Refractory macrocytic anemias in patients with clonal hematopoietic disorders and isolated mutations of the spliceosome gene ZRSR2

Although mutations in RNA splicing genes occur frequently in patients with clonal cytopenias of unknown significance (CCUS) and myelodysplastic syndromes (MDS), very often additional common myeloid gene driver mutations are present at diagnosis. Thus, the clinical significance of isolated mutations in the most commonly mutated RNA splicing genes remains unknown. Here we report five unusual patients with an isolated mutation causing a loss of function of ZRSR2, a protein required for recognition of a functional 3′ splice site. Two of the patients had a diagnosis of CCUS and three patients had an MDS disorder characterized by low risk features and absence of complex cytogenetic abnor-malities. Notably, all five cases were characterized predominantly by macrocytic anemia. In addition, one CCUS patient followed for more than 15 years with a transfusion dependent macrocytic anemia was found to have an inactivating ZRSR2 mutation with an allele frequency of >60%. We conclude that the common clinical features of patients with an isolated mutation of ZRSR2 are a macrocytic anemia without leukopenia, thrombocytopenia or an increase in marrow blast percentage. At least in some cases, the presence of an isolated ZRSR2 mutation can accompany a dominant hematopoietic clone with a low risk for transformation to frank dysplasia or acute leukemia.