Human Hypoxanthine-guanine Phosphoribosyltransferase Gene Research Articles

Prioritization of Deleterious Variations in the Human Hypoxanthine-Guanine Phosphoribosyltransferase Gene

Prioritization of Deleterious Variations in the Human Hypoxanthine-Guanine Phosphoribosyltransferase Gene

Identification of Novel Mutations in the Human HPRT Gene

Inherited mutation of the purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT) gives rise to Lesch–Nyhan syndrome (LNS) or Lesch–Nyhan variants (LNVs). We report three novel independent mutations in the coding region of HPRT gene: exon 3: c.141delA, p.D47fs53X; exon 5: c.400G>A, p.E134K; exon 7: c.499A>G, p.R167G from three LNS affected male patients.

Novel Mutations in the Human HPRT Gene

Inherited mutation of a purine salvage enzyme, hypoxanthine guanine phosphoribosyltransferase (HPRT), gives rise to Lesch-Nyhan Syndrome (LNS) or HPRT-related gout. Here, we report five novel independent mutations in the coding region of the HPRT gene from five unrelated male patients manifesting different clinical phenotypes associated with LNS: exon 2: c.133A > G, p.45R > G; c.35A > C, p.12D > A; c.88delG; exon 7: c.530A > T, p.177D > V; and c.318 + 1G > C: IVS3 + 1G > C splice site mutation.

Spontaneous frequency of exon skipping in the human HPRT gene

In order to elucidate the mechanisms of mRNA splicing fidelity and the mutagenic potential of aberrant mis-spliced transcripts we have investigated the frequency of spontaneous exon skipping in the human hypoxanthine-guanine phosphoribosyl transferase ( HPRT) gene in well characterized human primary fibroblasts isolated from two different individuals. In these cells, coexisting with the WT species, we also detected three aberrant HPRT transcripts missing exon IV, VII, or VIII. We were unable to detect transcripts missing exon II, III, V, or VI. Significantly, all the exons affected by skipping do not generate new stop codons more than 50 nucleotides upstream from the 3′ most exon–exon junction. Exon VIII was the most prone to skipping with a relative frequency to WT of 0.019±0.004 (approximately one aberrant transcript per 50 WT transcripts). Exon IV exhibited a relative frequency of skipping of 0.006±0.002 ((approximately one aberrant transcript per 150 WT transcripts) and exon VII exhibited a relative frequency of skipping of 0.003±0.002 ((approximately one aberrant transcript per 300 WT transcripts). These data demonstrate that aberrant transcripts with exon skipped are generated spontaneously in humans and some appear to persist in the cell.

Mapping of an origin of DNA replication near the transcriptional promoter of the human HPRT gene.

A quantitative PCR method was used to map a functional origin of DNA replication in the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene in normal human fibroblasts. This PCR method measures the abundance of specific sequences in short fragments of newly replicated DNA from logarithmically growing cells. Quantitative measurements rely on synthetic molecules (competitors) that amplify with the same primer sets as the target molecules, but generate products of different sizes. This method was first utilized to determine the position of the replication origin near the lamin B2 gene (Giacca et al. [1994] Proc. Natl. Acad. Sci. U S A. 91:7119-7123). In the present study, primer sets were tested along a 16-kb region near exon 1 of the HPRT gene. The most abundant fragment was found to be located in the first intron of HPRT, just downstream of the promoter and exon 1 of the gene, and approximately 3.5 kb upstream of a previously reported autonomously replicating sequence (Sykes et al. [1988] Mol. Gen. Genet. 212:301-309).

A new T-lymphocyte cloning assay for detection of in vivo mutations in the human hypoxanthine-guanine phosphoribosyltransferase gene

The X-linked hypoxanthine-guanine phosphoribosyl transferase (hprt) gene is a target of analyses of in vivo mutation frequencies in circulating T-lymphocytes. We established a novel, accessory cell-free cloning method of T-lymphocytes with a hprt mutation by a combined use of recombinant interleukin-2, conditioned medium from activating T-lymphocytes and culture plates coated with anti-CD3 monoclonal antibody. Using the method, we examined mutation frequencies of the hprt gene in T-lymphocytes from six healthy individuals, nine patients with colon cancer including two patients from different families with hereditary nonpolyposis colon cancer and six cancer-free relatives of the patients. In six healthy individuals, the mean cloning efficiency and mutation frequency (MF) of the hprt gene in T-lymphocytes were 0.51 +/- 0.28 and 9.4 +/- 7.5 x 10(-6), respectively. These data were similar to the reported values. The mean MFs in the nine colon cancer patients (10.6 +/- 7.3 x 10(-6)) were not significantly different from those of the 12 cancer-free individuals (11.6 +/- 9.4 x 10(6)). The correlation between mutation frequencies and age of the individuals was significant regardless of the presence or absence of cancers. The single-strand conformation polymorphism analyses of nested RT-PCR products of hprt mRNA were done in 33 mutant clones from five members of a family of which MF values were high. All the analyzed mutant clones show a genetic aberration in the coding region of the hprt gene. At least 28 of 33 mutants were independent. Our method provides a new versatile tool for in vivo analysis for mutations of the hprt gene.

HPRT Yeast Artificial Chromosome Transfer into Human Cells by Four Methods and an Involvement of Homologous Recombination

The transfer of a yeast artificial chromosome (YAC) into mouse and human cell lines was effected by four methods, and the efficiency and integrity of the incorporated YAC DNA were compared. A 500 kb YAC containing the human hypoxanthine-guanine phosphoribosyl transferase (HPRT) gene was transferred more efficiently by polyethylene glycol-mediated fusion than by lipofection, electrofusion, or electroporation. Southern blot analysis demonstrated that PEG fusion lines yielded fragments of the size of the original YAC clone, whereas lipofection and electroporation did not. Two of 53 fusion lines showed 6-thioguanine resistance and confirmatory disruption of the HPRT gene in the YAC DNA, that the YAC DNA was integrated by homologous recombination with the HPRT gene region.

Software for the analysis of mutations at the human hprt gene

Mutations at the human hypoxanthine-guanine phosphoribosyl transferase gene ( hprt) are currently of great interest because mutations at this locus are being used as a biomonitor of human mutagenic exposure. Not only can somatic hprt mutants arising in vivo in humans be recovered and sequenced, but there is also a considerable body of information about the in vitro mutational spectra of different carcinogens at this locus. Previously, we reported the creation of a computerized database containing DNA-sequence information on human hprt mutants (Cariello et al. (1992) Environ. Mol. Mutagen., 20, 81–83). In the present manuscript, software for the analysis of mutations in the hprt database is described. Numerous routines have been developed for the analysis of single-base substitutions, including programs to (i) determine if two mutational spectra are different, (ii) display the number of mutations and mutable sites in each exon, (iii) determine if mutations show a DNA-strand bias, (iv) determine the frequency of transitions and transversions, (v) display the number and kind of mutations observed at each base in the coding region, (vi) perform nearest-neighbor analysis and (vii) display mutable amino acids in the hprt protein. The software runs only on IBM-compatible machines with MS-DOS. The software and hprt database is freely available via the INTERNET using remote file-transfer protocol. These programs simplify the analysis of the rapidly increasing information about hprt mutation. The programs permit the facile comparison between in vitro and in vivo data, as well as the identification of mutational patterns that may be of importance to experimenters using hprt as a biomonitor and of importance to researchers studying mechanisms of mutation.

Identification of two new nucleotide mutations (HPRTUtrecht and HPRTMadrid) in exon 3 of the human hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene

Mutations in the X-linked hypoxanthine-guanine phosphoribosyl transferase gene (HPRT) result in deficiencies of HPRT enzyme activity, which may cause either a severe form of gout or Lesch-Nyhan syndrome depending on the residual enzyme activity. Mutations leading to these diseases are heterogeneous and include DNA base substitutions, DNA deletions, DNA base insertions and errors in RNA splicing. Identification of mutations has been performed at the RNA and DNA level. Sequencing genomic DNA of the HPRT gene offers the possibility of direct diagnostic analysis independent on the expression of the mature HPRT mRNA. We describe a Dutch and a Spanish family, in which the Lesch-Nyhan syndrome and a severe partial HPRT-deficient phenotype, respectively, were diagnosed. Direct sequencing of the exons coding for the HPRT gene was performed in both families. Two new exon 3 mutations have been identified. At position 16676, the normally present G was substituted by an A in the Dutch kindred (HPRTUtrecht), and led to an arginine for glycine change at residue 70. At position 16680, the G was substituted by a T in the Spanish family (HPRTMadrid); this substitutes a valine for glycine at residue 71. These new mutations are located within one of the clusters of hotspots in exon 3 of the HPRT gene in which HPRTYale and HPRTNew Haven have previously been identified.

A germ line mutation within the coding sequence for the putative 5-phosphoribosyl-1-pyrophosphate binding site of hypoxanthine-guanine phosphoribosyltransferase (HPRT) in a Lesch-Nyhan patient: missense mutations within a functionally important region probably cause disease.

Lesch-Nyhan syndrome caused by a complete deficiency of hypoxanthine guanine phosphoribosyltransferase (HPRT) is the result of a heterogeneous group of germ line mutations. Identification of each mutant gene provides valuable information as to the type of mutation that occurs spontaneously. We report here a newly identified HPRT mutation in a Japanese patient with Lesch-Nyhan syndrome. This gene, designated HPRT Tokyo, had a single nucleotide change from G to A, as identified by sequencing cDNA amplified by the polymerase chain reaction. Allele specific oligonucleotide hybridization analysis using amplified genomic DNA showed that the mutant gene was transmitted from the maternal germ line. This mutation would lead to an amino acid substitution of Asp for Gly at the amino acid position 140 located within the putative 5-phosphoribosyl-1-pyrophosphate (PRPP) binding region. Missense mutations in human HPRT deficient patients thus far reported tend to accumulate in this functionally active region. However, a comparison of the data suggested that both missense and synonymous mutations can occur at any coding sequence of the human germ line HPRT gene, but that a limited percentage of all the missense mutations cause disease. The probability that a mutation will cause disease tends to be higher when the missense mutation is within a functionally important sequence.

Mutations which alter splicing in the human hypoxanthine-guanine phosphoribosyltransferase gene.

A large proportion of mutations at the human hprt locus result in aberrant splicing of the hprt mRNA. We have been able to relate the mutation to the splicing abnormality in 30 of these mutants. Mutations at the splice acceptor sites of introns 4, 6 and 7 result in splicing out of the whole of the downstream exons, whereas in introns 1, 7 or 8 a cryptic site in the downstream exon can be used. Mutations in the donor site of introns 1 and 5 result in the utilisation of cryptic sites further downstream, whereas in the other introns, the upstream exons are spliced out. Our most unexpected findings were mutations in the middle of exons 3 and 8 which resulted in splicing out of these exons in part of the mRNA populations. Our results have enabled us to assess current models of mRNA splicing. They emphasize the importance of the polypyrimidine tract in splice acceptor sites, they support the role of the exon as the unit of assembly for splicing, and they are consistent with a model proposing a stem-loop structure for exon 8 in the hprt mRNA.

Herpes simplex virus-mediated human hypoxanthine-guanine phosphoribosyltransferase gene transfer into neuronal cells.

The virtually complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a devastating neurological disease, Lesch-Nyhan syndrome. Transfer of the HPRT gene into fibroblasts and lymphoblasts in vitro and into hematopoietic cells in vivo has been accomplished by other groups with retroviral-derived vectors. It appears to be necessary, however, to transfer the HPRT gene into neuronal cells to correct the neurological dysfunction of this disorder. The neurotropic virus herpes simplex virus type 1 has features that make it suitable for use as a vector to transfer the HPRT gene into neuronal tissue. This report describes the isolation of an HPRT-deficient rat neuroma cell line, designated B103-4C, and the construction of a recombinant herpes simplex virus type 1 that contained human HPRT cDNA. These recombinant viruses were used to infect B103-4C cells. Infected cells expressed HPRT activity which was human in origin.

Herpes simplex virus-mediated human hypoxanthine-guanine phosphoribosyltransferase gene transfer into neuronal cells.

The virtually complete deficiency of the purine salvage enzyme hypoxanthine-guanine phosphoribosyltransferase (HPRT) results in a devastating neurological disease, Lesch-Nyhan syndrome. Transfer of the HPRT gene into fibroblasts and lymphoblasts in vitro and into hematopoietic cells in vivo has been accomplished by other groups with retroviral-derived vectors. It appears to be necessary, however, to transfer the HPRT gene into neuronal cells to correct the neurological dysfunction of this disorder. The neurotropic virus herpes simplex virus type 1 has features that make it suitable for use as a vector to transfer the HPRT gene into neuronal tissue. This report describes the isolation of an HPRT-deficient rat neuroma cell line, designated B103-4C, and the construction of a recombinant herpes simplex virus type 1 that contained human HPRT cDNA. These recombinant viruses were used to infect B103-4C cells. Infected cells expressed HPRT activity which was human in origin.

AN ALTERED HPRT PROTEIN, CREATED BY AN IN VITRO MUTAGENESIS OF THE GENE, CAN BE RECOGNIZED SPECIF: CALLY BY AN ANTIPEPTIDE ANTIBODY: 235

Lesions in hypoxanthine guanine phosphoribosyl transferase (HPRT) gene are associated with the devastating Lesch-Nyhan syndrome and forms of gouty arthritis in human patients. We are interested in developing a retroviral vector by which the human HPRT gene can be introduced into mice, and its expression can be detected by immunocytochemical methods. Due to the nearly identical amino acid sequences of mouse and human HPRT, antibodies against human HPRT cross react with mouse HPRT. To overcome this problem, we constructed a retroviral vector which codes for a mutated HPRT protein by site-directed in vitro mutagenesis. The termination codon of human HPRT gene was mutated so that six extra amino acids (glu-asp-glu-ser-ser-ser) are added to the C-terminus of normal HPRT protein during translation. In DNA transfection, the mutated protein worked as well as the normal protein in rescuing HPRT− cells during HAT selection. The isoelectrofocusing point of this mutated protein is also altered on gels due to the addition of three acidic amino acids. A polyclonal antibody, directed against the last eight amino acids of C-terminus of the mutated protein, was raised. In immunoprecipitation this antibody can bring down the mutated human HPRT protein, but not normal human HPRT protein or mouse HPRT protein.

834 STABILITY OF GENE TRANSFER IN LESCH NYHAN CELLS

One theoretical criterion for effective and efficient gene therapy for human genetic disease is stability of a functional gene transferred into defective cells. We are studying the deficiency of hypoxanthine guanine phosphoribosyl transferase (HPRT) in the Lesch Nyhan disease as a model system for the development of methods for gene therapy, and have used retroviral vectors to introduce a functional wild-type human HPRT gene into cultured cells derived from Lesch Nyhan patients. Recipient cells express the foreign gene to a variable extent and demonstrate partial phenotypic reversion of several parameters of aberrant purine biosynthesis characteristic of enzyme-deficient cells. Cells grown without selection and revertants to the HPRT-negative phenotype show the continued presence, loss or rearrangement of the foreign gene and of sequences derived from the vector. The introduction of the HPRT gene into random cellular sites by means of some retroviral vectors therefore clearly leads to variability not only in the degree of gene expression but also in its stability, and we conclude that in the presence or absence of selection pressure, the expression and stability of the newly introduced gene depend on the site of integration into the host cell genome.