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<p><strong>Severe Combined Immunodeficiency</strong>, or <strong>SCID</strong>, is a genetic disorder in which both "arms" (<a title="B cell" href="http://en.wikipedia.org/wiki/B_cell">B cells</a> and <a title="T cell" href="http://en.wikipedia.org/wiki/T_cell">T cells</a>) of the <a title="Adaptive immune system" href="http://en.wikipedia.org/wiki/Adaptive_immune_system">adaptive immune system</a> are crippled, due to a defect in one of several possible genes. SCID is a severe form of heritable <a title="Immunodeficiency" href="http://en.wikipedia.org/wiki/Immunodeficiency">immunodeficiency</a>. 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 <a title="David Vetter" href="http://en.wikipedia.org/wiki/David_Vetter">David Vetter</a>.</p>
<p>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 <a title="Pneumocystis carinii pneumonia" href="http://en.wikipedia.org/wiki/Pneumocystis_carinii_pneumonia"><em>Pneumocystis jiroveci</em></a> pneumonia, and profuse oral <a title="Candidiasis" href="http://en.wikipedia.org/wiki/Candidiasis">candidiasis</a> commonly occur. <a title="" href="http://en.wikipedia.org/wiki/Severe_combined_immunodeficiency#Treatment">Treatment</a> options are much improved since <a title="David Vetter" href="http://en.wikipedia.org/wiki/David_Vetter">David Vetter</a>, and living in a bubble is no longer necessary.</p>
<p><script typespan class="text/javascriptmw-headline">//<![CDATA[ if (window.showTocToggle) { var tocShowText font size= "show5"; var tocHideText = "hide"; showTocToggle(); } //]]></script><a id="Types" name="Types"></afont></p><h2><span class="mw-headline">Types</span></h2><p><a id="JAK3" name="JAK3"></a></p>
<h3><span class="mw-headline">JAK3</span></h3>
<p>Janus kinase-3 (JAK3) is an enzyme that mediates transduction downstream of the γ<sub>c</sub> signal. Mutation of its gene also causes SCID.</p>
<p><a id="V.28D.29J_recombination" name="V.28D.29J_recombination"></a> </p>
<h3><span class="mw-headline">V(D)J recombination</span></h3>
<p>The manufacture of <a title="Immunoglobulin" href="http://en.wikipedia.org/wiki/Immunoglobulin">immunoglobulins</a> requires recombinase enzymes derived from the recombination activating genes RAG-1 and RAG-2. These enzymes are involved in the first stage of <a title="V(D)J recombination" href="http://en.wikipedia.org/wiki/V%28D%29J_recombination">V(D)J recombination</a>, the process by which segements of a <a title="B cell" href="http://en.wikipedia.org/wiki/B_cell">B cell</a> or <a title="T cell" href="http://en.wikipedia.org/wiki/T_cell">T cell</a>'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 <a title="V(D)J recombination" href="http://en.wikipedia.org/wiki/V%28D%29J_recombination">V(D)J recombination</a>, causing SCID.</p>
<p><a id="Adenosine_deaminase" name="Adenosine_deaminase"></a> </p>
<h3><span class="mw-headline">Adenosine deaminase</span></h3>
<p>Another well-known form of SCID is caused by a defective enzyme, <a title="Adenosine deaminase" href="http://en.wikipedia.org/wiki/Adenosine_deaminase">adenosine deaminase</a> (ADA), necessary for the breakdown of <a title="Purine" href="http://en.wikipedia.org/wiki/Purine">purines</a>. 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.</p>