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X-linked Severe Combined Immunodeficiency (X-SCID) as its name suggests, is an immunodeficiency disease which causes lacks lymphocytes, cells that help protect our bodies. This mutation occurs in the gene responsible for the functionality of the interleukin 2 receptor, also known as or IL2RG. Also, X-SCID is an X-linked recessive trait.
Persons afflicted with X-SCID often have infections very early in life, before three months of age. This is followed by pneumonitis, an inflammation of the lung which produces common symptoms such as cough, fever, chills, and shortness of breath. In addition, moniliasis, a type of fungal infection, is a telltale sign of X-SCID. Moniliasis involves moist areas of the body such as skin, the mouth, respiratory tract, and vagina. Symptoms of moniliasis include difficulty in swallowing, pain on swallowing and oral lesions. Recurrent eczema-like rashes are also a common symptom. X-SCID is usually fatal in the first years of life.
X-SCID is X-linked mutation, which means males born from a female carrier (either heterozygous or homozygous), will inherit X-SCID, and thus, express it. Many times, X-linked mutations can be passed by a healthy, unaffected mother. A heterozygous woman carrier will have a 50% chance of transmitting the disease-causing mutation. Females who are heterozygous are carriers, but will not be affected.
Interleukins, a range of naturally occurring polypeptides, are produced by lymphocytes, among other cell types, and are released in response to antigenic and non-antigenic stimuli. Interleukin, belong to a, family of cytokines that act as intercellular mediators, specifically, they maintain balance in the immune system. IL2RG, which normally promotes growth and differentiation of T-cells, B cells, natural killer cells, glioma cells, and cells of the monocyte lineage, is mutated in X-SCID. Its mutation is caused by large deletions in the IL2RG gene, that disable the interleukin 2 receptor so that it is unable to form molecules with other small proteins that affect cell communication, behavior, and/or interaction. The gene is located on Xq13, with a DNA length of 4.2 kb. Analysis has shown that the mRNA length is 3.6 kb long. IL2RG has 369 amino acids, and contains eight exons and seven introns.
Diagnosis of X-SCID is possible through observation and investigation of immune system. One possible test is a lymphocyte count. A healthy immune system should contain large amounts of lymphocytes, but individuals with X-SCID will contain unusually small amounts of T-cells, non-functional B-cells, and relatively, no natural killer cells. There are also lymphocyte functional tests. These tests introduce agents to the immune system and observation, one can see how the lymphocytes react. Antibody responses to introduced vaccines or infections are absent, and T-cell responses to mitogens, substances that stimulate lymphocyte transformation, are deficient. Immunoglobulins, substances that aid in fighting off infections, are very low. Also, the thymic shadow of the thymus is absent on chest X-rays. Since the mutation in X-SCID is X-linked, there are genetic tests for detecting carriers in XSCID pedigrees. One method is to look for family-specific IL2RG mutations. Finally, if none of those options are available, there is an unusual pattern of nonrandom X-chromosome inactivation on lymphocytes in carriers, thus looking for such inactivation would prove useful.
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