Ribosomal protein haploinsufficiency occurs in diverse human diseases including Diamond-Blackfan anemia (DBA),1,2 congenital asplenia,3 and T-cell leukemia.4 Yet how mutations in such ubiquitously expressed proteins result in cell-type and tissue specific defects remains a mystery.5 Here, we show that GATA1 mutations that reduce full-length protein levels of this critical hematopoietic transcription factor can cause DBA in rare instances. We show that ribosomal protein haploinsufficiency, the more common cause of DBA, can similarly reduce translation of GATA1 mRNA - a phenomenon that appears to result from this mRNA having a higher threshold for initiation of translation. In primary hematopoietic cells from patients with RPS19 mutations, a transcriptional signature of GATA1 target genes is globally and specifically reduced, confirming that the activity, but not the mRNA level, of GATA1 is reduced in DBA patients with ribosomal protein mutations. The defective hematopoiesis observed in DBA patients with ribosomal protein haploinsufficiency can be at least partially overcome by increasing GATA1 protein levels. Our results provide a paradigm by which selective defects in translation due to mutations in ubiquitous ribosomal proteins can result in human disease.

Genome-wide association studies (GWASs) have identified a genetic variant of moderate effect size at 6p21.1 associated with erythrocyte traits in humans. We show that this variant affects an erythroid-specific enhancer of CCND3. A Ccnd3 knockout mouse phenocopies these erythroid phenotypes, with a dramatic increase in erythrocyte size and a concomitant decrease in erythrocyte number. By examining human and mouse primary erythroid cells, we demonstrate that the CCND3 gene product cyclin D3 regulates the number of cell divisions that erythroid precursors undergo during terminal differentiation, thereby controlling erythrocyte size and number. We illustrate how cell type-specific specialization can occur for general cell cycle components-a finding resulting from the biological follow-up of unbiased human genetic studies.

Significance Mouse models have been instrumental in advancing our understanding of blood cell production. Although many studies have suggested specific differences between human and mouse red cell production (erythropoiesis), a global study of such similarities and differences has been lacking. By computationally comparing global gene expression data from adult human and mouse erythroid precursors representing the distinct stages of maturation, we showed that, while the overall transcriptional landscape has changed, critical erythroid gene signatures and transcriptional regulators have remained conserved. Importantly, these analyses can serve as a tool to integrate data between human and mouse erythropoiesis research, explain why certain human blood diseases are not faithfully recapitulated in mouse models, and highlight hurdles in translating therapeutic findings from mice to humans.