Mutations in Ribosomal Protein Genes L9 and L15 Link to DBA with Specific Clinical Phenotypes

Diamond-Blackfan anemia (DBA) is a rare inherited bone marrow failure disorder linked predominantly to ribosomal protein (RP) gene mutations. The European (EuroDBA) consortium reports here new mutations identified in RP genes L15 and L9 in patients with DBA. Although point mutations have not been previously reported for RPL15 in DBA, we identified 6 unrelated children of whom 4 carried truncating mutations c.242dupA; p.Tyr81X (N=3) and c.85C>T; p.Gln29X (N=1). Additional 2 patients carried RPL15 missense variants c.29T>C; p.Leu10Pro and c.458A>C; p.Lys153Thr. Notably, of the 4 patients with truncating mutations, 3 manifested prenatally with severe hydrops fetalis and required between 4-9 intrauterine transfusions. Even more remarkable is the observation that 3/4 patients with this mutation type rapidly became treatment-independent and maintained normal blood counts until the last follow-up (age 5-16 years). This finding suggests that a previously unknown correlation between genotype and hematological phenotype may underlie the tendency of some DBA patients to undergo treatment independence. Additionally we report a pathogenic splice donor mutation c. -2+1G>C in RPL9, a gene that has not been previously implicated in DBA pathogenesis. The patient presented with DBA and severe colitis at the age of 6 months, and showed dysmorphic features including microcephaly and bilateral thumb abnormality.

Cells carrying either RPL15 or RPL9 mutations revealed pre-rRNA processing defects, reduced 60S ribosomal subunit formation, a reduction of de novo protein synthesis, and stabilization of the TP53 tumor suppressor. Zebrafish models reducing either RP recapitulated a loss of hemoglobin-expressing cells in early development. Red cell culture assays of RPL15 - or RPL9 -mutated primary erythroblast cells revealed reductions of cell proliferation and delayed erythroid differentiation, elevated TP53 activity, and increased apoptosis. Colon tissue from patient with RPL9 mutation revealed intense epithelial TP53 staining, CD3+ lymphocytic infiltrates, and insufficient crypt regeneration

This study identifies RPL9 as a new DBA-linked gene, identifies recurrent truncating hotspot mutation in RPL15, reaffirms a role for TP53 stabilization in disease pathophysiology, and suggests that certain DBA genotypes may be used for risk stratification. To our knowledge, this is the first report on multiple DBA patients presenting with hydrops fetalis or colitis due to haploinsufficiency of specific RP genes.