Suppression of the Hematopoietic Defect in TF-1 Cells Depleted of Shwachman-Diamond Syndrome Protein: Correlation with Decreased eIF6 Levels

Abstract 1270

Shwachman Diamond syndrome (SDS) is a rare autosomal recessive bone marrow failure syndrome mainly characterized by neutropenia, exocrine pancreatic insufficiency and an increased risk of myelodysplastic syndrome and leukemia. The phenotype in patients is variable for unclear reasons, but approximately 90% of patients have biallelic mutations in the SBDS gene. At least one action of the SBDS protein is to couple with the GTPase ELF1 to facilitate release of the eIF6 protein from the 60S ribosome subunit, thus enabling joining of the 60S and 40S ribosome subunits, a function that has prompted many to consider SDS a “ribosomopathy”. We created a cellular model of SDS using TF-1 erythroleukemia cells transduced with lentiviral vectors containing two different shRNAs against SBDS or a scrambled sequence. Clones were grown under puromycin selection and a clone from each shRNA was selected. In each clone, knockdown of SBDS was verified at the protein level by western blot, and expression levels of SBDS were less than 1%. Both clones underwent differentiation to either myeloid or erythroid colonies by culturing in GM-CSF or erythropoietin, respectively. The 2–12 clone had a significant decrease in the number and size of both myeloid and erythroid colonies (see Table) when compared with the scrambled shRNA control. In contrast, the 1–7 clone had the same number of myeloid and erythroid colonies as the control. Previous work by other investigators in SDS yeast models revealed that missense mutations in the anti-association factor, Tif6 suppress the slow growth phenotype of SDS-mutant yeast cells. In exploring the molecular basis for the difference in phenotype observed in our TF-1 cells, we therefore focused on eIF6, the human ortholog of Tif6. The 2–12 clone had similar expression of the eIF6 protein when compared to the scrambled control. However, the 1–7 clone had a significantly decreased amount of eIF6 protein compared to the control. DNA sequencing did not reveal any mutations in the eIF6 gene, and quantitative RT-PCR showed similar levels of eIF6 mRNA transcripts, suggesting that the differences in eIF6 protein levels may be due to post-translational modifications. Pressato and colleagues (Br J Haematol 157:503, 2012) have recently speculated that the relatively benign course of SDS patients with a deletion of chromosome 20q may be due to loss of the eIF6 protein (whose gene is located on 20q). Our findings add to the hypothesis that antagonizing eIF6 may modify or rescue the SDS phenotype, possibly by reducing the requirement of SBDS in giving rise to 60S subunits lacking eIF6.