Fetal Erythropoiesis Is Defective in Rpl11 Heterozygous Mice and Increases in Severity in Young Animals

Diamond Blackfan Anemia (DBA) is a congenital red blood cell aplasia usually caused by haploinsufficiency of selected ribosomal protein genes. Approximately 10% of DBA patients are anemic at birth, and 80% of DBA patients present by 6 months. These and other data suggest that the DBA erythroid defect is exacerbated by the switch from fetal to adult hematopoiesis. We hypothesize that unique molecular or cellular features initiate DBA pathogenesis during fetal hematopoiesis that manifests after the switch to adult erythropoiesis as severe anemia.

To test this hypothesis, we developed a mouse model with constitutive Rpl11 haploinsufficiency in fetal and adult hematopoietic tissues driven by VavCre. Rpl11 is a commonly mutated ribosomal gene in DBA. In adult mice, reduced Rpl11 expression results in severe anemia by 6 weeks, with markedly reduced hemoglobin, increased red cell volume and elevated erythroid adenosine deaminase. Flow cytometric analysis of the bone marrow (BM) revealed no change in the frequency of megakaryocyte-erythroid progenitors (MEP) but a significant increase in both BFU-E and CFU-E, with a corresponding decrease in maturing Ter119⁺ cells. Functional progenitor assays showed reduced and delayed proliferation with an impaired ability to produce viable CD71⁺ Ter119⁺ cells. Treatment of purified Rpl11^+/- BFU-E with dexamethasone rescued this proliferative defect. Thus, our data indicate that a differentiation block and functional impairment at the BFU-E stage of adult erythropoiesis contribute to Rpl11 haploinsufficient anemia.

At the molecular level, Rpl11 haploinsufficiency in BM resulted in the accumulation of 32S and 12S pre-rRNA. Polysome profiling of total BM suggested reduced polysome assembly and impaired translation. Transcriptional profiling of normal and Rpl11^+/- MEP, BFU-E, CFU-E and Ter119⁺ cells identified 13 differentially expressed genes across DBA erythropoiesis including known and novel targets such as CDKN1A, TNFSF4 and TSPAN14 . Enrichment of the p53 pathway was unique to BFU-E and CFU-E progenitors. Interestingly, TGFB1 and EGF pathways were also selectively affected within Rpl11^+/- BFU-E and CFU-E progenitors, suggesting p53-independent self-renewal and proliferative defects in these populations.

In contrast to adult hematopoiesis, fetal hematopoiesis defects are less severe. For example, fetal BFU-E are not significantly increased within E14 fetal livers and functionally generate a similar number of maturing erythroblasts in vitro . Lodish and colleagues have established that fetal liver BFU-E comprise at least two distinct subpopulations, termed "early" and "late". Early BFU-E have high self-renewing capacity whereas late BFU-E have low self-renewing capacity. To determine whether the balance between early and late BFU-E is affected by Rpl11 haploinsufficiency, and to identify gene expression changes in early and late fetal Rpl11^+/- BFU-E, we performed single cell RNA-sequencing.

Normal fetal livers contained similar proportions of early and late BFU-E, whereas normal adult bone marrows only contained late BFU-E. Surprisingly, Rpl11^+/- fetal livers primarily contained late BFU-E. One explanation for this observation is that early BFU-E are depleted from Rpl11^+/- fetal livers as this population compensates for reduced function of non-renewing late BFU-E. Interestingly, late BFU-E from Rpl11^+/- BM have a stronger "late" signature than their normal counterparts suggesting a reduced self-renewing capacity of BM BFU-E in DBA. We are currently testing this hypothesis by overexpressing early genes associated with BFU-E self-renewal in Rpl11^+/- BFU-E and reassessing erythroid phenotypes.

In conclusion, our studies show that erythropoiesis is defective in Rpl11^+/- fetal liver at the cellular and molecular level, although functional impairment is not as marked as in adult mice. Moreover, single cell RNA-sequencing of BM and fetal BFU-E in our Rpl11^+/- mouse model has uncovered a novel hypothesis for DBA pathogenesis that the loss of early, self-renewing BFU-E in fetal liver is a harbinger of adult erythroid failure.