Adenosine Deaminase 2 Deficiency As a Cause of Pure Red Cell Aplasia Mimicking Diamond Blackfan Anemia

Diamond Blackfan anemia (DBA) is an inherited pure red cell aplasia. Most cases present in the first year of life with elevation in erythrocyte adenosine deaminase (eADA) and frequently with increased mean corpuscular volume (MCV) and hemoglobin F (hgb F). Approximately 70 percent of cases are due to a mutation in one of several ribosomal protein (RP) genes or in GATA1, whereas the remaining cases are genetically uncharacterized. Here we report a child born with severe anemia and diagnosed with DBA at 2 months of age. His bone marrow was normocellular with a paucity of erythroid progenitors and scattered lymphocytes. An eADA level was not obtained prior to the first red cell transfusion. He was red cell transfusion dependent and his anemia did not respond to a steroid trial. His 4 year old sister, who had normal hemoglobin, MCV, hgb F, and eADA measurements, served as his HLA identical donor for hematopoietic stem cell transplantation (HSCT). HSCT resulted in 100% donor chimerism, but red cell engraftment was not achieved. He subsequently underwent a mismatched unrelated HSCT with trilineage engraftment.

Ten years later, at the age of 14 years, the sister presented with profound hypoproductive normocytic anemia. The bone marrow showed absence of erythroid precursors and presence of lymphoid aggregates. Findings of immunodeficiency included numerous cutaneous warts, recurrent aphthous ulcers, Epstein Barr virus (EBV) reactivation, low IgM, and low numbers and percentages of CD19+ and CD3-56+16+ lymphocytes. The anemia and reticulocytopenia persisted despite resolution of EBV reactivation. Upon her presentation, levels of iron, ferritin, transferrin saturation, and liver transaminases were elevated. A liver biopsy obtained after transfusion of a total of 60 ml/kg packed red blood cells demonstrated 4.8 mg Fe/g dry liver weight with stage 2 portal fibrosis.

Targeted DNA sequencing studies performed on the affected sister were negative for single nucleotide variants in any of 12 RP genes previously implicated in DBA and a genome wide chromosome microarray was normal. Whole exome analysis of her and her parents demonstrated that she carried compound heterozygous variants in CECR1 (cat eye syndrome chromosome region, candidate 1). The variant p.R169Q had been previously reported as pathogenic, while the p.G358R variant was of uncertain significance. These variants are present at frequencies of 4.9X10^-4 and 2.6X10^-5 in the Exome Aggregation Consortium database, respectively. Analysis of buccal swab DNA of the proband showed the same biallelic variants. An unaffected 16-year-old sibling had a normal genotype. CECR1 encodes adenosine deaminase 2 (ADA2) and ADA2 levels in the plasma of the affected sister were markedly low, consistent with a deficiency state.

CECR1 is highly expressed in cells of myeloid origin and ADA2 is a secreted protein implicated in macrophage differentiation and proliferation. Deficiency of ADA2 (DADA2) results in aberrant monocyte differentiation favoring M1 over M2 macrophages, thereby resulting in a proinflammatory state. Recent descriptions of patients with DADA2 due to CECR1 mutations reported a spectrum of phenotypes including intermittent fevers, lacunar stroke in childhood, livedoid rash, polyarteritis nodosa, and immunodeficiency with B lymphopenia and low IgM levels. Our cases are similar to the report of one of two brothers, homozygous for CECR1 p.R169Q, described by van Montfrans, et al,. (NEJM, 2014). The eldest was given a diagnosis of atypical DBA (refractory pure red cell aplasia) in infancy and underwent a HSCT from his asymptomatic, HLA identical brother. This HSCT resulted in non-engraftment, necessitating a subsequent unrelated donor HSCT. The younger sibling donor later developed hepatosplenomegaly, profound lymphopenia, and evidence of an inflammatory state.

Together, these three cases support pure red cell aplasia as a presentation of DADA2 and that this may be confused with DBA, particularly when manifest in infancy. We propose DADA2 should be considered in patients with genetically uncharacterized DBA. Differentiating features to suggest DADA2 may include normal eADA, MCV, and hgb F levels and findings of associated immunodeficiency. Additionally, the macrophage activation due to DADA2 may have played a role in the iron overload observed in our second patient prior to any red cell transfusion.