Adenine Ribonucleotide (AXP) Depletion Due to Inherited Deficiency of Adenosine Deaminase 1 (ADA)

Adenosine deaminase 1-deficient severe combined immune deficiency (ADA-SCID) has widely been attributed to the toxic accumulation of adenine deoxyribonucleotides (dAXP) derived from 2’-deoxyadenosine (dAdo). The presence of dAXP in red blood cells (RBC) is pathognomonic, and higher dAXP levels track with phenotypic severity. A less well-known consequence of ADA1 deficiency, depletion of adenine ribonucleotides (AXP), was discovered >40 years ago and documented in RBC of several patients with ADA-SCID. To better characterize the degree of AXP depletion and relationship to dAXP levels and phenotype, we have assessed adenine nucleotide levels in RBC of >700 patients from North America tested for ADA1 deficiency in our laboratory since 2010. AXP and dAXP were measured in RBC from EDTA blood sent overnight at ambient temperature. AXP in RBC of the 120 ADA patients averaged 35% lower than in 592 who were not deficient (ND), P < 0.00001. dAXP were absent in ND RBC, but in ADA-RBC, they averaged 1.1 ± 0.5 µmol/mL, or 48.4 ± 20% of total adenine nucleotides (TAN = [AXP] + [dAXP]). AXP and dAXP levels were reciprocal, with a stronger negative correlation between AXP and %dAXP than with absolute dAXP concentration. TAN in ADA patients averaged ∼29% higher than in ND patients (in whom dAXP = 0). AXP depletion was restricted to infants <1 year old and patients with >20-25% dAXP—characteristics of ADA-SCID. Older ADA patients and those with <20% dAXP, typical of delayed or late onset, showed minimal AXP depletion. In 68 ADA patients who received enzyme replacement therapy, RBC AXP levels normalized with dAXP clearance. Our findings show that RBC AXP depletion is a biomarker for ADA-SCID and are consistent with in vitro evidence that dAXP accumulation activates AXP catabolism. ADA-SCID has been widely attributed to dATP-mediated allosteric inhibition of ribonucleotide diphosphate reductase, an enzyme essential for DNA replication. As ATP is an activator and binds to the same site as dATP, AXP depletion should synergize with increased dATP to potentiate reductase inhibition. Thus, the combined effects of elevated dAXP and depleted AXP may account for the more profound lymphocytopenia associated with ADA-SCID than with later onset phenotypes.