Aberrantly spliced mRNAs of the 3-hydrixy-3-methylglutaryl coenzime A lyase (HL) gene with a donor splice-site point mutation produce hereditary HL deficiency

BUESA C; PIE J; BARCELO A; MASCARO C; CASALS N; CASALE CH; HARO D; DURAN M; SMEITINK, J. A. M.; HEGARDT FG

JLR PAPERS IN PRESS

Año: 1996 vol. 37 p. 2420 - 2432

0022-2275

A novel point mutation in the 3-hydroxy-3-methylglutaryl coenzyme A lyase gene was found in a Turkish patient with homozygous 3-hydroxy-3-methylglutaric acidemia. Amplification by RT-PCR of the mRNA using six different pairs of oligonucleotides produced no differences in four of the fragments amplified with respect to the control, but generated two fragments of different size. One was representative of a deletion of 126 bp and the other of an insertion of 78 bp. These abnormal mRNAs resulted from a G + C transversion at the nucleotide +1 of an intron, which changed the invariant GT dinucleotide of the 5’ donor splice site. This was associated with the occurrence of an alternative splicing, which led to the skipping of the whole exon of 126 bp, and also with the activation of one cryptic donor splice site in the same intron. These aberrant spliced mRNAs are predicted to encode two abnormal HMGCoA lyase proteins: the first results in a protein with an internal deletion of42 amino acids, whose enzyme activity is largely abolished, as the catalytic site was completely removed; the second contains 17 missense amino acids that precede a stop codon. Northern blot analysis showed that the overall content of these aberrantly spliced mRNAs in proband fibroblasts was the same as that found in control fibroblasts. However, hardly any transcript was observed corresponding to the inserted mutated mRNA when it was examined by a specific probe. To quantify the relative proportion of the two mRNAs, a quantitative RT-PCR (the DNA-mimic PCR reaction) was carried out. Results show that the proportion of the inserted mRNA with respect to the deleted mRNA is only 1.2%. The father, mother, and two brothers of the proband were heterozygous in the G -+ C mutation in the + 1 nucleotide of the intron considered, while the two alleles of another brother were free of the mutation.-