Identification of microphthalmia-associated transcription factor isoforms in dogs.

Microphthalmia-associated transcription factor (MITF) belongs to the basic helix-loop-helix leucine zipper (bHLH-LZ) family of transcription factors, and plays an important role in the development and differentiation of melanocytes, mast cells, and osteoclasts. To investigate canine MITF isoforms, cDNA cloning with 5'-rapid amplification of cDNA ends was performed. Three canine isoforms, the counterparts of human MITF-M, MITF-H, and MITF-A, were determined. Canine MITF-M, MITF-H, and MITF-A were highly homologous to the nucleotide sequences in isoform-specific 1M, 1H, and 1A with 100%, 98%, and 97% identity in humans, respectively. When scanning of the canine genome was performed to identify the homologous regions to human MITF isoforms, in addition to MITF-M, MITF-H, and MITF-A isoforms, the homologous sequences to all the other isoforms observed in humans existed in the canine genome sequence of chromosome 20. Comparison of the homologous sequences for MITF isoforms among dogs and humans suggested that MITF-D, MITF-E and MITF-J may be expressed in canine adult tissues in addition to canine MITF-M, MITF-H, and MITF-A. However, RT-PCR showed that canine MITF-E and MITF-J, but not canine MITF-D were expressed. Although the mRNAs encoding canine MITF-H and MITF-A were expressed widely, MITF-M, MITF-E and MITF-J were expressed in tissue-specific patterns. Two types of mRNAs with and without an 18-base insert were detected for five MITF isoforms in canine tissues. MITF-M distal enhancer (MDE) was also located upstream from canine exon 1M.