Variegated expression of the endogenous immunoglobulin heavy-chain gene inthe absence of the intronic locus control region

The expression of chromosomally integrated transgenes usually varies greatl y among independent transfectants, This variability in transgene expression has led to the definition of locus control regions (LCRs) as elements whic h render expression consistent, Analyses of expression in single cells reve aled that the expression of transgenes which lack an LCR is often variegate d, i.e., on in some cells and off in others. In many cases, transgenes whic h show variegated expression were found to have inserted near the centromer e. These observations have suggested that the LCR prevents variegation by b locking the inhibitory effect of heterochromatin and other repetitive-DNA-c ontaining structures at the insertion site and have raised the question of whether the LCR plays a similar role in endogenous genes, To address this q uestion, we have examined the effects of deleting the LCR from the immunogl obulin heavy-chain locus of a mouse hybridoma cell line in which expression of the immunoglobulin mu heavy-chain gene is normally highly stable, Our a nalysis of mu expression in single cells shows that deletion of this LCR re sulted in variegated expression of the mu gene, That is, in the absence of the LCR, expression of the mu gene in the recombinant locus could be found in either of two epigenetically maintained, metastable states, in which tra nscription occurred either at the normal rate or not at all. In the absence of the LCR, the on state had a half-life of similar to 100 cell divisions, while the half-life of the off state was similar to 40,000 cell divisions, For recombinants with an intact LCR, the half-life of the on state exceede d 50,000 cell divisions. Our results thus indicate that the LCR increased t he stability of the on state by at least 500-fold.