Article: Severe combined immunodeficiency

Severe Combined Immunodeficiency, or SCID, is a genetic disorder in which both "arms" (B cells and T cells) of the adaptive immune system are crippled, due to a defect in one of several possible genes. SCID is a severe form of heritable immunodeficiency. It is also known as the "bubble boy" disease because its victims are extremely vulnerable to infectious diseases and must live (if untreated) in a completely sterile environment. The most famous case is the boy David Vetter.

SCID affects about 1 in 100,000 live births. These babies, if untreated, usually die within 1 year due to severe, recurrent infections. Chronic diarrhea, ear infections, recurrent Pneumocystis jiroveci pneumonia, and profuse oral candidiasis commonly occur. Treatment options are much improved since David Vetter, and living in a bubble is no longer necessary.

Types

JAK3

Janus kinase-3 (JAK3) is an enzyme that mediates transduction downstream of the γ_c signal. Mutation of its gene also causes SCID.

V(D)J recombination

The manufacture of immunoglobulins requires recombinase enzymes derived from the recombination activating genes RAG-1 and RAG-2. These enzymes are involved in the first stage of V(D)J recombination, the process by which segements of a B cell or T cell's DNA are rearranged to create a new T cell receptor or B cell receptor (and, in the B cell's case, the template for antibodies). Certain mutations of the RAG-1 or RAG-2 genes prevent V(D)J recombination, causing SCID.

Adenosine deaminase

Another well-known form of SCID is caused by a defective enzyme, adenosine deaminase (ADA), necessary for the breakdown of purines. Lack of ADA causes accumulation of dATP. This metabolite will inhibit the activity of ribonucleotide diphosphate reducatase, which is the enzyme that synthesizes DNA and RNA. Since lymphocytes require a large amount of DNA and RNA during proliferation, accumulation of dATP will effect them in a dramatic fashion.

Detection

Standard testing of SCID is not performed for newborns due to the rarity of the disease and the cost of the testing. SCID can be detected by sequencing fetal DNA if a known history of the disease exists. Otherwise, SCID is not detected until about six months of age, usually indicated by recurrent infections. The delay in detection is due to the fact that newborns carry their mother's antibodies for the first few weeks of life.

Treatment

The most common treatment for SCID is bone marrow transplantation, which requires matched donors (a sibling is generally best). David Vetter, the original "bubble boy," endured several failed transplantations, and finally died because of an unscreened virus, Epstein-Barr, in his newly-transplanted bone marrow from his sister. Today, transplants done in the first three months of life have a high success rate.

More recently, gene therapy has proved useful. Transduction of the missing gene to hematopoietic stem cells using viral vectors is being tested in ADA SCID and X-linked SCID. The first gene therapy trials were performed in 1990, with peripheral T cells. In 2000, the first gene therapy "success" resulted in SCID patients with a functional immune system. These trials were stopped when it was discovered that three of eleven patients in one trial had developed leukemia resulting from the insertion of the gene-carrying retrovirus near an oncogene. Work is now focusing on correcting the gene without triggering an oncogene. No leukemia cases have yet been seen in trials of ADA-SCID, which does not involve the gamma c gene which may be oncogenic when expressed by a retrovirus.

Trial treatments of SCID have been gene therapy's only success; since 1999, gene therapy has restored the immune systems of at least 17 children with two forms (ADA-SCID and X-SCID) of the disorder.