Alternative splicing of BRAF transcripts and characterization of C‐terminally truncated B‐Raf isoforms in colorectal cancer

The BRAF proto‐oncogene is mutated in a subset of human tumors, including colorectal cancer. A splicing variant lacking exons 14 and 15 ( BRAF del E14/15) has been described recently. However, the frequency of the variant, the kinase activity of the protein isoform, its biological function, and which allele it is derived from remains unknown. BRAF mRNA from colorectal cancer cell lines and colonic epithelium was reversely transcribed, subcloned, and screened for alternative splicing. New transcript variants and allelic origin of alternatively spliced transcripts were analyzed by DNA sequencing. Kinase activity of the B‐Raf isoforms was determined by Western blotting after transfections with expression constructs of the different BRAF variants. Four additional BRAF transcript variants resulting in C‐terminal truncation of the gene product were found. Alternative splicing was found at frequencies from 4.7 to 16.7% in normal and neoplastic colorectal cells. Alternative transcripts were shown to be derived from both wild‐type and V600E alleles. All nonconsensus B‐Raf protein variants were found to be kinase‐dead and failed to coactivate full‐length B‐Raf. In conclusion, we present a highly sensitive method for the detection of aberrantly spliced transcripts. Alternative splicing of exons 14, 15, 15b, 16b and 16c occurs in a considerable fraction of BRAF mRNA in normal colon and colorectal cancer cells and is independent of the V600E mutational status of the parental allele. Splicing of nonfunctional transcripts affects overall cellular B‐Raf activity and might represent a mechanism to decrease sensitivity to growth signals.