Deletion of p16(INK4A)/CDKN2 and p15(INK4B) in human somatic cell hybrids and hybrid-derived tumors

Deletion or epigenetic inactivation of the tumor suppressor gene p16(INK4A) /CDKN2 (p16) has been observed in multiple human tumors. We assayed hybrid cell lines between human diploid fibroblasts and fibrosarcoma cells for p16 allelic status and expression and found that p16 was expressed in the pare ntal diploid fibroblast cell lines used, whereas the parental fibrosarcoma cell line HT1080.6TG exhibited homozygous deletion of p16. Most immortalize d hybrid cell lines derived from these parent cell lines, whether tumorigen ic or nontumorigenic, exhibited loss of fibroblast-derived p16 alleles, All p16-negative hybrid cell lines also exhibited deletion of p15(INK4B) (p15) Hybrid cell lines yielded tumors upon s.c. injection into athymic nude mic e regardless of p16/p15 status. Tumors derived from six p16/p15-positive hy brid cells, however, revealed deletions of both p16 and p15. When human dip loid fibroblasts were fused with A388.6TG squamous cell carcinoma cells, wh ich exhibit aberrant methylation of p16, the resulting hybrids again exhibi ted deletion of the unmethylated fibroblast-derived p16 alleles, Transfecti on of both HT1080.6TG and A388.6TG cells with wild-type p16 expression vect or resulted in decreased clonogenicity in culture. Although the determinant s directing genetic versus epigenetic inactivation of p16 and p15 remain un clear, these results demonstrate that p16-mediated growth suppression could be abrogated by either mechanism in somatic cell hybrids.