Vol. 12, Issue 11, 3563-3572, November 2001

Chromosomal G-dark Bands Determine the Spatial Organization of Centromeric Heterochromatin in the Nucleus

Célia Carvalho,^*§ Henrique M. Pereira,^§ João Ferreira,^* Cristina Pina,^* Denise Mendonça, Agostinho C. Rosa, and Maria Carmo-Fonseca^*¶

 ^*Instituto de Histologia e Embriologia, Faculdade de Medicina, Universidade de Lisboa, 1649-028 Lisboa;  Instituto de Sistemas e Robótica, Instituto Superior Técnico, Lisboa;  Instituto de Ciências Biomédicas Abel Salazar, Universidade do Porto, Portugal

Gene expression can be silenced by proximity to heterochromatin blocks containing centromeric -satellite DNA. This has been shown experimentally through cis-acting chromosome rearrangements resulting in linear genomic proximity, or through trans-acting changes resulting in intranuclear spatial proximity. Although it has long been been established that centromeres are nonrandomly distributed during interphase, little is known of what determines the three-dimensional organization of these silencing domains in the nucleus. Here, we propose a model that predicts the intranuclear positioning of centromeric heterochromatin for each individual chromosome. With the use of fluorescence in situ hybridization and confocal microscopy, we show that the distribution of centromeric -satellite DNA in human lymphoid cells synchronized at G₀/G₁ is unique for most individual chromosomes. Regression analysis reveals a tight correlation between nuclear distribution of centromeric -satellite DNA and the presence of G-dark bands in the corresponding chromosome. Centromeres surrounded by G-dark bands are preferentially located at the nuclear periphery, whereas centromeres of chromosomes with a lower content of G-dark bands tend to be localized at the nucleolus. Consistent with the model, a t(11; 14) translocation that removes G-dark bands from chromosome 11 causes a repositioning of the centromere, which becomes less frequently localized at the nuclear periphery and more frequently associated with the nucleolus. The data suggest that "chromosomal environment" plays a key role in the intranuclear organization of centromeric heterochromatin. Our model further predicts that facultative heterochromatinization of distinct genomic regions may contribute to cell-type specific patterns of centromere localization.