Tissue-specific expression of the human tropomyosin gene involved in the generation of the trk oncogene

The trk oncogene is a human transforming gene generated by the fusion of tropomyosin gene sequence to a truncated tyrosine kinase receptor gene. We have now characterized the normal tropomyosin gene from which the trk oncogene is derived. At least two different transcripts are expressed by this gene using a tissue-specific alternative messenger RNA splicing mechanism: a 2.5-kilobase (kb) mRNA encoding a 248-amino-acid tropomyosin in human fibroblasts and a 1.3-kb mRNA encoding a 285-amino-acid tropomyosin in human skeletal muscle. The rearrangement which generates the trk oncogene preserves most of the tropomyosin-coding sequences of the normal gene, including exons alternatively spliced in muscle and non-muscle tissue. We therefore expect the trk oncogene to show a tissue-specific pattern of transforming activity. Correct expression of the trk oncogene can occur only in non-muscle tissues. In muscle tissue the oncogene would almost certainly be inactive, as splicing according to the alternative muscle pattern aborts synthesis of the tyrosine kinase domain.