Model for alternative RNA processing in human calcitonin gene expression

Abstract

The alternative RNA processing pathways in human calcitonin gene (CALC-I gene) expression were investigated using steady state RNA isolated from human medullary thyroid carcinoma (MTC) and from a culture line derived from this tumor. On Northern blots the mature 1.0 kilobases (Kb) calcitonin (CT) - and 1.1 Kb calcitonin gene-related peptide (CGRP) mRNAs were detected with CALCI gene specific probes as well as high molecular weight poly (A) containing RNAs of 2.1, 2.3, 3.3, 4.2, 5.0 and 5.7 Kb. The 5.7 Kb RNA was identified as the poly(A) tailed primary transcript containing sequences corresponding to all 6 exons and 5 introns of the CALC-I gene. From the composition of the other RNAs the splicing order of the different introns could be deduced. The results suggest the following model. First all introns not involved in alternative processing (introns 1, 2 and 5) are spliced from the 5.7 Kb RNA in rapid successive reactions yielding a 3.3 Kb RNA, which accumulates. From this 3.3 Kb RNA, the last common intermediate in the alternative processing pathway, CT mRNA is formed by splicing of intron 3 and poly(A) addition at exon 4, in this order or the reverse order via 2.3 Kb or 2.1 Kb RNA intermediates respectively. Alternatively, the whole intron 3-exon 4-intron 4 region is spliced from the 3.3 Kb RNA yielding CGRP mRNA. The temporal sequence of poly(A) addition at exons 4 and 6 may relate to the observed structural differences between the poly(A) addition signals at these sites. The ratio of CT- to CGRP mRNA may relate also to the differences in the primary structures of the intron 3- and intron 4 splice acceptor sites.

CGRP and its receptors

“…While the term CGRP is widely used, there are actually two forms of this peptide – αCGRP and βCGRP – which are derived from two separate genes. CALCA encodes αCGRP and calcitonin in a tissue‐specific manner and CALCB encodes βCGRP . Other gene products encoded by the CALCA gene include procalcitonin, katacalcin, and N/C‐terminal peptides from pro‐CGRP .…”

Peptides from the calcitonin genes: molecular genetics, structure and function

“…In the case of the a-calcitonin/CGRP gene, it was originally proposed that the pattern was regulated at the level of polyadenylation selection . However, it is now clear that the splicing together of the exons generally precedes polyadenylation (Bovenberg et al, 1986).…”

Biology of Peptides From the Calcitonin Genes

“…They have also reported that polyadenylation precedes and determines the choice of splicing. Recently Lips' group has confirmed that there is alternative processing, but that polyadenylation at the calcitonin site generally follows the removal of the introns (Bovenberg, van de Meerendonk, Baas, Steenbergh, Lips & Jansz, 1986). Solnick (1985) has reported alternative splicing in vivo and in vitro due to RNA secondary structure.…”