Mutation of Murine Rpl5 reveals a New Model for Diamond Blackfan Anemia Characterized By Defective Erythropoiesis

Background

Ribosome dysfunction is implicated in several abnormal developmental conditions and diseases in humans. Heterozygous germline ribosomal protein (RP) variants are found in the majority of individuals with Diamond Blackfan anemia (DBA) with RPL5 (uL18) being the second most frequently mutated gene. Patients with RPL5 mutations are for unknown reasons at increased risk of congenital defects when compared to other genotypes. There is currently no available Rpl5 mouse model with anemia and therefore hematopoiesis in this genotype has not been well studied.

Methods

We characterized a novel mutant mouse with a growth and kinky tail defect, and increased mortality. The mouse was generated in a chromosome 11 balancer ENU mutagenesis screen. Following genetic mapping and whole exome sequencing, we identified a heterozygous intronic Rpl5 missense variant, which segregated with all affected mice. We first performed molecular analyses to validate the impact of this variant on RPL5 and ribosome levels, and then examined the development and physiology of multiple tissues by histology, micro-CT, echocardiogram and measurement of erythroid differentiation to determine the etiology of the observed increased mortality in newborn Rpl5 mutant mice.

Results

This variant led to decreased Rpl5 mRNA and protein, and consequential polysome defects (decreased 60S and 80S ribosome subunits) consistent with RPL5 haploinsufficiency. All Rpl5+/- neonates exhibited deficient terminal erythroid differentiation. A fraction of the mutants progressed to severe anemia leading to increased mortality while others appeared to undergo spontaneous remission. Surviving adult/aged mice exhibited no anemia although preliminary studies indicate reduced numbers of hematopoietic progenitors. Most newborn mutant mice had large ventricular septal defects (VSD) while surviving adult mice had small VSDs on echocardiogram. Rpl5+/- mice were significantly smaller without craniofacial abnormalities as assessed by micro-CT or histology. Bone and cartilage staining of newborn mice revealed that the kinky tail defect resulted from delayed endochondral ossification.

Conclusion

This model recapitulates the major developmental and erythroid differentiation defects of DBA in an animal model. We propose that this novel Rpl5 mutant provides an opportunity to study the mechanism of the variable penetrance of anemia and the congenital defects normally encountered in DBA.