Part of “CHAPTER 3 – BIOSYNTHESIS AND SECRETION OF PEPTIDE HORMONES“

A major example of the generation of diversity is the calcitonin and calcitonin gene–related peptide (CGRP) system. In this system, the C cell of the thyroid expresses a calcitonin-CGRP transcript that initially contains six exons. In the C cell, tissue-specific factors determine the use of the polyadenylation site in the fourth exon, but in the brain, transcription through the sixth exon, which encodes CGRP and the alternative polyadenylation site present in that exon, provides the alternative splicing and deletion of the fourth exon, which encodes calcitonin. The C cells express mostly calcitonin and not much CGRP; conversely, the hypothalamus produces mostly CGRP but not much calcitonin (see Chap. 53 and Fig. 53-1). Other examples of alternative splicing yielding different polypeptides include the synthesis of the alternate human growth hormone form, substance P, substance K, and protooncogenes.90,91