Studies on histocompatibility mutations in mouse tumour cells using isogenic strains of mice
Authors: Dhaliwal S. S. 
Published: September 2009

The feasibility of using heterozygous tumours induced in hybrids between Snell's isogenic resistant (IR) strains of mice for mutation studies was examined. The system has been described by Mitchison (1956) and Klein, Klein & Révész (1957).

Fourteen hybrid sarcomas (A × A.SW F1; H-2 genotype a/s) were tested in mice of the parental strains. Seven were specific for the original F1 hybrid genotype and failed to give any variants. Four tumours gave variants that were compatible with one of the parental strains. The variants grew progressively in all mice of the parental strain, even in pre-immunized mice, and failed to grow in mice of foreign genotypes. Of these, one tumour, as. 14 gave true variants towards both the parental strains. One tumour, as.7, gave variants regularly in 35·5% of the A.SW mice but only in 7·7% of pre-immunized A.SW mice. It failed to give variants towards the other parental strain.

The effect of triethylenemelamine (TEM) and X-ray treatment on the rate of variant production in hybrid tumours was studied. The effect of the treatment on the viability of tumour cells was first examined by titrating tumour cell suspensions on groups of mice or on the CAMs of groups of developing chick embryos. The TD50 values (50% end-points) were calculated. Comparable results were obtained by titrating on mice and on the CAM, although titration on mice gave more consistent results. Approximately 102 viable, untreated cells were needed to produce a tumour in mice or on the CAM.

Tumour as.7 was tested in the parental strains after treatment with TEM or X-rays. Using comparable cell doses in the untreated controls and treated series, the percentage of variants produced in one of the parental strains (A.SW) was significantly increased after treatment with TEM or X-rays. With the exception of one experiment, no variants were produced in the other parental strain. In one experiment, four variants were produced towards the A strain after treatment with TEM, three of which were tested: only one was found to be specific for the A strain. Three of the A.SW variants, originating after treatment with TEM, were tested: two were specific while one showed incomplete specificity for the A.SW strain. Treatment of specific tumours with TEM or X-rays did not give variants towards either of the parental strains.

Two of the hybrid tumours and their derived variants were examined cytologically. Variants originating spontaneously and after treatment with TEM were examined. While most of the original tumours had chromosome numbers with an exactly diploid modal number, the spontaneous and TEM-treated variants had modal numbers which were hyperdiploid. The TEM-treated variants also had a larger proportion of cells which were triploid or hypotetraploid. This would explain the incomplete specificity of the variants isolated after treatment with TEM.

While it appears possible to use heterozygous tumours for isolating genetical variants specifically compatible with one of the parental strains, the actual nature of the mechanism of variant production remains dubious.