Cloning of in vivo-derived thioguanine-resistant human B cells

Similarity of in vivo somatic mutations at an autosomal adenine phosphoribosyltransferase locus between T- and B-cells in human peripheral blood

In vivo somatic mutations have been detected at several human loci by using clonal cultures of peripheral blood T-cells. It has not been fully understood whether or not the somatic mutations in T-cells are similar to those of other cell types. To address this issue, we cloned, from human peripheral blood, T- and B-cells with mutations at an autosomal adenine phosphoribosyltransferase (APRT) locus. For the efficient detection of somatic mutations at the APRT locus, a blood sample from a human individual heterozygous for germline APRT deficiency was used. T- and B-cells deficient in APRT enzyme activity were cloned from peripheral blood mononuclear cells using a selecting agent, 2,6-diaminopurine. The APRT-deficient mutant frequencies were on the order of 10(-4) in both T- and B-cells. The single-strand conformation polymorphism analysis of the APRT DNA of mutant B-cell clones suggested that the molecular mechanisms leading to the APRT deficiency in B-cells were similar to those in T-cells. Our observations suggest that both the frequency and the mode of in vivo somatic mutations occurring spontaneously at general autosomal loci in B-cells are similar to those in T-cells.

Selection against blood cells deficient in hypoxanthine phosphoribosyltransferase (HPRT) in Lesch-Nyhan heterozygotes occurs at the level of multipotent stem cells

Lesch-Nyhan syndrome is caused by a severe genetic deficiency of hypoxanthine phosphoribosyltransferase (HPRT) and is characterized by central nervous system disorders, gout, and in some cases, macrocytic anemia. Women heterozygous for HPRT deficiency are healthy but their somatic cells are mosaic for enzyme deficiency owing to random inactivation of the X chromosome. Frequencies of red blood cells and T cells deficient in HPRT are significantly lower than the expected 50% in heterozygotes, suggesting that HPRT-negative blood cells are selected against in heterozygotes. To determine at which stage of hematopoiesis such selection occurs, we determined the frequencies of HPRT-negative T, B and erythroid precursor cells in three heterozygotes. Since the cloning efficiencies of T and B cells and colony forming efficiency of burst-forming unit erythroid (BFU-E) for sample from Lesch-Nyhan patients were similar to those of normal cells, HPRT deficiency does not seem to render the differentiated cells less efficient for proliferation. However, the frequencies of HPRT-negative T and B cells, and BFU-E were all less than 10% in each of the three heterozygotes. Although the frequencies of HPRT-negative cells showed tenfold variations between the heterozygotes, each heterozygote had similar frequencies of HPRT-negative cells in the three cell types. These results suggest that HPRT is important at early stages of hematopoiesis, but less so after the cells have differentiated into T cells, B cells and erythroid precursor cells.

Somatic cell mutation frequency at the HPRT, T-cell antigen receptor and glycophorin A loci in Cockayne syndrome

Skin fibroblasts of patients with Cockayne syndrome (CS) are hypersensitive to the lethal or mutagenic effect of ultraviolet light, which may cause genetic instability. Up to now, however, no systematic study of in vivo somatic cell mutation in CS cells has been reported. This article describes our investigation of the mutation frequencies (Mfs) at three different loci, i.e. hypoxanthine-guanine phosphoribosyl transferase (HPRT), T-cell antigen receptor (TCR) and glycophorin A (GPA), in six patients with CS. Mfs at the HPRT and TCR loci were found to be within the normal range as determined in age-matched controls. In the GPA locus of two patients, there was a slight increase, but it was much smaller than that reported in other DNA repair deficient syndromes. The frequency of spontaneous HPRT mutation in Epstein-Barr virus transformed B-lymphoblastoid cells derived from CS patients was similar to that in cells from normal children. The molecular characterization of the representative HPRT mutant T cell clones from CS patients did not show any structural alterations. These results may explain, at least in part, why CS is not associated with predisposition to cancer.

High somatic mutation frequencies in a LacZ transgene integrated on the mouse X-chromosome

To study spontaneous and induced mutagenesis in vivo we recently constructed a series of transgenic mice harboring different numbers of bacteriophage lambda shuttle vectors, provided with a LacZ mutational target gene, integrated in their genome. The transgenic mice enabled analysis of spontaneous and induced mutation frequencies in postmitotic tissues like liver and brain. The obtained data indicated spontaneous mutation frequencies in the order of 10(-5)-10(-6). Here we report a 25-100 times higher spontaneous mutation frequency in liver and brain DNA of mice from strain 35.5, with the lambda-gt10LacZ concatemer integrated on the X-chromosome. These results indicate the presence of a mutational 'hot spot' in the mammalian somatic genome in vivo.

Review of the molecular characteristics of gene mutations of the germline and somatic cells of the human

Molecular analyses of the limited number of de novo germinal mutations identified in humans indicate that an array of alterations in gene structure can be generated. Similar conclusions are derived from the large data set obtained from molecular analyses of alleles that segregate in the human population and cause genetic diseases. The molecular alterations include nucleotide substitutions as well as insertions, deletions and other rearrangements of the DNA. The lesions may be located in the coding or the noncoding regions of genes or may involve the flanking sequences. The insertions and deletions involve fragments ranging from single nucleotides to many kilobases, and involve both unique sequences and repetitive elements. The nature of the lesions observed to date as either de novo mutations or segregating variants suggests there are locus-specific characteristics of the alterations in DNA structure that are recovered as genetic diseases. Differences in mutation spectra among genetic loci appear to reflect both the structure of the target sequences and the relationship between gene structure and gene function. No induced germinal mutations have been identified, thus no data are available that reveal the relationships between mutagenic exposures and the molecular fingerprints of the lesion induced in the human germ cell and transmitted to the subsequent generations. In contrast, the prospects for analyzing the roles of genetic target, exposure history and individual responsiveness to exposure in creating particular molecular lesions in somatic cells are excellent, both for alterations of single nucleotides and for major alterations of gene structure.(ABSTRACT TRUNCATED AT 400 WORDS)