Identification of two novel, kinase‐deficient variants of discoidin domain receptor 1: differential expression in human colon cancer cell lines

Discoidin domain receptor 1 belongs to a subfamily of tyrosine kinase receptors activated by various types of collagen. Alternative splicing in the juxtamembrane region of the receptor results in the insertion of 37 amino acids. Here we used primer flanking the juxtamembrane region to search for additional isoforms. From human colon carcinoma Colo 206F cells, we identified two novel isoforms, DDR1d and DDR1e. In the DDR1d sequence, exon 11 and 12 are deleted, which results in a frame‐shift mutation and premature termination of translation. DDR1e arises from an alternative usage of a splice acceptor site in exon 10 and deletion of exon 11 and12. Both new isoforms are predicted to be membrane‐anchored, but kinase‐deficient, receptors. Transcripts for DDR1d and DDR1e were present to various degrees in a panel of human colon cancer cell lines. Using antibodies to several different receptor epitopes, we found correlating expression of DDR1d and DDR1e protein with the expected molecular weight of 68 and 95 kDa in colon carcinoma cell lines. Despite the abundant expression of truncated DDR1d in Colo 206F cells, stimulation with collagen induced full‐length receptor phosphorylation, thereby suggesting a lack of dominant‐negative inhibition by DDR1d. Exogenous overexpression of DDR1d in transient transfection assays supported a nondominant negative mechanism of signal modulation.