8. Breeding, Genetics and Cytogenetics O F the Soft-Furred Rat, Millardia Meltada. V Karyological Study on a Methylchoranthrene Induced Tumor*'

28 Proc. Japan Acad., 61, Ser. B (1985) [Vol. 61(B),

8. Breeding, Genetics and Cytogenetics o f the Soft-Furred Rat, Millardia meltada. V Karyological Study on a Methylchoranthrene Induced Tumor*'

By Tosihide H. YosIDA National Institute of Genetics,Misima, Japan (Communicated by Sajiro MAKINO,M. J. A., Jan. 12, 1985)

The karyological analysis of the mammary tumors developed in the male specimens of the soft-furred rat showed that all tumors except one had the normal karyotype (2n=50) of this species, but one of them had a trisomy of the no. 16. The mammary tumors were diagnosed to be the benign type depending on the male hormone. The normal karyotype seemed to be due to the benign characteristic of the tumor as seen in the other benign type tumors in the other species (Yosida 1985). To compare the benign type with the malignant type tumors of the soft-furred rat, a tumor was induced by injection of the methylchoranthrene, and the karyotype of the tumor cells was analysed. In the present paper the karyotypes of the methylchoranthrene induced tumor will be described in comparison with those of the benign type mammary tumor of the soft-furred rats. Material and method. The soft-furred rat (Millardia meltada) bred in our laboratory by the brother and sister mating was used in the present study. Two mg of the 3-methylchoranthrene dissolved in 1 ml of the olive oil were in- jected subcutaneously to the inguinal region in 5 specimens (~ 3 : a 2). The tumor developed about 5 months after the injection at the injection site in a female (MM 1510a). The tumor was examined histopathologically through the courtesy of Dr. Yasuaki Nishizuka, the Aichi Cancer Center and it was diagnosed to be the fibrosarcoma. The tumor pulps were cultured in vitro to observe the chromosomesby our routine procedure described in the previous paper (Yosida 1985). The tumor was successfully transplanted to the other soft-furred rats. Results and discussion. In the normal female 50 chromosomes consisting of 24 autosome pairs and XX complements were observed. Among them nos. 1 and 2 autosome pairs were easily recognized due to the large subtelocentric and the X chromosomeswere also recognized by the large metacentrics (Fig. 1). From the 50 tumor cells obtained from the in vitro culture, the chromosome number was observed (Table I). As seen in the table the number of chromosomeswas widely distributed from 40 to 150 or more, without any sharp mode of the distribution. The cells with 50 chromosomes which were the normal chromosome number in this species were observed only in 3 cells. The deviation of the chromosomenumbers in this tumor seemed to mainly be due to the nondisjunction in some chromosomes and also the translocation between two acrocentrics. As it is difficult to show the chromosomes of all tumor cells observed, three cells selected randomly were demonstrated here. In one cell with 46 chromosomes, two X's were observed, but one Robertsonian fusion newly occurred. One of the

*' Contribution no . 1594 from the National Institute of Genetics, Japan. No. 1] Karyology of a Methylchoranthrene Induced Tumor in Millardia 29

Figs. 1-2. 1: Metaphase of the normal cell with 50 chromosomes in the soft-furred rat. Two X's, no. 1 and no. 2 chromosome pairs are observed. 2: A metaphase cell with 46 chromosomes from the tumor developed by the methylchoranthrene injection. Two X's and no. 1 pair are observed, but the no. 2 is unpaired. One Robertsonian metacentric is included.

Table I. Distribution of chromosome numbers in a methylchoranthrene induced tumor i n the soft-furred rat

no. 2 disappeared from the metaphase (Fig. 2). A cell with 49 chromosomes had also two X's, but one Robertsonian fusion was observed as similar to the above cell (Fig. 3). The chromosome constitution of a cell with 94 chromosomes was more complicated. Two X's and one Robertsonian fusion were observed similar to the above two cells, but a tetrasomy of the no. 1 chromosome and a trisomy of the no. 2 chromosome were clearly demonstrated. In the other chromosomes the tetrasomy and the trisomy would be included, although the identification of these chromosomes was difficult (Fig. 4). The chromosomes of the tumors induced by the methylchoranthrene were observed by several investigators in the mouse, rat and the other animals. The methylchoranthrene induced tumors showed usually the chromosome alterations. In the Indian spiny mouse, Mus platythrix, the aberrant chromosomes could easily be identified, because the chromosome number of that species was 2n=26 and each chromosome pair was easily demonstrated. In the Indian spiny mouse the alteration of the chromosomes with the nucleolar organizer region (NOR) was preferentially included in the tumor cells (Yosida 1981). In the soft-furred rat, however, it was difficult to know the relation between the NOR and tumor de- velopment, because this species had a high chromosome number and moreover 30 T. H. Y0sIDA [Vol. 61(B),

Figs. 3-4. 3 : A metaphase with 49 chromosomes from the tumor. Two X's, no. 1 and no. 2 pairs are included, but one Robertsonian metacentric is obserevd. 4. A metaphase cell with 94 chromosomes from the tumor. Two X's are observed, but four no. 1 chromosomes (tetrasomy) and three no. 2 chromosomes (trisomy) are demonstrated. One Robertsonian metacentric is also included. the methylchoranthrene induced tumor had a wide distribution of the chromosome numbers. In comparison with the chromosomes in the methylchoranthrene induced tumor and those of the mammary tumors described previously (Yosida 1985), it is interesting because the mammary tumor had always the normal karyotype, but the tumor developed by the carcinogen had the enormous variation of the chromosome constitution. In the rat the thyroid tumor depending on the thyroid hormone had the normal karyotype, but the autonomous thyroid tumors had the chromosome alterations (Al Saadi and Mizej ewski 1972). The difference of the chromosome constitution between the benign type mammary tumor depending on the male hormone and the malignant tumor induced by the methylchoranthrene in the soft-furred rats is interesting to elucidate the mechanism of the malignant transformation. Based on the comparison of the chromosomes between these two tumors, it is suggested that the malignant transformation of the mammary tumor should occur following the chromosome alterations. Summary. Chromosomes of a methylchoranthrene induced tumor in the soft-furred rat are analysed. The chromosome numbers are distributed widely from 40 to 150 or more. In this tumor the chromosome number variation is due to the nondisjunction and the Robertsonian fusion. The malignant tumor with the anormous chromosome alteration is in comparison with the benign mammary tumors with the normal karyotype observed in this animal. Acknowledgements. The present author is grateful to Emeritus Professor, Sajiro Makino, M. J. A., for his critical reading of this manuscript. He is also indebted to Dr. Yasuaki Nishizuka, the Aichi Cancer Center, for his histopatho- logical examination of this tumor. No. 1] Karyology of a Methylchoranthrene Induced Tumor in Millardia 31

References

Al Saadi, A., and Mizejewski, G. J.: Cancer Res., 32, 501 -505 (1972). Yosida, T. H.: Cytogenet. Cell Genet., 3, 211-220 (1981). Pron. Japan Acad., 61B, 24-27 (1985).