Chromosome assignment of a murine glucocorticoid receptor gene (Grl-1) using intraspecies somatic cell hybrids

Abstract Hybrids between two mouse lymphoma cell lines, the glucocorticoid sensitive S49.1 and the resistant EL4 line, are sensitive to the cytolytic steroid effect as long as they retain the S49.1 specific glucocorticoid receptor and the complete chromosome complements contributed by both parental lines. With the use of the semisynthetic glucocorticoid dexamethasone, resistant segregants (dex R ) have been isolated which have lost the S49.1 specific receptor but retain the EL4 type of receptor. On the average these segregants have also lost one chromosome. We have carried out a detailed karyotype analysis of both parental cell lines and their hybrids using trypsin-Giemsa banding for chromosome identification. S49.1 cells contain 40 apparently normal chromosomes with monosomy X and trisomy 1. EL4 cells have 39 chromosomes, half of which are structurally abnormal. In addition to low levels of random karyotypic variability in S49.1 × EL4 hybrids we have observed one consistent difference between sensitive (dex s ) and dex R sublines: sensitive hybrids contained three chromosomes 18 while resistant segregants only had two. We were able to distinguish both chromosomes 18 contributed by the S49.1 parent and the one from the EL4 cell line by structural variations involving the centromeric heterochromatin and the nucleolar organizer regions. A specific chromosome 18 derived from S49.1 was consistently absent in 3 dex R segregant lines derived from different dex s hybrid clones. Since the dex R segregant hybrids have lost the S49.1 specific glucocorticoid receptor we conclude that the gene for the receptor is located on that chromosome 18 which is consistently lost. The other S49.1 derived 18 presumably carries a mutant or silent receptor gene, as S49.1 has previously been shown to have hemizygous levels of the receptor. This study demonstrates the usefulness of intraspecies somatic cell hybrids for mapping purposes. The overall karyotypic stability of such hybrids allows the identification of specific chromosomes eliminated by selection pressure.