Sandwiching of a gene within 12 kb of a functional telomere and alpha satellite does not result in silencing.

Pericentric heterochromatin and telomeres have been shown to be capable of repressing the expression of genes located in close proximity. The effect of adjacent structural sequences on gene expression will be important in the design of mammalian artificial chromosomes. In the process of using telomere-containing constructs to generate a deletion panel of the long arm of the human X chromosome, several cell lines were produced which appeared by in situ hybridization to be broken in Xq at or near the centromere. After analysis of end clones rescued from these cell lines, only two produced data consistent with breaks in the alpha satellite array without accompanying rearrangements. The mitotic stability of an X chromosome, with at least 750 kb of the alpha satellite array deleted, was compared to controls where the alpha satellite array remained intact. No significant change in the stability of the chromosome was observed, suggesting that the truncated chromosome has a fully functional mitotic centromere. There was no detectable change in the expression of the hygromycin resistance gene, which is located between a functional centromere and telomere, in this cell line. This study indicates that structural elements flanking a mammalian selectable marker do not result in silencing.