IDENTIFICATION OF A NOVEL VARIANT FORM OF FIBROBLAST GROWTH-FACTOR RECEPTOR-3 (FGFR3 IIIB) IN HUMAN COLONIC EPITHELIUM

Although several tyrosine kinase-type growth factor receptors have bee n demonstrated in human colonic epithelial cells, the full spectrum of growth factor receptors has not been identified. Low stringency scree ning of a complementary DNA library prepared from the human colon canc er-derived cell line HT-29 with a probe containing the tyrosine kinase domain of human c-src kinase led to the identification and isolation of a clone containing a receptor class tyrosine kinase. This putative receptor was found to be identical to the human fibroblast growth fact or receptor 3 (FGFR3) except for a region of 150 nucleotides (50 amino acids) encoding the presumptive ligand-binding domain, where it exhib ited only 32% homology with the previously described FGFR3. The varian t domain corresponded precisely to the splicing junctions of the exon encoding the carboxyl terminal half of the third immunoglobulin-like d omain, suggesting that two forms of FGFR3 result from splicing of alte rnate exons in a manner similar to that previously found for FGFR1 and FGFR2. By prior convention, the previously reported form of FGFR3 was designated me due to its high degree of homology with the me domain o f FGFR1 (83% homology) and the me domain of FGFR2 (81% homology). Howe ver, the ligand-binding domain of FGFR3 found in the HT-29 cell line w as more highly divergent from all pre,iously reported FGFR immunoglobu lin-like domain IIIs than any other two members of this receptor famil y. Therefore, me propose to designate the newly reported form as the F GFR3 IIIb variant. Genomic polymerase chain reaction confirmed that th e IIIb-containing exon occupies a position 5' relative to the me-conta ining exon within the FGFR3 gene. Northern blot analysis using a probe encompassing sequences unique to the FGFR3 IIIb mRNA confirmed the ex pression of a 4.4-kilobase transcript in two colon cancer-derived cell lines as well as normal human colonic mucosa. Using a technique combi ning reverse transcriptase polymerase chain reaction with restriction endonuclease digestion, cell lines, primary cells, and tissues were as sessed for IIIb and IIIc transcripts; expression of the IIIb variant w as associated with an epithelial lineage, while the IIIc variant was e xpressed predominantly in nonepithelial cells and tissues.