Carrier detection of the X-linked primary immunodeficiency diseases using X-chromosome inactivation analysis

Carrier detection of three of the X-linked primary immunodeficiency diseases (X-linked agammaglobulinemia, X-linked severe combined immunodeficiency disease, and the Wiskott-Aldrich syndrome) is possible by analyzing patterns of X-chromosome inactivation in those cells affected by the disorder. Normal women have balanced patterns of X-chromosome inactivation; that is, in a given population of cells, approximately half of their active X chromosomes are of paternal origin and half of their active X chromosomes are of maternal origin. In contrast, female carriers of these X-linked immunodeficiency disorders have an unbalanced pattern of X-chromosome inactivation in those cell lineages that are affected by the disorder; that is, all the active X chromosomes in affected cell lineages are the X chromosomes that carry the normal allele. Two techniques are available for X-chromosome inactivation analysis. One technique depends on methylation differences between the active and inactive X chromosome, and the other technique uses somatic cell hybrids that selectively retain the active X chromosome. In either case, carrier detection can be performed in individuals from families in which only one member of the family has been affected, since neither of these methods depends on linkage analysis.