Evolutionary pathways of the calcitonin (CALC) genes

Recombinant DNA techniques have made it possible to establish the structure of various genes encoding polypeptide hormones. Comparison of nucleotide sequences of the calcitonin (CALC) genes in man has revealed surprising similarities and variations. These findings and the homologies among the sequences in different species offered an opportunity for speculation about relationships between these genes and about their evolutionary origin. The first gene (CALC-I) directing the synthesis of calcitonin (CT) or CT gene-related peptide (CGRP) comprises six exons and gives rise to two mRNAs by an alternative RNA-processing mechanism. The homology between CGRP and CT reflects their common origin. The human genome contains a second gene (CALC-II) that is structurally related to the CALC-I gene. The CALC-II RNA transcripts do not appear to be differentially processed, as only preproCGRP-II mRNA and not preproCT-II is detected. The first and second CT/CGRP genes probably have evolved from a common ancestor gene early in evolution. Meanwhile, a third genomic locus containing nucleotide sequences highly homologous to exons 2 and 3 of both CALC genes was detected and probably generated by duplication of a part of CALC-II. This locus is not likely to encode a CT- or CGRP-related polypeptide hormone. The CALC genes and this last (pseudo) gene are located on the short arm of chromosome 11. Recently, islet- or insulinoma-amyloid polypeptide (IAPP) was isolated as a major constituent of amyloid present in human insulinoma and in pancreatic islet amyloid in noninsulin-dependent diabetes mellitus. IAPP shows 46% amino acid sequence homology with human CGRP-II.(ABSTRACT TRUNCATED AT 250 WORDS)