Structure of human succinic semialdehyde dehydrogenase gene: identification of promoter region and alternatively processed isoforms.

Mitochondrial NAD(+)-dependent succinic semialdehyde dehydrogenase (ALDH5A1, SSADH) represents the last enzyme in the GABA catabolism and irreversibly oxidizes SSA to succinate. In human, SSADH deficiency results in 4-hydroxybutyric aciduria, an autosomal recessive disorder due to an accumulation of GABA and 4-hydroxybutyric acid in the CNS. We already identified SSADH gene on human chromosome 6p22 and characterized the coding region. Furthermore, we described the first two mutations causing the disease. We report here the complete cDNA and genomic structure of the gene. A single transcription start site was identified by RNase protection 122 bp upstream of the ATG. EST database search and reporter gene constructs of the 3(') genomic region showed that the two major SSADH mRNA isoforms are due to alternative polyadenylation sites. The two mRNAs of 1827 and 5225 nt were analyzed for differential stability and translation efficiency. The analysis of mRNA turnover showed that both SSADH transcripts are equally stable. Similarly, a measurement of polysomal association capability of the two GFP-SSADH reporter mRNAs (containing the 3' UTR regions of the two SSADH mRNAs) did not reveal any difference. However, we cannot exclude the fact that differential properties could be restricted to particular physiological conditions and/or specific tissues. We have also identified an alternatively spliced small exon, which may lead to a novel isoform of the enzyme. Furthermore, we report here on naturally occurring missense variants, which may significantly contribute to inter-individual variation of SSADH activity, possibly influencing GABA and GHB endogenous levels.