquired uniparental disomy (11q-aUPD) cooperate with RUNX1 mutation to develop CMML .
Recently, Yoshimi et al. have shown that somatic mutations in CDC25C gene are found in more than half of the patients with FPD/AML in Japan . Mutant CDC25C disrupts the G2/M checkpoint and promotes cell cycle progression. CDC25C mutation is considered as an early genetic event that generates pre-leukemic clones. This might allow acquiring subsequent genetic events, resulting in overt leukemia progression . In some FPD/AML patients who lack CDC25C mutation, Sakurai et al. identified TET2 mutation or abnormal chromosome 21 (trisomy 21) with chromosomal deletion encompassing RUNX1 locus . Since both genetic events are involved in clonal expansion of HSCs and progenitors (HSPCs) [38, 39, 40, 41], TET2 mutation or trisomy 21 could cooperate with RUNX1 haploinsufficiency to generate pre-leukemic clones. Those patients further acquired additional mutations, such as mutations in RB1, ZRSR2, and BCOR, and eventually developed overt MDS .
On the other hand, a high frequency of RUNX1 biallelic mutations in the patients with FPD/AML has been reported from a French group [42, 43]. Besides RUNX1 biallelic mutations, a variety of mutations were identified at the AML stage, such as mutations in FLT3, KRAS, CBL, TP53, SRSF2, SF3B1, TET2, and DNMT3A . However, CDC25C mutations were not identified in this study . CDC25C mutations were also not detected in the two affected patients with FPD/AML from the Ireland group  and 13 individuals of FPD/AML families from the United States . The difference of ethnic origin of FPD/AML patients is suggested. After the acquisition of secondary genetic events, affected individuals of FPD/AML develop myeloid malignancies in many cases. However, some affected individuals also develop lymphoid neoplasms [20, 46, 47, 48].
Accumulating evidences have indicated that there are age-related hematopoietic clones which have somatic mutations associated with hematological malignancies, namely clonal hematopoiesis of indeterminate potential (CHIP) [49, 50, 51]. Several hematological malignancy-related mutations (such as DNMT3A, TET2, and ASXL1) were identified in around 10–20% of healthy individuals >65–70 years of age, while the detectable mutations were very rare in a younger cohort <40 years of age. On the other hand, a recent analysis of familial MDS/AML revealed that clonal hematopoiesis was detected in 67% of asymptomatic RUNX1 mutant carriers <50 years of age . Further study with a larger cohort will be required to determine whether detecting clonally skewed clones could predict a risk for leukemic transformation.
Recent advance on FPD/AML research using human patient-derived iPS cells
Although studies using transgenic mice have revealed many critical roles of RUNX1 in hematopoiesis, there is a clear difference between mice and humans. For instance, haploinsufficiency of RUNX1 causes thrombocytopenia in the individuals of FPD/AML families. On the other hand, Runx1 haploinsufficiency does not generate observable phenotypes in the megakaryocytes and platelets [29, 52], while Runx1 homozygous deletion results in thrombocytopenia in mice [4, 5]. Considering the difference of the RUNX1 gene dosage effect on phenotypes between humans and mice, there is no animal model that faithfully recapitulates clinically relevant pathogenesis of human FPD/AML. To overcome this problem, studies using human cells were conducted. Sakurai et al. successfully established induced pluripotent stem cells (iPSCs) from three pedigrees of FPD/AML (FPD-iPSCs) . FPD-iPSCs presented with the defect of megakaryocyte differentiation. This defective phenotype was rescued by overexpression of wild-type RUNX1 in FPD-iPSCs. Connelly et al. and Iizuka et al. also established FPD-iPSCs from the patients in FPD/AML families and found defective megakaryocytic differentiation in FPD-iPSCs [54, 55]. Importantly, this megakaryocyte phenotype was able to be rescued by gene-targeting correction of RUNX1 mutation [54, 55]. Patient-derived iPSCs would be a useful tool to dissect the pathogenesis of FPD/AML and the process of leukemic transformation in human hematopoietic cells . Gene correction of RUNX1 mutations might also hold the promise of establishing a new therapeutic strategy for FPD/AML patients [54, 55].
Clinical management for FPD/AML
Currently, the guidelines of clinical management for patients and their families with familial MDS/AL predisposition syndromes are proposed . At diagnosis of FPD/AML, complete blood count (CBC), bone marrow biopsy and aspiration with cytogenetic analysis, and HLA typing of both the patients and their siblings are recommended. During follow-up, annual testing of CBC and clinical examination (every 6 months), bone marrow biopsy (when any worrying changes in CBC are observed), and consultation with hematology prior to any invasive procedures are recommended . When a patient is identified as an affected individual of FPD/AML and needs to undergo allogeneic stem cell transplantation, a sibling donor, who also has RUNX1 mutations, should not be chosen. Indeed, it has been reported that the use of donors who carry the same germ line mutations results in poor outcome [20, 57]. In such a case, the use of HLA-matched unrelated donors should be considered .
FPD/AML is a rare autosomal dominant inherited disorder, which is derived from germ line RUNX1 mutations. However, the awareness of FPD/AML is increasing. The incidence might still be underestimated due to its diversity of disease onset and clinical features. Similar to the cases of sporadic MDS or AML, acquisition of additional mutations and clone selection are critical steps for disease progression in FPD/AML. Prominent clonal hematopoiesis is also observed in FPD/AML. Further elucidation for clonal expansion and disease progression in FPD/AML will provide a better understanding of FPD/AML pathogenesis and may lead to the establishment of better management and therapeutic strategies for not only FPD/AML patients but also many other individuals of CHIP or hematopoietic malignancies.
We thank G. Freudiger (Cincinnati Children’s Hospital Medical Center) for critically reading the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare no conflict of interest.
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