Redundant and Unique Roles of Two Enhancer Elements in the TCR Locus in Gene Regulation and T Cell Development

in the TCR␤ locus abolished both germline transcription of the D␤-J␤ gene segments and ␤ gene rearrangement (Mathieu et al., 2000). Deletion of either the Ig␬ intronic Summary enhancer or the 3Ј enhancer had modest effects on rearrangement, whereas expression was largely normal Many mammalian genes, including those encoding an-in both cases (Xu et al., 1996; Gorman et al., 1996). In the tigen receptors, contain more than one enhancer ele-Ig heavy chain locus, deletion of the intronic enhancer ment. Deleting one element often does not prevent mildly impaired recombination and substantially re-expression, but functional redundancy has never been duced transcription (Sakai et al., 1999). Deletion of differ-directly demonstrated by gene targeting of multiple ent 3Ј enhancers had mild effects on these processes, elements. We demonstrate that simultaneous deletion but did strongly impair class switching (Manis et al., of two enhancer/LCR-like elements in the TCR C␥1 1998; Pinaud et al., 2001). Deletion of the TCR␣ enhancer cluster, HsA and 3؅E C␥1 , severely diminishes TCR␥ tran-blocked rearrangement of TCR␣ genes and substantially scription, selectively impairs development of ␥␦ thy-impaired transcription of both TCR␣ and TCR␦ genes mocyte subsets, but only modestly reduces TCR␥ (Sleckman et al., 1997). Deletion of the TCR␦ enhancer gene rearrangement, while deletion of each element blocked TCR␦ rearrangement but not transcription separately has little effect. In contrast to these results (Monroe et al., 1999). A deletion of several hypersensi-in thymocytes, deletion of HsA alone reduces tran-tive sites in the TCR␣ LCR did not have a substantial scription of one V␥ gene specifically in peripheral ␥␦ impact on TCR␣ gene rearrangement or expression T cells. Thus, the two elements exhibit functional re-(Hong et al., 1997). dundancy in thymocytes but also have unique func-To explain the instances where the deletion of positive tions in other settings. elements had little or no effect, it has been proposed that the multiple elements in the locus are at least partially Introduction redundant in function. Consistent with this possibility, examples have been reported of two elements acting The development of B and T cells depends on the so-at least partially redundantly in ectopically integrated matic assembly of immunoglubulin (Ig) and T cell recep-transgenes (Baker et al., 1999). However, results with tor (TCR) genes, respectively, from V, D, and J gene transgenes are in some instances not borne out by com-segments and subsequent transcription of the genes. parable analysis performed by gene targeting. Few …