Tissue specific expression of angiotensin converting enzyme: a new way to study an old friend.

Angiotensin-converting enzyme (ACE) plays a central role in blood pressure regulation by producing the vasoconstrictor angiotensin II. When ACE knockout mice were studied, they presented with a complicated phenotype, including cardiovascular, reproductive, hematologic and developmental defects. The complexity of an ACE knockout mouse emphasizes the advantages and disadvantages of the classic knockout strategy. An animal lacking all ACE is very different from a wild type animal, and can be modeled as representing an extreme phenotype. To understand the role of ACE in a tissue and organ specific fashion, our group used targeted homologous recombination to create mouse models in which a promoter swapping strategy results in very restricted tissue patterns of ACE expression. Mice with ACE expression only in the heart, termed ACE 8/8 mice, present with atria enlargement and electrical conduction defects, but normal ventricular function. Mice with ACE expression only in monocytes and macrophages, termed ACE 10/10 mice, have a marked resistance to the growth of melanoma due to an enhanced immune response characterized by increased tumor specific CD8+ T cells and increased proinflammatory cytokines. These mice may define a new means of augmenting the immune response, potentially useful in human clinical situations. The promoter swapping strategy permits scientific investigation of questions unapproachable by other experimental approaches.