In vivo tumour x host cell fusion in spontaneous Syrian hamster metastasis

The HSV-2 tumour system was originally derived from an in vitro transformation of HEF by inactivated HSV-2. When injected s.c. these cells produce spindle-cell sarcomas which are metastatic at a low level. Detailed cytogenetic studies have provided evidence of tumour x normal host cell fusion in two of seven cell lines derived from metastatic lung deposits (Met D and Met G). This is the first report, in an unselected, intraspecific system, of in vivo cell fusion in spontaneous metastasis. The cells of Met D consisted of a heterogeneous population of fused and unfused tumour cells, whereas those of Met G were a homogeneous population of hybrid cells. Fusion, therefore, is likely to have occurred after metastasis in Met D and prior to, or at, metastasis in Met G. The fused cells of Met D showed comparatively little chromosome loss, while in Met G there was loss of approaching one haploid set of chromosomes. The generation of metastatic variants by cell fusion contributes to genetic diversity and emphasizes the importance of tumour heterogeneity in malignancy.