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Genes crucial for cancer development can be mutated via various mechanisms, which may reflect the nature of the mutagen. In thyroid papillary carcinomas, mutations of genes coding for effectors along the MAPK pathway are central for transformation. BRAF point mutation is most common in sporadic tumors. By contrast, radiation-induced tumors are associated with paracentric inversions activating the receptor tyrosine kinases RET and NTRK1. We report here a rearrangement of BRAF via paracentric inversion of chromosome 7q resulting in an in-frame fusion between exons 1-8 of the AKAP9 gene and exons 9-18 of BRAF. The fusion protein contains the protein kinase domain and lacks the autoinhibitory N-terminal portion of BRAF. It has elevated kinase activity and transforms NIH3T3 cells, which provides evidence, for the first time to our knowledge, of in vivo activation of an intracellular effector along the MAPK pathway by recombination. The AKAP9-BRAF fusion was preferentially found in radiation-induced papillary carcinomas developing after a short latency, whereas BRAF point mutations were absent in this group. These data indicate that in thyroid cancer, radiation activates components of the MAPK pathway primarily through chromosomal paracentric inversions, whereas in sporadic forms of the disease, effectors along the same pathway are activated predominantly by point mutations.
Nonstandard abbreviations used: BAG, bacterial artificial chromosome; FISH, fluorescence in situ hybridization; MEK, MAPK/ERK kinase; ORF, open reading frame; PAC, P1 artificial chromosome; PKA, protein kinase A; RACE, rapid amplification of cDNA ends; RBD, RAS-GTP binding domain; RII, type II regulatory subunit.
Introduction
Thyroid papillary carcinoma is the most common type of endocrine malignancy. In this tumor, mutations of genes coding for effectors along the MAPK pathway are central for transformation. Indeed, activating mutations of BRAF, RET, or RAS are found in 70% of all cases and rarely overlap in the same tumor (1-3).
The RET gene codes for a cell membrane receptor tyrosine kinase (4, 5). In papillary carcinomas, it is activated via chromosomal rearrangement, which results in fusion of the 3' portion of the gene coding for the tyrosine kinase domain to the 5' portion of various genes, creating chimeric oncogenes named RET/PTC (6, 7). The 2 most common rearrangement types, RET/PTC1 and RET/ PTC3, are paracentric inversions, since both RET and its respective fusion partner, H4 or...