Immunodeficiency: primary or acquired
It is an emerging paradigm that mitochondrial metabolism impacts the fate of hematopoietic stem and progenitor cells (HSPCs), however, the underlying mechanisms remain poorly understood. Biallelic mutations of a mitochondrial enzyme, adenylate kinase 2 (AK2), cause Reticular Dysgenesis (RD), one of the most severe forms of severe combined immunodeficiency, characterized by almost complete absence of neutrophils and lymphocytes, and sensorineural hearing loss. AK2 catalyzes the interconversion between adenine nucleotides (AMP, ADP, and ATP) and controls the availability of ADP for oxidative phosphorylation. We hypothesize that AK2 drives HSPC fate decisions through regulation of metabolites that lead to lineage-specific epigenetic modifications. To test the hypothesis, we have developed a novel AK2-depleted cell model system in primary human HSPCs by CRISPR/Cas9 genome editing. Using homologous recombination-mediated GFP and BFP reporters, we are able to track and enrich biallelic AK2-edited HSPCs. Compared to AAVS1-edited controls, AK2-/- HSPCs showed severely decreased potential to form colonies of myeloid and erythroid lineages. An in vitro neutrophil differentiation assay showed AK2-/- maturation arrest at the CD117+, HLA-DR- stage. These defects are consistent with observations in RD patients. Currently, using a targeted metabolomics approach and global methylation/acetylation quantifications, we are analyzing AK2-dependent metabolite changes and their impact on the epigenome. We are also examining AK2-dependent changes in gene expression and chromatin accessibility by RNA-seq and ATAC-seq. Understanding how metabolism governs differentiation and self-renewal of human HSPCs will advance our understanding of many immune disorders and has important translational implications to improve stem cell products and transplantation outcomes.