Structure of the mouse metalloprotease meprin β gene (Mep1b): Alternative splicing in cancer cells

The mouse meprin β gene encodes an integral membrane protease that is expressed in a tissue-specific manner in embryonic and adult epithelial cells, and in carcinoma cells. The meprin b mRNA in the embryo, kidney and intestinal cells is 2.5 kb, whereas the isoform in carcinoma cells (β' mRNA) is 2.7 kb. The work herein was initiated to explore the molecular mechanism responsible for the different isoforms. Overlapping fragments containing the Mep1b gene were obtained from a yeast artificial chromosome clone using polymerase chain reactions. The gene spans approximately 40 kb and consists of 18 exons and 17 introns. The first three exons are unique to the 5' end of β' mRNA; the next two exons correspond to the 5' end of β mRNA. The rest of the exons (13 total) encode the regions common to both β and β' messages. In conjunction with the cDNA sequences, the gene structure establishes that alternative splicing of 5' exons is responsible for the generation of the mRNA isoforms. The DNA regions between β'- and β-specific exons and upstream of the first β' exon have been completely sequenced to identify potential regulatory elements for β and β' transcription. There is significant homology between the two regions, indicating that a duplication event occurred during evolution of the Mep1b gene. Potential promoter elements and transcription factor-binding sites were identified from comparisons to sequences in the databanks. This is the first gene structure that has been completed for meprin subunits from all species. The work elucidates molecular mechanisms that regulate differential expression of the Mep1b gene.