A Zebrafish Model of Parn-Deficiency Reveals Conserved Mechanisms Underlying Marrow Failure and Opportunities for Therapeutic Amelioration of Hematopoiesis

Dyskeratosis congenita (DC) is a rare inherited bone marrow failure syndrome (IBMFS) and a telomere biology disorder that is characterized by physical malformations such as nail dystrophy, oral leukoplakia, reticular hyperpigmentation, multilineage cytopenia and immunodeficiency. Definitive treatment for the hematological complications is hematopoietic stem cell transplantation (HSCT), which is associated with high risk of organ toxicity and engraftment failure in this population. Further, DC patients are prone to developing myelodysplastic syndrome (MDS) and solid tumors. Germline mutations in the poly(A)-specific ribonuclease (PARN) gene are associated with Hoyeraal–Hreidarsson syndrome, the most severe form of DC. PARN encodes a deadenylase enzyme involved in the 3’-end processing of various RNAs including the telomerase RNA component (TERC). The clinical heterogeneity of phenotypes and the paucity of primary human samples necessitate the use of animal models such as zebrafish (Danio rerio), given their highly conserved genetic program, to elucidate underlying biology.

Zebrafish have a single PARN ortholog with 64% sequence identity at the amino acid level. Using CRISPR-Cas9 mutagenesis, we generated a 1.3 kb deletion in the zebrafish parn gene, predicted to result in premature protein truncation. To measure the hematopoietic output of the parn mutants, we performed whole-mount in situ hybridization (WISH) for key hematopoietic lineage markers. At 48 hours post-fertilization (hpf), we found a reduction in lcp1⁺ total leukocytes, mpx⁺ neutrophils and hbbe3⁺ mature erythrocytes. Impaired erythroid differentiation was accompanied by an accumulation of gata1⁺ erythroid progenitors. This finding was further corroborated by o-dianisidine staining, showing reduced hemoglobinized erythrocytes. Thus, erythrocyte and leukocyte differentiation are compromised in our zebrafish parn mutant, phenocopying the multilineage cytopenia observed in DC patients. Flow cytometry of the whole kidney marrow (WKM, human bone marrow equivalent) at 12 months of age revealed an increase in hematopoietic stem and progenitor cells in the parn mutants. Giemsa staining of WKM touchpreps further identified an increase in myeloid-biased progenitors.

Next, we investigated the consequences of Parn loss on telomerase activity. Using the telomere repeat amplification protocol, we found reduced telomerase activity in the WKMs of parn mutant zebrafish. Using 3’ rapid amplification of cDNA ends, we observed longer ends in the terc amplicons of the parn mutants, reminiscent of defective deadenylation of human TERC RNA in the setting of PARN mutations. Exogenous thymidine supplementation has been shown to support telomere elongation in human cells. Following treatment of zebrafish embryos with thymidine nucleoside, we observed an improvement in erythrocyte numbers in the parn mutants by o-dianisidine staining at 48 hpf. Collectively, our preliminary findings suggest that telomerase-dependent mechanisms may contribute to the erythropenia seen in parn-mutant zebrafish.

In summary, we generated a robust zebrafish model carrying a germline parn mutation that phenocopies the molecular and hematologic features seen in DC patients. Our model holds promise as a new in vivo tool for investigating DC-related disease mechanisms and as a platform for preclinical therapeutic screening.