Characterization of HAT- and HAsT-resistant HPRT mutant clones of V79 Chinese hamster cells

Monitoring for potential adverse effects of prenatal gene therapy: genotoxicity analysis in vitro and on small animal models ex vivo and in vivo

Gene delivery by integrating vectors has the potential to cause genotoxicity in the host by insertional mutagenesis (IM). Previously, the risk of IM by replication incompetent retroviral vectors was believed to be small. However, the recent observation of leukaemic events due to gamma retroviral vector insertion and activation of the LMO-2 proto-oncogene in patients enrolled in the French and British gene therapy trials for X-SCID demonstrates the need to understand vector associated genotoxicity in greater detail. These findings have led to the development of in vitro, ex vivo, and in vivo assays designed to predict genotoxic risk and to further our mechanistic understanding of this process at the molecular level. In vitro assays include transformation of murine haematopoietic stem cells by integrating retroviral (RV) or lentiviral (LV) vectors and measurement of cell survival resulting from transformation due to integration mainly into the Evi1 oncogene. Ex vivo assays involve harvesting haematopoietic stem cells from mice followed by gene transfer and re-infusion of RV or LV infected cells to reconstitute the immune system. Insertional mutagenesis is then determined by analysis of clonally dominant populations of cells. The latter model has also been made highly sensitive using cells from mice predisposed to oncogenesis by lack of the P53 and Rb pathways. Our investigations on fetal gene therapy discovered a high incidence of liver tumour development that appears to be associated with vector insertions into cancer-related genes. Many genes involved in growth and differentiation are actively transcribed in early developmental and are therefore in an open chromatin configuration, which favours provirus insertion. Some of these genes are known oncogenes or anti-oncogenes and are not usually active during adulthood. We found that in utero injection of primate HIV-1, HR'SIN-cPPT-S-FIX-W does not result in oncogenesis as opposed to administration of non-primate equine infectious anaemia virus (EIAV), SMART 2 lentivirus vectors and, most recently, the non-primate pLIONhAATGFP (FIV) vector, which both give rise to high frequency hepatocellular carcinoma. The peculiar integration pattern into cancer-related genes observed in this model makes the fetal mouse a sensitive tool, not only to investigate long-term vector-mediated gene expression, but also vector safety in an in vivo system with minimal immunological interference. The identification of distinct differences in genotoxic outcome between the applied vector systems i.e. EIAV or FIV vectors versus HIV may indicate a particular biosafety profile of the HIV-1-based vector, which renders it potentially suitable for safe prenatal gene therapy.

Mutational effects of retrovirus insertion on the genome of V79 cells by an attenuated retrovirus vector: implications for gene therapy

Attenuated retroviruses are currently the most widely used vectors in clinical gene therapy because of their potential to effect stable and permanent gene transfer. Since gene delivery is accompanied by random insertion of foreign genetic material into the recipient chromosomal DNA, the potential for insertional mutagenesis exists. In this study, we used a defective retrovirus vector containing a selectable marker, the hygromycin phosphotransferase gene, to investigate the mutagenic effects of vector integration on the mammalian genome. V79 Chinese hamster cells were infected with virus supernatants or by coculture with virus producer cells, and provirus insertion events occurred at low and high frequencies, respectively. The frequency of hprt mutagenesis was increased by a factor of 2.3 over the spontaneous hprt mutation frequency only following multiple provirus insertions/cell genome. Multiple provirus insertions (>3/genome) resulted in instability at the hprt locus in 63% of the virally induced hprt mutants, as indicated by rearrangements at the molecular level, whereas no rearrangements were found when the provirus copy number was 1-2/genome. To demonstrate direct proviral involvement in mutagenesis, the defective MLV vector was retrieved along with flanking genomic hprt sequences from one mutant, and localized within intron 5 of the hprt gene. These data suggest that provirus copy number is a key factor when considering the potential hazards of using retrovirus vectors for gene therapy.

Deletion-pattern analysis of alpha-particle and X-ray induced mutations at the HPRT locus of V79 Chinese hamster cells

To investigate the mutagenic mechanisms of low-energy alpha particles V79 Chinese hamster cells were irradiated with 241Am-alpha particles (mean LET of 112 keV/micron). Parallel experiments were performed using 300 kV X-rays. Cell inactivation and mutation induction cross sections were measured. At approximately 20%--survival level, DNA deletions were analysed at the HPRT locus by multiplex-PCR-analysis of all nine exons of 47 alpha-irradiated and 36 background mutants. 92 HPRT- mutants isolated after 300 kV-X-irradiation were analysed similarly for comparison, along with 15 corresponding background mutants. The resulting mutant deletion-pattern distributions were corrected for background mutations. alpha Particles induced a larger fraction of deletions than X-rays. Furthermore, non-contiguous partial deletions were present among the alpha-induced mutants, a type not found after X-irradiation.

Characterization of mutants involving partial exon duplications in the hprt gene of Chinese hamster V79 cells

Sequencing of hprt cDNA revealed that three spontaneous mutants in V79 Chinese hamster cells exhibit tandem duplications of exon(s), i.e., either exons 2 and 3 or exon 7. Sequences of different sizes (4.5-8 Kb) were found to be duplicated and inserted in tandem into the hprt gene. These mutants demonstrated spontaneous reversion frequencies which were about 40-fold higher than those observed with other types of spontaneous mutants, but on the same order of magnitude as spontaneous reversions in Sp5, a mutant with a duplication insertion involving exon 2 in this gene. These data suggest that all of the duplications found have the same genetic instability, regardless of the type, size or position of the duplicated fragment. The coding sequence of the hprt cDNA and the restriction pattern of the revertants were virtually identical to the wild-type, indicating restoration of a functional hprt gene by precise deletion of the duplicated fragment.

International Commission for Protection Against Environmental Mutagens and Carcinogens. Working paper no. 2. Spontaneous mutations in mammalian cells

Spontaneous or background mutation in mammals plays an important role in both medical and evolutionary contexts. However, establishing mutation frequencies or rates has not always been easy. When the field of mammalian mutagenesis was in its infancy, the word "variant" rather than "mutant" was often used because the genetic nature of the observed phenotypic alterations could not be adequately proven. Nowadays numerous target genes have been identified in which mutant frequencies can be measured, and occasionally even rates can be estimated. Indeed, the genetic basis for 'variants' now often comes from direct DNA sequencing. This review describes the most often used and best understood genetic markers for mutation research and examines their usefulness. In addition, mutational specificity is compared for several loci and the use of DNA-sequence data in determining the origins of spontaneous mutation is also discussed. An important observation is that spontaneous mutation frequencies of similarly sized genes can vary by more than an order of magnitude. Chromosomal location, the nature of the gene product and mutational specificity may offer a partial explanation.

Spectrum of spontaneously occurring mutations in the hprt gene of V79 Chinese hamster cells

A total of 76 independent spontaneous mutants in the hprt gene of V79 Chinese hamster cells have been analyzed. These mutants were obtained in two different laboratories, 17 and 59 mutants in sets 1 and 2, respectively, under different cell culture conditions. Mutation analysis was performed by amplification of hprt cDNA with the polymerase chain reaction and direct sequencing of the products. The data obtained showed similar spectra of spontaneous mutations in both sets of mutants, suggesting that culture does not play a major role in spontaneous mutagenesis. The majority of the mutations were base substitutions (greater than 60%), with twice as many transversions as transitions. Base changes were evenly distributed throughout the structural gene, including the splice junctions. All types of base substitutions appeared in comparable frequencies, except for A.T to T.A transversions, which were almost absent. The fraction of deletion mutations was low (13%). A striking feature of the observed mutation spectra is that one third of the spontaneous mutations analyzed involved aberrant splicing of the hprt primary transcript, with exon 4 being affected most frequently, indicating that splice mutations are a common mechanism of mutation in the hprt gene.