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The thiopurines azathioprine and 6-mercaptopurine (6-MP) are effective immune modulators and cytotoxic agents extensively used in the treatment of autoimmune diseases, graft rejection, and cancer. There is compelling epidemiologic evidence that thiopurine treatment increases the risk for a variety of tumors by mechanisms that are unclear. We investigated the in vivo mutagenicity of long-term thiopurine treatment by determining the frequency and spectra of somatic mutation events at the hypoxanthine phosphoribosyltransferase (HPRT) locus in peripheral T lymphocytes as well as the prevalence of mutant clonal proliferation in a cross-sectional analysis of data from 119 children and adults with inflammatory bowel disease (IBD). ANOVA and regression were performed to assess relationships among the frequency and spectra of HPRT mutations with disease, duration of illness, duration of treatment, and total therapeutic dose of azathioprine and 6-MP. We observed a significant increase in the frequency of somatic mutations in 56 subjects treated with thiopurines for IBD compared with 63 subjects not treated with thiopurines. This increase was related to both total dose (P < 0.001) and duration of treatment (P < 0.001). Comparative mutation spectra analysis of 1,020 mutant isolates revealed a significant increase in the proportion of all transitions (P < 0.001), particularly G:C to A:T transitions (P < 0.001). Combined analyses of two signatures for mutant clonality, HPRT mutation, and T-cell receptor β CDR3 region unique gene sequence also showed a significant thiopurine-dependent increase in mutant cell clonal proliferation (P < 0.001). These findings provide in vivo evidence for mutation induction as a potential carcinogenic mechanism associated with chronic thiopurine intervention. [Cancer Res 2009;69(17):7004–23]

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.

The somatic mutant frequency (Mf) of the hypoxanthine phosphoribosyl transferase (HPRT) gene has been widely used as a biomarker for the genotoxic effects of exposure but few studies have found an association with environmental exposures. We measured background Mfs in 49 current and former residents of Dover Township, New Jersey, who were exposed during childhood to industrially contaminated drinking water. The exposed subjects were the siblings of children who developed cancer after residing in Dover Township, where the incidence of childhood cancer has been elevated since 1979. Mfs from this exposed group were compared to Mfs in 43 age-matched, presumably unexposed residents of neighboring communities with no known water contamination and no increased cancer incidence. Statistical comparisons were based on the natural logarithm of Mf (lnMF). The mean Mf for the exposed group did not differ significantly from the unexposed group (3.90 × 10−6 vs. 5.06 × 10−6; P = 0.135), but unselected cloning efficiencies were higher in the exposed group (0.55 vs. 0.45; P = 0.005). After adjustment for cloning efficiency, lnMf values were very similar in both groups and age-related increases were comparable to those previously observed in healthy children. The results suggest that HPRT Mf may not be a sensitive biomarker for the genotoxic effects of environmental exposures in children, particularly when substantial time has elapsed since exposure. Environ. Mol. Mutagen., 2005. © 2005 Wiley-Liss, Inc.

Survival rates of children treated for cancer have increased dramatically over the last 25 years following the development of risk-directed multi-modality treatment protocols. As a result, there is a rapidly growing population of children and young adult cancer survivors in which the long-term genotoxic effects of chemotherapeutic intervention is unknown. We have previously observed that children treated for acute lymphocytic leukemia (ALL) have significantly increased somatic mutant frequencies (Mfs) (30- to 1300-fold higher) at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) reporter gene in their non-malignant peripheral T cells compared with children at diagnosis or controls. In order to gain insight into the etiology of the observed dramatic increase in Mfs following antineoplastic therapy, we investigated the prevalence of microsatellite instability (MSI), reflective of a defect in DNA mismatch repair (MMR), in children with ALL at diagnosis, during and after chemotherapy and compared them with healthy age-matched controls. MSI analysis using five microsatellite markers was performed on 167 T cell isolates from 40 healthy children and on 842 T cell isolates from 50 patients treated for ALL. High-frequency MSI (MSI-high) was identified in 2 healthy children (5%) and in 2 of 20 ALL subjects at the time of disease recurrence (relapse) (10%). There was no statistically significant difference between the prevalence of MSI-high in patients at the time of ALL relapse and healthy children, nor between the children with ALL at other time points and healthy children. These data indicate that MMR defects, represented by MSI, are not a significant contributor to the elevated HPRT Mfs seen in children treated for ALL. However, in a small number of patients chemotherapy may play a role in the selection of cells with defects in MMR that may have long-term clinical implications.

The rearrangement of immunoglobulin (Ig) and T-cell receptor (TCR) genes in lymphocytes by V(D)J recombinase is essential for immunological diversity in humans. These DNA rearrangements involve cleavage by the RAG1 and RAG2 (RAG1/2) recombinase enzymes at recombination signal sequences (RSS). This reaction generates two products, cleaved signal ends and coding ends. Coding ends are ligated by non-homologous end-joining proteins to form a functional Ig or TCR gene product, while the signal ends form a signal joint. In vitro studies have demonstrated that RAG1/2 are capable of mediating the transposition of cleaved signal ends into non-specific sites of a target DNA molecule. However, to date, in vivo transposition of signal ends has not been demonstrated. We present evidence of in vivo inter-chromosomal transposition in humans mediated by V(D)J recombinase. T-cell isolates were shown to contain TCRalpha signal ends from chromosome 14 inserted into the X-linked hypo xanthine-guanine phosphoribosyl transferase locus, resulting in gene inactivation. These findings implicate V(D)J recombinase-mediated transposition as a mutagenic mechanism capable of deleterious genetic rearrangements in humans.

The link between exposure to environmental mutagens and the development of cancer is well established. Yet there is a paucity of data on the relationship between gene–environment interactions and the mechanisms associated with the somatic mutational events involved with malignant transformation, especially in children. To gain insight into somatic mutational mechanisms in children who develop cancer, we determined the background mutant frequency (Mf) in the hypoxanthine phosphoribosyl transferase (HPRT) reporter gene of peripheral blood lymphocytes from pediatric cancer patients at the time of diagnosis and prior to therapeutic intervention. We studied 23 children with hematologic malignancies and 31 children with solid tumors prior to initial therapeutic intervention. Children with solid tumors, specifically sarcomas, and Hodgkin's disease were significantly older and had elevated HPRT Mfs (6.1 × 10−6 and 3.7 × 10−6, respectively) at the time of diagnosis, compared to normal controls (2.3 × 10−6) and other pediatric tumor groups including children with acute lymphocytic leukemia and non-Hodgkin's lymphoma (ALL/NHL, 1.7 × 10−6), central nervous system tumors (CNS, 3.6 × 10−6), and neuroblastoma (1.9 × 10−6). Of importance is that the significant differences observed in HPRT Mfs between these groups no longer existed after correcting for the effects of age. These data demonstrate that in children who develop cancer there appears to be no significant increase in background HPRT Mf that would indicate significant exposure to genotoxic chemicals or an underlying DNA repair defect resulting in genomic instability. In addition, these data demonstrate the importance of correcting for the effect of age when comparing the frequency of somatic mutations in children and should provide baseline data for future longitudinal biomonitoring studies on the genetic effects of chemotherapy in children treated for cancer. Environ. Mol. Mutagen. 42:44–49, 2003. © 2003 Wiley-Liss, Inc.

Limited information is available regarding the frequency, spectrum, and clinical relevance of somatic mutations in the developing fetus. The goal of this study was to determine somatic mutant frequencies (Mfs) at the hypoxanthine phosphoribosyltransferase (HPRT) reporter gene in cord blood T lymphocytes from preterm infants to gain insight intoin uteromutational events. Mf determinations were made by using theHPRTT cell cloning assay on cord blood samples from 52 preterm infants. Natural logarithm Mfs (lnMfs) from preterm infants were compared with results from our database for full-term infants. Our analysis revealed higher lnMfs in cord blood T lymphocytes from preterm compared with full-term infants (P= 0.008). In addition, preterm females had significantly higher lnMfs compared with full-term females (P< 0.001), whereas preterm males were found to have significantly lower lnMfs than preterm females (P= 0.005). Regression analyses also demonstrate a significant relationship between lnMf and gestational age for preterm females that does not exist for preterm males. These results demonstrate the gender-specific association between Mf and age in humans.

Background Microsatellites are nucleotide sequences of tandem repeats occurring throughout the genome, which have been widely used in genetic linkage analysis, studies of loss of heterozygosity, determination of lineage and clonality, and the measurement of genome instability or the emergence of drug resistance reflective of mismatch repair deficiency. Such analyses may involve the parallel evaluation of many microsatellite loci, which are often limited by sample DNA, are labor intensive, and require large data processing. Results To overcome these challenges, we developed a cost-effective high-throughput approach of microsatellite analysis, in which the amplifications of microsatellites are performed in miniaturized, multiplexed polymerase chain reaction (PCR) adaptable to 96 or 384 well plates, and accurate automated allele identification has been optimized with a collective reference dataset of 5,508 alleles using the GeneMapper software. Conclusions In this investigation, we have documented our experience with the optimization of multiplex PCR conditions and automated allele identification, and have generated a unique body of data that provide a starting point for a cost-effective, high-throughput process of microsatellite analysis using the studied markers.

The generation of TCR proteins is the result of V(D)J recombinase-mediated genomic rearrangements at recombination signal sequences (RSS) in human lymphocytes. V(D)J recombinase can also mediate rearrangements at nonimmune or “cryptic” RSS in normal and leukemic human peripheral T cells. We previously demonstrated age- and gender-specific developmental differences in V(D)J coding joint processing at cryptic RSS within the HPRT locus in peripheral T cells from healthy children (Murray et al. 2006. J. Immunol. 177: 5393–5404). In this study, we investigated developmentally specific V(D)J recombinase TCRβ immune gene rearrangements and coding joint processing at RSS in peripheral T cells in the same pediatric population. This approach provided a unique opportunity to investigate site-specific V(D)J recombinase rearrangements and coding joint processing at immune and nonimmune genes from the same individual T cell population. We determined the genomic sequence of 244 TCRβ coding junctions from 112 (63 male, 49 female) subjects from the late stages of fetal development through 9 y of age. We observed both age- and gender-specific V(D)J recombinase-mediated TCRβ gene usage and coding joint processing at immune RSS. To the best of our knowledge, these data represent the first description of age- and gender-specific developmental differences in TCR gene usage and coding joint processing that could directly influence TCR diversity and immune specificity. It will be important for future studies to ascertain the mechanistic etiology of these developmental and gender differences in TCR diversity and specificity, as well as their importance with respect to the age and gender risks for infectious and autoimmune diseases in humans.

In this paper, we have compared mutant frequency data at the hprt locus in circulating T-lymphocytes from four large datasets obtained in the UK (Sussex), the USA (Vermont), France (Paris) and The Netherlands (Leiden). In total, data from > 500 non-exposed individuals ranging in age from newborns (cord blood samples) to > 80 years old have been included in the analysis. Based on raw data provided by the four laboratories, a model is presented for the analysis of mutant frequency estimations for population monitoring. For three of the laboratories, a considerable body of data was provided on replicate estimates of mutant frequency from single blood samples, as well as estimates from repeat blood samples obtained over a period of time from many of the individual subjects. This enabled us to analyse the sources of variation in the estimation of mutant frequency. Although some variation was apparent in the results from the four laboratories, overall the data were in general agreement. Thus, in all laboratories, cellular cloning efficiency of T-cells was generally high (> 30%), although in each laboratory considerable variation between experiments and subjects was seen. Mutant frequency per clonable T-cell was in general found to be inversely related to cloning efficiency. With the exception of a few outliers (which are to be expected), mutant frequencies at this locus were in the same range in each dataset; no effect of subject gender was found, but an overall clear age effect was apparent. When log mutant frequency was analysed vs log (age + 0.5) a consistent trend from birth to old age was seen. In contrast, the effect of the smoking habit did differ between the laboratories, there being an association of smoking with a significant increase in mutant frequency in the Sussex and Leiden datasets, but not in those from the Vermont or Paris datasets. Possible reasons for this are discussed. One of the objectives of population monitoring is an ability to detect the effect of accidental or environmental exposure to mutagens and carcinogens among exposed persons. The large body of data from non-exposed subjects we have analysed in this paper has enabled us to estimate the size of an effect that could be detected, and the number of individuals required to detect a significant effect, taking known sources of variation into account.(ABSTRACT TRUNCATED AT 400 WORDS)

V(D)J recombinase normally mediates recombination signal sequence (RSS) directed rearrangements of variable (V), diversity (D), and joining (J) germline gene segments that lead to the generation of diversified T cell receptor or immunoglobulin proteins in lymphoid cells. Of significant clinical importance is that V(D)J-recombinase-mediated rearrangements at immune RSS and nonimmune cryptic RSS (cRSS) have been implicated in the genomic alterations observed in lymphoid malignancies. There is growing evidence that exposure to DNA-damaging agents can increase the frequency of V(D)J-recombinase-mediated rearrangements in vivo in humans. In this study, we investigated the frequency of V(D)J-recombinase-mediated rearrangements of an extrachromosomal V(D)J plasmid substrate following exposure to alkylating agents and ionizing radiation. We observed significant dose- and time-dependent increases in V(D)J recombination frequency (V(D)J RF) following exposure to ethyl methanesulfonate (EMS) and methyl methanesulfonate (MMS) but not a nonreactive analogue, methylsulfone (MeSulf). We also observed a dose-dependent increase in V(D)J RF when cells were exposed to gamma radiation. The induction of V(D)J rearrangements following exposure to DNA-damaging agents was not associated with an increase in the expression of RAG 1/2 mRNA compared to unexposed controls or an increase in expression of the DNA repair Ku70, Ku80 or Artemis proteins of the nonhomologous end joining pathway. These studies demonstrate that genotoxic alkylating agents and ionizing radiation can induce V(D)J rearrangements through a cellular response that appears to be independent of differential expression of proteins involved with V(D)J recombination. Environ. Mol. Mutagen., 2007. © 2007 Wiley-Liss, Inc.

The development of risk-directed treatment protocols over the last 25 years has resulted in an increase in the survival rates of children treated for cancer. As a consequence, there is a growing population of pediatric cancer survivors in which the long-term genotoxic effects of chemotherapy is unknown. We previously reported that children treated for acute lymphocytic leukemia have significantly elevated somatic mutant frequencies at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) gene in their peripheral T cells. To understand the molecular etiology of the increase in mutant frequencies following chemotherapy, we investigated the HPRT mutation spectra and the extent of clonal proliferation in 562 HPRT T cell mutant isolates of 87 blood samples from 47 subjects at diagnosis, during chemotherapy, and postchemotherapy. We observed a significant increase in the proportion of CpG transitions following treatment (13.6-23.3%) compared with healthy controls (4.0%) and a significant decrease in V(D)J-mediated deletions following treatment (0-6.8%) compared with healthy controls (17.0%). There was also a significant change in the class type percentage of V(D)J-mediated HPRT deletions following treatment. In addition, there was a >5-fold increase in T cell receptor gene usage–defined mean clonal proliferation from diagnosis compared with the completion of chemotherapeutic intervention. These data indicate that unique genetic alterations and extensive clonal proliferation are occurring in children following treatment for acute lymphocytic leukemia that may influence long-term risks for multifactorial diseases, including secondary cancers. (Cancer Res 2006; 66(17): 8455-61)

The V(D)J recombinase enzyme complex is responsible for the development of a diverse immune system by catalyzing intra-molecular rearrangements of immunoglobulin (Ig) and T cell receptor (TCR) genes at specific recombination signal sequences (RSSs). This enzyme complex has also been implicated in mediating pathologic and non-pathologic intra- and inter-molecular genomic rearrangements at cryptic (Ψ) RSSs outside the immune system loci in lymphoid cells. We describe here two V(D)J recombinase mediated genomic rearrangements resulting in alterations at the HPRT locus in human T-cells. These are inter-chromosomal insertions in which DNA fragments are inserted at breakpoints generated by V(D)J recombinase cleavage at Ψ RSS sites in the HPRT locus at Xq26. In the first, a TCR signal ended segment from chromosome 14q11 is inserted at a Ψ RSS in intron 1 of the HPRT locus. In the second, a DNA fragment from 9q22 is integrated between the coding ends generated by a V(D)J recombinase mediated HPRT deletion. Identification of these in vivo V(D)J mediated inter-chromosomal insertions at Ψ RSSs in the HPRT gene supports the accumulating evidence that V(D)J recombinase can mediate mutagenic rearrangements in humans with potential pathologic consequences. Published 2006 Wiley-Liss, Inc.

There is growing evidence linking somatic mutational events during fetal development and childhood to an increasing number of multifactorial human diseases. Despite this, little is known about the relationship between endogenous and environmentally induced exogenous mutations during human development. Here we describe a comparative spectral analysis of somatic mutations at the hypoxanthine-guanine phosphoribosyltransferase (HPRT) reporter gene locus in healthy children. We observed an age-specific decrease in the proportion of large alterations and a corresponding increase in the proportion of small alterations with increasing age following birth (P

There is continued controversy as to the sequential steps and mechanism(s) responsible for the in vivo acquisition of multiple mutations during neoplastic transformation. We investigated the in vivo clonality and mutational spectra of hypoxanthine-guanine phosphoribosyltransferase (HPRT) mutations in T cells from children with acute lymphocytic leukemia (ALL) to gain insight into the mutagenic mechanisms associated with leukemogenesis. We observed several instances of multiple, independent HPRT mutations accumulating in vivo in T cell receptor (TCR) gene defined clones that had undergone extensive pre- and/or post-thymic expansion following chemotherapy. In addition, we also detected the accumulation of multiple unique single mutations within distinct expanding post-thymic T cell clones. This pattern of clonally restricted hypermutability is compatible with extensive cell proliferation and selection alone without postulating genomic instability. These observations provide a paradigm for a continuum of cellular events that eventually results in the clonal accumulation of mutations in selected populations of cells in vivo and may provide insight into the primary genetic events associated with leukemogenesis, as well as the development of second malignancies and drug resistance following chemotherapy.

Perinatal treatment with 3'-azido-3'-deoxythymidine (AZT) has been found to reduce the rate of maternal-infant transmission of HIV; however, AZT is genotoxic in mammalian cells in vitro and induces tumors in the offspring of mice treated in utero. The purpose of the present study was to investigate the relationships between incorporation of AZT into DNA, and the frequency and spectrum of mutations at the HPRT locus of the human lymphoblastoid cell line, TK6, following in vitro exposures to AZT. Cells were cultured in medium containing 0 or 300 microM AZT for 1, 3, or 6 day(s) (n = 5/group). The effects of exposure duration on incorporation of AZT into DNA and HPRT mutant frequency were determined using an AZT radioimmunoassay and a cell cloning assay, respectively. AZT accumulated in DNA in a supralinear manner, approaching a plateau at 6 days of treatment (101.9 +/- 14.7 molecules AZT/10(6) nucleotides). After 3 days of AZT exposure, HPRT mutant frequency was significantly increased (1.8-fold, p = 0.016) compared to background (mutant frequency = 3.78 x 10(-6)). Multiplex PCR amplification of genomic DNA was used to determine the frequency of exon deletions in HPRT mutant clones from untreated cells versus AZT-treated cells. Molecular analyses of AZT-induced mutations revealed a significant difference in the frequency of total gene deletions (44/120 vs. 18/114 in controls, p = 0.004 by the Mann-Whitney U-statistic). In fact, the Chi-square test of homogeneity demonstrate that the differences between the control and AZT-treatment groups is attributed mainly to this increase in total gene deletion mutations (p = 0.00001). These data indicate that the primary mechanism of AZT mutagenicity in human TK6 cells is through the production of large deletions which occur as a result of AZT incorporation into DNA and subsequent chain termination. The data imply that perinatal chemoprophylaxis with AZT may put children of HIV-infected women at potential risk for genetic damage.

Somatic mutant frequencies (Mf) were determined using the HPRT T-cell cloning assay of peripheral blood T-lymphocytes from 14 children with juvenile onset dermatomyositis (JDM). Serologic parameters, specifically muscle enzyme determinations in JDM subjects, were correlated with residual lnMf (delta) in these patients to compare T-cell activation with clinical parameters associated with JDM. In addition TCR analysis was performed to determine T-cell proliferation and clonality on 12 HPRT mutant isolates from two individuals with JDM. Statistically significant correlations were found between residual lnMf and the following serologic parameters: aldolase (r = 0.771, P = 0.015); CPK (r = 0.602, P = 0.023); and SGOT (r = 0.656, P = 0.011) in children with JDM. In addition, identical TCR gene rearrangements were identified in 86 and 40% of the HPRT mutant isolates from the two patient samples analyzed, which is a significantly higher level of clonality than the 10-15% expected in normal individuals. These data suggest that determining HPRT Mf can be a useful antigen-independent method of selecting clonally expanding T-lymphocytes in autoimmune disease where relevant antigens are unknown. Future analysis of HPRT mutant isolates from children with active myositis may increase our understand of the activated T-cells involved in this disease.

We have investigated the molecular effects of passive maternal cigarette exposure in a newborn population and consider the possible implications of the observed genetic changes in the development of neoplastic diseases in children. We present a distribution analysis of somatic mutational events in a reporter gene, HPRT, in cord blood T lymphocytes from newborns after transplacental exposure to cigarette smoke. Analysis of 30 HPRT mutant isolates from 12 newborn infants born to mothers with no evidence of environmental exposure to cigarette smoke and 37 HPRT mutant isolates from 12 infants born to mothers exposed to passive cigarette smoke showed a significant difference in the HPRT mutational spectrum in those exposed in utero to cigarette smoke. The most notable change was an increase in 'illegitimate' genomic deletions mediated by V(D)J recombinase, a recombination event associated with hematopoietic malignancies in early childhood. Recent epidemiological studies of maternal and paternal cigarette smoke exposure and childhood cancers may need to be re-interpreted, given these results.

People with Down syndrome are 10–30 fold more likely to develop leukemia than the normal population. To date, little is known regarding the molecular mechanisms underlying this phenomenon. We have previously demonstrated that the spontaneous somatic mutant frequency (Mf) at a reporter gene, hypoxanthine-guanine phosphoribosyl transferase ( HPRT ), from a normal population showed a strict age dependency with an exponential increase in Mf from birth to late adolescents with a subsequent linear 2—5% increase per year in adults. In this study, we compared HPRT Mf in children and adults with Down syndrome using the HPRT T-cell cloning assay. We determined the Mf at the HPRT locus in 27 subjects with Down syndrome from ages 6 months to 53.4 years. Results demonstrated that background somatic Mf at the HPRT locus in children and adults with Down syndrome are not dependent on age as seen in a normal control population. Results also show that adults with Down syndrome have a significantly lower Mf than normal adults, and that children with Down syndrome have a significantly higher Mf than normal children, although the latter appears to be due to a decreased cloning efficiency (CE). These observations demonstrate that the frequency of spontaneous somatic mutations in children and adults with Down syndrome are atypical compared to normal controls, and suggest that the genetic mechanisms associated with background somatic mutational events in children and adults with Down syndrome may be different.

We utilized the hprt T-cell cloning assay to prospectively determined the somatic mutant frequency at the hprt locus of fetal T-lymphocytes exposed in utero to maternal active and passive cigarette smoke. In addition, a maternal questionnaire was administered to evaluate a number of social and medical parameters that may effect hprt mutant frequency. Newborn cord blood plasma cotinine levels were determined on all subjects to compare in utero tobacco metabolite levels with maternal smoking histories. A total of 63 newborns were enrolled and placed into four groups: Group I ( n =21), newborns whose mothers had no history of active or passive cigarette exposure during the pregnancy; Group II ( n =12), newborns whose mothers actively smoked cigarettes throughout the pregnancy; Group III ( n =8), newborns whose mothers actively smoked cigarettes during first trimester only; and Group IV ( n =22), newborns whose mothers were exposed only to passive cigarette smoke. Our analysis showed no statistically significant difference in hprt mutation frequency between any of the four groups. A significant increase in plasma cord blood cotinine was detected in Group II, newborns whose mothers were active cigarette smokers throughout the pregnancy. Our data indicate that exposure to active and passive maternal cigarette smoke in utero does not result in a significant increase in somatic mutant frequency as determined by the hprt T-cell cloning assay.

Somatic cell mutant frequencies at the hprt locus of the X-chromosome were measured with the T-lymphocyte cloning assay in a healthy pediatric population. Assays were performed on 49 subjects (29 males and 20 females) ranging in age from 0.08 to 15.2 years. A statistical analysis of the thioguanine-resistant (TGr) mutant frequency (MF), unselected cloning efficiency (CE) and age was performed using data obtained in this study and those previously obtained in our laboratory on 66 newborn umbilical cord blood samples and 230 adult blood samples. For statistical comparisons pediatric subjects were divided into 4 groups. Group I included cord blood samples (age 0 years); Group II were subjects between 0 and 5 years; Group III were between 6 and 11 years and Group IV were between 12 and 17 years. The ln MF of Groups I and II were significantly lower than Groups III and IV (p < 0.05). The mean ln MF for each of Groups I-IV was significantly lower than the adult value. The cloning efficiency for Group I was significantly lower than that for Groups II-IV and adults. The relationships among the ln MF, unselected CE and age were expressed by the equations: ln (MF) = 0.945 -2.453 CE (p < 0.001) and ln (MF) = 0.114 + 0.063 age (p 0.004). The slope coefficients for unselected CE and age were significantly different from adults (p < 0.05). Regression analysis of combined data from Groups I-IV and adults were performed using both age and unselected CE as well as terms to reflect differences in their relationships with ln MF in adults and children. The results showed that the intercept and the age coefficients differ significantly for children and adults after adjustment for CE and yielded the following equations: ln (MF) = 0.548 -1.676 CE + 0.075 age, (Groups I-IV) and ln (MF) = 2.263 -1.676 CE + 0.014 age (adults). An alternative statistical model using ln (age ), ln (MF) = 0.381 -1.767 CE + 0.673 ln (age + 1), (p < 0.001), describes the rapid increase in MF with age that levels off in late adolescence. These findings demonstrate the changing influence of age on mutant frequency in the pediatric population as compared to the adult populations. These studies also illustrate that the increase in background somatic mutant frequencies at the hprt locus in T-lymphocytes is not linear from birth to adolescence and is significantly different from that seen in the adult population.(ABSTRACT TRUNCATED AT 400 WORDS)