The frequency of illegitimate TCRbeta/gamma gene recombination in human lymphocytes: influence of age, environmental exposure and cytostatic treatment, and correlation with frequencies of t(14;18) and hprt mutation

Analysis of In Vivo Mutation in the Hprt and Tk Genes of Mouse Lymphocytes

Determining mutant frequencies in endogenous reporter genes is a tool for identifying potentially genotoxic environmental agents, and discovering phenotypes prone to genomic instability and diseases, such as cancer. Here, we describe a high-throughput method for identifying mouse spleen lymphocytes with mutations in the endogenous X-linked hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the endogenous autosomal thymidine kinase (Tk) gene. The selective clonal expansion of mutant lymphocytes is based upon the phenotypic properties of HPRT- and TK-deficient cells. The same procedure can be utilized for quantifying Hprt mutations in most strains of mice (and, with minor changes, in other mammalian species), while mutations in the Tk gene can be determined only in transgenic mice that are heterozygous for inactivation of this gene. Expanded mutant clones can be further analyzed to classify the types of mutations in the Tk gene (small intragenic mutations vs. large chromosomal mutations) and to determine the nature of intragenic mutation at both the Hprt and Tk genes.

Analysis of in vivo mutation in the Hprt and Tk genes of mouse lymphocytes

Assays measuring mutant frequencies in endogenous reporter genes are used for identifying potentially genotoxic environmental agents and discovering phenotypes prone to genomic instability and diseases, such as cancer. Here, we describe methods for identifying mouse spleen lymphocytes with mutations in the endogenous X-linked hypoxanthine guanine phosphoribosyl transferase (Hprt) gene and the endogenous autosomal thymidine kinase (Tk) gene. The selective clonal expansion of mutant lymphocytes is based upon the phenotypic properties of HPRT- and TK-deficient cells. The same procedure can be utilized for quantifying Hprt mutations in most strains of mice (and, with minor changes, in other mammalian species), while mutations in the Tk gene can be determined only in transgenic mice that are heterozygous for inactivation of this gene. Expanded mutant clones can be further analyzed to classify the types of mutations in the Tk gene (small intragenic mutations vs. large chromosomal mutations) and to determine the nature of intragenic mutation in both the Hprt and Tk genes.

Illegitimate recombination between T cell receptor genes in humans and pigs (Sus scrofa domestica)

T cell receptor (TCR) genes (TRA/TRD, TRB and TRG) reside in three regions on human chromosomes (14q11.2, 7q34 and 7p14, respectively) and pig chromosomes (7q15.3-q21, 18q11.3-q12 and 9q21-22, respectively). During the maturation of T cells, TCR genes are rearranged by site-specific recombination. Occasionally, interlocus recombination of different TCR genes takes place, resulting in chromosome rearrangements. It has been suggested that the absolute number of these "innocent" trans-rearrangements correlates with the risk of lymphoma. The aims of this work were to assess the frequencies of rearrangements with breakpoints in TCR genes in domestic pig lymphocytes and to compare these with the frequencies of corresponding rearrangements in human lymphocytes by using fluorescence in situ hybridization with chromosome painting probes. We show that frequencies of trans-rearrangements involving TRA/TRD locus in pigs are significantly higher than the frequency of translocations with breakpoints in TRB and TRG genes in pigs and the frequencies of corresponding trans-rearrangements involving TRA/TRD locus in humans. Complex structure of the pig TRA/TRD locus with high number of potential V(D)J rearrangements compared to the human locus may account for the observed differences. Furthermore, we demonstrated that trans-rearrangements involving pig TRA/TRD locus occur at lower frequencies in γδ T cells than in αβ T lymphocytes. The decrease of the frequencies in γδ T cells is probably caused by the absence of TRA recombination during maturation of this T cell lineage. High numbers of innocent trans-rearrangements in pigs may indicate a higher risk of T-cell lymphoma than in humans.

Biological markers of carcinogenic exposure in the aluminum smelter industry--a systematic review

Exposure monitoring programs have been used in the aluminum smelter industry for decades to decrease the risk of cancer from exposure to polycyclic aromatic hydrocarbons (PAHs). Biological monitoring of PAHs incorporates all routes of exposure. Measuring postshift urinary 1-hydroxypyrene (1OHP), a metabolite of pyrene, determines worker's daily PAH exposures, while measuring DNA adducts reflect chronic exposures to PAHs. We reviewed the scientific literature to identify changes over time in (1) 1OHP levels, (2) DNA adduct levels, and (3) other contributing factors associated with 1OHP and DNA adduct levels in the aluminum smelter industry. No trends were observed in 1OHP and DNA adduct levels. This could be due to variable selection of study populations and poorly identified job tasks that prevent comparison of jobs across plants and times, unassessed worker exposure variability, and the impact of cumulative exposures. Thus, it cannot be demonstrated that the use of biological monitoring to estimate PAH exposures has brought about an exposure reduction in the industry. Future studies should be aimed at follow-up in workplaces where dermal and inhalation exposure interventions have been employed. Inconsistent findings were also observed in the analysis of CYP1A1, GSTM1, and GSTP1 polymorphisms and their effect on biomarker levels.

Chromosome damage and cancer risk in the workplace: The example of cytogenetic surveillance in Croatia

The use of cytogenetic assays in the surveillance of populations occupationally exposed to genotoxic carcinogens originates from the assumption that chromosomal alterations might be causally involved in early stages of carcinogenesis. Historical cohort studies have since 1990s consistently reported an association between the level of chromosomal aberrations (CA) in peripheral lymphocytes of healthy subjects and the risk of cancer. Only in few cases, have these results been transformed into a regulatory tool for improving occupational safety. The cytogenetic surveillance program adopted for more than two decades in the Republic of Croatia is one of these few examples. Croatian workers exposed to genotoxic agents were systematically screened for CA, to identify occupational settings needing a priority intervention. Significant increases of mean CA frequency were observed in groups exposed to ionizing radiation, chemical agents, and mixed exposures when compared with a group of unexposed referents. CA data on 736 men and 584 women, monitored between 1987 and 2000, have been associated with cancer incidence. Although the small size of the cohort did not allow for reaching statistical significance, the medium tertile of the CA frequency distribution was associated with a doubling of cancer incidence rate ratio (IRR=2.40; 95% CI 0.85-6.77) when compared with the lowest tertile. For chromosome-type CA, IRR was non-significantly increased for both the medium (IRR 1.53, 95% CI 0.58-3.99) and high categories (IRR 1.69; 95% CI 0.61-4.72). Recommendations for future strategies comprise the inclusion of predictive biomarkers in surveillance programs, the definition of a regulatory framework, and their possible use for the identification of individual risk profiles.

Analysis of mutagenic V(D)J recombinase mediated mutations at the HPRT locus as an in vivo model for studying rearrangements with leukemogenic potential in children

Pediatric acute lymphocytic leukemia (ALL) is a multifactorial malignancy with many distinctive developmentally specific features that include age specific acquisition of deletions, insertions and chromosomal translocations. The analysis of breakpoint regions involved in these leukemogenic genomic rearrangements has provided evidence that many are the consequence of V(D)J recombinase mediated events at both immune and non-immune loci. Hence, the direct investigation of in vivo genetic and epigenetic features in human peripheral lymphocytes is necessary to fully understand the mechanisms responsible for the specificity and frequency of these leukemogenic non-immune V(D)J recombinase events. In this review, I will present the utility of analyzing mutagenic V(D)J recombinase mediated genomic rearrangements at the HPRT locus in humans as an in vivo model system for understanding the mechanisms responsible for leukemogenic genetic alterations observed in children with leukemia.

Lymphoma- and leukemia-associated chromosomal translocations in healthy individuals

Chromosomal translocations (CTs) are hallmark mutations of hematopoietic malignancy that result in the deregulated expression of oncogenes or the generation of novel fusion genes. The polymerase chain reaction (PCR) can be used to detect illegitimate recombinations of genomic DNA sequences as a more sensitive assay than cytogenetics for determining the presence of CTs. Both direct DNA-PCR and reverse transcriptase-PCR were used to examine healthy individuals for lymphoma- and leukemia-associated CTs. Two oncogene-activating CTs [t(14;18)(q32;q21) and t(8;14)(q24;q32)] and one fusion-gene CT [t(2;5)(p23;q35)] from lymphomas and five fusion-gene CTs from leukemia [t(9;22)(q34;q11), t(4;11)(q21;q23), t(15;17)(q22;q11), t(12;21)(p13;q22), t(8;21)(q22;q22)] were detected in such studies. The biological implication is that CTs associated with malignant tumors may also be found in cells that are not neoplastic. CTs are characteristic attributes of neoplastic clones but are by themselves insufficient to cause malignant transformation. A better understanding of the special biology of non-neoplastic CT-bearing cells will provide insight into their putative role as tumor precursors. Prospective epidemiological studies are needed to determine whether such cells in healthy individuals may, in some instances, become clonogenic founders of lymphoma or leukemia.

DNA sequence analysis of interlocus recombination between the human T-cell receptor gamma variable (GV) and beta diversity-joining (BD/BJ) sequences on chromosome 7 (inversion 7)

V(D)J recombinase-mediated recombination between the T-cell receptor (TCR) gamma variable (GV) genes at chromosome 7p15 and the TCR beta joining (BJ) genes at 7q35 leads to the formation of a hybrid TCR gene. These TCR gamma/beta interlocus rearrangements occur at classic V(D)J recombination signal sequences (RSS) and, because the loci are in an inverted orientation, result in inversion events that are detectable in the chromosome structure as inv(7)(p15;q35). Similar rearrangements involving oncogenes and either TCR or immunoglobulin genes mediated by the V(D)J recombinase are found in lymphoid malignancies. Oligonucleotide primers that allow polymerase chain reaction (PCR) amplification across the inv(7) genomic recombination junction sequence have been described. Southern blot analysis has been primarily used to confirm the GV/BJ hybrid nature of the product, with limited information on the DNA sequence of these recombinations. We have modified this PCR method using total genomic DNA from the mononuclear cells in peripheral blood samples to increase specificity and to allow direct sequencing of the translocation junction that results from the recombination between the GV1 and BJ1 families of TCR genes in 25 examples from 11 individuals (three adults, one child, six newborns, and one ataxia telangiectasia (AT) patient). We focused on samples from newborns based on previous studies indicating that the predominant hypoxanthine-guanine phosphoribosyl transferase (HPRT) mutations in newborns are V(D)J recombinase-mediated deletion events and that the frequency of these mutations decreases with increasing age. Although the dilution series-based PCR assay utilized does not yield sharply defined quantitative endpoints, results of this study strongly suggest that inv(7) recombinations in newborns occur at equal or lower frequencies than those seen in adults. Consistent with the PCR primer pairs, all sequenced products contain a GV1 and a BJ1 segment and most also contain a BD1 segment. GV1s2 and 1s4 were the most frequently found GV1 genes (8 and 9 examples, respectively) and BJ1s5 and 1s6 were the most frequently found BJ1 genes (9 and 10 examples, respectively). These results demonstrate the effectiveness of this methodology for assessing GV/BJ interlocus rearrangements mediated by V(D)J recombinase.

Analysis of Vgamma/Jbeta trans-rearrangements in paediatric patients undergoing chemotherapy

The frequency of the hybrid Vgamma/Jbeta trans-rearrangement in peripheral blood lymphocytes (PBLs) was analysed in a transversal study of paediatric patients (n = 210) with acute lymphoblastic leukaemia (ALL) and solid tumours (ST). Different amounts of DNA were used as the template for a nested polymerase chain reaction to evaluate the frequency of hybrid Vgamma/Jbeta genes, using silver-stained gels. The frequency of the rearrangement was evaluated in groups before, during and after therapy. A greatly increased frequency of Vgamma/Jbeta trans-rearrangement was found in PBLs of both groups of patients during exposure to chemotherapeutic agents compared with patients before chemotherapy. In patients who had finished treatment, the frequency of the rearrangement fell promptly to the baseline levels in ST but showed a slow decrease in ALL in those in whom increased levels could be found until 4 years after the end of treatment. We hypothesize that the chemotherapeutic agents are able to induce the Vgamma/Jbeta trans-rearrangement, but this is transient in most cases. The exact relationship between the persistence of the rearrangement and the occurrence of secondary leukaemia remains to be determined.