Disclosed are genetic expression cassettes, and vectors comprising them useful for the delivery of isolated nucleic acid segments including those expressing or encoding one or more selected therapeutic constructs (including, without limitation, therapeutic peptides, polypeptides, ribozymes, or catalytic RNA molecules), to one or more selected cells or tissues of a vertebrate animal. Methods employing the disclosed genetic constructs in the development of gene therapy-based viral vector systems are also disclosed. The expression cassettes and viral vectors disclosed herein provide new tools for methods of treating mammalian, and in particular, human diseases, disorders, and/or dysfunctions. The disclosed compositions and methods find particular utility in a variety of investigative, diagnostic, and therapeutic regimens, including, for example, in the treatment or amelioration of symptoms of a variety of mammalian, and particularly, human conditions.