Titin diversity--alternative splicing gone wild

Abstract

Titin is an extremely large protein found in highest concentrations in heart and skeletal muscle. The single mammalian gene is expressed in multiple isoforms as a result of alternative splicing. Although titin isoform expression is controlled developmentally and in a tissue specific manner, the vast number of potential splicing pathways far exceeds those described in any other alternatively spliced gene. Over 1 million human splice pathways for a single individual can be potentially derived from the PEVK region alone. A new splicing pattern for the human cardiac N2BA isoform type has been found in which the PEVK region includes only the N2B type exons. The alterations in splicing and titin isoform expression in human heart disease provide impetus for future detailed study of the splicing mechanisms for this giant protein.

Figure 1

Titin location and arrangement in the cardiac sarcomere.

Figure 2

Domain arrangement of different titin isoforms. Arrows indicate the exons spliced, solid line connections denote consecutive exons. Cross-hatch pattern in PEVK indicates variable patterns of skipped exons.

Figure 3

Exon inclusion in clones from the human titin PEVK region. Data from left ventricle samples of five different individuals—three normal (NH-1, NH-2, NH-3), one restrictive cardiomyopathy (RCM), and one dilated cardiomyopathy (DCM). Exon number is listed across the top. Filled boxes denote exon inclusion. Letter designations in each block refer to separate clones from each individual. Clones were obtained by PCR amplification after RT-PCR using primers from exons 108 and 225. Full details on the methods have been published [38]. Data from sample NH-1 is from [38] and for NH-2 clones from [37].

Figure 4

Exon inclusion in clones from rat left ventricle of wild type and mutant rats. The percent of clones that contain the exons shown is plotted versus exon position. Only exons 115, 224, and 225 were found in all wild type and mutant clones (heterozygote [Ht] and homozygote [Hm]). Data was pooled from four different ages [47].