Current approaches to leukemia treatment predominantly aim to improve outcomes by optimizing treatment after the malignancy is diagnosed. Instead, we wanted to investigate opportunities for pre-emptive treatment. In our recently published study in Leukemia, we focused on Shwachman-Diamond syndrome (SDS), a type of inherited bone marrow failure syndrome (IBMFS), with a high predisposition for myeloid malignancies such as myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). Using a novel zebrafish line carrying mutations in dnajc21, a poorly characterized SDS gene with canonical functions in ribosomal biogenesis(1), we unravel previously unknown functions for Dnajc21 in regulating nucleotide metabolism and propose the use of nucleoside supplementation to alleviate neutropenia in SDS.  

How can studying SDS help us find pre-emptive treatments for leukemia?

Patients with SDS have a 36% higher risk of developing myeloid malignancies by 30 years of age compared to the normal population(2,3). Hematological presentations are predominated by neutropenia, but also anemia and thrombocytopenia, all pointing to a failing bone marrow. It is thus perplexing as to how a failing marrow in SDS converts to a highly proliferative one in AML. We hypothesized that altered metabolism may be the key to this dramatic functional change. Furthermore, pancreatic insufficiency and nutritional malabsorption make the case for metabolic adaptations stronger in SDS-AML. We reasoned that therapeutically targeting these alterations may be a viable strategy for rescuing cytopenia and preventing subsequent malignant transformation.

What were the major findings of our study?

We generated the first animal model for dnajc21 mutations using the zebrafish. Our mutants accurately recapitulated salient SDS patient phenotypes such as cytopenia (neutropenia, anemia and lymphopenia), short stature and reduced global protein synthesis. Dnajc21 loss inhibited hematopoietic differentiation, increased endogenous DNA damage and reduced cell proliferation, leading to cytopenia. Like the homozygous loss of DNAJC21 tolerated in patients, our homozygous mutant fish were viable to adulthood, providing an opportunity to study malignant transformation. dnajc21 mutant marrows displayed characteristics of MDS, including erythroid dysplasia and expanded immature progenitors, both of which worsened with the introduction of a second mutation in the tp53 gene. By performing transcriptomics and metabolomics on dnajc21 mutant embryos and marrows, we identified numerous alterations in nucleotide biosynthesis, leading to deficiencies in both purine and pyrimidine nucleotides. Supplementing mutant embryos with either uridine or thymidine nucleosides improved growth and ameliorated neutropenia.

Where to from here?

Our study unveils a novel role for Dnajc21 in regulating nucleotide biosynthesis and its importance for neutrophil differentiation. We provide the first preliminary evidence for pyrimidine nucleoside supplementation in rescuing neutropenia in dnajc21-mutant SDS. Targeting the purine biosynthesis pathway may be similarly beneficial. Further validation using mice and patient-derived models will increase the translational potential of our findings and help establish dosing and toxicity profiles, ultimately paving the way for therapeutic studies in SDS patients.

References

1. Tummala H, Walne AJ, Williams M, Bockett N, Collopy L, Cardoso S, et al. DNAJC21 Mutations Link a Cancer-Prone Bone Marrow Failure Syndrome to Corruption in 60S Ribosome Subunit Maturation. Am J Hum Genet. 2016 Jul 7;99(1):115–24.
2. Donadieu J, Leblanc T, Bader Meunier B, Barkaoui M, Fenneteau O, Bertrand Y, et al. Analysis of risk factors for myelodysplasias, leukemias and death from infection among patients with congenital neutropenia. Experience of the French Severe Chronic Neutropenia Study Group. Haematologica. 2005 Jan;90(1):45–53.
3. Burroughs L, Woolfrey A, Shimamura A. Shwachman Diamond Syndrome – a review of the clinical presentation, molecular pathogenesis, diagnosis, and treatment. Hematol Oncol Clin North Am. 2009 Apr;23(2):233–48.