Aetiology and outcomes of secondary myelofibrosis occurring in the context of inherited platelet disorders: A single institutional study of four patients

Abstract

Primary myelofibrosis (PMF) is a clonal stem cell disorder that usually presents in adulthood with cytopenias, leukoerythroblastosis, extramedullary hematopoiesis, bone marrow (BM) fibrosis and a tendency for transformation to acute myeloid leukaemia.1 Driver mutations encountered in PMF include V617F in JAK2 (50–60%) and variants in CALR (10–15%) and MPL (5–10%).1 Myelofibrosis occurrence in children and young adults is rare, often non-neoplastic and difficult to diagnose.2 Several reports have described paediatric patients with myelofibrosis occurring in the setting of inherited platelet disorders (IPD).2-7 Prominent examples include the gray platelet syndrome (GPS), with variants in NBEAL2 or GFI1B, as well as other platelet disorders associated with variants in MPIG6B (anaemia, thrombocytopenia and myelofibrosis), Src family kinase (SFK) member SRC (thrombocytopenia, myelofibrosis, bleeding and bone pathologies) and VPS45 (neutropenia, neutrophil dysfunction, myelofibrosis, organomegaly and thrombasthenia).3-8 Our study describes the characteristics, aetiology, and outcomes of patients with IPD who developed BM fibrosis at our institution. After institutional review board approval, we identified patients with IPD from our platelet disorder database who developed BM fibrosis. Most patients had undergone a detailed work-up that included platelet aggregation studies, light microscopy, platelet transmission electron microscopy (PTEM) and flow cytometry (Supplementary Methods). All patients underwent BM biopsies with morphology, reticulin staining, cytogenetics and molecular genetics where indicated. We only included patients who were subjected to platelet-specific gene panel testing or research-based whole exome sequencing (WES) (Supplemetary Methods). We identified four female patients with IPD and BM fibrosis (Table I). Three patients (75·0%) had GPS. The median age at diagnosis of GPS was 15 years, while BM fibrosis was first objectively detected at a median age of 44 years (range, 39–49). All patients included had light microscopic and/or PTEM features consistent with GPS, with genetic confirmation. WES on one patient revealed two heterozygous variants in NBEAL2 (NM_015175·2:c. [3384 + 5G>A];[5965G> A] - p.[?];[(Glu1989Lys)]). These variants were reported as variants of uncertain significance (VUS) following the American College of Medical Genetics and Genomics guideline.9 However, RNA sequencing performed on whole blood indicated that the c.3384 + 5G>A variant impacts the splicing of the transcript (Table SI). This allowed us to reclassify the variant as likely pathogenic. We could not perform parental testing to confirm that the variants were in opposite alleles. Two other patients were found to have homozygous NBEAL2 variants (NM_015175·2:c.7604del,p.(Gly2535Valfs*5) by WES and c.1823G> A,p.(Trp608*) by platelet-specific gene panel).10 BM fibrosis was graded as moderate with a score of +2 (0–3 scale) in one patient (33·3%) and mild fibrosis with a score of +1 in two patients (66·7%). Two patients (66·7%) had subtle to mild dysplasia (not diagnostic of myelodysplastic syndrome) in one or more cell lines. BM biopsies were only performed to assess worsening cytopenias. Two patients (66·7%) had palpable splenomegaly. Two patients (66·7%) have since been clinically observed, while one patient is being evaluated for allogeneic haematopoietic cell transplantation (HCT). Figure S1 shows the trend of haemoglobin, platelet count and white blood cell count over years where longitudinal follow-up data are available. Markedly hypercellular; subtle dyserythropoiesis & dysgranulopoiesis; grade 1/3 reticulin fibrosis 9 3·7 Observation 0·8 Heterozygous NBEAL2 variant: c.5965G> A, p.(Glu1989Lys) One woman in our cohort was diagnosed with MPIG6B-related IPD at age 40 years. She is of European descent and from a consanguineous family. She was diagnosed with Evans syndrome at age five years after presenting with symptoms of easy bruising. Over the years, she was treated with several agents, including systemic glucocorticoids, intravenous immunoglobulin therapy and cyclophosphamide with weekly vincristine. At age 17 years, she underwent splenectomy with a brief improvement in her platelet counts. At age 26 years, the patient was seen at our institution and had a platelet count of 57 × 109/l, haemoglobin of 11·8 g/dl and white blood cell count of 7.9 × 109/l. BM biopsy showed 80% cellularity with adequate megakaryocytes and mild grade (+1) reticulin fibrosis. Testing for JAK2 V617F and variants in exon 9 of CALR was negative. PTEM showed an increased number of large platelets with complex canalicular membrane systems and a markedly decreased concentration of alpha granules. WES identified a homozygous pathogenic variant in MPIG6B (c.469G> A, p.(Gly157Arg)). Repeat BM biopsy at age 41 years (platelet count 29 × 109/l) demonstrated a hypercellular BM with increased erythropoiesis and granulopoiesis, decreased megakaryopoiesis and moderate grade (+2) reticulin fibrosis (Fig 1). She is currently being evaluated for allogeneic HCT. This is the first individual of European descent with MPIG6B-related myelofibrosis reported in the literature. Early-onset BM fibrosis is usually acquired. Potential underlying aetiologies include autoimmune disorders (systemic lupus erythematosus, scleroderma, mixed connective tissue disease and polymyositis), vitamin D deficiency with secondary hyperparathyroidism, renal osteodystrophy and infections (visceral leishmaniasis and tuberculosis).11-13 Although rare, BM fibrosis associated with IPD should be considered in patients with antecedent personal or family history of quantitative or qualitative platelet abnormalities. BM fibrosis in GPS is thought to be related to the premature release of profibrotic substances, including transforming growth factor β1 and platelet-derived growth factor, from megakaryocytes into the BM.14 Promotion of collagen synthesis may be compounded by the release of platelet factor 4, an inhibitor of collagenase.14 Based on previous series, in most patients with GPS, BM fibrosis is absent during childhood and develops in adulthood, starting in the third decade with subsequent progressive worsening.15 Progressive BM fibrosis in IPD often leads to worsening cytopenias, splenomegaly and an increase in transfusion requirements. The latter can result in alloimmunization, potentially interfering with HCT plans. Although human G6b has been shown to rescue the perimegakaryocytic pattern of BM fibrosis in G6b knockout mice, the mechanism of BM fibrosis in MPIG6B-related IPD has not been clearly delineated.16 In our institutional series, GPS was the most common monogenic IPD associated with BM fibrosis. In both GPS and MPIG6B-related IPD, we observed that BM fibrosis leads to worsening blood counts and increasing transfusion needs. Therefore, early identification of IPD, prompt recognition of the potential for BM fibrosis and close follow-up of blood counts are key elements for timely diagnosis and optimal management of these patients. The authors have no conflicts of interest to disclose. A.N.S., D.C., A.F., F.P.V., M.E.S. and M.M.P. performed the research. A.N.S. and M.M.P. analysed the data and wrote the manuscript draft. D.C. and M.E.S. contributed the figure for the study. A.N.S., R.P., D.C., A.F., F.P.V., E.W.K., M.E.S., N.G., A.T., A.M., A.B., A.D.B., A.H., J.M. and M.M.P. designed the research study and wrote and reviewed the paper. All authors were involved in reviewing and editing the manuscript. All authors approved the final version. No external funding was used. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

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