| Research Abstract |
1. Examination on suppression of thymomagenesis in BUF/Mna-rnu/+ rats : (1) A congenic nude rat strain was established by 20 sequential backcrossings of BUF/Mna rats to (N : NIH-rnu/+ X BUF/Mna) F1 rats. (2) A large decrement of thymus weight by the introduction of rnu gene was confirmed in rats sequentially backcrossed for 19 generations. Furthermore, a complete suppression of thymomagenesis was reconfirmed in 18 months old BUF/Mna-rnu/+rats sequentially backcrossed for 17 generations.
2. (1) We examined the expression of receptor-type tyrosine kinase genes in seven cell lines which were established from benign (2 lines) or malignant (2 lines) thymomas of BUF/Mna rats and normal thymic epithelial cells of ACI/NMs rats. Poly A^+ RNAs were purified from these cell lines and analyzed by Northern blotting using EGFR, c-erbB-2, insulin receptor, ret and kit cDNAs as probes. As a result, only the EGFR gene was found to be expressed in all cell lines, although the level of its expression in malignant thymoma cell lines was several-fold lower than that in benign thymoma and normal thymic epithelial cell lines. Two major transcripts of 10 and 5.8 kb, which were the same as those observed in normal liver RNA, were detected in them. (2) We isolated 17 clones of 92/93bp monomer and 8 clones of 185bp dimer of satellite I DNA from thymoma DNAs of BUF/Mna rats. Based on the nucleotide sequences, 16 clones of the monomer were classified into four subgroups (A, B, C and D) and 8 clones of the dimer into three (AB, BC, and CD). However, one clone of the monomer was unclassifiable. Nucleotide sequence indicated that this clone was generated by DNA deletion which occurred during unequal sister chromatid exchange and by gene conversion between subgroups of the monomer.
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