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In order to identify the genetic factors associated with the radiation-resistant human laryngeal carcinoma cell line (SQ-20B), tumor cell DNA was transfected into NIH/3T3 cells. A high incidence (six out of six) of raf sequences was found in transfected NIH/3T3 clones and the tumorigenic potential of SQ-20B DNA could be linked to genomic fragments that represent most of the kinase domain of human c-raf-1. An apparently unaltered 3.5-kilobase pair (kb) human c-raf transcript was identified in SQ-20B cells but was not observed in the transfected NIH/3T3 cell clones. Two new transcripts (4.2 kb and 2.6 kb) were found in tumorigenic clones; the large transcript was missing in a very poorly tumorigenic clone. Cytogenetic analysis indicated that the normal autosomes of chromosome 3 were absent in SQ-20B karyotypes and had formed apparently stable marker chromosomes. Unlike the recipient NIH/3T3 cell line, 30 percent of the transformed clone-1 metaphases had minute and double-minute chromosomes representative of amplified DNA sequences. The frequency of the c-raf-1 identification by NIH/3T3 transfection of SQ-20B DNA suggests the presence of some genetic abnormality within this locus.
Kasid et al. (Fri,) studied this question.