A NONSENSE MUTATION IN THE 3-HYDROXY-3-METHYLGLUTARYL-COA LYASE GENE PRODUCES EXON SKIPPING IN 2 PATIENTS OF DIFFERENT ORIGIN WITH 3-HYDROXY-3-METHYLGLUTARYL-COA LYASE DEFICIENCY

A novel nonsense mutation associated with the skipping of constitutive exon 2 of the 3-hydroxy-3-methylglutaryl-CoA lyase gene was found in two patients, from Portugal and Morocco, with 3-hydroxy-3-methylglutar ic acidemia. By reverse transcriptase PCR and single-strand conformati onal polymorphism a G-T transversion was located, at nucleotide 109, o f the 3-hydroxy-3-methylglutaryl-CoA lyase cDNA, within exon 2. Two mR NAs were produced as a result of this nonsense mutation: one of the ex pected size that contains the premature stop codon UAA, and the other with a deletion of 84 bp corresponding to the whole of exon 2. This de letion produced the loss of the last seven amino acids of the leader p eptide and the first 21 amino acids of the mature protein. The nonsens e mutation was found in a purine-rich GGAAG sequence, which is equal t o, or similar to, others reported to be exonic splicing enhancers (ESE ). We suggest that the nonsense mutation may affect a possible ESE on exon 2, which would hinder the splice site selection and facilitate an aberrant splice with the skipping of this exon. Determination by quan titative PCR shows that the ratio of mRNA with the nonsense mutation t o the mRNA with the deletion is approx. 3:1.