Tissue-specific extinguisher loci in the human genome: a screening study based on random marking and transfer of human chromosomes.

Expression of many liver-specific genes is extinguished when cultured hepatoma cells are fused with fibroblasts, but liver genes can be reexpressed in hybrid segregants that have lost fibroblast chromosomes. To map extinguisher loci involved in this process, hepatoma microcell hybrids retaining single fibroblast chromosomes have been employed. Two different, transdominant loci that affect liver gene expression have been defined in this way. To determine whether other monochromosomal extinction phenotypes could be observed, we inserted a selectable marker into many human chromosomal sites and transferred the marked human chromosomes into rat hepatoma recipient cells by microcell fusion. Nearly 200 microcell hybrid clones were isolated and screened for expression of liver-specific mRNAs. Most liver transcripts continued to be expressed. However, PEPCK mRNA was extinguished in 12 hybrid clones. Some of these hybrids contained human TSE1, the previously characterized extinguisher locus on chromosome 17, but others contained a novel extinguishing function that mapped to human chromosome 14. The implications of these findings are discussed.