Oncogenic AKAP9-BRAF fusion is a novel mechanism of MAPK pathway activation in thyroid cancer

TLDR

In thyroid papillary carcinoma, MAPK pathway mutations—most commonly BRAF point mutations in sporadic tumors—drive transformation, whereas radiation‑induced tumors typically involve paracentric inversions activating receptor tyrosine kinases such as RET and NTRK1. A paracentric inversion of chromosome 7q fuses exons 1–8 of AKAP9 to exons 9–18 of BRAF, producing a protein that retains the kinase domain but lacks BRAF’s autoinhibitory N‑terminus. The AKAP9‑BRAF fusion exhibits hyperactive kinase activity, transforms NIH3T3 cells, and is predominantly present in radiation‑induced papillary carcinomas, demonstrating that radiation activates MAPK signaling through chromosomal inversions rather than point mutations.

Abstract

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.