AK2 Deficiency In Zebrafish Recapitulates Human Reticular Dysgenesis, An Autosomal Recessive Form Of Severe Combined Immunodeficiency

The adenylate kinase (AK) gene family consists of 7 different members that contribute to energy cell metabolism by converting ATP+AMP to 2ADP. AKs are critical players in ensuring cellular energy homeostasis in all tissues. Mutations in the AK2 gene are responsible for reticular dysgenesis (RD), an autosomal recessive form of severe combined immunodeficiency (SCID). RD is characterized by an early differentiation arrest in the granulocyte lineage and impaired lymphoid maturation and it represents less than 2% of total SCID. Affected children succumb to overwhelming infections early in life unless their immune system is successfully restored with allogeneic hematopoietic stem cells transplant (HSCT). The mechanisms underlying the pathophysiology of RD remain unclear. The phenotype of AK2 deficient animals has never been reported in the literature, but murine lines carrying homozygous inactivating retroviral insertions are embryonically lethal (our personal observations). We used the zebrafish model to perform a comprehensive study of the effects of AK2 deficiency using Morpholino oligomers injections and two different kinds of AK2 mutants (a ENU-induced T371C/L124P missense mutant and two null mutant lines generated using zinc-finger nuclease technology). In situ hybridization analyses of AK2-deficient embryos indicated that only erythroid development was affected during primitive hematopoiesis. Conversely, during definitive hematopoiesis, the loss of function of AK2 resulted in abnormalities distributed along all hematopoietic lineages suggesting an impairment of hematopoietic stem cell (HSC) development. Moreover, we observed that the AK2 deficiency induced oxidative stress and consequent apoptosis in both primitive erythroid cells and definitive HSCs. Importantly, antioxidant treatment of AK2 mutant embryos rescued the hematopoietic phenotypes as indicated by the recovered expression of HSC and lymphoid markers (such as c-myb and rag1). Overall, our data indicate that zebrafish represents a good model for studying the molecular mechanisms involved in RD and testing of new therapeutic interventions. To date, our mutant lines remain the only animal model of this rare and lethal human disease.