Mutation frequency in human blood cells increases with age

Somatic Mutations in KEAP1-NRF2 Complex in Breast Cancer

Breast cancer remains the leading cause of cancer deaths for women. Long-term estrogen exposure is considered carcinogenic due to semiquinone production and to compromised detoxification. Metabolic regulator polymorphisms, such as KEAP1 (rs1048290) and NRF2 (rs35652124, rs6721961, rs6706649), can be valuable in understanding the individual cytoprotection profile. Thus, we aim to genotype these polymorphisms in blood, tumours and surrounding tissue, to identify somatic mutations and correlate it to prognoses. A total of 23 controls and 69 women with histological confirmed breast cancer were recruited, and DNA from blood/surrounding/tumour tissue was genotyped. Genotyping and clinicopathological data were correlated. We verified that rs35652124 presents different genotype distribution between the blood/surrounding tissue (p-value = 0.023) and tumour/surrounding tissues (p-value = 0.041). Apart from rs35652124 and considering the histological grade, the other four polymorphisms have different distributions among different tissues. There is a tendency towards the loss of heterozygosity in the surrounding tissue when compared to blood and tumour tissues, and higher genotype variability in histologic grade 2. These somatic mutations and different distribution patterns may indicate a heterogeneous and active microenvironment, influencing breast cancer outcome. Additionally, it would be pertinent to evaluate the predictive value of the histologic grade 2 considering somatic mutation profiles and distributions.

Occupational exposure to formaldehyde and early biomarkers of cancer risk, immunotoxicity and susceptibility

Formaldehyde (FA) is a high-volume production chemical manufactured worldwide to which many people are exposed to both environmentally and occupationally. FA was recently reclassified as a human carcinogen. Several epidemiological studies have revealed an increased risk of cancer development among workers exposed to FA. Although FA genotoxicity was confirmed in a variety of experimental systems, data from human studies are conflicting. The aim of the present study was to evaluate the occupational exposure to FA in a multistage approach relating the exposure with different biomarkers (dose and effect) and individual susceptibility. Air monitoring was performed to estimate the level of exposure to FA during shift work. Eighty-five workers from hospital anatomy-pathology laboratories exposed to FA and 87 controls were tested for cytogenetic alterations in lymphocytes (micronucleus, MN; sister-chromatid exchange, SCE) and T-cell receptor (TCR) mutation assay. The frequency of MN in exfoliated buccal cells, a first contact tissue was also assessed. Percentages of different lymphocyte subpopulations were selected as immunotoxicity biomarkers. The level of formic acid in urine was investigated as a potential biomarker of internal dose. The effects of polymorphic genes of xenobiotic metabolising enzymes and DNA repair enzymes on the endpoints studied were determined. The mean level of FA exposure was 0.38 ± 0.03 ppm. MN (in lymphocytes and buccal cells) and SCE were significantly increased in FA-exposed workers compared to controls. MN frequency positively correlated with FA levels of exposure and duration. Significant alterations in the percentage of T cytotoxic lymphocytes, NK cells and B lymphocytes were found between groups. Polymorphisms in CYP2E1, GSTP1 and FANCA genes were associated with increased genetic damage in FA-exposed subjects. The obtained information may provide new important data to be used by health and safety care programs and by governmental agencies responsible for setting the acceptable levels for occupational exposure to FA.

Somatic mutations - Evolution within the individual

With the rapid advancement of sequencing technologies over the last two decades, it is becoming feasible to detect rare variants from somatic tissue samples. Studying such somatic mutations can provide deep insights into various senescence-related diseases, including cancer, inflammation, and sporadic psychiatric disorders. While it is still a difficult task to identify true somatic mutations, relentless efforts to combine experimental and computational methods have made it possible to obtain reliable data. Furthermore, state-of-the-art machine learning approaches have drastically improved the efficiency and sensitivity of these methods. Meanwhile, we can regard somatic mutations as a counterpart of germline mutations, and it is possible to apply well-formulated mathematical frameworks developed for population genetics and molecular evolution to analyze this 'somatic evolution'. For example, retrospective cell lineage tracing is a promising technique to elucidate the mechanism of pre-diseases using single-cell RNA-sequencing (scRNA-seq) data.

Exploring Genetic Outcomes as Frailty Biomarkers

Frailty has emerged as a reliable measure of the aging process. Because the early detection of frailty is crucial to prevent or even revert it, the use of biomarkers would allow an earlier and more objective identification of frail individuals. To improve the understanding of the biological features associated with frailty as well as to explore different biomarkers for its early identification, several genetic outcomes-mutagenicity, different types of genetic damage, and cellular repair capacity-were analyzed in a population of older adults classified into frail, prefrail, and nonfrail. Besides, influence of clinical parameters-nutritional status and cognitive status-was evaluated. No association of mutation rate or primary DNA damage with frailty was observed. However, DNA repair capacity showed a nonsignificant tendency to decrease with frailty, and persistent levels of phosphorylated H2AX, as indicative of DNA breakage, increased progressively with frailty severity. These results support the possible use of H2AX phosphorylation to provide information regarding frailty severity. Further investigation is necessary to determine the consistency of the current findings in different populations and larger sample sizes, to eventually standardize biomarkers to be used in clinics, and to fully understand the influence of cognitive impairment.

Aging and the rise of somatic cancer-associated mutations in normal tissues

DNA mutations are inevitable. Despite proficient DNA repair mechanisms, somatic cells accumulate mutations during development and aging, generating cells with different genotypes within the same individual, a phenomenon known as somatic mosaicism. While the existence of somatic mosaicism has long been recognized, in the last five years, advances in sequencing have provided unprecedented resolution to characterize the extent and nature of somatic genetic variation. Collectively, these new studies are revealing a previously uncharacterized aging phenotype: the accumulation of clones with cancer driver mutations. Here, we summarize the most recent findings, which converge in the novel notion that cancer-associated mutations are prevalent in normal tissue and accumulate with aging.

Epigenetic Loss of MLH1 Expression in Normal Human Hematopoietic Stem Cell Clones is Defined by the Promoter CpG Methylation Pattern Observed by High-Throughput Methylation Specific Sequencing

Normal human hematopoietic stem and progenitor cells (HPC) lose expression of MLH1, an important mismatch repair (MMR) pathway gene, with age. Loss of MMR leads to replication dependent mutational events and microsatellite instability observed in secondary acute myelogenous leukemia and other hematologic malignancies. Epigenetic CpG methylation upstream of the MLH1 promoter is a contributing factor to acquired loss of MLH1 expression in tumors of the epithelia and proximal mucosa. Using single molecule high-throughput bisulfite sequencing we have characterized the CpG methylation landscape from −938 to −337 bp upstream of the MLH1 transcriptional start site (position +0), from 30 hematopoietic colony forming cell clones (CFC) either expressing or not expressing MLH1. We identify a correlation between MLH1 promoter methylation and loss of MLH1 expression. Additionally, using the CpG site methylation frequencies obtained in this study we were able to generate a classification algorithm capable of sorting the expressing and non-expressing CFC. Thus, as has been previously described for many tumor cell types, we report for the first time a correlation between the loss of MLH1 expression and increased MLH1 promoter methylation in CFC derived from CD34⁺ selected hematopoietic stem and progenitor cells.

Flow cytometric quantification of mutant T cells with altered expression of the T-cell receptor: detecting somatic mutants in humans and mice

Spontaneously generated mutant T cells defective in T-cell receptor (TCR) gene expression are detectable at the frequency of 2×10(-4) in vivo, and the mutant fractions are dose dependently increased by exposure to genotoxic agents such as ionizing radiation. Mutant cells with altered expression of TCRα or -β among CD4(+) T cells can be detected as CD3(-)/CD4(+) cells by two-color flow cytometry using anti-CD3 and anti-CD4 monoclonal antibodies labeled with different fluorescent dyes, because incomplete TCRαβ/CD3 complexes cannot be transported to the cellular membrane. This flow cytometric mutation assay can be applied to CD4(+) T cells from human peripheral blood and mouse spleen. Methods for both preparation of target cells and detection of the mutant cells are described.

Follow-up study of genotoxic effects in individuals exposed to oil from the tanker Prestige, seven years after the accident

The accident with the oil tanker Prestige in November 2002 resulted in a major spill of about 63,000 tons of heavy fuel oil. More than 300,000 people participated in the clean-up activities, which lasted for up to 10 months. Previous studies reported increases in genotoxicity endpoints in individuals exposed to Prestige oil, both at the moment of exposure [DNA breakage, micronuclei (MN), sister chromatid exchange] and two years later (chromosomal aberrations). In this work we carried out for the first time the follow-up of genotoxic effects in subjects exposed to an oil spill seven years after the exposure. The main objective was to determine the possible persistence of genotoxic damage in individuals exposed to Prestige oil seven years after the accident. The exposed group was composed of 54 residents of Galician villages in Spain that were heavily affected by the spill. This group was involved in clean-up labor for at least two months in the period November 2002-September 2003. They were compared with 50 matched controls. Primary DNA damage was evaluated by the comet assay, mutagenicity by the T-cell receptor (TCR) mutation assay, and MN frequency was determined both by the cytokinesis-block test and by flow cytometry. The results obtained showed no significant differences between the exposed and the controls in the comet assay, the TCR mutation assay and the cytokinesis-block MN test. An unexpected and significant decrease was observed in the exposed group for the results of the MN test evaluated by flow cytometry, probably influenced by modifying factors - other than age, sex and smoking - not considered in this study. Our results show no evidence of the persistence of genotoxic damage in individuals exposed to Prestige oil seven years later. Nevertheless, the need to plan biomonitoring studies on people participating in clean-up activities in case a new oil spill occurs should be established.

Genotoxic effect of exposure to metal(loid)s. A molecular epidemiology survey of populations living and working in Panasqueira mine area, Portugal

Previous studies investigating the exposure to metal(loid)s of populations living in the Panasqueira mine area of central Portugal found a higher internal dose of elements such as arsenic, chromium, lead, manganese, molybdenum and zinc in exposed individuals. The aims of the present study were to evaluate the extent of genotoxic damage caused by environmental and occupational exposure in individuals previously tested for metal(loid) levels in different biological matrices, and the possible modulating role of genetic polymorphisms involved in metabolism and DNA repair. T-cell receptor mutation assay, comet assay, micronucleus (MN) test and chromosomal aberrations (CA) were performed in a group of 122 subjects working in the Panasqueira mine or living in the same region. The modifying effect of polymorphisms in GSTA2, GSTM1, GSTP1, GSTT1, XRCC1, APEX1, MPG, MUTYH, OGG1, PARP1, PARP4, ERCC1, ERCC4, and ERCC5 genes was investigated. Significant increases in the frequency of all biomarkers investigated were found in exposed groups, however those environmentally exposed were generally higher. Significant influences of polymorphisms were observed for GSTM1 deletion and OGG1 rs1052133 on CA frequencies, APEX1 rs1130409 on DNA damage, ERCC1 rs3212986 on DNA damage and CA frequency, and ERCC4 rs1800067 on MN and CA frequencies. Our results show that the metal(loid) contamination in the Panasqueira mine area induced genotoxic damage both in individuals working in the mine or living in the area. The observed effects are closely associated to the internal exposure dose, and are more evident in susceptible genotypes. The urgent intervention of authorities is required to protect exposed populations.

Somatic mutations in aging, cancer and neurodegeneration

The somatic mutation theory of aging posits that the accumulation of mutations in the genetic material of somatic cells as a function of time results in a decrease in cellular function. In particular, the accumulation of random mutations may inactivate genes that are important for the functioning of the somatic cells of various organ systems of the adult, result in a decrease in organ function. When the organ function decreases below a critical level, death occurs. A significant amount of research has shown that somatic mutations play an important role in aging and a number of age related pathologies. In this review, we explore evidence for increases in somatic nuclear mutation burden with age and the consequences for aging, cancer, and neurodegeneration. We then review evidence for increases in mitochondrial mutation burden and the consequences for dysfunction in the disease processes.

Metal(loid) levels in biological matrices from human populations exposed to mining contamination--Panasqueira Mine (Portugal)

Mining activities may affect the health of miners and communities living near mining sites, and these health effects may persist even when the mine is abandoned. During mining processes various toxic wastes are produced and released into the surrounding environment, resulting in contamination of air, drinking water, rivers, plants, and soils. In a geochemical sampling campaign undertaken in the Panasqueira Mine area of central Portugal, an anomalous distribution of several metals and arsenic (As) was identified in various environmental media. Several potentially harmful elements, including As, cadmium (Cd), chromium (Cr), manganese (Mn), nickel (Ni), lead (Pb), and selenium (Se), were quantified in blood, urine, hair, and nails (toe and finger) from a group of individuals living near the Panasqueira Mine who were environmentally and occupationally exposed. A group with similar demographic characteristics without known exposure to mining activities was also compared. Genotoxicity was evaluated by means of T-cell receptor (TCR) mutation assay, and percentages of different lymphocyte subsets were selected as immunotoxicity biomarkers. Inductively coupled plasma-mass spectrometry (ICP-MS) and inductively coupled plasma-atomic emission spectrometry (ICP-AES) analysis showed elevated levels of As, Cd, Cr, Mn, and Pb in all biological samples taken from populations living close to the mine compared to controls. Genotoxic and immunotoxic differences were also observed. The results provide evidence of an elevated potential risk to the health of populations, with environmental and occupational exposures resulting from mining activities. Further, the results emphasize the need to implement preventive measures, remediation, and rehabilitation plans for the region.

Review of retrospective dosimetry techniques for external ionising radiation exposures

The current focus on networking and mutual assistance in the management of radiation accidents or incidents has demonstrated the importance of a joined-up approach in physical and biological dosimetry. To this end, the European Radiation Dosimetry Working Group 10 on 'Retrospective Dosimetry' has been set up by individuals from a wide range of disciplines across Europe. Here, established and emerging dosimetry methods are reviewed, which can be used immediately and retrospectively following external ionising radiation exposure. Endpoints and assays include dicentrics, translocations, premature chromosome condensation, micronuclei, somatic mutations, gene expression, electron paramagnetic resonance, thermoluminescence, optically stimulated luminescence, neutron activation, haematology, protein biomarkers and analytical dose reconstruction. Individual characteristics of these techniques, their limitations and potential for further development are reviewed, and their usefulness in specific exposure scenarios is discussed. Whilst no single technique fulfils the criteria of an ideal dosemeter, an integrated approach using multiple techniques tailored to the exposure scenario can cover most requirements.

Geno- and immunotoxic effects on populations living near a mine: a case study of Panasqueira mine in Portugal

Mining industry is a vital economic sector for many countries but it is also one of the most hazardous activities, both occupationally and environmentally. Existing studies point to several adverse effects on communities' health living near mines, effects such as mesothelioma and respiratory illnesses. Results achieved in a geochemical sampling campaign undertaken in the vicinity of São Francisco de Assis village showed an anomalous distribution of some heavy metals in soils and waters. To evaluate the effects of mining activities on human health produced by these conditions, a group of 28 individuals from São Francisco de Assis village was examined for some biological endpoints. A nonexposed group (30 individuals) with the same demographic characteristics without exposure to genotoxic compounds was also studied and data obtained from both groups compared. Results of the T-cell receptor mutation assay and micronucleus (MN) test showed significant increases in the frequencies of both mutations and MN in exposed subjects compared to controls. Data obtained in the analysis of the different lymphocyte subsets demonstrated significant decreases in percentages of CD3+ and CD4+ cells, and a significant increase in percentage of CD16/56+ cells, in exposed individuals. The results of the present study indicate an elevated risk of human environmental contamination resulting from mining activities, emphasizing the need to implement preventive measures, remediation, and rehabilitation plans. This would lead to a reduction in cancer risk not only for this particular population but for all populations exposed under similar conditions.

Mutation in aging mice occurs in diverse cell types that proliferate postmutation

To determine the relationship between aging, cell proliferation and mutation in different cell types, hearts, brains and kidneys from G11 PLAP mice between 1 week and 24 months of age were examined. Mutant cells were detected in tissue sections by staining for Placental Alkaline Phosphatase (PLAP) activity, an activity that marks cells that have sustained a frameshift mutation in a mononucleotide tract inserted into the coding region of the human gene encoding PLAP. The number of PLAP(+) cells increased with age in all three tissues. The types of cells exhibiting a mutant phenotype included cells that are proliferative, such as kidney epithelial cells, and cells that do not frequently replicate, such as cardiac muscle cells and neurons. In the brain, PLAP(+) cells appeared in various locations and occurred at similar frequencies in different regions. Within the cerebellum, PLAP(+) Purkinje cell neurons appeared at a rate similar to that seen in the brain as a whole. PLAP(+) cells were observed in kidney-specific cell types such as those in glomeruli and collecting tubules, as well as in connective tissue and blood vessels. In the heart, PLAP(+) cells appeared to be cardiac muscle cells. Regardless of tissue and cell type, PLAP(+) cells occurred as singletons and in clusters, both of which increased in frequency with age. These data show that age-associated accumulation of mutant cells occurs in diverse cell types and is due to both new mutation and proliferation of mutant cells, even in cell types that tend to not proliferate.

Studies of thioguanine-resistant lymphocytes induced by in vivo irradiation of mice

The frequency of Hprt-deficient lymphocytes in mice after in vivo gamma irradiation, has been found to vary as a function of time elapsed after exposure and irradiation dose. The frequency of mutant lymphocytes in spleen was determined using an in vitro, clonogenic assay for thioguanine-resistant T-lymphocytes. Mice were exposed to single doses of 0-400 cGy from cesium-137 or to eight daily doses of 50 cGy. The time to maximum-induced mutant frequency was 3 weeks. The dose response was strikingly curvilinear at 3-5 weeks after irradiation, but less precisely defined for 10-53 weeks after exposure, being fit by either linear or quadratic dependence. Three weeks after eight daily 50 cGy exposures, mutant frequency was elevated above controls and mice exposed to 50 cGy (which were not distinct from the nonirradiated controls), but only 17% in that of mice given a single 400 cGy fraction. This fractionation effect and the curvilinearity of the early dose-response curve suggested that saturation of repair increased the yield of mutations at higher acute doses. The decline of spleen mutant frequency in mice observed between 5 and 10 weeks after irradiation may reflect selection against some mutants. The marked variation of mutant frequency, as a function of time after irradiation and of dose rate, emphasize the need to evaluate these variables carefully and consistently in future studies.

Elevated levels of somatic mutation in a manifesting BRCA1 mutation carrier

Homozygous loss of activity at the breast cancerpredisposing genes BRCA1 and BRCA2 (FANCD1) confers increased susceptibility to DNA double strand breaks, but this genotype occurs only in the tumor itself, following loss of heterozygosity at one of these loci. Thus, if these genes play a role in tumor etiology as opposed to tumor progression, they must manifest a heterozygous phenotype at the cellular level. To investigate the potential consequences of somatic heterozygosity for a BRCA1 mutation demonstrably associated with breast carcinogenesis on background somatic mutational burden, we applied the two standard assays of in vivo human somatic mutation to blood samples from a manifesting carrier of the Q1200X mutation in BRCA1 whose tumor was uniquely ascertained through an MRI screening study. The patient had an allele-loss mutation frequency of 19.4 x 10(-6) at the autosomal GPA locus in erythrocytes and 17.1 x 10(-6) at the X-linked HPRT locus in lymphocytes. Both of these mutation frequencies are significantly higher than expected from age-matched disease-free controls (P < 0.05). Mutation at the HPRT locus was similarly elevated in lymphoblastoid cell lines established from three other BRCA1 mutation carriers with breast cancer. Our patient's GPA mutation frequency is below the level established for diagnosis of homozygous Fanconi anemia patients, but consistent with data from obligate heterozygotes. The increased HPRT mutation frequency is more reminiscent of data from patients with xeroderma pigmentosum, a disease characterized by UV sensitivity and deficiency in the nucleotide excision pathway of DNA repair. Therefore, this BRCA1-associated breast cancer patient manifests a unique phenotype of increased background mutagenesis that likely contributed to the development of her disease independent of loss of heterozygosity at the susceptibility locus.

An in vitro model of T cell receptor revision in mature human CD8+ T cells

V(D)J recombination is a mechanism peculiar to the somatic rearrangement of antigen receptor genes. It requires both expression of the RAG-1 and RAG-2 recombinases and accessibility of the substrate to its recombinase and post-cleavage/DNA repair stage. TCR revision is a genetic correction mechanism that changes T cell specificity by re-activating V(D)J recombination in peripheral T cells. This process is now well described in both normal or pathological murine and human settings. Many of its features, such as the question of whether it occurs in truly mature T cells, remain to be elucidated. Its occurrence in human CD8+ T cells is also an open question. We have therefore established an in vitro model of TCR revision in mature human CD8+ T cells to determine whether down-regulation of the TCR/CD3 complex from the cell surface in the presence of IL7 as a factor favouring chromatin remodelling initiates a TCR revision pathway. Only mature CD8+ T cells carrying already-formed antigen receptors were used. CD8+ T cells treated with anti-CD3 and IL7 showed rearrangement intermediates and expressed new Vbeta-chains on their surface. Investigation of the molecular pathway thus induced disclosed up-regulation of the RAG-2 transcript, but absence of the 'canonical' RAG-1 mRNA. A surprising finding was the demonstration of alternative splice forms of this mRNA, already expressed in untreated CD8+ T cells, encoding for the full-length RAG-1 protein, which was increased three-fold in the treated cells. All the V(D)J requirements were thus fulfilled when mature human CD8+ T cells were stimulated with anti-CD3 and IL7. Induction of TCR revision in vitro in mature T cells is an easily controllable system that could be employed in further studies to elucidate the molecular pathways involved in secondary V(D)J rearrangements in peripheral cells.

Age-related accumulation of autosomal mutations in solid tissues of the mouse is gender and cell type specific

Most cancers in solid tissues increase with age and invariably contain causal mutations eliminating expression of one or more autosomal tumor suppressor genes. However, very little is known about the effect of age on autosomal mutations, often large in size, in cells of solid tissues. In this study, the frequency and spectrum of autosomal mutations were examined as a function of age for kidney epithelial cells and ear mesenchymal cells in B6D2F1 mice heterozygous for the selectable Aprt locus. Aprt mutant frequencies were found to increase with age in the kidneys of both male and female mice, but at all ages the mutant frequencies were approximately twice as high in the females, which in this strain have shorter lifespans than the males. An age-related increase in Aprt mutant frequencies was also observed for ear cells from female mice, but no significant increases in mutant frequencies were observed for the ear cells of male mice. A molecular analysis showed that the kidney and ear mutational spectra were distinct and that the age-related increases in mutant frequencies did not involve significant shifts in the mutational spectra. In total, the results demonstrate both gender and cell-type-specific patterns of autosomal mutational accumulation as a function of age in two solid tissues of the mouse.

Genotype profiles of loci encoding DNA repair enzymes in newborn and elderly populations: no evidence of association with longevity

The comparison of genotype frequencies between neonates and elderly populations can aid in the identification of loci, and polymorphisms within those loci, that affect longevity. Here we have compared genotype frequencies of seven polymorphisms at four loci involved in DNA repair between a cohort of newborns (n = 290) and a retired population (average age at sampling 70.02 years; n = 430) who have suffered a lifetime of DNA damage from normal, metabolic processes, and on whom selection on DNA repair gene variants may be expected to have acted. No differences in genotype frequencies at the four SNP loci were seen, indicating that there is no evidence of association with longevity in this population. Significant differences in frequency of certain repeat sizes at three microsatellite loci were detected. However, since there is no known functional consequence of these repeat lengths, the action of selection cannot yet be ascribed.

Investigating genetic damage in workers occupationally exposed to methotrexate using three genetic end-points

Genetic damage in workers occupationally exposed to an antineoplastic drug was studied using the micronucleus (MN) test, the comet assay, the hprt gene mutation assay and the TCR gene mutation assay. The subjects were divided into two groups: (i) 21 workers from a plant producing methotrexate (MTX); (ii) 21 controls were matched according to age, gender and smoking. Fresh blood samples were collected from the workers and controls. The results of the MN test showed that the mean micronuclei rate (MNR) and mean micronucleated cell rate (MCR) in workers were 10.10 +/- 0.95 per thousand and 8.05 +/- 0.75 per thousand, respectively, which were significantly higher than those (5.48 +/- 0.82 per thousand and 4.38 +/- 0.58 per thousand) in controls (P < 0.01). It was found in the comet assay that the mean tail length (MTL) of workers and controls were 1.30 +/- 0.06 microm and 0.07 +/- 0.01 microm, respectively. There was a significant difference between workers and controls for MTL (P < 0.01), but the difference between the mean tail moment (MTM, 0.23 +/- 0.03) of workers and MTM (0.17 +/- 0.04) of controls was not significant (P > 0.05). The results of hprt gene mutation assay showed that the average mutation frequency (Mf-hprt) of hprt in workers was 1.00 +/- 0.02 per thousand, which was significantly higher than that (0.86 +/- 0.01 per thousand) in controls (P < 0.01). Meanwhile, the results of TCR gene mutation assay indicated that Mfs-TCR gene mutation frequencies of workers and controls were 6.87 +/- 0.52 x 10(-4) and 1.67 +/- 0.14 x 10(-4), respectively, which were significantly different (P < 0.01). The results of our experiment suggest that genetic damage is detectable in the 21 workers occupationally exposed to methotrexate.

Individual Variation of Somatic Gene Mutability in Relation to Cancer Susceptibility: Prospective Study on Erythrocyte Glycophorin A Gene Mutations of Atomic Bomb Survivors

It has previously been reported that hemizygous mutant fraction (Mf) at the glycophorin A (GPA) locus in erythrocytes increased with radiation dose in heterozygotes among Hiroshima and Nagasaki atomic bomb survivors. In the present study, we analyzed the relationship between GPA Mf and cancer risk using newly developed cancers among previously cancer-free subjects whose GPA Mf had been measured between 1988 and 1996. Among 1,723 survivors (1,117 in Hiroshima and 606 in Nagasaki), we identified 186 subjects who developed a first cancer by the end of 2000. We compared the radiation dose responses of GPA Mf between cancer and cancer-free groups using a linear-quadratic model fit by multiple regression analysis in combination with age, sex, and city. The slope of the GPA Mf dose-response curve was significantly higher in the cancer group than in the cancer-free group among Hiroshima subjects. Moreover, no significant difference of GPA Mf between cancer and cancer-free groups was found in unexposed controls in the two cities. The same conclusions were obtained using a linear dose-response model and by further analysis using Cox regression of cancer incidence. These findings suggest that there might be interindividual variation in mutability of somatic genes and that Hiroshima survivors who have higher mutability in response to radiation exposure would be expected to have a higher probability of suffering radiation-related cancer.

Use of the glycophorin A somatic mutation assay for rapid, unambiguous identification of Fanconi anemia homozygotes regardless of GPA genotype

A 7-year-old girl was hospitalized with pancytopenia requiring blood transfusion. She and an older brother with suspicious symptoms were referred for laboratory testing to confirm a clinical diagnosis of Fanconi anemia (FA). Blood samples from these two children and one parent were examined with the GPA somatic mutation assay. The patient's total GPA somatic mutation frequency of 1.4 x 10(-4) was determined despite the confounding effects of her recent transfusion, and was greater than 10-fold higher than that of a population of pediatric controls, consistent with the known FA phenotype. Her brother was not informative for the standard GPA assay, which requires heterozygosity for the MN blood group, but was analyzed with a modified assay that measured only allele loss mutation. His mutation frequency, 6.8 x 10(-4) was also supportive of a diagnosis of FA. Both analyses also showed evidence of ongoing mutation through terminal erythroblast differentiation, a characteristic of patients with DNA repair syndromes which further confirmed the diagnoses. These conclusions were confirmed with traditional DEB-induced chromosome breakage studies. The quantitative and qualitative aspects of the GPA assay relevant for applying this test for FA diagnosis, and perhaps for carrier detection, are discussed.

Mutations of the T-cell receptor constant region after in utero stem cell transplantation

Like the immunoglobulin genes, the T-cell receptor genes are generated by rearrangements of non-contiguous genomic V, D and J regions, but unlike the immunoglobulin genes, somatic hypermutation is an infrequent event in T-cell receptor genes. Here, we describe the occurrence of spontaneous mutations in the constant regions of the T-cell receptor beta chains of T lymphocytes obtained from two babies who underwent in utero transplantation because of severe combined immunodeficiency. In view of the fact that in babies receiving transplants before birth, hematopoietic chimerism is consistently present, the lymphocytes are likely to be under chronic activation, which may represent a relevant biologic stimulus for generating the observed T-cell receptor hypermutation. This possibility is supported by the finding that the highest number of mutations was identified in clonally expanded T cells. These results provide further support indicating that hypermutation of the T-cell receptor genes may indeed occur, given the necessary conditions.

Dose response for T-cell receptor (TCR) mutants in patients repeatedly treated with 131I for thyroid cancer

The T-cell receptor mutant frequency (TCR-Mf) was measured in 53 young adults, who were treated with radioiodine for thyroid cancer. Patients came from the southern part of Belarus. This region had suffered the most from the Chernobyl Disaster. TCR-Mf was determined by flow cytometry before and after 1 to maximal 10 treatments. Before treatment, TCR-Mf of patients was 2.0 x 10(-14). This Mf value is in the same range as that of young healthy students. After radioiodine therapy (RIT), TCR-Mf increases within about half a year to a maximum. The increase per one mGy to red marrow was 8.7 x 10(-7). After the maximum TCR-Mf declines exponentially. The half-life of TCR mutants was found to be 3.2 years. On the basis of these data, a calibration curve for the use of TCR-Mf as a biological dosimeter is given.

Dear old dad

The origin and frequency of spontaneous mutations that occur with age in humans have been a topic of intense discussion. The mechanisms by which spontaneous mutations arise depend on the parental germ line in which a mutation occurs. In general, paternal mutations are more likely than maternal mutations to be base substitutions. This is likely due to the larger number of germ cell divisions in spermatogenesis than in oogenesis. Maternal mutations are more often chromosomal abnormalities. Advanced parental age seems to influence some mutations, although it is not a factor in the creation of others. In this review, we focus on patterns of paternal bias and age dependence of mutations in different genetic disorders, and the various mechanisms by which these mutations arise. We also discuss recent data on age and the frequency of these mutations in the human male germ line and the impact of these data on this field of research.

Unique tissue-specific level of DNA nucleotide excision repair in primary human mammary epithelial cultures

DNA repair is essential for the maintenance of genomic integrity and stability. Nucleotide excision repair (NER) is a major pathway responsible for remediation of damage caused by UV light, bulky adducts, and cross-linking agents. We now show that NER capacity is differentially expressed in human tissues. We established primary cultures of peripheral blood lymphocytes (PBLs: N = 33) and foreskin fibroblasts (FF: N = 6), as well as adult breast tissue (N = 22) using a unique culture system, and measured their NER capacity using the unscheduled DNA synthesis (UDS) functional assay. Relative to FF, primary cultures of breast cells exhibited only 24.6 +/- 2.1% of NER capacity and PBLs only 8.9 +/- 1.2%. Cells from the breast therefore have a unique and distinctive DNA repair capacity. The NER capacities of all three cell types had similar coefficients of variation in the range of 10%-15%, which should be taken into account when running controls for this contextual assay. Unlike previous studies and speculation in the field, we found that NER was not affected by cell morphology, donor age, or proliferation as measured by the S phase index. While the NER capacity of the transformed lymphoblastoid cell line TK6 was within the range of our PBL samples, the breast tumor-derived MDA MB-231 cell line was four-fold higher than normal breast tissue. These studies show that analysis of baseline DNA repair in normal human cell types is critical as a basis for evaluation of the effects of "mutator" genes as etiological factors in the development of cancer.

Analysis of secondary V(D)J rearrangements in mature, peripheral T cells of ataxia-telangiectasia heterozygotes

Ataxia-telangiectasia (AT) is a rare recessive disease with pleiotropic involvement of the nervous and lymphoid systems. AT heterozygotes have a population frequency of about 1%, and although not manifesting any overt clinical symptoms, they have an increased mortality, mainly because of cancer and ischemic heart disease. We and others have described a mature T lymphocyte population with an altered T cell receptor surface expression ("TCR variant") that reactivates the recombination activating genes (RAG) and is expanded in the blood of patients with AT. In view of the known role of V(D)J recombination in the onset of tumorigenic translocations, we proposed that the increased RAG activity was responsible for the predisposition of AT homozygotes to develop mature-type T leukemia/lymphoma. In the present report, we used cytofluorimetry to quantify the TCR variant population and the memory/naïve T-cell compartments in the blood of AT heterozygotes compared with AT patients and controls. We assessed the expression of different recombinase genes through RT-PCR/oligotyping and cytofluorometric analysis and searched for rearrangement intermediates by ligase-mediated PCR in T-cell lines from four heterozygous carriers. We found the TCR variant population was increased on average 2x in AT heterozygotes (vs 10x in homozygotes) compared with controls, and naïve CD4(+) T lymphocytes were reduced on average 0.5x (vs 0.1x in homozygotes). We were able to demonstrate recombinase gene expression in all four heterozygous T-cell lines, and rearrangement intermediates, indicative of ongoing V(D)J recombination, in two. These rearrangements were compatible with V-gene replacement, a mechanism of receptor editing described for Ig and TCRalpha genes, to our knowledge not previously documented for TCRbeta. In conclusion, we found that RAG reactivation and secondary V(D)J rearrangements, potential risk factors of mature-type leukemia in AT homozygotes, also take place in AT heterozygous carriers and might place this large population fraction at an increased risk of leukemia/lymphoma.

Somatic cell mutations at the glycophorin a locus in erythrocytes of radiation workers from the Sellafield nuclear facility

The glycophorin A (GPA) somatic mutation assay for N0 and NN mutant erythrocytes was performed on 245 current and 48 retired workers who had been occupationally exposed to radiation at the British Nuclear Fuels plc facility at Sellafield. A positive association with increasing age was found for current workers for both N0 and NN frequencies of 0.14 +/- 0.05 x 10(-6) (P = 0.012) and 0.25 +/- 0.07 x 10(-6) (P = 0.0003) per year, respectively. No association with age was found for the retired workers. In a comparison of ever-smokers with never-smokers, no difference was observed for N0 frequencies for current workers, but a significantly higher frequency was found for ever-smokers in the retired group (P = 0.001). NN mutant frequencies were slightly higher in ever-smokers than in never-smokers for both current and retired workers, but in neither case was the increase significant. In age-adjusted analyses for N0 mutant frequencies, a slight positive radiation dose response was found for current workers (1.6 +/- 3.8 x 10(-6) per Sv), for retired workers (2.9 +/- 2.5 x 10(-6) per Sv), and in the combined analysis (2.6 +/- 2.2 x 10(-6) per Sv), but in no case did this reach significance. Similar analyses for NN mutant frequencies revealed a positive dose response for current workers (4.7 +/- 4.6 x 10(-6) per Sv) and a negative response for retired workers (-2.4 +/- 3.6 x 10(-6) per Sv) that was maintained in the combined analysis (-1.4 +/- 2.8 x 10(-6) per Sv), but none of these slopes was significantly different from zero. The results suggest that the GPA mutation assay is insufficiently sensitive to be used as a biological marker of low-dose chronic exposure and provide further evidence that, in contrast to high acute radiation exposure, protracted exposure is much less effective at inducing somatic mutations in vivo.

Polymorphisms of the DNA repair gene XRCC1 and the frequency of somatic mutations at the glycophorin A locus in newborns

Two DNA polymorphisms in the XRCC1 gene, a microsatellite repeat region in the 3' un-translated region (3'UTR) of the gene and a G-->A substitution resulting in an Arg to Gln amino acid change in codon 399, were examined in 189 newborns who had previously been studied for glycophorin A (GPA) N0 and NN variant frequencies (Vfs) in cord blood erythrocytes. The GPA analysis had revealed that 14 of the 189 had extreme NN Vfs ranging from 40 x 10(-6) to 1787 x 10(-6). Mean Vfs for the remaining 175 were N0=(4.8+/-2.80)x10(-6) and NN=(2.62+/-2.01)x10(-6). Seven alleles of a polymorphic tandem [AC](n) region of the XRCC1 gene were identified. No association between [AC](n) genotype and either N0 or NN Vfs was found amongst the group of 175 nor was the distribution of genotypes unusual for the group of 14 with extreme NN Vfs. Analysis of the 399Gln polymorphism revealed that for the group of 175, 36.0% were Arg/Arg, 49.7% Arg/Gln and 14.3% Gln/Gln and genotype had no influence on N0 and NN Vfs. However, the distribution of genotypes was significantly different in the group of 14 with extreme NN Vfs, 14.3% being Arg/Arg, 42.8% Arg/Gln and 42.8% Gln/Gln. The 14 newborns with extreme NN Vfs may represent a sub-group with an unidentified genotoxic exposure and/or predisposition to gene-duplication mutations or alternatively the high values could have arisen by increased clonal expansion of haemopoietic precursor cells carrying NN mutations. Our results suggest that carriers of the Gln/Gln genotype are over represented in this group but the role that the genotype has in the derivation of high NN Vfs remains to be resolved.

Loss of heterozygosity, microsatellite instability, and mismatch repair protein alterations in the radial growth phase of cutaneous malignant melanomas

Little is known about genomic alterations during development of the radial growth phase (RGP) of cutaneous malignant melanomas (CMMs). In this investigation polymerase chain reaction-based microsatellite assays were applied to analyze 13 RGP-CMMs with 18 microsatellite markers at six chromosomal regions: 1p, 3p, 4q, 6q, 9p, and 10q. Loss of heterozygosity (LOH) was found in eight cases (62%), at 9p22, 1p36, and 10q11, suggesting the presence of tumor-suppressor genes at these regions. LOH was encountered frequently at the interferon-alpha (31%) and D10S249 loci (15%). Low-level microsatellite instability (MSI) (11-16% of investigated loci unstable) was noted in three cases (23%). Two MSI banding patterns were seen: band shift and the presence of additional bands. To investigate the underlying mechanisms of the low-level MSI pattern, we analyzed the lesions for expression of mismatch repair (MMR) proteins with immunoperoxidase methods and mouse monoclonal antibodies. The average percentages of positively stained cells for human MutL homolog 1 (hMLH1), human MutS homolog 2 (hMSH2), and human MutS homolog 6 (hMSH6) in RGP-CMM (75.6 +/- 3.4%, 67.20 +/- 7.71%, and 76.6 +/- 2.1%, respectively) were reduced compared with benign nevi. No statistically significant differences in MMR protein expression were found between microsatellite-stable and low-level MSI lesions (P = 0.173, P = 0.458, and P = 0.385 for hMLH1, hMSH2, and hMSH6, respectively). There was a direct correlation between values for percentages of positively stained cells for hMSH2 and hMSH6 (r = +0.9, P = 0.03), suggesting that common mechanisms regulate their expression. In conclusion, LOH, MSI, and reduced MMR protein expression appear to be present in at least some RGP-CMMs and may play a role in their pathogenesis. Further studies are necessary to support these finding and to determine their diagnostic and prognostic significance.

Building bridges in cancer: mismatch repair and microsatellite instability

Mismatch repair (MMR) defects and microsatellite instability (MSI) are two genetic alterations that have been documented in a wide variety of human cancers, including some that involve the skin. Available evidence indicates that these two features are sometimes directly related, although their connection seems to be indirect or nonexistent in other instances. The purposes of this review are to summarize the variable relations between MMR and MSI as deduced from analysis of a diverse array of human neoplasms and to give brief insights as to the other molecular mechanisms potentially involved in the maintenance of genomic stability.

Estimation of mutation rate at human glycophorin A locus in hematopoietic stem cell progenitors

Surveys of human mutant cells exhibit a few individuals with relatively high "outlying" values, which might be explained by rare mutations occurring during development. To estimate how commonly this occurs, mutant red cell frequencies at the glycophorin A locus in 135 neonates and 109 children and adolescents from three research centers are compared with simulations in which mutations arise from successive cycles of binary fission. The simulations predict the data most accurately when the mutation rate in stem cell precursors is about 2-4 x 10(-7) per division cycle, which is similar to previous estimates from adult stem cell divisions. If these mutation rates are accurate, and the number of stem cell divisions during adult life is as low as previously estimated, it is predicted that up to one-sixth of mutant stem cells over a lifetime arose in early life. However, these mutant stem cells would be difficult to detect in surveys because their distribution within the general population is so skewed.

The fundamental prevalence of chronic myeloid leukemia-generating clonogenic cells in the light of the neutrality theory of evolution

A variety of normal human tissues have been reported to harbor small cell populations carrying potentially oncogenic gene rearrangements. This backdrop of mutant cells may be present in the majority of healthy individuals and is apparently weakly selected against. This may provide empirical support for the concept of global neutrality, or near-neutrality (very weak selection), of many somatic mutations. Many healthy individuals, as well as patients with chronic myeloid leukemia, manifest the BCR-ABL fusion gene in blood cells. The presumed neutrality of the BCR-ABL rearrangement-carrying pluripotential hematopoietic stem cells and the relative uniformity of the incidence rate of CML worldwide were used to estimate the extent of the background of BCR-ABL-positive stem cells and the numerical size of the human pluripotential hematopoietic stem cell pool. Three different approaches (molecular-epidemiological, statistical, and population genetical) were employed. Each resulted in very similar estimates of the size of the stem cells carrying the BCR-ABL allele fusions (1.4 x 10(4) cells) and the size of the total human stem cell pool (1.6 x 10(9) cells per individual). The implication of these estimates in the context of the hierarchical nature of the stem cell pool is also considered. The presumptive smaller-sized population of CD34(-) stem cells could not be characterized by any of the approaches used as a "founding" population, representing an ultimate source of all hematopoietic progenitors, or as a subset of stem cells comprising a deeper "kinetic" segment of the total (10(9)-sized) stem cell compartment.

Direct separation of in vivo and in vitro am3 revertants in transgenic mice carrying the phiX174 am3, cs70 vector

Target genes in most transgenic systems have higher spontaneous mutation frequencies than do endogenous mammalian genes. Spontaneous mutations in transgenes predominantly arise from three sources: (1) mutations fixed in the animals, (2) mutations arising from replication errors caused by damage to the DNA that may have occurred in vivo or in vitro and then was fixed during amplification of the vector in vitro, and (3) mutations arising during replication of non-revertant phages in non-permissive bacteria. An assay based on single bursts was developed to directly distinguish between the in vivo and in vitro origins of revertants. The size of the aliquot is determined by mutant frequency and is adjusted so that ideally no more than 10 to 20% of the aliquots contain a bacterial cell transformed with a mutant phage. Mutations are detected as revertants of an amber mutation (am3) in phiX174 am3, cs70. The minimum burst size of non-revertant phiX am3, cs70 from splenic DNA on a permissive bacterial strain was larger than 30 plaque-forming units (pfu). Based on this observation, a burst size of 31 plaque-forming revertants was chosen as the minimum burst size of a fixed mutation. The single burst assay was tested on DNA from spleens of animals that were treated with 150 mg/kg 1-ethyl-1 nitrosurea. Only the fraction of aliquots with single bursts of revertants (> 30) increased in the treated animals compared to the controls. In contrast, there was no difference between treated and control animals for revertant frequencies calculated for burst sizes < or =30 pfu. Among the spontaneous mutations, only 30% were caused by mutations fixed in animals (i.e. burst size >30 pfu). Total average revertant frequency measured in DNA from treated animals was less than twofold more than the average spontaneous frequency (P = 0.048). When frequencies were based on burst sizes >30, there was a 4.6-fold increase among treated animals compared with controls (P = 0.026). The single burst-assay resulted in a more sensitive test for mutagenicity because it eliminated noise from in-vitro mutations.

Cancer chemotherapy and somatic cell mutation

The occurrence of a second neoplasm is one of the major obstacles in cancer chemotherapy. The elucidation of the genotoxic effects induced by anti-cancer drugs is considered to be helpful in identifying the degree of cancer risk. Numerous investigations on cancer patients after chemotherapy have demonstrated: (i) an increase in the in vivo somatic cell mutant frequency (Mf) at three genetic loci, including hypoxanthine-guanine phosphoribosyl-transferase (hprt), glycophorin A (GPA), and the T-cell receptor (TCR), and (ii) alterations in the mutational spectra of hprt mutants. However, the time required for and the degree of such changes are quite variable among patients even if they have received the same chemotherapy, suggesting the existence of underlying genetic factor(s). Accordingly, some cancer patients prior to chemotherapy as well as patients with cancer-prone syndrome have been found to show an elevated Mf. Based on the information obtained from somatic cell mutation assays, an individualized chemotherapy should be considered in order to minimize the risk of a second neoplasm.

Dietary restriction during murine development provides protection against MNU-induced mutations

The developmental stage is the most rapid period for the accumulation of somatic mutations. Epidemiological studies have also suggested a significant role of early life for cancer susceptibility, showing a protective effect of modest dietary restriction early in life. To determine if mutation rate, diet, and cancer risk are related, we have investigated the effect of dietary restriction on somatic mutations early in life. The diet of mouse dams was restricted during pregnancy and lactation by 10% from ad libitum control. F(1) pups (SWRxMutaMouse) were weaned at 3 weeks of age. Pups from dams that were on a restricted diet were kept under dietary restriction (40% until 5 weeks of age and then 20% until sacrifice). Only females from litters of seven or eight were used in this study. A portion of pups from both groups were treated with N-methyl-N-nitrosourea (MNU, 50mg/kg, i.p.) at 5 weeks of age and all mice were sacrificed at 10 weeks of age. The frequency of induced mutations was reduced by about 30% at the three loci studied, lacZ (P=0.028) and cII (P=0.042) and Dlb-1 (P=0.032) in the small intestine in the restricted group. A similar decrease in the lacZ mutant frequency was observed in the bone marrow, but the results did not reach statistical significance (P=0.074). Few differences in the lacZ mutant frequency were observed in the colon and the mammary epithelium, but variability of the mutant frequencies was such that an effect of similar magnitude could not be excluded statistically. Analysis of 47 cII mutants revealed that the majority of MNU-induced mutations were G:C to A:T transition at non-CpG sites, with no difference in the mutation spectrum between the two dietary groups.

The origins, patterns and implications of human spontaneous mutation

The germline mutation rate in human males, especially older males, is generally much higher than in females, mainly because in males there are many more germ-cell divisions. However, there are some exceptions and many variations. Base substitutions, insertion-deletions, repeat expansions and chromosomal changes each follow different rules. Evidence from evolutionary sequence data indicates that the overall rate of deleterious mutation may be high enough to have a large effect on human well-being. But there are ways in which the impact of deleterious mutations can be mitigated.

Analysis of genomic instability using multiple assays in a patient with Rothmund-Thomson syndrome

We report on a patient with Rothmund-Thomson syndrome (RTS) whose cytogenetic evaluation showed a normal karyotype with no evidence of trisomy mosaicism or chromosomal rearrangements. Cultured lymphocytes from the patient, her mother, and a control exposed to mitomycin C and diepoxybutane did not show increased sensitivity to the dialkylating agents. Unlike some previous reports, we found no evidence of a deficiency in nucleotide excision repair, as measured with the functional unscheduled DNA synthesis assay. Glycophorin A analysis of red blood cells for somatic mutation revealed suspiciously high frequencies of both allele loss and loss-and-duplication variants in the blood of the patient, a pattern consistent with observations in other RecQ-related human diseases, and evidence for clonal expansion of a mutant clone in the mother. Discrepant results in the literature may reflect true heterogeneity in the disease or the fact that a consistent set of tests has not been applied to RTS patients.

Somatic cell mutations: can they provide a link between aging and cancer?

Cancers increase during aging in mammals, and an accumulating body of evidence suggests that mutational events too do likewise. Mutational events are intimately involved in the malignant process. One current view is that mutator phenotypes are required in malignant cells for a sufficient number of critical target genes to be affected. These mutator phenotypes are believed to result from underlying deficiencies in genes necessary to maintain genomic stability. This review will provide a framework for a discussion of cancer and aging by detailing with a pair of wise approach studies that address the relations between aging, cancer, and mutations. Results from these studies will be used to suggest that a mutator phenotype develops in the cells of older individuals in the absence of an underlying genetic deficiency. Instead, it is proposed that a mixture of chromosomal aberrations, DNA damage, and chronic exposure to genotoxic forces, including oxidative stress, provide the basis for this age-accelerated mutator phenotype.

Modelling haemopoietic stem cell division by analysis of mutant red cells

Data describing the number of human red cells mutated at the glycophorin A locus, measured flow cytometrically, are reported for 752 adults and 49 neonates. The variance increases with age more rapidly than the approximately linear increase in mean. It is postulated that this discrepancy is explained by the known property of asymmetric stem cell division, so that the division of a single mutant stem cell may result in zero, one or two progeny stem cells. A mathematical analysis allows description of this process with three parameters: stem cell number, mean division rate and mutation rate per division. The values of these parameters can not be deduced from the data presented here. However, estimates of either stem cell number or mutation rate from other sources enable deduction of the two other parameters. The mean number of divisions per stem cell per lifetime was estimated to be about 70. This analysis therefore implies that the rate at which blood cell telomeres shorten with age acts as a direct measure of stem cell turnover. Furthermore, it is argued that this low figure implies that mutations occurring during early life, including organogenesis, are relatively important in initiating stem cell-derived malignancy. Finally, the number of human stem cell divisions per lifetime is similar to shorter-lived mammals, suggesting this number is important in the ageing process.

Targeted disruption of an erythrocyte binding antigen in Plasmodium falciparum is associated with a switch toward a sialic acid-independent pathway of invasion

Erythrocyte invasion by Plasmodium requires molecules present both on the merozoite surface and within the specialized organelles of the apical complex. The Plasmodium erythrocyte binding protein family includes the Plasmodium falciparum sialic acid-binding protein, EBA-175 (erythrocyte binding antigen-175), which binds sialic acid present on glycophorin A of human erythrocytes. We address the role of the conserved 3'-cysteine rich region, the transmembrane, and cytoplasmic domains through targeted gene disruption. Truncation of EBA-175 had no measurable effect on either the level of EBA-175 protein expression or its subcellular localization. Similarly, there appears to be no impairment in the ability of soluble EBA-175 to be released into the culture supernatant after schizont rupture. Additionally, the 3'-cys rich region, transmembrane, and cytoplasmic domains of EBA-175 are apparently non-essential for merozoite invasion. In contrast, erythrocyte invasion via the EBA-175/glycophorin A route appears to have been disrupted to such a degree that the mutant lines have undergone a stable switch in invasion phenotype. As such, EBA-175 appears to have been functionally inactivated within the truncation mutants. The sialic acid-independent invasion pathway within the mutant parasites accounts for approximately 85% of invasion into normal erythrocytes. These data demonstrate the ability of P. falciparum to utilize alternate pathways for invasion of red blood cells, a property that most likely provides a substantial survival advantage in terms of overcoming host receptor heterogeneity and/or immune pressure.

Cutting edge: recombinase-activating gene expression and V(D)J recombination in CD4+CD3low mature T lymphocytes

The recombinase-activating genes, RAG-1 and RAG-2, can be expressed by a subset of B cells within germinal centers, where they mediate secondary V(D)J rearrangements. This receptor revision mechanism could serve either receptor diversification or tolerance-induced functions. Alternatively, it might rescue those cells the receptors of which have been damaged by somatic mutation. Less is known about the occurrence of similar mechanisms in T cells. Here we show that mature T cells with defective TCR surface expression can express RAG genes and are capable of initiating secondary V(D)J rearrangements. The possibility that a cell rescue mechanism based on the generation of a novel Ag receptor might be active in peripheral T cells is envisaged.

The incidence of acute promyelocytic leukemia appears constant over most of a human lifespan, implying only one rate limiting mutation

It is believed that most malignancies become more common with increasing age due to the requirement for several mutations to accumulate and subsequently interact. The age specific incidence of acute promyelocytic leukemia (APL) was investigated using population-based data from 77 million subject years of observation, yielding 149 consecutive cases. The incidence appears approximately constant with respect to age, an observation not previously reported with any other malignancy. These findings are most easily explained by there being only one rate limiting genetic event required to initiate the disease, although other, non-rate limiting mutations may also be necessary for disease development. It is also argued that this mutation is probably restricted to cells committed to differentiation, which may explain why APL is curable by chemotherapy.

Age-associated increase of spontaneous mutant frequency and molecular nature of mutation in newborn and old lacZ-transgenic mouse

Accumulation of mutation has long been hypothesized to be a cause of aging and contribute to many of the degenerative diseases, which appear in the senescent phase of life. To test this hypothesis, age-associated changes in spontaneous mutation in different tissues of the body as well as the molecular nature of such changes should be examined. This kind of approach has become feasible only lately with a development of new transgenic mice suitable for mutation assay. Here, using one of these transgenic mice harboring lacZ gene, we have shown that the age-associated increase in spontaneous mutant frequency is common to all tissues examined; spleen, liver, heart, brain, skin and testis, while the rates of increase in mutant frequency differed among the tissues. DNA sequencing of the 496 lacZ mutants recovered from the tissues of newborn and old mice has revealed that spectra of mutations are similar at the two age points with G:C to A:T transition at CpG site being a predominant type of mutation. Furthermore, some mutations in old tissues are complex type and not found in tissues of newborn mice. These results suggest that similar mechanisms may be operating for mutation induction in fetal and postnatal aging process. In addition, the appearance of complex types of mutations in the old tissues suggests a unique cause for these mutations in aging tissues.

Interindividual variation in mitotic recombination

Mitotic recombination (MR) between homologous chromosomes is a mutational event that results in loss of heterozygosity in half of the segregants at mitosis. Loss of heterozygosity may have important biological consequences. The purpose of this study was to describe human variation in the spontaneous frequency of MR. Using an immunoselection technique for isolating the somatic mutations that result in loss of expression of one of the codominant alleles at the HLA-A locus, we have measured the frequency and molecular basis of somatic mutations in lymphocytes from a population of young adults. Mutations were classified as being the result of intragenic changes, major deletions, or MR. Here we show that the MR mutation frequency in females was significantly greater than that in males but that intragenic mutation frequency showed no association with sex. Individual variation in MR frequency ranged over more than two orders of magnitude and was not normally distributed. Furthermore, the observed number of individuals from whom no mutants resulting from MR were obtained was significantly greater than was expected. The endogenous level of MR may be under genetic control. Given the association of loss of heterozygosity with cancer initiation and progression, low endogenous MR may confer a reduced lifetime risk of cancer, and the converse may apply.

Diagnosis of ataxia telangiectasia with the glycophorin A somatic mutation assay

There are no widely applied definitive laboratory tests for the diagnosis of ataxia telangiectasia (AT). We, and others, have previously reported significantly elevated levels of in vivo somatic mutation in blood samples from known AT patients, observations that might form the basis for a useful prospective laboratory test for confirmation of a clinical diagnosis of AT. In the present case, a 4 1/2-year-old black female was suspected of having AT based on ataxic gait and chronic upper respiratory infections. Blood work-up showed low IgG2 and elevated alpha-fetoprotein (AFP), consistent with the AT phenotype. Her peripheral blood karyotype was normal, however, with no spontaneous breakage observed among 100 solid stained metaphases. Lymphocytes from AT patients often show elevated levels of chromosome rearrangement, especially at sites of immunoglobulin and T-cell receptor genes. Therefore, a blood sample was analyzed with the glycophorin A (GPA) in vivo somatic mutation assay. The GPA assay detects and quantifies the phenotypically variant erythrocytes resulting from loss of heterozygosity for the MN blood group. The patient had a 10-fold increased frequency of variant erythrocytes with a phenotype consistent with simple loss of the N allele, which is characteristic of AT. In addition, the variant cell distribution for this patient showed three other, more qualitative hallmarks of AT: a normal frequency of allele loss and duplication events, a unique ridge of cells of intermediate phenotype between the normal and mutant peaks, and evidence of similar ongoing mutational loss of the M allele. Together with clinical data, these distinctive qualitative and quantitative features of the GPA assay allow for a diagnosis of AT with a projected accuracy of 95%. Therefore, we suggest that the GPA assay, which can be performed on < 1 ml of blood and completed in less than a day, be considered as a confirmatory laboratory test for a clinical diagnosis of AT.

Influence of sex, smoking and age on human hprt mutation frequencies and spectra

Examination of the literature for hprt mutant frequencies from peripheral T cells yielded data from 1194 human subjects. Relationships between mutant frequency, age, sex, and smoking were examined, and the kinetics were described. Mutant frequency increases rapidly with age until about age 15. Afterward, the rate of increase falls such that after age 53, the hprt mutant frequency is largely stabilized. Sex had no effect on mutant frequency. Cigarette smoking increased mean mutant frequency compared to nonsmokers, but did not alter age vs. mutant frequency relationships. An hprt in vivo mutant database containing 795 human hprt mutants from 342 individuals was prepared. No difference in mutational spectra was observed comparing smokers to nonsmokers, confirming previous reports. Sex affected the frequency of deletions (>1 bp) that are recovered more than twice as frequently in females (P = 0. 008) compared to males. There is no indication of a significant shift in mutational spectra with age for individuals older than 19 yr, with the exception of A:T --> C:G transversions. These events are recovered more frequently in older individuals.