MOLECULAR-CLONING AND CHARACTERIZATION OF A HUMAN PAX-7 CDNA EXPRESSED IN NORMAL AND NEOPLASTIC MYOCYTES

The myogenic basic helix-loop-helix proteins are essential components of the regulatory network controlling vertebrate myogenesis. However, determined myoblasts appear in the limb buds which do not initially ex press any member of this transcription factor family. In a search for potential novel regulators of myogenesis, a human PAX-7 cDNA was isola ted from primary myoblasts. Analysis of the DNA-binding properties of the Pax-7 paired domain revealed that it binds DNA in a sequence-speci fic manner indistinguishable from that of the paralogous Pax-3 protein . Each of the two proteins also binds to palindromic homeodomain-bindi ng sites by cooperative dimerization. Both Pax-3 and Pax-7, which are known to partially overlap in their expression during development, can also efficiently form heterodimers on these sites and stimulate repor ter gene transcription in transient transfection experiments which, in the case of Pax-7, is dependent on the transactivation function encod ed by the C-terminal sequences. Thus, the formation of heterodimers mi ght have important consequences for target gene recognition and regula tion during development. PAX-7 was found to be weakly expressed in nor mal human myoblasts, while PAX-3 could not be detected in these cells at all. However, transcripts for either PAX-3 and/or PAX-7 were expres sed at elevated levels in tumorigenic rhabdomyosarcoma cell lines. Hen ce, overexpression of these PAX genes may be involved in the genesis o f myogenic tumors.