Review Question - QID 106688

This patient presents with severe combined immunodeficiency (SCID) which can be caused by adenosine deaminase deficiency leading to reduced levels of inosine, thus preventing DNA synthesis.

SCID is a B and T cell disorder characterized by early onset (within first 3 months of life) of failure to thrive, chronic diarrhea, thrush, and recurrent viral, bacterial, fungal and protozoal infections. Findings include absence of the following: thymic shadow, germinal centers on lymph node biopsy, and B cells on peripheral blood smear. The most common cause of SCID is a defective IL-2 receptor and the second most common cause is adenosine deaminase deficiency. Adenosine deaminase catalyzes the conversion of adenosine to inosine in the purine salvage pathway. Deficiency of this enzyme results in accumulation of adenosine and deficiency of inosine (Illustration A). This imbalance prevents DNA synthesis and thus decreases lymphocyte count as seen in SCID.

Reust reviews the evaluation of primary immunodeficiency diseases in children. When an immunodeficiency disease is suspected, initial laboratory screening should include a complete blood count with differential and measurement of serum immunoglobulin and complement levels. The presence of lymphocytopenia on complete blood count suggests a T-cell disorder, whereas a finding of neutropenia suggests a phagocytic disorder.

Rivers et al. review the diagnosis and management of SCID. They state there has been increasing interest in newborn screening for SCID, which has the potential to significantly improve outcomes through early diagnosis and implementation of prophylactic medications. Definitive treatments such as hematopoietic stem cell transplantation and gene therapy have also made major advances over the last decade and promise to improve the overall outcome for SCID with reduced long-term toxicities.

Figure A demonstrates a patient with thrush as is often observed in SCID. Illustration A graphically represents the purine salvage pathway and demonstrates where an adenosine deaminase defect would exert its effect.

Incorrect Answers:
Answer 2: Adenosine levels would be increased if a defect in adenosine deaminase was present.
Answers 3-5: Cytosine, thymine, and uracil are pyrimidines and thus not involved in the purine salvage pathway. Thus, their levels should not be affected by a defect in adenosine deaminase.