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Changes in Replication, Nuclear Location, and Expression of the Igh Locus after Fusion of a Pre-B Cell Line with a T Cell Line¹

Jie Zhou^*, Shireen Saleque^*, Olga Ermakova^*, Manuel A. Sepulveda^*, Qiaoxin Yang^*, Laurel A. Eckhardt, Carl L. Schildkraut^* and Barbara K. Birshtein^2,*

^* Department of Cell Biology, Albert Einstein College of Medicine, Bronx, NY 10461; and Department of Biological Sciences, Hunter College, and Graduate School of City University of New York, New York, NY 10021

We have previously observed that replication and nuclear location of the murine Igh locus are developmentally regulated during B cell differentiation. In non-B, B, and plasma cells, sequences near the 3' end of the Igh locus replicate early in S while upstream Vh sequences replicate late in S, and the Igh locus is located near the nuclear periphery. In fact, in MEL non-B cells, replication of a 500-kb segment containing Igh-C and flanking sequences occurs progressively later throughout S by 3' to 5' unidirectional fork movement. In contrast, in pro- and pre-B cells, the entire 3-Mb Igh locus is located away from the nuclear periphery and replicates early in S by forks progressing in both directions. In this study, using an 18-81 (pre-B) x BW5147 (T) cell fusion system in which Igh expression is extinguished, we found that in all Igh alleles, Vh sequences replicated later in S than 3' Igh sequences (similar to that detected in BW5147), but the Igh locus was situated away from the nuclear periphery (similar to that observed in 18-81). Thus, pre-B cell-derived Igh genes had changes in replication timing, but not in nuclear location, whereas T cell-derived Igh genes changed their nuclear location but not their replication timing. These data are consistent with the silencing of a pre-B cell-specific replication program in the fusion hybrid cells and independent regulation of the nuclear location of Igh loci.