Mechanism of suppression of malignancy in hybrids between Y79 retinoblastoma and NIH3T3 cells.

Hybrids were prepared between Y79 retinoblastoma (RB) and nonmalignant NIH3T3 cells and studied to confirm the presumed recessiveness of RB. Twenty hybrids containing both of the dominant gene markers, pSV2neo and pSV2GPT, initially transfected into the parent cells were isolated. All of the hybrids showed a fibroblastic morphology and anchorage-dependent growth. None of the tested hybrids or the parent NIH3T3 cells showed growth in soft agar; the parent RB cells showed a 15% growth in soft agar. The results indicate suppression of the malignant phenotype in the hybrids, confirming the recessiveness of the malignant phenotype of RB at the cellular functional level. Karyotyping of selected hybrids and the parent cells indicated a cumulative representation of all of the Y79 chromosomes in the hybrids, excluding loss of a specific Y79 chromosome causing the suppression of malignancy. Northern analysis of RNA from the hybrids demonstrated the mRNA of the reported putative mouse RB gene, consistent with the complementation of the Y79 RB gene defect by the normal mouse RB gene in the hybrids. Such a complementation may be a factor in the suppression of the malignant phenotype. Interestingly, the abnormal Y79 RB mRNA was absent in the hybrids, suggesting a possible negative feedback control by the normal mouse RB gene product.