X inactivation and somatic cell selection rescue female mice carrying a Piga-null mutation

A somatic mutation in the X linked PIGA gene is responsible for the deficie ncy of glycosyl phosphati dylinositol (GPI)-anchored proteins on blood cell s from patients with paroxysmal nocturnal hemoglobinuria. No inherited form of GPI-anchor deficiency has been described. Because conventional Piga gen e knockout is associated with high embryonic lethality in chimeric mice, we used the Cre/loxP system. We generated mice in which two loxP sites flank part of Piga exon 2, After crossbreeding with female mice of the EIIa-cre s train, the flexed allele undergoes Cre-mediated recombination with high eff iciency during early embryonic development, Because of X chromosome inactiv ation, female offspring are mosaic for cells that express or lack GPI-linke d proteins. Analysis of mosaic mice showed that in heart, lung, kidney, bra in, and liver, mainly wild-type Piga is active, suggesting that these tissu es require GPI-Iinked proteins. The salient exceptions mere spleen, thymus, and red blood cells, which had almost equal numbers of cells expressing th e wild-type or the recombined allele, implying that GPI-linked proteins are not essential for the derivation of these tissues. PIGA(-) cells had no gr owth advantage, suggesting that other factors are needed for their clonal d ominance in patients with paroxgsmal nocturnal hemoglobinuria.