Detection of an Abnormal Myeloid Clone by Flow Cytometry in Familial Platelet Disorder With Propensity to Myeloid Malignancy

Diagnostic work-up of hematological malignancies with underlying germline predisposition disorders (GPD)

Hematological malignancies with underlying germline predisposition disorders have been recognized by the World Health Organization 5th edition and International Consensus Classification (ICC) classification systems. The list of genes and the associated phenotypes are expanding and involve both pediatric and adult populations. While the clinical presentation and underlying molecular pathogenesis are relatively well described, the knowledge regarding the bone marrow morphologic features, the landscape of somatic aberrations associated with progression to hematological malignancies is limited. These pose challenges in the diagnosis of low-grade myelodysplastic syndrome (MDS) to hematopathologists which carries direct implication for various aspects of clinical management of the patient, donor selection for transplantation, and family members. Here in, we provide a focused review on the diagnostic work-up of hematological malignancies with underlying germline predisposition disorders with emphasis on the spectrum of hematological malignancies associated with each entity, and characteristic bone marrow morphologic, somatic cytogenetic and molecular alterations at the time of diagnosis of hematological malignancies. We also review the key clinical, morphologic, and molecular features, that should initiate screening for these entities.

Germline and somatic drivers in inherited hematologic malignancies

Inherited hematologic malignancies are linked to a heterogenous group of genes, knowledge of which is rapidly expanding using panel-based next-generation sequencing (NGS) or whole-exome/whole-genome sequencing. Importantly, the penetrance for these syndromes is incomplete, and disease development, progression or transformation has critical clinical implications. With the earlier detection of healthy carriers and sequential monitoring of these patients, clonal hematopoiesis and somatic driver variants become significant factors in determining disease transformation/progression and timing of (preemptive) hematopoietic stem cell transplant in these patients. In this review, we shed light on the detection of probable germline predisposition alleles based on diagnostic/prognostic 'somatic' NGS panels. A multi-tier approach including variant allele frequency, bi-allelic inactivation, persistence of a variant upon clinical remission and mutational burden can indicate variants with high pre-test probability. We also discuss the shared underlying biology and frequency of germline and somatic variants affecting the same gene, specifically focusing on variants in DDX41, ETV6, GATA2 and RUNX1. Germline variants in these genes are associated with a (specific) pattern or over-/underrepresentation of somatic molecular or cytogenetic alterations that may help identify the underlying germline syndrome and predict the course of disease in these individuals. This review is based on the current knowledge about somatic drivers in these four syndromes by integrating data from all published patients, thereby providing clinicians with valuable and concise information.

RUNX1-mutated families show phenotype heterogeneity and a somatic mutation profile unique to germline predisposed AML

First reported in 1999, germline runt-related transcription factor 1 (RUNX1) mutations are a well-established cause of familial platelet disorder with predisposition to myeloid malignancy (FPD-MM). We present the clinical phenotypes and genetic mutations detected in 10 novel RUNX1-mutated FPD-MM families. Genomic analyses on these families detected 2 partial gene deletions, 3 novel mutations, and 5 recurrent mutations as the germline RUNX1 alterations leading to FPD-MM. Combining genomic data from the families reported herein with aggregated published data sets resulted in 130 germline RUNX1 families, which allowed us to investigate whether specific germline mutation characteristics (type, location) could explain the large phenotypic heterogeneity between patients with familial platelet disorder and different HMs. Comparing the somatic mutational signatures between the available familial (n = 35) and published sporadic (n = 137) RUNX1-mutated AML patients showed enrichment for somatic mutations affecting the second RUNX1 allele and GATA2. Conversely, we observed a decreased number of somatic mutations affecting NRAS, SRSF2, and DNMT3A and the collective genes associated with CHIP and epigenetic regulation. This is the largest aggregation and analysis of germline RUNX1 mutations performed to date, providing a unique opportunity to examine the factors underlying phenotypic differences and disease progression from FPD to MM.

Secondary leukemia in patients with germline transcription factor mutations (RUNX1, GATA2, CEBPA)

Recognition that germline mutations can predispose individuals to blood cancers, often presenting as secondary leukemias, has largely been driven in the last 20 years by studies of families with inherited mutations in the myeloid transcription factors (TFs) RUNX1, GATA2, and CEBPA. As a result, in 2016, classification of myeloid neoplasms with germline predisposition for each of these and other genes was added to the World Health Organization guidelines. The incidence of germline mutation carriers in the general population or in various clinically presenting patient groups remains poorly defined for reasons including that somatic mutations in these genes are common in blood cancers, and our ability to distinguish germline (inherited or de novo) and somatic mutations is often limited by the laboratory analyses. Knowledge of the regulation of these TFs and their mutant alleles, their interaction with other genes and proteins and the environment, and how these alter the clinical presentation of patients and their leukemias is also incomplete. Outstanding questions that remain for patients with these germline mutations or their treating clinicians include: What is the natural course of the disease? What other symptoms may I develop and when? Can you predict them? Can I prevent them? and What is the best treatment? The resolution of many of the remaining clinical and biological questions and effective evidence-based treatment of patients with these inherited mutations will depend on worldwide partnerships among patients, clinicians, diagnosticians, and researchers to aggregate sufficient longitudinal clinical and laboratory data and integrate these data with model systems.

Bone Marrow Morphology Associated With Germline RUNX1 Mutations in Patients With Familial Platelet Disorder With Associated Myeloid Malignancy

Germline mutations in RUNX1 result in autosomal dominant familial platelet disorder with associated myeloid malignancy (FPDMM). To characterize the hematopathologic features associated with a germline RUNX1 mutation, we reviewed a total of 42 bone marrow aspirates from 14 FPDMM patients, including 24 cases with no cytogenetic clonal abnormalities, and 18 with clonal karyotypes or leukemia. We found that all aspirate smears had ≥10% atypical megakaryocytes, predominantly characterized by small forms with hypolobated and eccentric nuclei, and forms with high nuclear-to-cytoplasmic ratios. Core biopsies showed variable cellularity and variable numbers of megakaryocytes with similar features to those in the aspirates. Granulocytic and/or erythroid dysplasia (≥10% cells per lineage) were present infrequently. Megakaryocytes with separate nuclear lobes were increased in patients with myelodysplastic syndrome (MDS) and acute leukemia. Comparison to an immune thrombocytopenic purpura cohort confirms increased megakaryocytes with hypolobated eccentric nuclei in FPDMM patients. As such, patients with FPDMM often have atypical megakaryocytes with small hypolobated and eccentric nuclei even in the absence of clonal cytogenetic abnormalities; these findings are related to the underlying RUNX1 germline mutation and not diagnostic of MDS. Isolated megakaryocytic dysplasia in patients with unexplained thrombocytopenia should raise the possibility of an underlying germline RUNX1 mutation.

High-grade B-cell lymphomas with TdT expression: a diagnostic and classification dilemma

Mature B-cell neoplasms and immature or precursor B-cell neoplasms need to be distinguished because these patients usually require different therapeutic approaches. B-cell neoplasms that express TdT without unequivocal other features of immaturity may therefore present a diagnostic challenge. We describe 13 patients with TdT-positive aggressive B-cell lymphoma. The clinicopathologic features of these patients were highly heterogeneous, but for the purpose of this study we grouped these cases as follows: (1) de novo high-grade B-cell lymphoma with MYC, BCL2, and/or BCL6 rearrangements (double-hit or triple-hit lymphoma) with TdT expression. In this group we included two cases of de novo composite lymphoma in which there were components of diffuse large B-cell lymphoma and TdT-positive blastic B-cell lymphoma; (2) TdT-positive aggressive B-cell lymphoma arising in patients who previously had follicular lymphoma; (3) initial relapse of TdT-negative aggressive B-cell lymphoma in patients who previously had follicular lymphoma, followed by relapses in which the neoplasm acquired TdT expression; and (4) mature B-cell lymphomas that acquired TdT expression at relapse. This group included one case of EBV-positive diffuse large B-cell lymphoma and one case of pleomorphic variant mantle cell lymphoma. All patients in this study had an aggressive clinical course and a dismal outcome despite appropriate therapy. Rather than "squeezing" these cases into current World Health Organization classification categories, we suggest the use of a descriptive term such as high-grade B-cell lymphoma with TdT expression. In these tumors, the cytogenetic findings and poor prognosis of this patient subgroup suggest that these neoplasms need to be distinguished from B-lymphoblastic leukemia/lymphoma. Segregation of these neoplasms also may foster additional research on these neoplasms.

Bone marrow pathologic abnormalities in familial platelet disorder with propensity for myeloid malignancy and germline RUNX1 mutation

A subset of patients with familial platelet disorder with propensity to myeloid malignancy and germline RUNX1 mutation develops hematological malignancies, often myelodysplastic syndrome/acute myeloid leukemia, currently recognized in the 2016 WHO classification. Patients who develop hematologic malignancies are typically young, respond poorly to conventional therapy, and need allogeneic stem cell transplant from non-familial donors. Understanding the spectrum of bone marrow morphologic and genetic findings in these patients is critical to ensure diagnostic accuracy and develop criteria to recognize the onset of hematologic malignancies, particularly myelodysplastic syndrome. However, bone marrow features remain poorly characterized. To address this knowledge gap, we analyzed the clinicopathologic and genetic findings of 11 patients from 7 pedigrees. Of these, 6 patients did not develop hematologic malignancies over a 22-month follow-up period; 5 patients developed hematologic malignancies (3 acute myeloid leukemia; 2 myelodysplastic syndrome). All patients had thrombocytopenia at initial presentation. All 6 patients who did not develop hematologic malignancies showed baseline bone marrow abnormalities: low-for-age cellularity (n=4), dysmegakaryopoiesis (n=5), megakaryocytic hypoplasia/hyperplasia (n=5), and eosinophilia (n=4). Two patients had multiple immunophenotypic alterations in CD34-positive myeloblasts; 1 patient had clonal hematopoiesis. In contrast, patients who developed hematologic malignancies had additional cytopenia(s) (n=4), abnormal platelet granulation (n=5), bone marrow hypercellularity (n=4), dysplasia in ≥2 lineages including megakaryocytes (n=3) and acquired clonal genetic aberrations (n=5). In conclusion, our study demonstrated that specific bone marrow abnormalities and acquired genetic alterations may be harbingers of progression to hematological malignancies in patients with familial platelet disorder with germline RUNX1 mutation.

RUNX1 deficiency (familial platelet disorder with predisposition to myeloid leukemia, FPDMM)

In this review, we discuss disease-causing alterations of RUNT-related transcription factor 1 (RUNX1), a master regulator of hematopoietic differentiation. Familial platelet disorder with predisposition to myeloid leukemia (FPDMM) typically presents with (1) mild to moderate thrombocytopenia with normal-sized platelets; (2) functional platelets defects leading to prolonged bleeding; and (3) an increased risk to develop myelodysplastic syndromes (MDS), acute myeloid leukemia (AML), or T-cell acute lymphoblastic leukemia (T-ALL). Hematological neoplasms in carriers of a germline RUNX1 mutation need additional secondary mutations or chromosome aberrations to develop. If a disease-causing mutation is known in the family, it is important to prevent hematopoietic stem cell transplantation from a sibling or other relative carrying the familial mutation. First experiments introducing a wild-type copy of RUNX1 into induce pluripotent stem cells (iPSC) lines from patients with FPDMM appear to demonstrate that by gene correction reversal of the phenotype may be possible.

Role of RUNX1 in hematological malignancies

RUNX1 is a member of the core-binding factor family of transcription factors and is indispensable for the establishment of definitive hematopoiesis in vertebrates. RUNX1 is one of the most frequently mutated genes in a variety of hematological malignancies. Germ line mutations in RUNX1 cause familial platelet disorder with associated myeloid malignancies. Somatic mutations and chromosomal rearrangements involving RUNX1 are frequently observed in myelodysplastic syndrome and leukemias of myeloid and lymphoid lineages, that is, acute myeloid leukemia, acute lymphoblastic leukemia, and chronic myelomonocytic leukemia. More recent studies suggest that the wild-type RUNX1 is required for growth and survival of certain types of leukemia cells. The purpose of this review is to discuss the current status of our understanding about the role of RUNX1 in hematological malignancies.