HPRT gene expression differs in mutants derived from normal and Fanconi anemia cells: analysis of spontaneous and psoralen-photoinduced mutants

Mutations induced in the HPRT gene by X-irradiation during G(1) or S: analysis of base pair alterations, small deletions, and splice errors

Reverse transcriptase PCR was performed with mRNA obtained from HPRT mutants that had base pair alterations, or small deletions or insertions <20bp. The frequencies of mutants yielding RT-PCR products (mRNA) were the same when human EJ30 cells were irradiated in G(1) or S (3-4-fold higher for 6 than 3Gy). However, the frequencies of mutants that did not yield RT-PCR products were approximately 10-fold higher in the cells irradiated in G(1) than in those irradiated in S. Sequence analysis of RT-PCR products and genomic DNA showed that 40% of the RT-PCR products had splice errors (one or more exons not spliced into mRNA), with 64% of them due to 1-17bp deletions. Also, the distributions of molecular alterations in exons, acceptor sites, and donor sites for mutants having splice errors (observed in this study and reported by others) were similar to those reported for mutants not yielding RT-PCR products (isolated from Russian cosmonauts). In addition, we have found previously that large deletions which eliminated 1-9 exons were preferentially induced in G(1). Therefore, we postulate that the preferential induction of mutants not yielding mRNA is due primarily to splice errors that result from deletions preferentially induced during G(1). These splice errors would then result either in no message or a message that is rapidly degraded.

Is Fanconi anemia caused by a defect in the processing of DNA damage?

Fanconi anemia (FA) is an autosomal genetic disease characterized by a complex array of developmental disorders, a high predisposition to bone marrow failure and to acute myelogenous leukemia. The chromosomal instability and the hypersensitivity to DNA cross-linking agents led to its classification with the DNA repair disorders. This review aimed at establishing whether it is still appropriate to consider 1/approximately FA within a DNA repair framework taking into account the recently discovered genetic heterogeneity characteristics of the defect (eight complementation groups). We discuss the possibility that the FA proteins interact to form a complex which may control different functions, including the processing of specific DNA lesions. Such a complex may act as a sensor to initiate protective systems as well as transcription of specific genes specifying, among others proteins, growth factors. Such steps may be organized as a linear cascade or more likely under the form of a web network.

Molecular spectrum of mutations induced at the HPRT locus by a cross-linking agent in human cell lines with different repair capacities

Molecular characterization of mutations photoinduced by a cross-linking agent, 4,5',8-trimethylpsoralen (Me3Pso), in normal human lymphoblasts was conducted in parallel with lymphoblasts derived from Fanconi anemia patients. Such cells have been previously described to be impaired in repair of psoralen photolesions. The endogenous HPRT locus was used as a target gene. The treatment of cells with Me3Pso in combination with 365 nm irradiation leads to the formation of interstrand cross-links, and specific monoadducts. Our analysis revealed that the mutagenic processing of Me3Pso photoadducts in normal human cells results essentially in base substitutions (84%). These are localized to sequences shown previously to be favored for the formation of Me3Pso monoadducts. The mutagenic processing of the same lesions in Fanconi anemia cells results in fewer base substitutions (22%), with deletions (66%) being the predominant class of mutation. In contrast to prokaryotic systems, frameshifts are poorly represented among Me3Pso induced mutations in human cells. In spite of important differences between the kinds of mutations observed in the two cell lines, our analysis reveals similarities in the type of base substitutions and their sequence distribution. In both normal and Fanconi anemia cell lines mutations, mostly targeted on thymine residues, are preferentially located on the non-transcribed strand.