Randomly inserted and targeted Hox/reporter fusions transcriptionally silenced in Polycomb mutants.
Polycomb-group (Pc-G) proteins ensure late maintenance of transcriptional repression outside the expression domain of target genes in flies and vertebrates. They act in complexes, presumably by modulating chromatin structure. In Drosophila, they have been found to be associated with transcriptionally inactive loci but seem to be present in association with actively transcribed promoters as well, a feature which is not yet understood. In the mouse, mutations in several Pc-G genes result in an often subtle, local derepression of only a subset of the Hox genes rostral to their expression domains. We report here that Hox/reporter fusion genes, either randomly integrated as transgenes or as insertions within endogenous loci, are transcriptionally silenced in two mouse Pc-G-null mutants, Mel18 and rae28. Transcriptional silencing of Hox/reporter transgenes in Pc-G mutants was accompanied by increased DNA methylation in the promoter region. Gene silencing was observed at early developmental stages, long before Pc-G and trithorax-group proteins exert their function in maintenance of the Hox patterns. Although all five Hox genes tested as Hox/reporter fusions were silenced in the Pc-G mutants, transcription of the endogenous loci was mildly decreased in a subset of these Hox genes, and Hoxb1 was the most strongly affected. We discuss the possibilities that the observed negative effect of Pc-G mutations on Hox and Hox/reporter expression may reflect a positive involvement of the Pc-G epigenetic repressors in initial Hox gene transcription and that this requirement is exacerbated by the reporter insertion.
Hubrecht Laboratory, Netherlands Institute for Developmental Biology, Uppsalalaan 8, 3485 CT Utrecht, The Netherlands.
Wim
W
d Graaff
Daihachiro
D
Tomotsune
Tony
T
Oosterveen
Yoshihiro
Y
Takihara
Haruhiko
H
Koseki
Jacqueline
J
Deschamps
D000469:Alkaline Phosphatase / Q000378:metabolism; D000818:Animals; D002843:Chromatin / Q000378:metabolism; D019175:DNA Methylation; D029721:Drosophila Proteins / Q000235:genetics*; D020868:Gene Silencing; D017930:Genes, Reporter; D018398:Homeodomain Proteins / Q000235:genetics*; D017403:In Situ Hybridization; D051379:Mice; D008810:Mice, Inbred C57BL; D008808:Mice, Inbred CBA; D008817:Mice, Mutant Strains; D008822:Mice, Transgenic; D009154:Mutation; D063150:Polycomb Repressive Complex 1; D011401:Promoter Regions, Genetic; D011993:Recombinant Fusion Proteins / Q000378:metabolism*; D014158:Transcription, Genetic; D019076:Transgenes
D002843:Chromatin; D029721:Drosophila Proteins; D018398:Homeodomain Proteins; C066770:Pc protein, Drosophila; D011993:Recombinant Fusion Proteins; D063150:Polycomb Repressive Complex 1; D000469:Alkaline Phosphatase