Review Question - QID 216555

This patient who presents with failure to thrive (current weight less than birth weight), recurrent infections, candidal infections, chronic diarrhea, and an extremely low leukocyte count most likely has severe combined immunodeficiency. This disease is can be caused by a mutation in adenosine deaminase.

Severe combined immunodeficiency (SCID) is caused by a number of different mutations (common gamma chain, adenosine deaminase [ADA], JAK3, etc.) that result in deficiency of both B and T cells. This defect in the immune system leads to life-threatening infections within the first 3 months after birth. These include bacterial skin and soft tissue infections, viral upper respiratory infections, candidal mucosal infections, and chronic diarrhea. In addition, patients will present with failure to thrive and chronic weight loss. Laboratory testing will show decreased levels of leukocytes (<3000 /mm^3) and decreased levels of immunoglobulins. Management includes protective isolation and prophylaxis against infection. Definitive treatment is a hematopoietic stem cell transplant from a tissue-matched donor.

Reinhardt et al. present results from gene therapy for patients who lack functional adenosine deaminase. They show that reconstitution of the immune system using CD34+ cells allowed for protection against life-threatening infections. They recommend further study to weigh the benefit of gene therapy for ADA SCID against the risks of toxicity with gene therapy.

Incorrect Answers:
Answer 2: Bruton tyrosine kinase is defective in X-linked agammaglobulinemia, which can also present with recurrent skin and soft tissue infections; however, these patients will have normal levels of T-cells so lymphocyte counts will not be decreased as severely as in severe combined immunodeficiency. Usually, this manifests around 6 months of age when maternal Ig levels decline and presents with recurrent bacterial infections.

Answer 3: CD40 ligand is defective in hyper-IgM syndrome, which will also present with opportunistic infections such as mucosal candidiasis and chronic diarrhea; however, this disease will present with pancytopenia rather than an isolated decrease in leukocytes.

Answer 4: Lysosomal trafficking regulator is defective in Chediak-Higashi syndrome, which can present with recurrent pyogenic skin infections. However, these patients will also present with partial albinism and neurological symptoms.

Answer 5: Signal transducer activator of transcription 3 (STAT3) is defective in hyper IgE syndrome, which will present with coarse facies, cold abscesses, retained primary teeth, and dermatologic problems such as eczema.

Bullet Summary:
Severe combined immunodeficiency is most commonly caused by a mutation in the common gamma chain or adenosine deaminase.