GENOMIC STRUCTURE AND ISOFORM EXPRESSION OF THE MOUSE, RAT AND HUMAN CBFA1 OSF2 TRANSCRIPTION FACTOR/

Although the CBFA1 gene encodes an osteoblast-specific transcription f actor that regulates osteoblast differentiation, uncertainty exists ab out the organization of its 5' end and the relevance of a novel N-term inal sequence identified in the mouse Cbfa1/Osf2 isoform. We found the novel 5' Cbfa1/Osf2 sequence is encoded by a previously unrecognized upstream exon, designated exon -1, which is highly conserved in mouse, rat and human. In addition, two splice donor sites may be utilized to generate Cbfa1/Osf2 cDNAs containing different N-terminal sequences. The first ATG and splice donor site in exon -1 is predicted to transcr ibe a cDNA containing the unique Osf2 5' sequence, whereas a second do nor splice site gives rise to cDNAs that contain sequences encoding an 11 amino acid insert. In the human CBFA1 gene, an additional 2-bp nuc leotide insert shifts the reading frame and results in stop codons in the cDNA sequence derived from exon -1. The 5'-most exon of the human CBFA1 gene, therefore, contains the 5' non-coding region rather than a human OSF2 homolog. The absence of a homologous OSF2 coding sequence in the human CBFA1 cDNA suggests that the novel mouse N-terminal Osf2 sequence is not essential for functioning of the CBFA1 gene product. I n addition, multiple transcripts derived from a single CBFA1/Cbfa1 gen e were detected in osteoblasts by Northern analysis and RT-PCR, includ ing additional Cbfa1/Osf2 isoforms containing deletions of exons 1 and 4. Thus, the alternative use of transcription start sites and splicin g leads to the genesis of CBFA1/Cbfa1 isoforms with possible differenc es in transactivation potentials. (C) 1998 Elsevier Science B.V. All r ights reserved.