Dysregulation of the TGF β Pathway in Induced Pluripotent Stem Cells (iPSCs) Generated from Patients with Diamond Blackfan Anemia (DBA)

DBA is an inherited bone marrow failure syndrome that usually develops in the first year of life with clinical features of red cell aplasia and sometimes variable developmental abnormalities. Most affected patients have heterozygous loss of function mutations of one of the 11 ribosomal proteins (RPs) or mutations in the GATA1 gene which encodes an erythroid specific transcription factor. We have previously demonstrated that induced pluripotent stem cells (iPSCs) from fibroblast of DBA patients with RPS19 or RPL5 mutations recapitulate the pathogenesis of DBA, with the mutant lines showing abnormal ribosome biogenesis and altered erythropoiesis. The mechanism whereby haploinsufficiency for RPs causes failure of erythropoiesis and the other DBA features is still unknown. We investigated the pathways that are affected in these DBA iPSCs using an Affymetrix human exon array, and we observed the striking dysregulation of the TGF β pathway in DBA lines. The TGF β downstream target genes, such as DKK1, BAMBI, FN1, COL3A1, COLA1A1 and PAI-1 significantly increased in the DBA iPSCs. The TGF β signaling is complex and can occur via a canonical pathway or by a number of non-canonical pathways. We measured levels of a number of intermediates in these pathways by western blot, and observed a significant increase in the levels of p-JNK, a mediator of a non-canonical pathway, in the DBA iPSCs. Moreover, when the mutant cells were corrected by ectopic expression of WT RPS19 or RPL5, levels of p-JNK returned to normal. We also investigated the SMAD family, which are mediators of the TGF β canonical pathway and are known to negatively regulate the regeneration of hematopoietic stem cells. We observed a drastic decrease in SMAD4, but no change in p-SMAD2. Again corrected lines showed normal expression levels of SMAD4. Our data suggests that the activation of a non-canonical TGF β pathway in the DBA iPSCs may lead increased expression of the downstream genes; and the decrease of anti-proliferative factor SMAD4 may explain how DBA iPSCs maintain their growth. We conclude that the mutations of RPS19 or RPL5 both affect ribosome biogenesis and TGF β signaling, which can cause the failure of erythropoiesis at the stem cell stage. We further suggest that the suppression of SMAD4 may be used as a therapeutic target for DBA treatment.