Somatic cell hybridization of Roberts syndrome and normal human fibroblasts transfected with plasmids carrying dominant selection markers

Roberts syndrome (RS) is a rare human recessive disorder involving, in the chromosomes of some patients, a characteristic puffing or splitting apart of the constitutive heterochromatin (the RS effect). We carried out somatic cell hybridizations between an RS cell strain (R22) with the heterochromatin abnormality and a hypoxanthine phosphoribosyltransferase-deficient cell strain (GM1662) with normal chromosome structure to determine if the presence of the normal genome would correct the RS effect in the hybrid cells. In order to provide the fibroblast strains with dominant selection markers for the hybridizations, GM1662 was transfected with the plasmid pSV3neo which conferred resistance to the antibiotic G418, and R22 was transfected with the plasmid pSV3gpt which provided resistance to mycophenolic acid. Two somatic cell hybridizations were carried out: (1) R22 X GM1662 pSV3neo and (2) R22 pSV3gpt X GM1662 pSV3neo. The RS effect was found to be absent in 95% and 92%, respectively, of the 200 hybrid cells examined in each experiment. This indicated that the GM1662 genome was able to correct the RS effect. The presence of the RS effect in a few of the hybrid cells was attributed to the unstable karyotype resulting from pSV3 transfection which presumably caused the loss of the normal allele(s) of the RS gene in these hybrid cells.