Carrier identification in X-linked immunodeficiency diseases

Objective: Carrier identification in X-linked immunodeficiency disorders can be based on the demonstration of non-random X inactivation (NRXI) in affected blood cell lineages when growth is impaired in cells expressing the abnormal gene. We examined the utility of seeking evidence of NRXI¹ to test the carrier status of women in families affected by X-linked severe combined immunodeficiency (XSCID) and X-linked hypogammaglobulinaemia (XLH),² to identify as carriers the mothers of boys with SCID or hypogammaglobulinaemia whose phenotype suggested X-linkage and³ to infer X-linkage in boys with SCID or hypogammaglobulinaemia whose disease was not clearly X-linked on the basis either of family history or clinical and immunological characteristics. Methodology: A polymerase chain reaction-based method was used to amplify a polymorphic CAG repeat in the first exon of the androgen receptor gene after selective digestion of the active X chromosome with a methylation-sensitive enzyme, Hpall to distinguish between the paternal and maternal alleles and to identify their methylation status. Results: Heterozygosity was found in 24 of 31 female subjects (77%). As anticipated, NRXI could be demonstrated in all lymphoid cells studied from obligate carriers of XSCID and an obligate carrier of XLH but not on a carrier of X-linked immunodeficiency with hyper- IgM. The finding of NRXI in the mother of a boy with a SCID variant showed her to be a carrier of XSCID and establishes that her son has XSCID, not otherwise evident from available data. Conclusions: This PCR assay provides a rapid method for carrier detection of X-linked immunodeficiencies, and has allowed us to expand the phenotype of XSCID.