Complete genomic structure of the human nebulin gene and identification of alternatively spliced transcripts.

The giant nebulin protein is a fundamental structural component of the thin filaments of the striated muscle sarcomere. Nebulin binds to actin and the size of nebulin correlates with actin filament length, suggesting that nebulin may determine the length of the thin filaments during myofibrillogenesis. We have previously described the genomic organization of the 3' end of the nebulin gene (NEB), and identified 18 different NEB mutations in patients with autosomal recessive nemaline myopathy. Here we present the genomic organization of the entire nebulin gene, and the identification of numerous alternatively spliced mRNAs. The gene comprises 183 exons spanning 249 kb of the genomic sequence. The translation initiation codon is in exon 3, and the stop codon and the 3' UTR are in exon 183. There are four regions with alternatively spliced exons, that is, exons 63-66, 82-105, 143-144 and 166-177, giving rise to a number of different transcripts. The alternatively spliced exons 143-144 give rise to two different transcripts varying between muscle types and between muscles of different developmental stages. The alternatively spliced exons 166-177 express at least 20 different transcripts in adult human tibialis anterior muscle alone. Preliminary results show several transcripts in both of the two remaining alternatively spliced regions. Extensive alternative splicing of NEB may explain why nemaline myopathy patients with homozygous truncating mutations show expression of the carboxy-terminus of the nebulin protein contrary to expectations. The use of alternative transcripts might also explain why severe phenotypes are rare among patients with two truncating mutations.