ANALYSIS OF LARGE AND SMALL COLONY L5178Y TK(- -) MOUSE LYMPHOMA MUTANTS BY LOSS OF HETEROZYGOSITY (LOH) AND BY WHOLE CHROMOSOME-11 PAINTING - DETECTION OF RECOMBINATION/

Analysis of 122 spontaneous large and small colony mutants derived fro m L5178Y tk(+/-) mouse lymphoma cells at 28 heteromorphic microsatelli te loci on chromosome 11 showed that extensive loss of heterozygosity (LOH) is common in both large colony and small colony mutants, elimina ting most chromosome 11 loci as candidates for a putative growth contr ol locus. These results, in conjunction with historical cytogenetic da ta, suggest that a putative growth control locus lies distal to the th ymidine kinase (Tk1) gene, near the telomere. Thirty seven mutants wer e hybridized with a chromosome Ii-specific whole chromosome painting p robe for analysis of rearrangements. Generally, painting confirmed ear lier observations that large colony mutants are karyotypically normal, whereas small colony mutants frequently have detectable rearrangement s. A point probe distal to Tk1 revealed no evidence of chromosome brea kage in small colony mutants that appeared normal on whole 11 painting and had no LOH. Therefore, the molecular difference between large and small colony mutants remains unknown. Models to explain large and sma ll colony mutants consistent with our findings are presented, includin g loss of a putative growth control gene, differential mechanisms of c hromosome breakage/repair and second site mutations as explanations fo r small colony mutants. Painting revealed translocations and aneuploid y and showed that non-disjunction was not a common explanation for com plete LOH. The most common finding was that large regions of LOH do no t result from deletions, demonstrating that these cells can detect rec ombination events as well as previously observed chromosomal rearrange ments, deletions and point mutations.