BCR/ABL preleukemic fusion gene in subpopulations of hematopoietic stem and progenitor cells from human UCB.

The BCR/ABL preleukemic fusion gene (PFG) is one of the most frequent fusion genes in acute lymphoblastic leukemia (ALL) and was also detected in hematopoietic cells from umbilical cord blood (UCB) of healthy newborns. Since hematopoietic stem/progenitor cells (HSPC) are considered to be a critical cellular target for origination of leukemia, we have studied the presence of BCR/ABL PFG in expanded subpopulations of HSPC and differentiated cells from UCB of those healthy newborns, who have previously been tested positive for BCR/ABL by screening of their UCB mononuclear cells using RT-qPCR and FISH methods. We isolated cells from human UCB samples positive for BCR/ABL and negative controls. The isolated cells were sorted into 5 hematopoietic and progenitor cell subpopulations. We analyzed BCR/ABL in sorted and expanded subpopulations of UCB using FISH and RT-qPCR. We found that the number of BCR/ABL positive cells was similar in each studied subpopulation and the same as in differentiated lymphocytes. Our data showed that there is no specific subpopulation of hematopoietic and progenitor stem cells with an increased leukemogenic potential due to the presence of higher copies of BCR/ABL.

Incidence of preleukemic fusion genes in healthy subjects.

The diagnostics of leukemia relies upon multi-parametric approach involving a number of different pathology disciplines such as flow cytometry, histopathology, cytogenetics and molecular genetics [fluorescent in situ hybridization (FISH) and polymerase chain reaction (PCR)]. Childhood leukemia is often determined by the presence of specific chromosomal translocation that entails the generation of preleukemic fusion genes (PFG). In the last two decades, several studies have reported observations that PFG are present in healthy population and not necessarily result in leukemia. The first such study by Limpens and colleagues on t(14/18)/ BCL2-JH [1] and next in line [2, 3] led to many questions regarding the significance of these chromosomal translocations in leukemogenesis. However, the data on the incidence of PFG are contradictive. This review aims to highlight the molecular genetic approaches used by various studies with regard to differences in diagnostics and incidence of PFG in healthy subjects. The focus is on the incidence and prevalence of the most common PFG such as TEL-AML1, MLL-AF4, BCR-ABL (p190), AML1-ETO, PML-RARA, and CBFB-MYH11 detected in umbilical cord blood, in neonatal blood spots (Guthrie cards (GC)), bone marrow, peripheral blood and tissues of amortized fetuses. We conclude that the incidence of PFG is significantly higher than incidence of leukemia and more sophisticated analysis of PFG in leukemogenic cell populations is warranted to relate the occurrence of PFG with leukemia. The emerging notion is that only those PFG may contribute to development of leukemia which arise in stem cells at specific time windows during development. Thus, screening of PFG in subpopulations of stem cells may be a challenge for assessment of predisposition to leukemia and for validation of cell transplant to minimize donor cell-derived leukemia.

Combined multiplex and monoplex RT-PCR as a reliable and cost-effective method for molecular diagnostics of pediatric acute lymphoblastic leukemia.

The precise diagnosis of acute lymphoblastic leukemia is essential for correct prognosis assessment and therapy regimen selection. At present, immunophenotyping, cytogenetics and molecular screening are major and complementary methods utilized in a routine leukemia diagnostics. The aim of this study was to validate the application of multiplex reverse transcription-polymerase chain reaction (RT-PCR) assay for molecular diagnosis of the most common pediatric acute lymphoblastic leukemia-associated fusion transcripts. Our data show that screening of bone marrow and/or peripheral blood by RT-PCR, consisting of multiplex and monoplex PCR, confirmed results of real-time quantitative PCR (RT qPCR). This screening may provide a reliable, specific and sensitive method amenable to standard laboratory practice and a cost-effective alternative to more complex and expensive RT qPCR techniques.

Novel and recurrent germline alterations in the MLH1 and MSH2 genes identified in hereditary nonpolyposis colorectal cancer patients in Slovakia.

Hereditary non-polyposis colorectal cancer (HNPCC) is associated with germline mutations in DNA mismatch repair genes, predominantly MSH2 and MLH1. Mutation carriers develop cancers in the colorectum, endometrium, ovary, stomach, small intestine and the upper urinary tract. We describe here the results of a mutational analysis of 11 unrelated HNPCC patients by direct genomic sequencing of MLH1 and MSH2. The alterations found include 7 novel changes and 4 different pathogenic mutations described previously in Poland, Moldavia, Finland, Germany, France and USA. Four novel pathogenic mutations in the MLH1 gene include two frameshift mutations (c.1150delG and c.1210_1211delCT), one missense mutation (c.793C>A) and one intron-exon border mutation (c.546- 2A>C). The last change resulted in the skipping of exon 7, as shown by sequencing of RT-PCR products. The only novel MSH2 pathogenic change was a nonsense mutation c.1129C>T. The novel intronic change c.381-41A>G in MLH1 was found in a patient carrying a previously-described mutation in the MSH2 gene. Interestingly, two unrelated patients carried also a novel change in the promoter region of MLH1 in one of the CpG islands (c.-269C>G). However, this alteration does not abrogate transcription, as shown by RT-PCR analysis. In summary, most (approximately 80%) pathogenic germline mutations detected in the studied group of patients by direct genomic sequencing of MLH1 and MSH2 were located in the MLH1 gene. These and previous data indicate that the majority of germline point mutations and small deletions/insertions in HNPCC families in Slovakia affect the MLH1 locus.

Effect of expression of the Escherichia coli nth gene in Saccharomyces cerevisiae on the toxicity of ionizing radiation and hydrogen peroxide.

PURPOSE: To examine the contribution of endonuclease III (Nth)-repairable lesions to the cytotoxicity of ionizing radiation (IR) and hydrogen peroxide (H2O2) in the yeast Saccharomyces cerevisiae. MATERIALS AND METHODS: A selectable expression vector containing the E. coli nth gene was transformed into two different wild-type strains (7799-4B and YNN-27) as well as one rad52 mutant strain (C5-6). Nth expression was verified by Western analysis. Colony-forming assay was used to determine the sensitivity to IR and H2O2 in both stationary and exponentially growing cells. RESULTS: The pADHnth-transformed wild-type (77994B) strain was considerably more resistant than vector-only transformants to the toxic effects of IR, in both stationary and exponential growth phases, although this was not the case in another wild-type strain (YNN-27). In contrast, there were no significant effects of nth expression on the sensitivity of the wild-type cells to H2O2. Moreover, nth expression caused no effects on the H2O2 sensitivity in the rad52 mutant cells, but it led to a slight increase in sensitivity in these cells following IR, particularly at the highest dose levels used. CONCLUSIONS: Whilst other damage-processing systems may play a role, DNA lesions that are substrates for Nth can also make a contribution to the toxic effects of IR in certain wild-type yeast. Hence, DNA double-strand breaks should not be considered the sole lethal lesions following IR exposure.

Approaches to identification of HNPCC suspected patients in Slovak population.

Patients with hereditary non-polyposis colorectal cancer (HNPCC) have a DNA mismatch repair defect (MMR) in their tumor tissue that results in instability of microsatellite DNA sequences (MSI). Thus, MSI analysis may effectively indicate this form of cancer that should be then proved by analysis of germline mutations in MMR genes. The aim of this study was to identify HNPCC suspected patients in the Slovak population by investigating microsatellite instability in colorectal tumor tissues. MSI was studied at 5-11 loci in matched tumor and normal DNA using radioactively labeled PCR products separated on sequencing gels. High microsatellite instability (MSI-H) was present only in patients younger than 50 years, in 100% of patients having two affected relatives by colorectal cancer and in 67% of patients with only one affected relative. In both groups of patients colorectal cancer was present in two successive generations. No MSI-H was found in the group of patients older than 50 years, even if they had positive family history for colorectal cancer. Among all markers used, the BAT26 mononucleotide repeat (100%), DI0S197 and D13S175 (62.5%) dinucleotide repeats were the most frequently altered in the tumor tissues. Retrospective analysis revealed that some of the patients having MSI-H tumors have had clinicopathological characteristics frequently reported to HNPCC. The family members of those patients with MSI-H are enrolled in preventive health care program until mutational analyses will enable to select carriers from non-carriers of mutated MMR genes.

Differential incision of bulky carcinogen-DNA adducts by the UvrABC nuclease: comparison of incision rates and the interactions of Uvr subunits with lesions of different structures.

The UvrABC nuclease system from Escherichia coli removes DNA damages induced by a wide range of chemical carcinogens with variable efficiencies. The interactions with UvrABC proteins of the following three lesions site-specifically positioned in DNA, and of known conformations, were investigated: (i) adducts derived from the binding of the (-)-(7S,8R,9R,10S) enantiomer of 7,8-dihydroxy-9, 10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(-)-anti-BPDE] by cis-covalent addition to N(2)-2'-deoxyguanosine [(-)-cis-anti-BP-N(2)-dG], (ii) an adduct derived from the binding of the (+)-(1R,2S,3S,4R) enantiomer of 1,2-dihydroxy-3,4-epoxy-1,2,3, 4-tetrahydro-5-methylchrysene [(+)-anti-5-MeCDE] by trans addition to N(2)-2'-deoxyguanosine [(+)-trans-anti-MC-N(2)-dG], and (iii) a C8-2'-deoxyguanosine adduct (C8-AP-dG) formed by reductively activated 1-nitropyrene (1-NP). The influence of these three different adducts on UvrA binding affinities, formation of UvrB-DNA complexes by quantitative gel mobility shift analyses, and the rates of UvrABC incision were investigated. The binding affinities of UvrA varied among the three adducts. UvrA bound to the DNA adduct (+)-trans-anti-MC-N(2)-dG with the highest affinity (K(d) = 17 +/- 2 nM) and to the DNA containing C8-AP-dG with the least affinity (K(d) = 28 +/- 1 nM). The extent of complex formation with UvrB was also the lowest with the C8-AP-dG adduct. 5' Incisions occurred at the eighth phosphate from the modified guanine. The major 3' incision site corresponded to the fifth phosphodiester bond for all three adducts. However, additional 3' incisions were observed at the fourth and sixth phosphates in the case of the C8-AP-dG adduct, whereas in the case of the (-)-cis-anti-BP-N(2)-dG and (+)-trans-anti-MC-N(2)-dG lesions additional 3' cleavage occurred at the sixth and seventh phosphodiester bonds. Both the initial rate and the extent of 5' and 3' incisions revealed that C8-AP-dG was repaired less efficiently in comparison to the (-)-cis-anti-BP-N(2)-dG and (+)-trans-anti-MC-N(2)-dG containing DNA adducts. Our study showed that UvrA recognizes conformational changes induced by structurally different lesions and that in certain cases the binding affinities of UvrA and UvrB can be correlated with the incision rates. The size of the bubble formed around the damaged site with mismatched bases also appears to influence the incision rates. A particularly noteworthy finding in this study is that UvrABC repair of a substrate with no base opposite C8-AP-dG was quite inefficient as compared to the same adduct with a C opposite it. These findings are discussed in terms of the available NMR solution structures.

The nucleotide excision repair protein UvrB, a helicase-like enzyme with a catch.

Nucleotide excision repair (NER) is a universal DNA repair mechanism found in all three kingdoms of life. Its ability to repair a broad range of DNA lesions sets NER apart from other repair mechanisms. NER systems recognize the damaged DNA strand and cleave it 3', then 5' to the lesion. After the oligonucleotide containing the lesion is removed, repair synthesis fills the resulting gap. UvrB is the central component of bacterial NER. It is directly involved in distinguishing damaged from undamaged DNA and guides the DNA from recognition to repair synthesis. Recently solved structures of UvrB from different organisms represent the first high-resolution view into bacterial NER. The structures provide detailed insight into the domain architecture of UvrB and, through comparison, suggest possible domain movements. The structure of UvrB consists of five domains. Domains 1a and 3 bind ATP at the inter-domain interface and share high structural similarity to helicases of superfamilies I and II. Not related to helicase structures, domains 2 and 4 are involved in interactions with either UvrA or UvrC, whereas domain 1b was implicated for DNA binding. The structures indicate that ATP binding and hydrolysis is associated with domain motions. UvrB's ATPase activity, however, is not coupled to the separation of long DNA duplexes as in helicases, but rather leads to the formation of the preincision complex with the damaged DNA substrate. The location of conserved residues and structural comparisons with helicase-DNA structures suggest how UvrB might bind to DNA. A model of the UvrB-DNA interaction in which a beta-hairpin of UvrB inserts between the DNA double strand has been proposed recently. This padlock model is developed further to suggest two distinct consequences of domain motion: in the UvrA(2)B-DNA complex, domain motions lead to translocation along the DNA, whereas in the tight UvrB-DNA pre-incision complex, they lead to distortion of the 3' incision site.

Crystal structure of UvrB, a DNA helicase adapted for nucleotide excision repair.

Nucleotide excision repair (NER) is a highly conserved DNA repair mechanism. NER systems recognize the damaged DNA strand, cleave it on both sides of the lesion, remove and newly synthesize the fragment. UvrB is a central component of the bacterial NER system participating in damage recognition, strand excision and repair synthesis. We have solved the crystal structure of UvrB in the apo and the ATP-bound forms. UvrB contains two domains related in structure to helicases, and two additional domains unique to repair proteins. The structure contains all elements of an intact helicase, and is evidence that UvrB utilizes ATP hydrolysis to move along the DNA to probe for damage. The location of conserved residues and structural comparisons allow us to predict the path of the DNA and suggest that the tight pre-incision complex of UvrB and the damaged DNA is formed by insertion of a flexible beta-hairpin between the two DNA strands.

Thermostable archaeal O6-alkylguanine-DNA alkyltransferases.

Archaea represent some of the most ancient organisms on earth, and they have relatively uncharacterized DNA repair processes. We now show, using an in vitro assay, that extracts of two Crenarchaeota (Sulfolobus acidocaldarius and Pyrobaculum islandicum) and two Euryarchaeota (Pyrococcus furiosus and Thermococcus litoralis) contain the DNA repair protein O6-alkylguanine-DNA alkyltransferase (ATase). The ATase activities found in the archaea were extremely thermostable, with half-lives at 80 degreesC ranging from 0.5 hr (S. acidocaldarius) to 13 hr (T. litoralis). The temperature optima of the four proteins ranged from approximately 75 to approximately 100 degreesC, although activity was seen at 37 degreesC, the temperature optimum of the Escherichia coli and human ATases. In all cases, preincubaton of extracts with a short oligonucleotide containing a single O6-methylguanine residue caused essentially complete loss of ATase activity, suggesting that the alkylphosphotriester-DNA alkyltransferase activity seen in some prokaryotes is not present in Archaea. The ATase from Pyrobaculum islandicum had an apparent molecular mass of 15 kDa, making it the smallest of these proteins so far described. In higher organisms, ATase is responsible for the repair of toxic and mutagenic O6-alkylguanine lesions in alkylated DNA. The presence of ATase in these primitive organisms therefore suggests that endogenous or exogenous exposure to agents that generate appropriate substrates in DNA may be an early event in evolution.

Expression of the E.coli ada gene in S.cerevisiae provides cellular resistance to N-methyl-N'-nitro-N-nitrosoguanidine in rad6 but not in rad52 mutants.

The Escherichia coli ada gene protein coding region under the control of the yeast alcohol dehydrogenase promoter in the extrachromosomally replicating yeast expression vectors pADHO6C and pVT103LO6C was introduced into the wild-type yeast strains, YNN-27 and FF-18733, and the repair deficient mutants LN-1 (rad1-1), VV-5 (rad6-1), C5-6 (rad52-1) and FF-18742 (rad52::URA3). This resulted in the expression of 3950, 1900, 1870, 1620, 1320 and 1420 fmol ada-encoded ATase/mg protein respectively: transformation with the parent vectors resulted in ATase activities of 3-17 fmol/mg protein. The wild-types, rad1-1 and rad6-1 yeast expressing the bacterial ATase showed increased resistance to the toxic and mutagenic effects of N-methyl-N'-nitro-N- nitrosoguanidine (MNNG). Expression of ATase in the rad52-1 and rad52::URA3 mutants neither complemented their sensitivity, nor reduced the mutagenic effects of this agent. These results suggest that whilst a portion of the toxic and mutagenic lesions induced by MNNG can be repaired in yeast by the E.coli Ada protein in a RAD1- and RAD6-independent manner, the RAD52 gene product may be essential for the complete functioning of the Ada ATase. This is the first suggestion of a possible cofactor requirement for ATase.

Site-directed mutagenesis of glutamic acid 172 to glutamine completely inactivated human O6-alkylguanine-DNA-alkyltransferase.

DNA repair by O6-alkylguanine-DNA-alkyltransferase involves the stoichiometric transfer of the O6-alkyl group from the guanine lesion to the active-site cysteine residues of the protein. Site-directed mutagenesis of glutamic acid 172 of human O6-alkylguanine-DNA-alkyltransferase (EC 2.1.1.63) to glutamine totally abolished the alkyltransferase activity of the protein. This suggests that glutamic acid 172 is crucial to the alkyl transfer. It may act as a general acid (as CO2H) or base (as CO2-), or have a role as a component of a salt-link (-CO2-.....+N-), vital for the structural integrity of the active site. This is the first mutational inactivation of a protein in this family of DNA repair molecules by means of a residue change outside the highly conserved pentet (PCHRV) which includes the active-site cysteine.

c-Ha-ras BamHI RFLP in human urothelial tumors and point mutations in hot codons.

High-molecular-weight DNAs from 30 bladder and renal cell carcinomas (RCC) were isolated and the c-Ha-ras gene BamHI RFLP was examined. Amplification of c-Ha-ras with normal localization with regard to the size of alleles was found only in one case. One of the normally localized c-Ha-ras allele termed RCC c-Ha-ras of a length of about 6.6 kbp was cloned and an oncogene-activating point mutation was identified using two restriction enzymes. After comparison of CfrI and Cfr10I cleavage maps of RCC c-Ha-ras to complete nucleotide sequences of EJ/T24 c-Ha-ras oncogene and its normal counterpart, a point mutation was identified within codon 11 or 12. The use of CfrI and Cfr10I is of value for clinical practice in identification of point mutations in c-Ha-ras PCR product in neoplasia accompanied by somatic mutation of c-Ha-ras. The correlation among c-Ha-ras allele, amplification/loss, presence of point mutation and progression of neoplasia is discussed.

Transfection of the Escherichia coli nth gene into radiosensitive Chinese hamster cells: effects on sensitivity to radiation, hydrogen peroxide, and bleomycin sulfate.

The Escherichia coli nth gene encodes endonuclease III, which catalyses the glycolytic removal of various oxidized thymine residues from DNA. A truncated version of nth, with the prokaryotic regulatory sequences removed, was ligated into the retrovirus-based vector pZipneoSV(X)1 and transfected into the radiosensitive Chinese hamster ovary cell line, xrs7. Following selection with G418, two clones (x7nth1 and x7nth6) were shown by Southern analysis to contain the nth gene. No substantial difference in gamma-ray sensitivity was detected between xrs7, clones x7nth1 and x7nth6, and the parent vector transfected clone (x7neo1). However, clones containing the nth gene were more resistant to hydrogen peroxide cytotoxicity [D0's for x7nth1 and x7nth6 were 0.072 microgram/ml (4 microM) and 0.046 microgram/ml, respectively, compared with D0's of 0.034 and 0.027 microgram/ml for xrs7 and x7neo1, respectively] but markedly more sensitive to bleomycin sulfate cytotoxicity than xrs7 and x7neo1 (e.g., 1D0's for x7nth6 and xrs7 were 0.05 and 0.12 microgram/ml, while 2D0's for x7nth1 and xrs7 were 0.35 and 0.48 microgram/ml, respectively). Alterations in sensitivity to hydrogen peroxide and bleomycin sulfate could not be explained by differences in the distribution of the cell-cycle phases and growth rate of nth-containing clones and control cell lines. These results are consistent with the hypothesis that modified thymine lesions are potentially cytotoxic. Hence, when cells incur a high level of endonuclease III-repairable damage relative to strand breakage, such as after treatment with hydrogen peroxide, increased repair capacity increases survival. Gamma radiation produces a lower level of endonuclease III-repairable damage relative to all the other types of lesions produced; hence increased repair capacity has no measurable effect on cell survival. The increased sensitivity of x7nth1 and x7nth6 to bleomycin sulfate toxicity may indicate that, when thymine damage and single-strand breaks are in close proximity on opposite strands of the DNA, endonuclease III, which incises DNA at the site of damaged residues, can increase the number of double-strand breaks and hence decrease the level of cell survival.

A recA-ada hybrid gene inducible by DNA damage.

A damage-inducible expression vector was constructed in which the original recA structural gene was replaced by the protein-coding region of the ada gene. The O6-alkylguanine-DNA alkyltransferase encoded by the ada gene can be measured by a rapid and highly sensitive assay. The introduction of this construct into an appropriate host cell provides an effective bacterial assay for genotoxins.

Expression of the E.coli ada gene in yeast protects against the toxic and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine.

The E.coli ada gene protein coding region has been ligated into an extrachromosomally replicating yeast expression vector downstream of the yeast alcohol dehydrogenase gene promoter region to produce pADH06C. The yeast strains SX46A, 7799-4B and VV-6 are deficient in endogenous O6-alkylguanine-DNA-alkyltransferase and transformation of these strains with this shuttle vector resulted in the expression of 1730, 1260 and 374 fmoles ada-encoded ATase/mg protein in stationary phase yeast: transformation with the parent vector had no effect on endogenous ATase activity which remained less than 2 fm/mg. In comparison with parent vector transformed yeast, all of the pADH06C-transformed strains showed an increase in the resistance to the toxic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG). In addition, 7799-4B and VV-6 were more resistant to the mutagenic effects of this agent. These results indicate that the toxic and mutagenic effects of MNNG in yeast are mediated, at least in part, by DNA lesions than can be repaired by the E.coli ada gene product.

Repression of damage-inducible (din) genes by the lexA3 mutation or by plasmid carrying the lexA gene; effect on pyrimidine dimer excision in UV-irradiated Escherichia coli.

Dimer excision was followed in Escherichia coli K-12 AB1157 DM49 lexA3 mutant (whose repressor is not cleavable with RecA protease), and in E. coli K-12 AB2497[pGC3] carrying the cloned lexA gene. In either case din genes could not be efficiently derepressed. In such cells ultraviolet (UV) irradiation caused an extensive DNA degradation, which was not observed in cells with derepressed din genes. Even after a high UV dose (70 J/m2) dimers were being excised efficiently. However, progressive DNA degradation interfered with the precise detection of unexcised dimers. We conclude that induction of din genes is required for filling some of the gaps and for prevention of DNA degradation, but not for excision itself.