CARRIER IDENTIFICATION IN X-LINKED IMMUNODEFICIENCY DISEASES

Objective: Carrier identification in X-linked immunodeficiency disorde rs can be based on the demonstration of non-random X inactivation (NRX I) in affected blood cell lineages when growth is impaired in cells ex pressing the abnormal gene. We examined the utility of seeking evidenc e of NRXI1 to test the carrier status of women in families affected by X-linked severe combined immunodeficiency (XSCID) and X-linked hypoga mmaglobulinaemia (XLH),(2) to identify as carriers the mothers of boys with SCID or hypogammaglobulinaemia whose phenotype suggested X-linka ge and(3) to infer X-linkage in boys with SCID or hypogammaglobulinaem ia whose disease was not clearly X-linked on the basis either of famil y history or clinical and immunological characteristics. Methodology: A polymerase chain reaction-based method was used to amplify a polymor phic CAG repeat in the first exon of the androgen receptor gene after selective digestion of the active X chromosome with a methylation-sens itive enzyme, Hpall to distinguish between the paternal and maternal a lleles and to identify their methylation status. Results: Heterozygosi ty was found in 24 of 31 female subjects (77%). As anticipated, NRXI c ould be demonstrated in all lymphoid cells studied from obligate carri ers 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 mot her of a boy with a SCID variant showed her to be a carrier of XSCID a nd establishes that her son has XSCID, not otherwise evident from avai lable data. Conclusions: This PCR assay provides a rapid method for ca rrier detection of X-linked immunodeficiencies, and has allowed us to expand the phenotype of XSCID.