X-linked Severe Combined Immunodeficiency - Molecular Biology

Molecular Biology

Interleukins are produced by lymphocytes, among other cell types, and are released in response to antigenic and non-antigenic stimuli. The gene IL2RG codes for the common gamma chain protein, which is a common subunit of the individual receptors for Interleukin 2, Interleukin 4, Interleukin 7, Interleukin 9, Interleukin 15 and Interleukin 21. Signalling from these receptors normally promotes growth and differentiation of T-cells, B cells, natural killer cells, glial cells, and cells of the monocyte lineage, depending on the cell type and receptor activated. The gene encoding the common gamma chain in these interleukins is mutated in X-SCID. The mutation leads to an absent or abnormally functioning common gamma chain. The mutation can occur through large, or even single nucleotide, deletions in the IL2RG gene, that disable the common gamma chain so that it is unable to bind with other receptor subunits and signal cytokine activation. Normally, when the interleukin binds to the trimeric receptor protein containing the alpha, beta, and gamma subunits, the common gamma subunit activates Janus Kinase 3 (JAK3), which leads to the phosphorylation of Signal Transducer and Activator of Transcription 5, STAT5. The STAT5 proteins dimerize and translocate to the nucleus, controlling subsequent downstream signalling. Due to the fact that the common gamma chain is absent or abnormal, this downstream pathway is inhibited. This change prevents the T-lymphocytes from signaling other cells, like B-lymphocytes and natural killer cells. Because these cells never receive these signals, they can never mature and differentiate into full grown immune cells. 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.