Somatic Mutations in Splicing Factor 3b, Subunit 1 (SF3B1) Are a Useful Biomarker to Differentiate Between Clonal and Non-Clonal Causes of Sideroblastic Anemia

Background: Sideroblastic anemia (SA) can present as congenital SA (CSA), acquired clonal SA, and acquired reversible SA. Patients (pts) with SA have anemia and ring sideroblasts (RS). Acquired clonal SA is often linked to myelodysplastic syndromes (MDS) or myelodysplastic/ myeloproliferative neoplasms (MDS/MPN). Clinico-pathologic overlap features, unmet morphologic and/or cytogenetic criteria complicate the diagnosis of SA leading to delayed therapies. Currently the diagnosis of SA is based on bone marrow (BM) examination and routine blood tests. There is a need to find easily testable biomarkers that can lead to faster diagnosis of clonal and non-clonal SA. Somatic mutation in splicing factor 3b, subunit 1 (SF3B1) are frequent in MDS-RS and MDS/MPN-RS and have been closely associated with RS.

Objective: SF3B1 mutations can be a useful diagnostic biomarker for pts with acquired clonal SA who present with cytopenias and/or minimal morphologic changes suspicious of MDS and MDS/MPN but not sufficient to make a definitive diagnosis.

Patients and Methods: Six pts with SA at presentation and seen at Cleveland Clinic were included in this study. The median age was 38 years (range, 6-75). Blood tests and BM biopsy showed persistent anemia [Hgb, 10.5 g/dL (range, 8.8-13)], RS [numerous (3 pts), 15% (1 pt), rare (1 pt) and >15% (2 pts)], 3/6 pts had minimal erythrodysplasia with 1 pt having a mild megakaryocytic dysplasia, 3 pts had hypercellular (60-90%), 2 pts had normocellular (50%, 80%) and 1 pt had hypocellular BM (30%) for age, and < 5% BM (2%=2 pts; 1%=1 pt; 3%=1 pt). Two pts had PLT count > 400 k/uL, 3 pts had > 100 k/uL, and 1pt <100 k/uL. Leukocytosis was observed in 2, leukopenia in 1, and neutropenia in 2 pts. Iron studies were unremarkable Molecular analysis was performed on SF3B1 (all exons), PRPF8 (exons 15, 25, 33), U2AF1 (exons 1, 6, 7), SRSF2 (exons 1, 2), DNMT3A (18-23), EZH2 (exons 18, 19) and IDH1/2 (exon 4). BCR/ABL p210 transcripts were assessed by RT-PCR. JAK2 (exon 14) was assessed by melting curve analysis.

Results: All pts presented with sustained anemia, RS w/o or with minimal BM dysplasia, and normal karyotype. SF3B1 mutations (K666N and K700E) were detected in 2 pts (pts#2 and 6) before the clonal disease manifested. The first pt was a 75-year-old man with leukocytosis, transfusion-independent anemia, and fatigue. At the time of SF3B1 molecular testing the BM was hypercellular (60%) with <10%, megakaryocytic dysplasia, 2% blasts and 15% RS. JAK2 and BCR-ABL were wild type (WT). Later on, the patient developed hepatomegaly, his cytopenias worsened requiring RBC transfusions, and a repeat BM biopsy showed more prominent dysplasia of erythroid cells and megakaryocytes. The second pt was a 51-year-old man with fatigue, low WBC= 3.0 k/uL, ANC=0.7 k/uL, and PLTs=113 k/uL and macrocytic anemia (MCV=106 fL and Hgb=12.2 g/dL). His spleen was borderline enlarged (14 cm). BM biopsy showed 90% cellularity, erythroid hyperplasia, rare dyserythropoiesis and 60% RS. Suspicion for a possible MDS was raised although morphology was inconclusive. SF3B1 mutation (K700E) confirmed the clonality of the disease. A few months later, the pt developed worsening counts and overt MDS was confirmed by BM biopsy. All SF3B1^WT pts were WT also for other genes known to be mutated in MDS and MDS/MPN like PRPF8, U2AF1, SRSF2, DNMT3A, IDH1/2, and EZH2. Only genetic tests showed an ALAS2 mutation in pt#1 and a paternally inherited duplication of 10q23.31 in pt#3. Both pts were diagnosed with CSA. Clinical history and work up assessed the same diagnosis for pt#4. Pt#5 presented with neutropenia and anemia. BM biopsy showed no dysplasia, 50% BM cellularity, trilineage maturation, and 25% RS. The presence of RS appeared to be induced by the use of an antibiotic prior the BM biopsy. Patient’s cytopenias and RS resolved after drug cessation.

Conclusion: This case series although conducted in few pts provides important insights in the diagnostic use of molecular genetics in clinical practice. Often SA pts come to the clinic with inconclusive morphologic features of MDS or MDS/MPN. The presence of SF3B1 mutations may serve as an additional tool that can help differentiating between clonal and non-clonal cases of SA. In both pts, SF3B1 mutations anticipated the subsequent overt manifestation of clinical MDS or MDS/MPN phenotype. The collection of a larger cohort of pts is ongoing in order to further confirm this finding.