The human glutamate dehydrogenase gene family: gene organization and structural characterization

Glutamate dehydrogenase is a mitochondrially located, key metabolic enzyme. In addition to its general metabolic role, GLUD is important in neurotransmission. Significant alterations in GLUD enzymatic activity have been associated with certain neurodegenerative human disorders. Although a single species of human GLUD cDNA molecule has been identified so far, both genomic DNA Southern and cytogenetic analyses have indicated the presence of a GLUD gene family. Screening of a human genomic lambda-phage library with the GLUD cDNA, led us to the isolation of several clones divided into five structurally distinct contigs. We have confirmed the presence of all GLUD-specific sequences in the human genome by detailed genomic Southern analysis. This study allowed the identification of the entire functional GLUD gene, named GLUD1. The GLUD1 gene is about 45 kb long and it is organized into 13 exons. Its nucleotide sequence, exon-intron boundaries, and transcription start sites were determined. Potential binding sites for various regulatory factors such as Sp1, AP-1, and AP-2 were recognized at the promoter region of the gene. The members of the other contigs showed an organization clearly different from GLUD1. Two distinct GLUD-specific gene loci, termed GLUDP2 and GLUDP3, possibly represent truncated pseudogenes. Their high degree of similarity to GLUD1 is limited to the region surrounding exons 2, 3, and 4. Finally, two additional GLUD-specific genomic sequences, termed GLUDP4 and GLUDP5, are structurally similar with the 3' part of the GLUD cDNA sequence. These loci probably represent truncated GLUD pseudogenes generated by retrotransposition. The data presented here suggest that all human GLUD pseudogenes have diverged recently in evolution.