Regulation of the fibroblast growth factor receptor 3 promoter and intron I enhancer by Sp1 family transcription factors

Fibroblast growth factor receptor 3 (FGFR3) has a complex spatial and temporal pattern of expression and is essential for the normal development of a diverse set of tissues. Recently, mutations have been identified in FGFR3 that result in constitutive tyrosine kinase activity and cause a number of different human skeletal disorders. To examine the regulatory mechanisms governing FGFR3 expression, the promoter for the FGFR3 gene was identified and characterized. It resides in a CpG island, which encompasses the 5' end of the FGFR3 gene and lacks classical cis-regulatory motifs. As little as 100 base pairs of sequence 5' to the initiation site can confer a 20-40-fold increase in transcriptional activity upon a promoter-less vector. The transcriptional activity of these cia-regulatory sequences is further stimulated by elements found within the first intron. Mapping of the enhancer activity found within intron I identified two purine-rich sequence moths between +340 and +395. Electrophoretic mobility shift assays demonstrated that sequences within this region bind members of the Spl family of transcription factors. In a background lacking Spl-like activity, we demonstrate that Spl can enhance transcription of the minimal promoter (which contains five classical Spl sites), whereas both Spl and Sp3 can enhance transcription through the elements found in intron I. Although these transcription factors are ubiquitously expressed, we demonstrate that the sequences between -220 and +609 of the FGFR3 gene are sufficient to promote the tissue-specific expression of a reporter gene in transgenic mice.