Diseases with DNA damage-processing defects

Immunodeficiency, centromeric region instability, facial anomalies syndrome (ICF)

The Immunodeficiency, Centromeric region instability, Facial anomalies syndrome (ICF) is a rare autosomal recessive disease described in about 50 patients worldwide and characterized by immunodeficiency, although B cells are present, and by characteristic rearrangements in the vicinity of the centromeres (the juxtacentromeric heterochromatin) of chromosomes 1 and 16 and sometimes 9. Other variable symptoms of this probably under-diagnosed syndrome include mild facial dysmorphism, growth retardation, failure to thrive, and psychomotor retardation. Serum levels of IgG, IgM, IgE, and/or IgA are low, although the type of immunoglobulin deficiency is variable. Recurrent infections are the presenting symptom, usually in early childhood. ICF always involves limited hypomethylation of DNA and often arises from mutations in one of the DNA methyltransferase genes (DNMT3B). Much of this DNA hypomethylation is in 1qh, 9qh, and 16qh, regions that are the site of whole-arm deletions, chromatid and chromosome breaks, stretching (decondensation), and multiradial chromosome junctions in mitogen-stimulated lymphocytes. By an unknown mechanism, the DNMT3B deficiency that causes ICF interferes with lymphogenesis (at a step after class switching) or lymphocyte activation. With the identification of DNMT3B as the affected gene in a majority of ICF patients, prenatal diagnosis of ICF is possible. However, given the variety of DNMT3B mutations, a first-degree affected relative should first have both alleles of this gene sequenced. Treatment almost always includes regular infusions of immunoglobulins, mostly intravenously. Recently, bone marrow transplantation has been tried.

Cockayne syndrome in three sisters with varying clinical presentation

We report three sisters showing the clinical features and investigational findings of Cockayne syndrome (CS). In the rehabilitation unit of Northwest Armed Forces Hospital (N.W.A.F.H.), Tabuk, Saudi Arabia, there was a 12-year-old girl with typical features of CS. The girl had no apparent problems until the end of the first year when growth and developmental delay prompted medical evaluation. Brain CT, bone X-rays, auditory and ophthalmological evaluation confirmed the clinical impression of Cockayne syndrome. Two of her 13 sibs, both sisters, were later found to have the same syndrome. The sisters varied in clinical severity, as two of them had cataracts and early global delay and died early of inanition and infection. The third showed the disease manifestations at a relatively later age, did not have cataract, exhibited milder manifestations of the disease, and remains alive. The parents are not related by any way and the father is married to two other wives with 11 unaffected children. This report documents variable degrees of manifestations in sibs who presumably have the same gene mutation.

The Mre11 complex is required for repair of hairpin-capped double-strand breaks and prevention of chromosome rearrangements

Inverted repeats (IRs) that can form a hairpin or cruciform structure are common in the human genome and may be sources of instability. An IR involving the human Alu sequence (Alu-IR) has been studied as a model of such structures in yeast. We found that an Alu-IR is a mitotic recombination hotspot requiring MRE11/RAD50/XRS2 and SAE2. Using a newly developed approach for mapping rare double-strand breaks (DSBs), we established that induction of recombination results from breaks that are terminated by hairpins. Failure of the mre11, rad50, xrs2, and sae2 mutants to process the hairpins blocks recombinational repair of the DSBs and leads to generation of chromosome inverted duplications. Our results suggest an additional role for the Mre11 complex in maintaining genome stability.

DNA damage processing defects and disease

Inherited defects in DNA repair or the processing of DNA damage can lead to disease. Both autosomal recessive and autosomal dominant modes of inheritance are represented. The diseases as a group are characterized by genomic instability, with eventual appearance of cancer. The inherited defects frequently have a specific DNA damage sensitivity, with cells from affected individuals showing normal resistance to other genotoxic agents. The known defects are subtle alterations in transcription, replication, or recombination, with alternate pathways of processing permitting cellular viability. Distinct diseases may arise from different mutations in one gene; thus, clinical phenotypes may reflect the loss of different partial functions of a gene. The findings indicate that partial defects in transcription or recombination lead to genomic instability, cancer, and characteristic disease phenotypes.

Development of radiation therapy optimization

The principal radiobiological problems in the treatment of advanced tumors and the solution of many of them by radiobiologically optimized intensity-modulated radiation therapy are presented. Considerable improvements of the treatment outcome using radiobiologically optimized intensity-modulated treatments are achieved by: (a) increasing the tumor dose and dose per fraction; (b) keeping constant or even reducing slightly the dose and dose per fraction to organs at risk, (c) reducing the overall treatment time and the number of treatment fractions. The merits of the new radiation modalities and advanced intensity-modulated treatment techniques are compared in terms of equipment costs per patient cured. It is predicted that the new development of radiobiologically optimized intensity-modulated radiation therapy will rapidly become an important clinical tool, increasing the efficiency of the collaboration between radiation physicists, radiation biologists and radiation oncologists. Not only does it allow the optimal treatment of every patient, but it also promotes an efficient feedback of treatment outcome and complication data to improve the accuracy of known dose response relations to further augment future treatment results. Equipment costs may go up during a transition period until efficient interfaces between new diagnostic equipment, treatment-planning systems and intensity-modulated treatment units are fully developed. From then onwards the cost of high quality biologically optimized intensity-modulated treatments will decrease and so will the treatment time and personnel requirements, at the same time as the treatment quality is greatly improved particularly for more advanced tumors.

Cell cycle control, checkpoint mechanisms, and genotoxic stress

The ability of cells to maintain genomic integrity is vital for cell survival and proliferation. Lack of fidelity in DNA replication and maintenance can result in deleterious mutations leading to cell death or, in multicellular organisms, cancer. The purpose of this review is to discuss the known signal transduction pathways that regulate cell cycle progression and the mechanisms cells employ to insure DNA stability in the face of genotoxic stress. In particular, we focus on mammalian cell cycle checkpoint functions, their role in maintaining DNA stability during the cell cycle following exposure to genotoxic agents, and the gene products that act in checkpoint function signal transduction cascades. Key transitions in the cell cycle are regulated by the activities of various protein kinase complexes composed of cyclin and cyclin-dependent kinase (Cdk) molecules. Surveillance control mechanisms that check to ensure proper completion of early events and cellular integrity before initiation of subsequent events in cell cycle progression are referred to as cell cycle checkpoints and can generate a transient delay that provides the cell more time to repair damage before progressing to the next phase of the cycle. A variety of cellular responses are elicited that function in checkpoint signaling to inhibit cyclin/Cdk activities. These responses include the p53-dependent and p53-independent induction of Cdk inhibitors and the p53-independent inhibitory phosphorylation of Cdk molecules themselves. Eliciting proper G1, S, and G2 checkpoint responses to double-strand DNA breaks requires the function of the Ataxia telangiectasia mutated gene product. Several human heritable cancer-prone syndromes known to alter DNA stability have been found to have defects in checkpoint surveillance pathways. Exposures to several common sources of genotoxic stress, including oxidative stress, ionizing radiation, UV radiation, and the genotoxic compound benzo[a]pyrene, elicit cell cycle checkpoint responses that show both similarities and differences in their molecular signaling.

The hMre11/hRad50 protein complex and Nijmegen breakage syndrome: linkage of double-strand break repair to the cellular DNA damage response

Nijmegen breakage syndrome (NBS) is an autosomal recessive disorder characterized by increased cancer incidence, cell cycle checkpoint defects, and ionizing radiation sensitivity. We have isolated the gene encoding p95, a member of the hMre11/hRad50 double-strand break repair complex. The p95 gene mapped to 8q21.3, the region that contains the NBS locus, and p95 was absent from NBS cells established from NBS patients. p95 deficiency in these cells completely abrogates the formation of hMre11/hRad50 ionizing radiation-induced foci. Comparison of the p95 cDNA to the NBS1 cDNA indicated that the p95 gene and NBS1 are identical. The implication of hMre11/hRad50/p95 protein complex in NBS reveals a direct molecular link between DSB repair and cell cycle checkpoint functions.

A DNA repair defect in a patient with ataxia, mental retardation, and short stature

A 12-year-old girl developed ataxia that gradually progressed. At age 18 the patient presented with mental retardation, cachectic dwarfism, microcephalus, and a progeroid appearance but no photosensitive skin lesions or deafness. On analysis of fibroblasts, unscheduled DNA synthesis was reduced to 50% of normal, but colony-forming ability after ultraviolet irradiation was normal. The symptoms and phenotype of the patient were distinguished from those in Cockayne syndrome and xeroderma pigmentosum. This case is interesting because the defect in DNA repair after ultraviolet irradiation was detected in a patient with neurologic disturbances but without photosensitive skin lesions.

The role of calcium in human lymphocyte DNA repair ability

DNA repair ability is reduced in a variety of pathologic conditions. In addition, in some of these diseases a disturbance in cellular Ca homeostasis occurs or cytosolic (Ca2+) responses to various stimuli are impaired. The leading environmental cause for genomal DNA damage is ultraviolet (UV) irradiation. The aims of the present study were (1) to evaluate a possible dependence of UV-induced DNA repair ability on cytosolic Ca2+ in human lymphocytes and (2) to assess the direct effect of UV irradiation on Ca2+ homeostasis in these cells. UV-induced DNA repair ability in lymphocytes was maximal at 1 mmol/L CaCl2 in the medium. Suppression of DNA repair ability occurred after elevation or reduction of cellular (Ca2+) when various methods were used, including changes in Ca2+ concentration in the medium, cellular Ca2+ depletion by ethyleneglycol-bis-(beta aminoethylether)-N,N,N',N'-tetraacetic acid, excessive Ca2+ concentration induced by ionophore, and shortening of Ca2+ presence time during repair synthesis. UV irradiation caused an immediate and significant rise in cytosolic (Ca2+) that was the result of both enhanced Ca2+ uptake and inhibition of plasma membrane Ca-adenosine triphosphatase activity. The tyrosine kinase inhibitor genistein inhibited both UV-induced DNA repair and UV-induced cytosolic (Ca2+) elevation. These results emphasize the importance of a precise cellular Ca2+ level regulation for the optimal DNA repair process. UV irradiation, by inducing cellular Ca2+ rise, may activate DNA repair as soon as DNA is damaged.

A simple method for diagnosing xeroderma pigmentosum variant

Patients with xeroderma pigmentosum variant have been diagnosed based on a post-replication repair assay using their cells together with their clinical manifestations. We present here an alternative simple method for the diagnosis of xeroderma pigmentosum variant that measures three cellular markers for DNA repair by autoradiography, unscheduled DNA synthesis, recovery of RNA synthesis, and recovery of replicative DNA synthesis after ultraviolet irradiation. Fibroblasts from a patient are plated on three coverslips parallel with normal cells (control). Unscheduled DNA synthesis, recovery of RNA synthesis, and recovery of replicative DNA synthesis after ultraviolet irradiation in the patient's cells are compared with those of adjacent normal cells by counting numbers of grains on nuclei for each coverslip. Of the hereditary photosensitive disorders including xeroderma pigmentosum, Cockayne syndrome, and newly established ultraviolet-sensitive syndrome, only xeroderma pigmentosum variant cells exhibit normal unscheduled DNA synthesis, normal recovery of RNA synthesis, but reduced recovery of replicative DNA synthesis (approximately 50% of that of control cells). This reduction of DNA synthesis is enhanced in the presence of caffeine. Because each disorder yields a different combination of these three markers, this method also provides a systematic basis for the diagnosis of these diseases.

Cockayne syndrome complementation group B associated with xeroderma pigmentosum phenotype

Two siblings have been reported whose clinical manifestations (cutaneous photosensitivity and central nervous system dysfunction) are strongly reminiscent of the DeSanctis-Cacchione syndrome (DCS) variant of xeroderma pigmentosum (XP), a severe form of XP. Fibroblasts from the siblings showed UV sensitivity, a failure of recovery of RNA synthesis (RRS) after UV-irradiation, and a normal level of unscheduled DNA synthesis (UDS), which were, unexpectedly, the biochemical characteristics usually associated with Cockayne syndrome (CS). However, no complementation group assignment in these cells has yet been performed. We here report that these patients can be assigned to CS complementation group B (CSB) by cell fusion complementation analysis. To our knowledge, these are the first patients with defects in the CSB gene to be associated with an XP phenotype. The results imply that the gene product from the CSB gene must interact with the gene products involved in excision repair and associated with XP.

Cranial MRI in ataxia-telangiectasia

We examined five males with laboratory-confirmed ataxia-telangiectasia (AT), aged 9-28 years, several times by MRI (9 examinations: 5 at 0.15 T, 3 at 0.5 T, 1 at 1.5 T). Intermediate, T1-, T2- and T2*-weighted spin-echo and gradient-echo sequences were performed. All patients showed vermian atrophy, enlarged fourth ventricle and cisterna magna; four showed cerebellar hemisphere atrophy; two enlarged infracerebellar subarachnoid spaces and four patients had sinusitis. No focal areas of abnormal signal were seen in the brain, diffuse high signal was found in the central cerebral white matter of the oldest patient. AT is an important human model of inherited cancer susceptibility and multisystem ageing; as in xeroderma pigmentosum and other "breakage syndromes", ionising radiation should be avoided. When imaging is necessary, MRI should be preferred to CT in patients known or suspected to have AT and those with undefined paediatric ataxias of nontraumatic origin. If atrophy of only the cerebellum, especially the vermis, is noted, laboratory research should be performed to confirm the diagnosis of AT.

Biological basis of germline mutation: comparisons of spontaneous germline mutation rates among drosophila, mouse, and human

Spontaneous mutation rates per generation are similar among the three species considered here--Drosophila, mouse, and human--and are not related to time, as is often assumed. Spontaneous germline mutation rates per generation averaged among loci are less variable among species than they are among loci and tests and between gender. Mutation rates are highly variable over time in diverse lineages. Recent estimates of the number of germ cell divisions per generation are: for humans, 401 (30-year generation) in males and 31 in females; for mice, 62 (9-month generation) in males and 25 in females; and for Drosophila melanogaster, 35.5 (18-day generation) in males and 36.5 (25-day generation) in females. The relationships between germ cell division estimates of the two sexes in the three species closely reflect those between mutation rates in the sexes, although mutation rates per cell division vary among species. Whereas the overall rate per generation is constant among species, this consistency must be achieved by diverse mechanisms. Modifiers of mutation rates, on which selection might act, include germline characteristics that contribute disproportionately to the total mutation rates. The germline mutation rates between the sexes within a species are largely influenced by germ cell divisions per generation. Also, a large portion of the total mutations occur during the interval between the beginning of meiosis and differentiation of the soma from the germline. Significant genetic events contributing to mutations during this time may include meiosis, lack of DNA repair in sperm cells, methylation of CpG dinucleotides in mammalian sperm and early embryo, gonomeric fertilization, and rapid cleavage divisions.

Genetic susceptibility to radiation and chemotherapy injury: diagnosis and management

Over 5% of the cancer patient population may be radiation sensitive due to genetics, and the sensitive patients may be greatly overrepresented among patients with cancer therapy complications. These individuals include not only rare ataxia telangiectasia (AT) homozygotes with up to three-fold normal radiation sensitivity, but also far more numerous patients with slight radiosensitivity conjectured to be carriers of AT or to have another inherited mutagen sensitivity. Procedures may eventually be used to reliably determine patient tolerance for radiation and antineoplastic agents before initiation or completion of therapy, to have the therapy approach but not exceed the radiation tolerance of the individual patient's irradiated normal tissue. Such procedures could include study of patient's cultured normal cells (e.g., fibroblasts, marrow cells, or lymphocytes) in much the same way that patients' cultured tumor cells may eventually be widely used in the human tumor stem cell assay to predict which course of radiotherapy or chemotherapy should be most useful for treating a cancer. Studies with the normal cells could include cytotoxicity assays, serially determined accumulated genetic damage over the course of therapy, or Southern blot analysis to identify carriers of DNA repair mutations. Such studies could permit more aggressive radiotherapy of most patients due to the noninclusion of a sensitive subpopulation of patients, with less radiotherapy of the relatively few radiation sensitive patients. The patient's tumor cells should have inherited any radiation (or chemotherapy) sensitivity mutations present in the patient's normal cells, so reducing the radiotherapy dose to compensate for the more radiosensitive patients' sensitivity will not necessarily result in undertreatment of the tumor.

A new UV-sensitive syndrome not belonging to any complementation groups of xeroderma pigmentosum or Cockayne syndrome: siblings showing biochemical characteristics of Cockayne syndrome without typical clinical manifestations

We report here on two siblings who show no clinical manifestations except for slight cutaneous photosensitivity and cutaneous pigmentation but have biochemical characteristics of Cockayne syndrome (CS). Fibroblasts derived from the patients (Kps2 and Kps3) were 3-4 times more sensitive to UV than normal cells. Although unscheduled DNA synthesis (UDS) in these cells was at a normal level, recovery of RNA synthesis (RRS) after UV irradiation was severely depressed. Microinjection of bacteriophage T4 endonuclease V into the cells corrected RRS after UV irradiation to a level near normal. These results indicate that DNA repair of cyclobutane-type pyrimidine dimers is impaired in the cells and the biochemical characteristics are similar to those of CS cells. However, cell fusion complementation tests with CS group A and B cells resulted in correction of RRS after UV irradiation. Cell fusion with XP group A, B, D, F and G cells also corrected RRS after UV irradiation, and microinjection of cell extracts prepared from Kps3 cells corrected UDS in XP group C and E cells, indicating that the patients do not belong to any complementation group of XP or CS. These results suggest that the patients have a new UV-sensitive syndrome with a biochemical phenotype of CS.

Xeroderma pigmentosum-Cockayne syndrome complex in two patients: absence of skin tumors despite severe deficiency of DNA excision repair

Two brothers had a complex combination of two DNA repair disorders: Cockayne syndrome and xeroderma pigmentosum. This rare combination has previously been observed in only two other patients. The clinical signs shared by these two brothers and the two other previously described patients include severe sun sensitivity, freckling, diminished stature, hearing and movement impairment, and neurologic degeneration. Although defective UV-induced unscheduled DNA synthesis has been demonstrated (5% of normal), no skin cancers have appeared in these 38- and 41-year-old brothers, whereas skin cancers developed at a relatively early age in the two previously described patients who also had defective UV-induced unscheduled DNA synthesis.

Radiation and chemotherapy injury: pathophysiology, diagnosis, and treatment

The text in general is not meant to represent the participants' entire presentations. The lecture presenters in general are not responsible for the summaries, and cannot necessarily be assumed to agree with all that is stated, but they deserve credit for providing the lecture and handout material on which the summaries are based, and in most cases have contributed far more to the summaries than I have.

The prevention of thymic lymphomas in transgenic mice by human O6-alkylguanine-DNA alkyltransferase

Nitrosoureas form O6-alkylguanine-DNA adducts that are converted to G to A transitions, the mutation found in the activated ras oncogenes of nitrosourea-induced mouse lymphomas and rat mammary tumors. These adducts are removed by the DNA repair protein O6-alkylguanine-DNA alkyltransferase. Transgenic mice that express the human homolog of this protein in the thymus were found to be protected from developing thymic lymphomas after exposure to N-methyl-N-nitrosourea. Thus, transgenic expression of a single human DNA repair gene is sufficient to block chemical carcinogenesis. The transduction of DNA repair genes in vivo may unravel mechanisms of carcinogenesis and provide therapeutic protection from known carcinogens.

Cutaneous markers of internal malignancy. I. Malignant involvement of the skin and the genodermatoses

Cutaneous findings often reflect the presence and course of an internal disease. Recognition of external clues is important to facilitate both early diagnosis and prompt treatment of the internal disorder. Early recognition is especially valuable in a patient with an internal malignant disease because intervention may significantly affect survival. In this two-part series, we review the spectrum of cutaneous markers of internal malignancy. Part I focuses on malignant involvement of the skin caused by either direct extension or metastases and the genodermatoses with malignant potential. Part II will be devoted to the paraneoplastic skin manifestations of internal malignancy as well as environmental carcinogens that produce cutaneous features. We also discuss some proposed but controversial associations between skin disease and internal malignancy.

Gene targeting using a mouse HPRT minigene/HPRT-deficient embryonic stem cell system: inactivation of the mouse ERCC-1 gene

A convenient system for gene targeting that uses hypoxanthine phosphoribosyltransferase (HPRT) minigenes as the selectable marker in HPRT-deficient mouse embryonic stem (ES) cells is described. Improvements to the expression of HPRT minigenes in ES cells were achieved by promoter substitution and the provision of a strong translational initiation signal. The use of minigenes in the positive-negative selection strategy for gene targeting was evaluated and the smaller minigenes were found to be as effective as a more conventional marker--the herpes simplex virus thymidine kinase gene. Minigenes were used to target the DNA repair gene ERCC-1 in ES cells. A new HPRT-deficient ES cell line was developed that contributes with high frequency to the germ line of chimeric animals. The ability to select for and against HPRT minigene expression in the new HPRT-deficient ES cell line will make this system useful for a range of gene-targeting applications.

Cranial CT and MRI in diseases with DNA repair defects

The CT and MRI appearances of 5 patients with Cockayne's syndrome, 5 with ataxia telangiectasia and 1 with Fanconi's anaemia are reported. These conditions, together with Bloom's syndrome and xeroderma pigmentosum are regarded as disorders of DNA repair. Characteristic CT and MRI features of Cockayne's syndrome include generalised atrophy, calcification in basal ganglia and dentate nuclei and white matter low density. Neuroradiological findings in the other DNA repair disorders are nonspecific.

Somatic cell hybridization of Roberts syndrome and normal lymphoblasts resulting in correction of both the cytogenetic and mutagen hypersensitivity cellular phenotypes

Roberts syndrome (RS) is a rare recessive condition of limb deformities, growth retardation, and developmental delay. Cultured cells from approximately half of RS patients exhibit a "puffing" of the constitutive heterochromatin and a hypersensitivity to mitomycin C (MMC). Patients exhibiting these cellular phenomena are designated RS+. Somatic cell hybridization with normal cells has been shown to correct the heterochromatin abnormality in RS+ cells. To determine if the MMC hypersensitivity could also be corrected by hybridization to normal cells, we fused two different RS+ lymphoblastoid cell lines (LCLs) to a ouabain-resistant, HAT-sensitive, normal LCL. Cytogenetic analyses of hybrid cell lines (HCLs) revealed complete correction of the heterochromatin abnormality. MMC cell killing assays revealed correction of the mutagen hypersensitivity as well. Five of the six HCLs tested exhibited D10 values (the dose at which 10% of the cells survive) that were not significantly lower than that of the normal parent but that were 6- to 18-fold greater than those of the RS+ parents. Correction of both of these cellular phenotypes in RS+ cells by fusion with normal cells supports the hypothesis that both of these phenomena are caused by a common defect in the Roberts syndrome gene (RBS).

Optimization of uncomplicated control for head and neck tumors

Almost 200 patients have been treated for head and neck tumors at two different dose levels. Based on the clinically observed probabilities for tumor control and fatal normal tissue complications at the two dose levels, the dose giving maximum uncomplicated control has retrospectively been calculated and compared with the clinical data. A Poisson statistical model for control and complications has been used including a correlation parameter, delta, to describe the fraction of patients where control and complications are statistically independent. The clinically observed probability of uncomplicated tumor control, P+, is consistent with only a small fraction of the patients treated being statistically independent (delta = 0.2 or 20%). Customarily, 100% of the patients are assumed to be statistically independent with regard to tumor control and normal tissue complications. More precisely, the clinical data are consistent, with almost 20% of the patients being significantly more sensitive to radiation since they gain local tumor control but simultaneously suffer fatal complications. An even larger fraction of the patients (almost 30%) seemed to be more resistant to radiation, showing neither serious treatment complications nor control of the local tumor growth. It is suggested that if these patient groups could be identified by a predictive assay for the radiation sensitivity of their normal tissues and preferably also for their tumors, the uncomplicated tumor control could be increased by about 20%. This figure is based on the actuarial survival of the patients and has been corrected for the inevitable uncertainty in dose delivery. It is also pointed out that about 20% of the patients can never be saved by a predictive assay because of the considerable statistical variance associated with the Poisson process and the eradication of the last clonogenic tumor cell. Finally, note that the possible existence of radiation sensitive and resistant patient groups is consistent with known genetic deficiencies such as ataxia telangiectasia for the sensitive patients and the existence of repair efficient head and neck tumors that are unusually efficient in repairing double strand breaks. If such sensitive and resistant patient groups do exist, it should be sufficient to perform a predictive assay on normal tissues alone avoiding the often impossible task of sampling the most radiation resistant tumor cell line.

Flowcytometric analysis of DNA pattern of cells derived from xeroderma pigmentosum A--hypersensitivity to vincristine, etoposide and methotrexate

Xeroderma pigmentosum complementation group A (XPA) is one of the DNA repair deficient syndromes. The cell biological features of XPA were examined by flowcytometry using Epstein Barr (EB) virus-transformed lymphoblastoid cells. Cellular sensitivity to vincristine (VCR), etoposide (VP-16) and methotrexate (MTX) were assayed by DNA pattern changes by flowcytometry. Recently, ataxia-telangiectasia (AT), one of the same kind of disorder, has been reported to have an increased sensitivity to VCR and VP-16. However, AT showed some resistance to MTX according to other reports. Our results showed that XPA had an increased sensitivity to VCR and also to VP-16. Moreover, different from AT, XPA showed some sensitivity to MTX. Thus there is some cell biological similarity between XPA and AT, as well as some difference of the abnormality in the DNA repair pathway.

Cancer predisposition of ataxia-telangiectasia heterozygotes

Ataxia-telangiectasia (A-T) is a progressive neurologic disorder in which there is varied immune dysfunction, an excess sensitivity to ionizing radiation, and a striking predisposition to cancer. It is the autosomal recessive syndrome for which there is the strongest evidence, derived from retrospective studies of cancer incidence and mortality in A-T families, that the heterozygote is predisposed to cancer. We present, in tabular form, the specific cancer sites or types most likely to be associated with A-T heterozygosity. These include solid tumors of the breast, pancreas, stomach, bladder, and ovary, and chronic lymphocytic leukemia. We also introduce a new method to test these associations. As soon as molecular probes for the A-T allele(s) are available, this new research design will be used to test rigorously each association, hypothesized on the basis of previous data, between a specific cancer site and A-T heterozygosity.

Heterogeneity of DNA repair: implications for human disease and oncology

DNA repair studies used to be confined to measurements representing an average over the entire mammalian genome. It is now possible to study repair processes at subgenomic levels including specific genes. We will describe such results and discuss the impact they may have on our understanding of important oncological processes. Also, we will describe and discuss some clinical conditions that may have some effect in DNA damage processing.

Mutations at six additional loci of Drosophila melanogaster cause alkylation hypermutability

8 mutagen-sensitive strains of Drosophila melanogaster were examined for their effects on alkylation-induced mutagenesis. Using methylnitrosourea as the DNA-damaging agent and the sex-linked recessive lethal test as the monitor of genetic endpoint, 6 of these strains were shown to be hypermutable following exposure to this alkylating agent. Previous studies of 6 other genes have demonstrated that strains exhibiting alkylation hypermutability are completely defective in repair replication following alkylation-induced DNA damage. The present observations suggest that at least 12 loci may be required for excision repair of alkylation DNA damage in this species.