【关键词】 b,cells
1 division of biological sciences, university of california, san diego, la jolla, california 92093, usa; 2 torrey pines institute for molecular studies, san diego, california 92121, usa
abstract
the immunoglobulin heavy-chain (igh) locus undergoes large-scale contraction in b cells poised to undergo igh v(d)j recombination. we considered the possibility that looping of distinct igh v regions plays a role in promoting long-range interactions. here, we simultaneously visualize three subregions of the igh locus, using three-dimensional fluorescence in situ hybridization. looping within the igh locus was observed in both b- and t-lineage cells. however, monoallelic looping of igh v regions into close proximity of the igh dj cluster was detected in developing b cells with significantly higher frequency when compared with hematopoietic progenitor or cd8+ t-lineage cells. looping of a subset of igh v regions, albeit at lower frequency, was also observed in rag-deficient pro-b cells. based on these observations, we propose that ig loci are repositioned by a looping mechanism prior to igh v(d)j rearrangement to facilitate the joining of ig variable, diversity, and joining segments.
[keywords: immunoglobulin heavy-chain; v(d)j rearrangement; locus contraction; long-range interactions; lymphocytes; e2a-deficient hematopoietic progenitors]
received november 29, 2004; revised version accepted december 10, 2004.
eukaryotic chromosomes are assembled into higher-order structures that are tightly packaged inside the nucleus. recent evidence strongly suggests that these complex chromatin structures are both dynamic and critical for appropriate regulation of gene expression. the organization of chromatin into a repressive or permissive state is regulated by noncoding elements such as promoters, enhancers, insulators, locus control regions, and matrix attachment regions (baxter et al. 2002; felsenfeld and groudine 2003). these cis-acting elements are selectively occupied by tissue- and stage-specific transcription factors that often serve as docking sites for chromatin remodeling factors. the precise mechanism regulating long-range interactions of cis-acting elements remains to be elucidated. recently, a looping model, in which regulatory elements separated by relative long distances in the -globin locus are brought into physical proximity, with the intervening dna looping out, has been suggested to establish an open chromatin domain and to activate -globin expression (carter et al. 2002; tolhuis et al. 2002; ostermeier et al. 2003).
