TISSUE-SPECIFIC EXTINGUISHER LOCI IN THE MURINE GENOME - A SCREENING STUDY BASED ON A RAT MOUSE MICROCELL HYBRID PANEL/

Extinction of tissue-specific traits in intertypic somatic cell hybrid s is a well-known phenomenon. In the past few rears, microcell hybrids have been used in attempts to dissect this phenotype genetically, and tissue-specific extinguisher loci have been mapped to two different m ouse chromosomes. When transferred from fibroblasts into hepatoma cell s by microcell fusion, these loci down-regulate expression of specific liver genes in trans. However, other liver genes that are extinguishe d in genotypically complete hybrids seem not to be extinguished in mon ochromosomal hybrids. To assess the generality of monochromosomal exti nction phenotypes, we assembled a collection of rat hepatoma/mouse fib roblast microcell hybrids that represent most of the mouse chromosome complement, and we screened them for expression of a large number of l iver-specific genes. Phosphoenolpyruvate carboxykinase gene expression was down-regulated in hybrids containing mouse chromosome 7 or mouse chromosome 11, but other extinction phenotypes were not readily appare nt. These results indicate that extinction of many liver genes may be a polygenic trait.