Localizing the genes for ataxia-telangiectasia: a human model for inherited cancer susceptibility

Expanding the genotype-phenotype correlation of childhood sensory polyneuropathy of genetic origin

Pure sensory polyneuropathy of genetic origin is rare in childhood and hence important to document the clinical and genetic etiologies from single or multi-center studies. This study focuses on a retrospective chart-review of neurological examinations and genetic and electrodiagnostic data of confirmed sensory polyneuropathy in subjects at a tertiary-care Children's Hospital from 2013 to 2019. Twenty subjects were identified and included. Neurological examination and electrodiagnostic testing showed gait-difficulties, absent tendon reflexes, decreased joint-position, positive Romberg's test and large fiber sensory polyneuropathy on sensory nerve conduction studies in all patients associated with lower-extremity spasticity (6), cardiac abnormalities or cardiomyopathy (5), developmental delay (4), scoliosis (3), epilepsy (3) and hearing-difficulties (2). Confirmation of genetic diagnosis in correlation with clinical presentation was obtained in all cases (COX20 n = 2, HADHA n = 2, POLG n = 1, FXN n = 4, ATXN2 n = 3, ATM n = 3, GAN n = 2, SPG7 n = 1, ZFYVE26 n = 1, FH n = 1). Our single-center study shows genetic sensory polyneuropathies associated with progressive neurodegenerative disorders such as mitochondrial ataxia, Friedreich ataxia, spinocerebellar ataxia type 2, ataxia telangiectasia, spastic paraplegia, giant axonal neuropathy, and fumarate hydratase deficiency. We also present our cohort data in light of clinical features reported for each gene-specific disease subtype in the literature and highlight the importance of genetic testing in the relevant clinical context of electrophysiological findings of peripheral sensory polyneuropathy.

Individual response to ionizing radiation

The human response to ionizing radiation (IR) varies among individuals. The first evidence of the individual response to IR was reported in the beginning of the 20th century. Considering nearly one century of observations, we here propose three aspects of individual IR response: radiosensitivity for early or late adverse tissue events after radiotherapy on normal tissues (non-cancer effects attributable to cell death); radiosusceptibility for IR-induced cancers; and radiodegeneration for non-cancer effects that are often attributable to mechanisms other than cell death (e.g., cataracts and circulatory disease). All the molecular and cellular mechanisms behind IR-induced individual effects are not fully elucidated. However, some specific assays may help their quantification according to the dose and to the genetic status. Accumulated data on individual factors have suggested that the individual IR response cannot be ignored and raises some clinical and societal issues. The individual IR response therefore needs to be taken into account to better evaluate the risks related to IR exposure.

Oligonucleotide therapeutics in cancer

Alterations in pre-mRNA splicing can have profound effects on gene expression and lead to cellular transformation. Oligonucleotide therapeutics are drugs that manipulate gene expression and improve the disease state. Antisense oligonucleotides hybridize with a target mRNA to downregulate gene expression via an RNase H-dependent mechanism. Additionally, RNase H-independent splice switching oligonucleotides (SSO) modulate alternative or aberrant splicing, to favor the therapeutically relevant splicing product. This chapter summarizes the progress made in the application of these oligonucleotide drugs in the treatment of cancer.

Effect of protein kinase C inhibitor (PKCI) on radiation sensitivity and c-fos transcription

PURPOSE

The human genetic disorder ataxia-telangiectasia (AT) is a multisystem disease characterized by extreme radiosensitivity. Although ionizing radiation was known to induce c-fos transcription and cellular protein kinase C (PKC) induces the expression of this immediate response gene, little is known about how mutated AT (ATM) or PKC-mediated signal transduction pathway modulates the c-fos gene transcription and gene expression. Here we have studied the effect of PKC inhibitor (PKCI) on radiation sensitivity and c-fos transcription in normal and AT cells, and also studied whether PKCI effect on c-fos occurs in Ras-dependent pathway.

METHODS AND MATERIALS

Normal (LM217) and AT (AT5BIVA) cells were transfected with PKCI expression plasmid and integration and overexpression of PKCI was evaluated by polymerase chain reaction and northern blotting, respectively. Cells were irradiated at a dose of 5 Gy/min with 137Cs irradiator and harvested 48 h after irradiation and investigated apoptosis with TUNEL method. The c-fos transcription activity was studied by performing compute assisted tomography (CAT) assay of reporter gene after transfection of c-fos CAT plasmid into LM and AT cells. Overexpression of Ras protein in transfected cells was shown by western blotting.

RESULTS

Our results demonstrated for the first time a role of PKCI on the radiation sensitivity and c-fos transcription in LM and AT cells. PKCI increased radiation induced apoptosis in LM cells (5% to 20%) but reduced apoptosis slightly in AT cells. The basal c-fos transcription activity is 70 times lower in AT cells than in LM cells. This c-fos transcription activity was repressed by overexpression of PKCI in LM cells but not in AT cells. After induction of c-fos by Ras protein, overexpression of PKCI repressed c-fos transcription in LM cells but not in AT cells.

CONCLUSIONS

Overexpression of PKCI increased radiation sensitivity and repressed c-fos transcription in LM cells but not in AT cells, and this is related with Ras. These results suggest that the effect of PKCI on c-fos transcription activity is related with Ras dependent signal transduction pathways and these mechanisms are different between normal fibroblasts, LM and ATM mutated, AT cells. The data obtained by this study provided evidence for novel transcriptional difference between LM and AT cells and this may be a reason for increased radiation sensitivity of AT cells.

Oxidative stress and cell cycle checkpoint function

Oxidative stress and the damage that results from it have been implicated in a wide number of disease processes including atherosclerosis, autoimmune disorders, neuronal degeneration, and cancer. Reactive oxygen species (ROS) are ubiquitous and occur naturally in all aerobic species, coming from both exogenous and endogenous sources. ROS are quite reactive and readily damage biological molecules, including DNA. While the damaging effects of ROS on DNA have been intensively studied, the effects of oxidative damage on cell cycle checkpoint function have not. Here will we review several biologically important ROS and their sources, the cell cycle, checkpoints, and current knowledge about the effects of ROS on initiating checkpoint responses.

Splicing defects in the ataxia-telangiectasia gene, ATM: underlying mutations and consequences

Mutations resulting in defective splicing constitute a significant proportion (30/62 [48%]) of a new series of mutations in the ATM gene in patients with ataxia-telangiectasia (AT) that were detected by the protein-truncation assay followed by sequence analysis of genomic DNA. Fewer than half of the splicing mutations involved the canonical AG splice-acceptor site or GT splice-donor site. A higher percentage of mutations occurred at less stringently conserved sites, including silent mutations of the last nucleotide of exons, mutations in nucleotides other than the conserved AG and GT in the consensus splice sites, and creation of splice-acceptor or splice-donor sites in either introns or exons. These splicing mutations led to a variety of consequences, including exon skipping and, to a lesser degree, intron retention, activation of cryptic splice sites, or creation of new splice sites. In addition, 5 of 12 nonsense mutations and 1 missense mutation were associated with deletion in the cDNA of the exons in which the mutations occurred. No ATM protein was detected by western blotting in any AT cell line in which splicing mutations were identified. Several cases of exon skipping in both normal controls and patients for whom no underlying defect could be found in genomic DNA were also observed, suggesting caution in the interpretation of exon deletions observed in ATM cDNA when there is no accompanying identification of genomic mutations.

Bleomycin sensitivity test in the exposed and reference human populations

Sensitivity to bleomycin was investigated in lymphocytes collected from three groups of males: 30 occupationally exposed cokery workers, 38 environmentally exposed Silesian citizen and 35 rural inhabitants. The data were analyzed at both the individual and group levels. The first analysis has revealed a substantial interindividual variability in the level of generated breaks (breaks per cell, b/c). This variability was independent of the age of the donor, smoking habit and X-ray exposure as tested in the multiple regression model. The means per group for the occupationally and environmentally exposed persons were almost the same with the values of 0.674 and 0.639, respectively. These two groups differed significantly from the rural population (b/c=0.448, p<0.001 by MANOVA). The reproducibility of the assay was satisfying (p>0.49 by the Wilcoxon matched paired test) after omitting 7 out of 49 repeatedly sampled donors. Those persons exhibited extremely high b/c rates in the first sampling.

Genetics of susceptibility to radiation-induced apoptosis in colon: two loci on chromosomes 9 and 16

Apoptosis, a mechanism for removal of genetically damaged cells and for maintenance of desired size of cell populations, has been implicated in tumor development. Previously, we defined polymorphic loci for susceptibility to apoptosis of thymocytes Rapop1, Rapop2, and Rapop3 on mouse Chromosomes 16, 9, and 3, respectively, using recombinant congenic CcS/Dem strains, each of which contains a random set of 12.5% STS/A genome in the genetic background of BALB/cHeA. The STS/A alleles at these loci confer lower susceptibility to radiation-induced apoptosis of thymocytes than the BALB/cHeA. In the present study, we tested susceptibility of colon crypt cells to radiation-induced apoptosis. In contrast to apoptosis in thymus, the STS/A mice were more susceptible to apoptosis in colon than the BALB/cHeA. Among the CcS/Dem strains, CcS-4, CcS-7, and CcS-16 were more susceptible to apoptosis in colon than the BALB/cHeA; in thymus, the CcS-7 mice are less susceptible, and the CcS-4 and CcS-16 are not different from the BALB/cHeA. Thus, individual CcS/Dem strains showed different apoptosis susceptibility in the two organs. Analysis of (CcS-7 x BALB/cHeA)F2 hybrids revealed linkage of susceptibility to radiation-induced apoptosis of colon crypt cells to two loci on Chrs 9 and 16, to which Rapop2 and Rapop1 are mapped. The STS/A allele at the locus on chromosome 9 results in high susceptibility to apoptosis of colon crypt cells in mice homozygous for the BALB/cHeA allele at the locus on Chr 16. Although these two loci may be identical to Rapop1 and Rapop2, they affect apoptosis in colon in a way different from that in thymus.

Cosmids and transcribed sequences from chromosome 11q23

To obtain cosmid markers and transcribed sequences from a specific chromosome region, a series of radiation-reduced hybrids (RHs) containing various regions of human chromosome 11 was prepared from microcell hybrid A9 (neo11) cells containing a normal human chromosome 11 tagged with pSV2neo at 11p11.2. Among 15 radiation hybrid clones isolated, RH(11)-9 which contains a q23 fragment in addition to the neo integration site, was used for the construction of a cosmid library. Cosmid clones having human DNA sequences were screened, and localized by Southern hybridization with the radiation hybrid panel. Fifty-nine cosmids were assigned to 11q23 and 6 cosmids to 11p11.2. Exon amplification proceeded with 23 of the 59 cosmids and 16 putative exons were cloned. Three of them were identical to those constituting a known gene which locates on q23 (ATDC), and the others were unknown. Thus, the RHs containing various subchromosomal fragments of chromosome 11 were useful for constructing region-specific DNA markers. The RH(11)-9 cells and putative exons also facilitate the positional cloning of genes in the 11q23 region.

A novel type of X-ray-sensitive Chinese hamster cell mutant with radioresistant DNA synthesis and hampered DNA double-strand break repair

It has been shown that the Chinese hamster cell mutant V-C8 is sensitive to different DNA damaging agents, such as mitomycin C (MMC), alkylating agents, UV light, and X-rays. We found that V-C8 is also sensitive to the following radiomimetic agents: bleomycin (approximately 2-fold, based on D10 values), H2O2 (approximately 2-fold), streptonigrin (approximately 11-fold), and etoposide (approximately 8-fold). Two independent spontaneous MMC-resistant revertants isolated from V-C8 cells show a level of cell killing by X-rays, EMS, and UV light which is similar to that of wild-type cells, suggesting that the observed pattern of cross-sensitivity of V-C8 cells to a wide spectrum of DNA damaging agents results from a single mutation. V-C8 cells also display radioresistant DNA synthesis following gamma-irradiation which, however, remained almost unchanged in the V-C8 revertants. The measurement of the level and rate of repair of DNA single- and double-strand breaks (SSBs and DSBs, respectively) by the DNA elution technique showed that the V-C8 mutant has a slower repair of DSBs induced by gamma-rays. The described unique phenotype of V-C8 cells suggested that V-C8 represents a novel type of mutant amongst X-ray-sensitive hamster cell mutants. To confirm this, complementation analysis with other X-ray-sensitive mutants was performed. V-C8 cells were fused with EM9, XR-1, xrs5, sxi-1, V-3, V-E5, irs3, and BLM2 mutant cells, representing different complementation groups. All the obtained hybrids regained X-ray resistance (or bleomycin resistance in the case of V-C8/BLM2 hybrids) similar to that of wild-type cells, indicating that V-C8 represents a new complementation group. The results presented indicate that V-C8 is defective in a gene involved in a pathway operating in the responses to different DNA damaging agents in mammalian cells.

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.

Risk factors and genetic susceptibility

Tobacco use is such a major determinant of head and neck cancer risk that classical epidemiologic techniques were more than adequate to document and characterize the etiologic association. However, the new emphasis in epidemiologic research is multidisciplinary, centering on the role of interindividual differences in susceptibility to carcinogenic exposures and particularly on the interaction of genetic susceptibility and environmental forces, that is, ecogenetics. For upper aerodigestive tract cancers, measurements of carcinogen metabolic activation and DNA repair capability are especially relevant. These susceptibility markers will enable the identification of high-risk population subgroups who can be targeted for the most intensive primary and secondary preventive strategies.

Human chromosome 11 complements ataxia-telangiectasia cells but does not complement the defect in AT-like Chinese hamster cell mutants

It has been shown that the X-ray-sensitive Chinese hamster V79 mutants (V-E5, V-C4 and V-G8) are similar to ataxia-telangiectasia (A-T) cells. To determine whether the AT-like rodent cell mutants are defective in the gene homologous to A-T (group A, C or D), human chromosome 11 was introduced to the V-E5 and V-G8 mutant cells by microcell-mediated chromosome transfer. Forty independent hybrid clones were obtained in which the presence of chromosome 11 was determined by in situ hybridization. The presence of the region of chromosome 11q22-23 was shown by molecular analysis using polymorphic DNA markers specific for the ATA, ATC and ATD loci. Seventeen of the obtained monochromosomal Chinese hamster hybrids contained a cytogenetically normal human chromosome 11, but only twelve hybrid cell lines were shown to contain an intact 11q22-23 region. Despite the complementation of the X-ray sensitivity by a normal chromosome 11 introduced to A-T cells (complementation group D), these twelve Chinese hamster hybrid clones showed lack of complementation of X-ray and streptonigrin hypersensitivity. The observed lack of complementation does not seem to be attributable to hypermethylation of the human chromosome 11 in the rodent cell background, since 5-azacytidine treatment had no effect on the streptonigrin hypersensitivity of the hybrid cell lines. These results indicate that the gene defective in the AT-like rodent cell mutants is not homologous to the ATA, ATC or ATD genes and that the human gene complementing the defect in the AT-like mutants seems not to be located on human chromosome 11.

Cellular characteristics of Chinese hamster cell mutants resembling ataxia telangiectasia cells

The radiosensitive Chinese hamster V79 cell mutants (V-C4, V-E5 and V-G8), isolated previously in our laboratory, have been shown to resemble human ataxia telangiectasia (A-T) cells. These hamster cell mutants were further characterized with respect to cross-sensitivity to different radiomimetic agents and to mutation induction by X-rays. The data on cell survival (D10 values) show that they are hypersensitive to adriamycin (2-3-fold increase), etoposide (3-fold for V-G8 and 6-fold for V-E5 and V-C4), calicheamicin gamma 1I (4-fold) and streptonigrin (3-fold for V-G8 and V-C4, and 12-fold for V-E5). The frequency of X-ray-induced hprt mutations is slightly enhanced in the hamster mutant cells treated with the same dose. However, the mutants show similar mutability as parental V79 cells when considering the same survival level. The overall conclusion from these studies is that these hamster cell mutants mimic the phenotypic characteristics observed in cultured cells from A-T patients and, therefore, may be defective in the same repair pathway as their human counterparts.

Genetic susceptibility to cancer

For any given level of exposure to a carcinogen, only a proportion of exposed individuals will develop cancer. Interindividual differences in susceptibility at some stage of the carcinogenic process must be postulated. One contributing factor is variation in the activity of metabolizing enzymes responsible for conversion of procarcinogens to proximate carcinogens. There is also a wide spectrum of DNA repair capability within the general population. At one end are the genetic instability syndromes characterized by extreme sensitivity to carcinogenic exposures and high rates of cancer in homozygotes of these traits. Less extreme differences in mutagen sensitivity can be demonstrated by a quantitative assay of chromosome breaks induced by in vitro mutagen exposure. Two case-control studies of patients with previously untreated upper aerodigestive tract cancers have demonstrated mutagen sensitivity to be an independent risk factor for the disease after controlling for the effects of tobacco and alcohol. Mutagen sensitivity also may have prognostic relevance. There was a fourfold elevated risk of developing multiple primary cancers in mutagen-sensitive patients. There are also data suggestive of familial aggregation of cancer in first-degree relatives of mutagen-sensitive patients (twofold risk for having one first-degree relative with cancer and sixfold risk for having two or more relatives with cancer). The preventive implications of identifying markers of carcinogen sensitivity are manifold.

Ataxia-telangiectasia. Review of the literature and a case report

Cutaneous vascular abnormalities are frequently detected on initial examination of newborns. Many of these lesions are common variations of normal lesions such as nevus simplex and strawberry hemangiomas. Some of the vascular abnormalities, however, are a feature of a number of syndromes with multisystemic involvement. These syndromes have been described under the heading of neurocutaneous diseases. Ataxia-telangiectasia is a neurocutaneous syndrome that appears with progressive cerebellar ataxia, oculocutaneous telangiectasias, and abnormalities of many other organs. Oral mucosa is also affected. Current concepts on the pathogenesis of ataxia-telangiectasia and one case of the disease are presented in this article.

Role of genetic factors in breast cancer susceptibility

Hereditary breast cancer is common and accounts for approximately 10-14% of all breast cancers. Knowledge of a family history of breast cancer may significantly influence diagnosis and therapy. Genetic heterogeneity has been demonstrated in familial breast cancer. Recently inherited mutations in the tumor suppressor gene p53, have been shown to be the underlying defect in the Li-Fraumeni syndrome. We have shown that defects in this gene also play a role in the predisposition to other familial breast cancers. The gene responsible for early onset familial breast and ovary cancer has recently been mapped to chromosome 17q21. For most of the sporadic breast cancers a multifactorial model, including variable genetic and environmental factors, has been considered. Two genetic risk factors which may predispose for a considerable portion of breast cancers are the gene causing ataxia telangiectasia (AT) and the gene that gives rise to proliferative breast disease (PBD). Identification of distinct genes enhancing the risk of breast cancer will give us the opportunity to identify high risk individuals. Such individuals may benefit from periodic examination affording the possibility of early diagnosis and treatment.