Expression of multiple γ-glutamyltransferase genes in man

Abstract

In clinical and pharmacological laboratories, the assay for gamma-glutamyltransferase (GGT) activity is an important diagnostic test, but one with high biological variability. Although the human genome contains multiple GGT genomic sequences, the diagnostic tests generally assume that only a single GGT gene is active. In the current study, segments encompassing parts of seven different potential human GGT genes have been molecularly cloned. Based on sequence determination of exons within these distinct genomic clones, oligonucleotide primers were designed which would prime and PCR-amplify putative mRNA of all seven potential GGT genes, if expressed. Gene-specific oligonucleotide probes were then utilized to assay the transcriptional status of the seven possible GGT genes in a wide variety of human RNAs. Our results show that a single GGT gene exhibits ubiquitous expression in all RNAs tested, including those from fetal and adult liver. A surprisingly large number of four additional GGT genes is expressed in man. Interestingly, these novel GGT genes are expressed in a tissue-restricted manner, which suggests that their corresponding gene products exhibit distinct functions in these specific tissues.

Cloning and expression of a novel type (III) of human γ‐glutamyltransferase truncated mRNA

“…The positive control for gene 1 hybridization was the gene 1 Bglll restriction fragment cloned in pSK and described by Courtay et al [3]. Blots were prehybridized for 2 h at 65°C in 3XSSC, 10xDen-hardt's, 50 gg/ml salmon sperm DNA and 0.1% SDS; hybridization was overnight at 55°C in the same solution with a type I (mut 6: 5'-CCAACGAGTI'TGGGGTACCC-3') or type III (mut 1 : 5'-CTCTC-GTCAATGTGCCCGAC-3') mRNA specific (7-32p)-labeled oligonucleotide.…”

“…Courtay et al [3] have partially described the genomic sequence encoding GGT type III mRNA, and they have shown a point mutation at the 5' end (the normal GT 5' donor site is replaced by AT) of an 81 bp intronic sequence that should result in an unspliced intron. The oligonucleotide Amp 5 hybridizes at the 5' end of this putative unspliced intron and therefore should be specific for type III GGT mRNAs, Thus, Amp 1 and Amp 5 were used as specific primers for RT-PCR reactions on placenta and sigrnoid RNA where the expression of this GGT gene has been previously shown [3].…”

“…Human GGT is a multigenic family composed of at least 7 loci [3]. Five of them are located on chromosome 22 [4] in the bcr region (break point cluster region) which is involved in chromosomal breaks and rearrangements associated with various diseases [5].…”

“…This ORF encodes for a single polypeptide precursor which upon cleavage during the post-translational process gives rise to the two non-identical subunits of the mature enzymes (large subunit 55-60 kDa, small subunit 21-30 kDa) [11]. These mRNAs, named type I mRNA, seem to be encoded by our ubiquitously expressed GGT gene (gene 6) [3].…”

“…It has been reported that at least three more GGT genes are expressed in human tissues and cell lines although no further characterization of the corresponding transcipts has been attempted [3]. Among them, gene 1 transcripts seem to be the most interesting as they were found in placenta, which presents a high GGT activity, in lung and sigmoid.…”