Functional overlap between Sgs1-Top3 and the Mms4-Mus81 endonuclease

The RecQ DNA helicases, human BLM and yeast Sgs1, form a complex with topoisomerase III (Top3) and are thought to act during DNA replication to restart forks that have paused due to DNA damage or topological stress. We have shown previously that yeast cells lacking SGS1 or TOP3 require MMS4 and MUS81 for viability. Here we show that Mms4 and Mus81 form a heterodimeric structure-specific endonuclease that cleaves branched DNA. Both subunits are required for optimal expression, substrate binding, and nuclease activity. Mms4 and Mus81 are conserved proteins related to the Rad1-Rad10 (XPF/ERCC1) endonuclease required for nucleotide excision repair (NER). However, the Mms4-Mus81 endonuclease is 25 times more active on branched duplex DNA and replication fork substrates than simple Y-forms, the preferred substrate for the NER complexes. We also present genetic data that indicate a novel role for Mms4-Mus81 in meiotic recombination. Our results suggest that stalled replication forks are substrates for Mms4-Mus81 cleavage-particularly in the absence of Sgs1 or BLM. Repair of this double-strand break (DSB) by homologous recombination may be responsible for the elevated levels of sister chromatid exchange (SCE) found in BLM(-/-) cells.

Requirement for three novel protein complexes in the absence of the Sgs1 DNA helicase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae Sgs1 protein is a member of the RecQ family of DNA helicases and is required for genome stability, but not cell viability. To identify proteins that function in the absence of Sgs1, a synthetic-lethal screen was performed. We obtained mutations in six complementation groups that we refer to as SLX genes. Most of the SLX genes encode uncharacterized open reading frames that are conserved in other species. None of these genes is required for viability and all SLX null mutations are synthetically lethal with mutations in TOP3, encoding the SGS1-interacting DNA topoisomerase. Analysis of the null mutants identified a pair of genes in each of three phenotypic classes. Mutations in MMS4 (SLX2) and SLX3 generate identical phenotypes, including weak UV and strong MMS hypersensitivity, complete loss of sporulation, and synthetic growth defects with mutations in TOP1. Mms4 and Slx3 proteins coimmunoprecipitate from cell extracts, suggesting that they function in a complex. Mutations in SLX5 and SLX8 generate hydroxyurea sensitivity, reduced sporulation efficiency, and a slow-growth phenotype characterized by heterogeneous colony morphology. The Slx5 and Slx8 proteins contain RING finger domains and coimmunoprecipitate from cell extracts. The SLX1 and SLX4 genes are required for viability in the presence of an sgs1 temperature-sensitive allele at the restrictive temperature and Slx1 and Slx4 proteins are similarly associated in cell extracts. We propose that the MMS4/SLX3, SLX5/8, and SLX1/4 gene pairs encode heterodimeric complexes and speculate that these complexes are required to resolve recombination intermediates that arise in response to DNA damage, during meiosis, and in the absence of SGS1/TOP3.

Bipartite structure of the SGS1 DNA helicase in Saccharomyces cerevisiae

SGS1 in yeast encodes a DNA helicase with homology to the human BLM and WRN proteins. This group of proteins is characterized by a highly conserved DNA helicase domain homologous to Escherichia coli RecQ and a large N-terminal domain of unknown function. To determine the role of these domains in SGS1 function, we constructed a series of truncation and helicase-defective (-hd) alleles and examined their ability to complement several sgs1 phenotypes. Certain SGS1 alleles showed distinct phenotypes: sgs1-hd failed to complement the MMS hypersensitivity and hyper-recombination phenotypes, but partially complemented the slow-growth suppression of top3 sgs1 strains and the top1 sgs1 growth defect. Unexpectedly, an allele that encodes the amino terminus alone showed essentially complete complementation of the hyper-recombination and top1 sgs1 defects. In contrast, an allele encoding the helicase domain alone was unable to complement any sgs1 phenotype. Small truncations of the N terminus resulted in hyper-recombination and slow-growth phenotypes in excess of the null allele. These hypermorphic phenotypes could be relieved by deleting more of the N terminus, or in some cases, by a point mutation in the helicase domain. Intragenic complementation experiments demonstrate that both the amino terminus and the DNA helicase are required for full SGS1 function. We conclude that the amino terminus of Sgs1 has an essential role in SGS1 function, distinct from that of the DNA helicase, with which it genetically interacts.

A hierarchy of SSB protomers in replication protein A

Replication Protein A (RPA) is a heterotrimeric single-stranded DNA-binding protein (SSB) found in all eukaryotic cells. RPA is known to be required for many of the same reactions catalyzed by the homotetrameric SSB of bacteria, but its origin, subunit functions, and mechanism of binding remain a mystery. Here we show that the three subunits of yeast RPA contain a total of four domains with weak sequence similarity to the Escherichia coli SSB protomer. We refer to these four regions as potential ssDNA-binding domains (SBDs). The p69 subunit, which is known to bind ssDNA on its own, contains two SBDs that together confer stable binding to ssDNA. The p36 and p13 subunits each contain a single SBD that does not bind stably, but corresponds to the minimal region required for viability in yeast. Photocross-linking of recombinant protein to ssDNA indicates that an SBD consists of approximately 120 amino acids with two centrally located aromatic residues. Mutation of these aromatic residues inactivates ssDNA binding and is a lethal event in three of the four domains. Finally, we present evidence that the p36 subunit binds ssDNA, as part of the RPA complex, in a salt-dependent reaction similar to the wrapping of ssDNA about E. coli SSB. The results are consistent with the notion that RPA arose by duplication of an ancestral SSB gene and that tetrameric ssDNA-binding domains and higher order binding are essential features of cellular SSBs.

Malignant fibrous histiocytoma: outcome and prognostic factors following conservation surgery and radiotherapy

PURPOSE

Malignant fibrous histiocytoma is the most common type of soft tissue sarcoma. This communication presents an analysis of outcome and prognostic factors based on a retrospective review of patients with this disease treated by conservation surgery and radiotherapy.

METHODS AND MATERIALS

From 1966 to 1991, 271 consecutive patients with malignant fibrous histiocytoma were treated with conservation surgery and radiotherapy. The outcome with local control, metastatic relapse, and survival as end points was evaluated by univariate and multivariate statistics to delineate independently significant prognostic factors.

RESULTS

Postoperative radiation at a mean dose of 62.8 Gy was used in 195 patients and preoperative radiation at a mean dose of 50 Gy was used in 76 patients. At a median follow-up of 7.3 years, 123 patients (45%) developed disease relapse at some site. Fifty-seven (21%) developed local recurrence leading to an actuarial local relapse rate of 26% at 10 years, 83 (31%) developed metastatic relapse for a 10-year actuarial metastatic rate of 33%, and the 5-, 10-, and 15-year survival rates were 68, 60, and 46%, respectively. For local control, prior local recurrence (in 53 patients) was identified as an adverse factor, yielding a 10-year recurrence rate of 42% compared to 22% for 218 patients without prior disease (p < 0.01). Also, a positive surgical margin (in 46 patients) was adverse with a 10-year local recurrence rate of 39% compared to a recurrence rate of 17% with negative margins (167) (p=0.01). Patients with pathologically undocumented resection margins (58) had a local recurrence rate similar to those with positive margins (41% at 10 years). Tumor site (extremity vs. nonextremity), location (proximal vs. distal), size (< or = 5 cm vs. > 5 cm), and histology (myxoid vs. nonmyxoid) were not significant determinants of local outcome. For metastatic relapse, the major determinants of outcome were histology (myxoid vs. nonmyxoid) and tumor size. Myxoid tumors (59 patients) had a low metastatic propensity (13% 10-year metastatic rate) compared to nonmyxoid tumors (212 patients) (40% 10-year metastatic rate) (p < 0.01). Size was an important covariate for metastases for both myxoid and nonmyxoid tumors. For nonmyxoid tumors the 10-year metastatic rates were 23 and 51% for lesions less than or greater than 5 cm. For myxoid tumors a significant metastatic rate appeared only for tumors exceeding 10 cm (10-year metastatic rates of 8% vs. 44% for tumors less than vs. greater than 10 cm). In this retrospective review we found no evidence that adjuvant chemotherapy decreased the metastatic rate. In multivariate analysis for metastatic relapse and survival, tumor histology (nonmyxoid vs. myxoid) and size (< 5 cm vs. > 5 cm) were the only independent determinants of outcome.

CONCLUSION

Malignant fibrous histiocytoma is a heterogeneous disease and its myxoid variant must be recognized as a distinct entity. Both variants are locally aggressive and require equally aggressive local therapy. Conservation surgery striving for negative margins with radiation therapy provides acceptable local control and is the treatment of choice for this disease. Patients with myxoid tumors do not require systemic therapy; patients with nonmyxoid disease exceeding 5 cm are at significant risk for metastases and the development of effective adjuvant treatment is an important research tool.

Establishing parasegments in Drosophila embryos: roles of the odd-skipped and naked genes

During Drosophila embryogenesis, a genetic cascade establishes repeating developmental units, the parasegments, along the anterior-posterior axis. Anterior and posterior boundaries of parasegments are defined by narrow stripes of cells expressing the segment polarity genes engrailed and wingless, respectively. Through single and double mutant analysis, we describe genetic interactions regulating the precise activation of engrailed and wingless in alternate parasegments. The pair-rule gene odd-skipped and the segment polarity gene naked are both required to restrict engrailed expression. odd-skipped represses expression of fushi tarazu, a known activator of engrailed. naked prevents activation of engrailed by fushi tarazu, without affecting fushi tarazu expression. engrailed expression is thus limited to narrow stripes of cells at the anterior boundaries of these parasegments. wingless expression is regulated by both odd-skipped and the pair-rule gene paired. odd-skipped represses wingless expression, while paired restricts the domain of expression of odd-skipped. wingless expression is thus allowed in narrow stripes of cells at the posterior boundaries of these parasegments. Accurate expression of engrailed and wingless is also required for cells within each parasegment to assume their proper positional identity. In odd-skipped mutants, the positional identities of particular cells are changed, creating mirror-image duplications of the body pattern. We present a model describing how the altered expression patterns of fushi tarazu, engrailed, and wingless generate the mutant phenotype.

Synovial sarcoma outcome following conservation surgery and radiotherapy

This retrospective review of 85 patients presents the prognostic factors and disease outcome for localized synovial sarcoma treated with conservation surgery and radiotherapy. Tumors were located in the lower extremity (48), upper extremity (20), trunk (11), and head and neck (6). All patients were treated with limited excision and radiotherapy. Sixty-seven patients had surgery followed by radiation and 18 had preoperative radiation. Postoperative radiation was delivered to a mean dose of 62 Gy, whereas the mean dose for preoperative radiation was 50 Gy. Thirty-five patients, mostly with tumors exceeding 5 cm, received adjuvant adriamycin-based chemotherapy (median adriamycin dose 406 mg/m2). At a median follow-up of 8.4 years the 5-, 10-, and 15-year survival rates were 76%, 63%, and 57%. Mortality was almost entirely due to metastatic relapse. Metastases developed in 36 patients yielding an actuarial 10-year metastatic rate of 48%. In multivariate analysis, tumor size was the dominant determinant of metastases with a lesser contribution due to patient age (those < or = 20 years having fewer metastases). Tumor site and histology were not independently significant. Ten-year metastatic rates according to tumor size, s, were s < or = 2 cm, 0%; 2 < s < or = 5 cm, 35%; 5 < s < or = 10 cm, 59%; 10 cm < s, 100%. Our retrospective data failed to reveal benefits for adjuvant chemotherapy either in univariate or multivariate analysis. Eight patients (9%) had local recurrence as the initial failure and 4 others had local failure after disease appeared elsewhere, yielding a 5-year actuarial local recurrence rate of 14%.(ABSTRACT TRUNCATED AT 250 WORDS)

The Vaccine Adverse Event Reporting System (VAERS)

Immunizations against most vaccine-preventable diseases will be needed indefinitely unless the disease is eradicated. Public acceptance of immunizations may be threatened as vaccine coverage increases and disease decreases, however, due to the increase in both causally and coincidentally related vaccine adverse events. The post-marketing surveillance for such events in the USA in response to the mandatory reporting requirements of the National Childhood Injury Act of 1986. While VAERS has many methodological limitations intrinsic to such systems, it can play an important role in helping to monitor vaccine safety and maintain public confidence in immunizations.

odd-paired: a zinc finger pair-rule protein required for the timely activation of engrailed and wingless in Drosophila embryos

The pair-rule gene, odd-paired (opa), is essential for parasegmental subdivision of the Drosophila embryo. In addition to its previously defined role in the activation of wingless (wg) in odd parasegments, we find that opa is required for the timely activation of wg in the remaining parasegments and for the timely activation of engrailed (en) in all parasegments. opa encodes a zinc finger protein with fingers homologous to those of the Drosophila segment polarity gene ciD, the human glioblastoma gene GLI and the Caenorhabditis elegans sex determination gene tra-1. Previous work showed that opa activity was essential for the establishment of alternate parasegments, suggesting opa expression or activity would be spatially restricted like other pair-rule genes. Instead, opa mRNA and protein are found throughout all segment primordia. Thus, opa does not act in a spatially restricted manner to establish the position of en and wg expression. Rather, opa must cooperate with other spatially restricted proteins to achieve proper subdivision of the Drosophila embryo.

Prevention of secondary transmission of pertussis in households with early use of erythromycin

To examine the effectiveness of erythromycin therapy and prophylaxis for pertussis, 17 households with one secondary case or more were compared with 20 households without secondary cases following a community-wide pertussis outbreak in Maricopa County, Arizona, in 1988. There were no significant differences between the two household groups in age distribution of members, size, crowding, race, proportion of children aged 7 months to 18 years with three or more diphtheria and tetanus toxoids and pertussis vaccine doses, or in the age distribution, vaccination status, or medical care of patients with primary cases. However, median intervals from onset of illness in primary cases to initiation of erythromycin therapy (for cases) and prophylaxis (for contacts) were 11 and 16 days, respectively, in households without secondary spread, vs 21 and 22 days, respectively, in households with secondary spread. These results provide additional evidence that erythromycin is effective in the medical management of pertussis and should be initiated promptly to minimize secondary spread.

Family history of convulsions and use of pertussis vaccine

To evaluate the risk of neurologic events after vaccination with diphtheria-tetanus-pertussis (DTP) vaccine, we used data from the Centers for Disease Control Monitoring System for Adverse Events Following Immunization to compare the family history of convulsions in persons reporting neurologic events with that in persons reporting nonneurologic events; these events have an onset within 3 days of immunization with DTP vaccine, given either alone or with oral poliovirus vaccine. Persons reporting neurologic events were 6.4 times more likely to report a prior personal history of convulsions than those reporting nonneurologic events (95% confidence interval 4.7 to 8.8), and were 2.4 times more likely to report a history of convulsions in first-degree family members, that is, siblings or parents (95% confidence interval 1.7 to 3.4). Similar risks were noted for subgroup analyses controlling for type of event (febrile vs nonfebrile convulsion), age at immunization, source of report, number of previous doses of DTP vaccine, and day of onset. Because the Centers for Disease Control monitoring system receives reports on a nonrandom sample of all adverse events after immunization, selection bias could not be ruled out. On the basis of these data, we conclude that children with a family history of seizures are at increased risk of neurologic events, primarily febrile convulsions, after DTP vaccination. However, this increase in risk may reflect a nonspecific familial tendency for convulsions rather than a specific vaccine effect. Considering the rare occurrence of neurologic events after DTP vaccination, the generally benign outcome of febrile convulsions (which make up the majority of these neurologic events), and the possible increased risk of pertussis in the general population if the estimated 5% to 7% of persons with a first-degree family history of convulsions were exempted from pertussis vaccination, we further conclude that a history of convulsions in siblings or parents should not be a contraindication to pertussis vaccination. Special care in the prevention of postvaccination fever may be warranted in children with a family history of seizures.

Identification and characterization of genes and mutants for an N-terminal acetyltransferase from yeast

A gene from Saccharomyces cerevisiae has been mapped, cloned, sequenced and shown to encode a catalytic subunit of an N-terminal acetyltransferase. Regions of this gene, NAT1, and the chloramphenicol acetyltransferase genes of bacteria have limited but significant homology. A nat1 null mutant is viable but exhibits a variety of phenotypes, including reduced acetyltransferase activity, derepression of a silent mating type locus (HML) and failure to enter G0. All these phenotypes are identical to those of a previously characterized mutant, ard1. NAT1 and ARD1 are distinct genes that encode proteins with no obvious similarity. Concomitant overexpression of both NAT1 and ARD1 in yeast causes a 20-fold increase in acetyltransferase activity in vitro, whereas overexpression of either NAT1 or ARD1 alone does not raise activity over basal levels. A functional iso-1-cytochrome c protein, which is N-terminally acetylated in a NAT1 strain, is not acetylated in an isogenic nat1 mutant. At least 20 other yeast proteins, including histone H2B, are not N-terminally acetylated in either nat1 or ard1 mutants. These results suggest that NAT1 and ARD1 proteins function together to catalyze the N-terminal acetylation of a subset of yeast proteins.

Extremely conserved histone H4 N terminus is dispensable for growth but essential for repressing the silent mating loci in yeast

Yeast histone H4 function was probed in vivo by deleting segments of this extremely conserved 102 amino acid protein. Deletions in the hydrophobic core of H4 are lethal and block chromosomal segregation. In contrast, deletions at the hydrophilic N terminus (residues 4-28) and C terminus (residues 100-102) are viable. However, N-terminal deletion alters normal chromatin structure and lengthens the cell cycle, especially G2. Surprisingly, removal of the H4 N terminus also derepresses the silent mating type loci, HML alpha and HMRa, disrupting mating. This activation is specific since other regulated genes (GAL10, PHO5, CUP1) are repressed and induced normally in these cells. Deletions of the hydrophilic N termini of H2A or H2B do not show this effect on mating. These experiments allow us to define a unique H4 function that is not shared by other histones (H2A and H2B).

Risk factors for community- and household-acquired pertussis during a large-scale outbreak in central Wisconsin

To identify risk factors associated with community- and household-acquired pertussis, we studied 61 households (HHs) with members with culture-positive illnesses and compared their characteristics with 58 neighborhood control-HHs and 62 randomly selected control-HHs. Case-HHs were more likely than either control group to have members 12-18 y of age (P less than .01); these individuals accounted for 34% of all primary cases. A history of exposure outside the home was the most important predictor of community-acquired infection (P less than .001), with adolescents being at higher risk than other age-groups (odds ratio, 3.2; P less than .001). After known exposure to a culture-positive case in the same HH, the risk of illness was unrelated to age; lengthy delays in initiating erythromycin therapy and prophylaxis were the only factors associated with secondary spread (P less than .01). The risk of pertussis may be related more to the likelihood of exposure than to age-related increases in susceptibility, and the risk can be reduced with appropriate use of erythromycin.

Monitoring system for adverse events following immunization

The Monitoring System for Adverse Events Following Immunization (MSAEFI) has collected data from the public sector nationwide on adverse events occurring during the 4-week period following administration of vaccine. From 1979 to 1984, 6483 reports were received. Although rates of reporting have increased throughout the 6-year period, increases were larger for less serious events (209%) than for more serious events (53%). The MSAEFI data have been used to evaluate risk factors for adverse events following immunization, will continue to provide information on the safety of both current and future vaccines and may identify other factors that increase the risks of adverse events following immunization.

Control of a measles outbreak in an elementary school. Baltimore County, Md. 1975

Between April 18 and May 20, 1975, 16 cases of measles occurred in pupils in an elementary school in Baltimore County, Md., and 1 case occurred in a sibling at a junior high school. Measles was serologically confirmed in 16 of these pupils. Attack rates were determined by grade and by vaccine status. The measles attack rate was 2.1 percent for the 377 children who had been given measles vaccine at 1 year of age or later. The rates were 27.8 percent (13 times higher) for those vaccinated at less than 10 months of age and 20.0 percent (10 times higher) for those with no definite history of vaccine. The higher attack rates for children who were vaccinated only before 10 months of age supports the 1972 recommendation of the Public Health Service Advisory Committee on Immunization Practices that children vaccinated before this age need to be revaccinated with live measles virus vaccine to assure full protection. The finding that 2 of 10 children with a history of measles became ill during the outbreak suggests that such histories are not a totally reliable indicator of immunity. Containment of the outbreak was attributed to the high level of immunity in the community and prompt initiation of control measures.