Characterizing Dyskeratosis Congenita Caused By Parn Mutations in the Zebrafish
Authors: Cordeiro Anna , Deveau Adam P , Dhanraj Santhosh , Dror Yigal , Berman Jason 
Published: November 2019

Inherited bone marrow failure syndromes (IBMFs) are a group of rare genetic disorders characterized by deficient hematopoiesis and extra-hematologic traits. Most known entities are related to a specific gene or group of genes, but others still remain unclassified. In many cases, the involved proteins are required for critical processes involved in cell survival, such as ribosome biogenesis, maintenance of telomere length, and DNA repair. Importantly, patients with IBMFs have higher risks of developing a variety of cancers from leukemia to solid tumours of the head and neck. In 2015, we published four patients from three different families with mutations in the Poly(A)-specific ribonuclease (PARN) gene. This gene encodes a ribonuclease which is involved in degradation of the poly(A) tails, which regulate mRNA turnover, and thus gene expression. Three of the patients presented with several degrees of mental illness and/or developmental delay. The fourth patient, harbored both a monoallelic deletion and a point mutation at the catalytic domain of the protein, and presented with bone marrow failure and hypomyelination, similar to a severe form of dyskeratosis congenital (DC) known as Hoyeraal-Hreidarsson syndrome.
In the last two decades, zebrafish (Danio rerio) has emerged as an excellent animal model for human disease, and is especially relevant in hematology, since many of the transcription factors and cell types are highly conserved. Zebrafish have a single PARN ortholog,with 64% sequence identity to the human gene. Using CRISPR-Cas9 genome editing and a combination of six sgRNAs, we generated a 1.2 kb deletion in the zebrafish parn ortholog extending from exon 5 to 13, causing a premature stop codon. Homozygous fish were generated by incrossing to replicate the complete loss-of-function observed in the patient with the DC-like phenotype.
Using whole-mount in situ hybridization (WISH) at 48 hours post-fertilization (hpf), we observed a decrease in the number of several mature myeloid cell lineages including neutrophils (labeled with mpx; p<0.0001), macrophages (lcp1; p=0.0005) and mast cells (cpa5; p=0.0005). We also observed a decrease in the amount of hemoglobin (o-dianisidine staining; p<0.0001). However, the number of hematopoietic stem cells (HSCs) was unchanged in parn mutants. This data parallels similar findings using parn directed splice site and translation start site morpholinos.
PARN is described as a protein involved in RNA processing, but has also been associated with telomere maintenance. This latter process is crucial for cell senescence and genome integrity, and is a known cause of several IBMFSs. The telomerase ribonucleoprotein complex is highly active in hematopoietic stem and progenitor cells (HSPCs) and plays a role in cell differentiation. This complex is composed of a reverse transcriptase (TERT), RNA template (TERC), and the dyskerin protein complex (DKC1), mutations of which represent a common cause of DC. qPCR analysis in zebrafish parn mutants revealed a 2.98-fold reduction in tert expression compared with the wild type fish.
Combined, these findings suggest that PARN plays an important role in HSC differentiation into myeloid and erythroid lineages, resulting in a bone marrow failure phenotype. Our model provides a unique in vivo platform to study the role of PARN in hematopoiesis and for identifying compounds that restore normal blood cell ratios, which may have the potential to prevent future leukemic transformation.

Disclosures
No relevant conflicts of interest to declare.