Chromosome rearrangements involved in the origin of trisomy 15 in spontaneous leukemia of AKR mice.

Trisomy 15, known to be the predominant chromosome abnormality in leukemic cells of the AKR strain, develops even in animals of the subline AKR/Rb1Ald with two constitutive Robertsonian translocation chromosomes (6.15). Among 10 leukemic animals of this subline exhibiting chromosomal anomalies, 6 showed trisomy of the whole Robertsonian translocation chromosome with the expression of combined trisomy 6 and 15 as the most frequent abnormality. Besides this, rearrangements were observed in five animals, most of them resulting from centric fission of the Rb(6.15) translocation chromosome. Trisomy 15 with two Rb(6.15) and an extra acrocentric 15 was found in 44% of spleen cells in one animal, and a new Rb translocation of chromosomes 11 and 15 was seen in 50% of spleen and thymus cells of another animal. In both cases trisomy 15 without simultaneous trisomy 6 resulted. Thus, the triplication of a whole Rb(6.15) is frequent in leukemic AKR/Rb(6.15)1Ald, and ensuing double trisomy 6 + 15 is tolerated by the leukemic cell. But the development of trisomy 15 combined with centric fission of Rb(6.15) without simultaneous trisomy 6 is another principle realized in leukemogenesis.