X-linked SCID is caused by mutations in the gene encoding the common y (y) chain shared by the receptors for the interleukins IL-2, IL-4, IL-7, IL-9, and IL-15 (see Chapters 4 and 9). X-linked SCID is characterized by impaired maturation of T cells and NK cells and greatly reduced numbers of mature T cells and NK cells, but the number of B cells is usually normal or increased. The humoral immunodeficiency in this disease is due to a lack of T cell help for antibody production. This disease is a result of the inability of the lymphopoietic cytokine IL-7, whose receptor uses the yc chain for signaling, to stimulate the growth of immature thymocytes. In addition, the receptor for IL-15, which is a potent stimulus for the proliferation of NK cells, also uses the yc signaling chain, and the failure of IL-15 function accounts for the deficiency of NK cells.
Heterozygous females are usually phenotypically normal carriers, whereas males who inherit the abnormal X chromosome manifest the disease. Because developing cells in females randomly inactivate one of the two X chromosomes, the normal allele encoding a functional yc protein will not be expressed in half the lymphocyte precursors in a female carrier. These cells will fail to mature, and consequently, all the mature lymphocytes in a female carrier will have inactivated the same X chromosome (carrying the mutant allele). In contrast, half of all nonlymphoid cells will have inactivated one X chromosome, and half the other. A comparison of X chromosome inactivation in lymphoid cells versus nonlymphoid cells may be used to identify carriers of the mutant allele. The nonrandom use of X chromosomes in mature lymphocytes is also characteristic of female carriers of other X-linked mutations of genes that affect lymphocyte development, as discussed later.
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