Alternative splicing: a mechanism for phenotypic rescue of a common inherited defect.

Abstract

Approximately 2% of Caucasians and African-Americans are homozygous for a nonsense mutation in exon 2 of the AMPD1 (AMP deaminase) gene. These individuals have a high grade deficiency of AMPD activity in their skeletal muscle. More than 100 patients with AMPD1 deficiency have been reported to have symptoms of a metabolic myopathy, but it is apparent many individuals with this inherited defect are asymptomatic given the prevalence of this mutant. Results of the present study provide a potential molecular explanation for "correction" of this genetic defect. Alternative splicing eliminates exon 2 in 0.6-2% of AMPD1 mRNA transcripts in adult skeletal muscle. Expression studies document that AMPD1 mRNA, which has exon 2 deleted, encodes a functional AMPD peptide. A much higher percentage of alternatively spliced transcripts are found during differentiation of human myocytes in vitro. Transfection studies with human minigene constructs demonstrate that alternative splicing of the primary transcript of human AMPD1 is controlled by tissue-specific and stage-specific signals. Alternative splicing of exon 2 in individuals who have inherited this defect provides a mechanism for phenotypic rescue and variations in splicing patterns may contribute to the variability in clinical symptoms. (J. Clin. Invest. 1993. 91:2275-2280

A novel exon in the cystic fibrosis transmembrane conductance regulator gene activated by the nonsense mutation E92X in airway epithelial cells of patients with cystic fibrosis.

“…Skipping ofexon 4 from CFTR transcripts has been reported in humans (21 ) and mice (37), but exon 4-transcripts represented only a very small proportion of total CFTR mRNA from human tissues (21), and did not appear to be associated with clinical symptoms. Exon skipping does not always cause disease; nonsense mutation-induced exon skipping has even been shown to function as a mechanism for phenotypic rescue of the common inherited defect of the AMPD1 (AMP deaminase) gene (38). Such a phenotypic rescue in case of the E92X mutation can probably not be realized by omission ofexon 4, given the severe clinical course in the AF508/E92X compound heterozygous patient, the high number of other known disease-causing CF mutations affecting this exon (29), and the postulated role of exon 4 in coding for a large part of the CFTR transmembrane pore (3).…”

Disruption of the splicing enhancer sequence within exon 27 of the dystrophin gene by a nonsense mutation induces partial skipping of the exon and is responsible for Becker muscular dystrophy.

“…However, these examples were not confirmed by in vitro experiments. In addition to the above, the nonsense-containing exon has been reported to be eliminated spontaneously by alternative splicing, a mechanism proposed to explain the phenotypic rescue of a common inherited defect (39).…”

ITPKC functional polymorphism associated with Kawasaki disease susceptibility and formation of coronary artery aneurysms

“…Because no amplification was observed from the templates without a reverse transcriptase step, a possible plasmid DNA contamination in the cDNA templates as the source of the 'unspliced' bands was excluded. To our knowledge, no splice variants of this gene using a different 5′ splice site, which could rescue splicing inefficiency 17 , have been reported in the literature or public databases. Furthermore, RT-PCR of the transcripts in PBMCs from individuals with the C allele did not detect such variants (data not shown).…”