Relationship between tumorigenicity and the dosage of lymphoma‐ vs . Normal‐parent‐derived chromosome 15 in somatic cell hybrids between lymphoma cells with rearranged pvt ‐1 gene and normal cells

Somatic cell hybrids were generated between YACUT, a doubly drug‐resistant subline of YAC‐I (a Moloney‐virus‐induced T‐cell lymphoma of strain A/Sn origin with 2 pro viral insertions near the pvt ‐1 locus) and normal diploid fibroblasts of CBAT6T6 origin. Three independent fusions were performed. Three uncloned hybrid cultures and 9 independently‐derived clones were tested for tumorigenicity by the inoculation of graded cell numbers into syngeneic hosts. One of 3 uncloned hybrid cultures and 3 of 9 clones were weakly tumorigenic (take incidence 0%), and 1 of 3 uncloned hybrid cultures and 6 clones were highly tumorigenic (take incidence >80%). One weakly tumorigenic hybrid and 3 weakly tumorigenic clones carried 3 copies of the tumor‐derived chromosome 15 and 2 copies of the normal fibroblast‐derived t(l4;l5) chromosomes. In contrast, 2 highly malignant hybrid clones lost one copy of the normal‐fibroblast‐derived t(14;15), but contained increased numbers (3.44‐4.44) of the tumor‐derived chromosome 15. Four tumorigenic segregants selected from the weakly tumorigenic fibroblast hybrids by in vivo inoculation showed the same cytogenetic change as the highly tumorigenic hybrid clones, in that the ratio of the normal:tumor‐derived chromosomes 15 changed from 1.18‐1.55 to 4.11‐5.71. Tumorigenicity was thus associated with a modified balance between the tumor vs. the normal‐parent‐derived 15‐chromosomes. Instead of the usual 3:2 ratio, the tumor‐derived 15‐chromosomes increased disproportionately, whereas the relative number of the normal‐parent‐derived 15‐chromosome decreased, as a rule. These results suggest that amplification of the lymphoma‐derived chromosome 15 favors tumorigenicity, but that this effect is counteracted by some influence emanating from the normal‐parent‐derived homologous chromosome.