PubMed Health⌕ Search

Biomedical subjects

S Koh

Publications and source records attributed to S Koh.

At least 37 records · Page 2Linked to original sources
PubMed1999

Hepatitis B virus variants with lamivudine-related mutations in the DNA polymerase and the 'a' epitope of the surface antigen are sensitive to ganciclovir.

Lamivudine is a new antiviral agent effective against hepatitis B viral (HBV) infections but can result in virus-drug resistance associated with mutations in the conserved 'YM552DD' motif of the HBV DNA polymerase. Due to their overlapping coding regions in the HBV genome, mutations in the DNA polymerase may result in substitutions in the hepatitis B surface antigen (HBsAg), albeit outside the antigenic 'a' epitope. Here we report the identification of a novel type of lamivudine-related mutations located in both the polymerase (YM552DD-->Y1552DD) and the 'a' epitope of HBsAg (Gly130-->Asp130). The same virus carried a HBsAg Gly145-->Arg145 mutation prior to therapy. Both the wild type HBV and lamivudine-related mutants with the Gly145-->Arg145 HBsAg mutation were suppressed following ganciclovir treatment, indicating a beneficial additive effect of both drugs against different forms of HBV mutants.

Adult↗
PubMed1999

Anterior lumbosacral radiculopathy after intrathecal methotrexate treatment.

Intrathecal chemotherapy with methotrexate or cytosine arabinoside is the standard approach to prophylaxis and treatment of central nervous system leukemia in children. Progressive paraplegia, one of the devastating neurologic complications related to this mode of treatment, has been attributed to spinal cord toxicity. Reported are three children who developed progressive paraparesis after intrathecal methotrexate administration followed by complete or partial recovery. Gadolinium enhancement of anterior lumbosacral spinal nerve roots was demonstrated in all three patients, and an elevation of cerebrospinal fluid immunoglobulin G synthesis was evident in two patients with more severe symptoms. The clinical data suggest that anterior lumbosacral radiculopathy is also a type of neurologic complication associated with intrathecal methotrexate treatment.

Antimetabolites, Antineoplastic↗
PubMed1999

Ac as a tool for the functional genomics of rice.

To examine whether the maize autonomous transposable element Ac can be used for the functional analysis of the rice genome, we used Southern blot analysis to analyze the behaviour of Ac in 559 rice plants of four transgenic families through three successive generations. All families showed highly active transposition of Ac, and 103 plants (18.4%) contained newly transposed Ac insertions. In nine of the 12 independent transpositions analyzed, their germinal transmission was detected. Partial sequencing of 99 Ac-flanking sequences revealed that 21 clones exhibited significant similarities with protein-coding genes in databases and four of them matched rice cDNA sequences. These results indicate preferential Ac transposition into protein-coding rice genes. To examine the feasibility of PCR-based screening of gene knockouts in rice Ac plants, we prepared bulked genomic DNA from the leaves of approximately 6000 rice Ac plants and pooled the DNA according to a three-dimensional matrix. Of 14 randomly selected genes, two gene knockouts were identified, and one encoding a rice cytochrome P450 (CYP86) gene was shown to be stably inherited to the progeny. Together, these results suggest that Ac can be efficiently used for the functional analysis of the rice genome.

Amino Acid Sequence↗
PubMed1999

The localizing value of ictal SPECT in children with tuberous sclerosis complex and refractory partial epilepsy.

Although multiple cortical tubers are a hallmark of tuberous sclerosis complex (TSC), seizures often originate from a single tuber, making excisional surgery a therapeutic option for intractable patients. To assess the role of ictal single photon emission computed tomography (SPECT) in defining the epileptogenic tuber, we reviewed videoelectroencephalography (V/EEG) data, magnetic resonance imaging (MRI) and SPECT scans of 15 patients (aged 3 months to 15 years, mean 5.1) with medically resistant partial seizures and TSC. SPECT scans were performed using Tc-99m-hexamethylpropyleneamine oxime (HMPAO) injected within 30 seconds of electrographic seizure onset and were graded on a scale from 1 to 5 (1 weakly perfused, 5 strongly perfused). The scalp EEG revealed localized seizure origin in ten patients; five had concordant hyperperfused SPECT regions consisting of comma-shaped areas surrounding hypoperfused areas in the candidate tuber. Strongly hyperperfused regions (grade 3-5) were noted in two patients whose ictal EEG patterns were characterized by sustained, rhythmic fast activity or spiking. The other five patients had nonlocalizing or poorly sustained ictal EEG patterns; one patient in this group had focal hyperperfusion. Invasive EEG recordings in two patients revealed propagated EEG patterns that correlated with the ictal SPECT findings. These findings indicate a strong correlation between ictal SPECT and ictal scalp EEG, especially when there are sustained rhythmic fast ictal EEG patterns.

Adolescent↗
PubMed1999

Biochemical characterization of a UDP-sugar pyrophosphorylase from Thermus caldophilus GK24.

An extremely thermostable UDP-GlcNAc pyrophosphorylase has been purified from Thermus caldophilus GK24 by chromatographic methods including ion-exchange, hydrophobic interaction, and affinity chromatographies. The specific activity of the enzyme was enriched 41.8-fold, with a recovery of 2%. The molecular mass of the enzyme was 41 kDa by SDS/PAGE and 45 kDa by gel-filtration chromatography. The activity was maximum at 86 degrees C and its half-life at 95 degrees C was 30 min. Its optimum pH was 6.9 in the presence of Mg2+ ions. A biochemical study showed that UDP-GlcNAc pyrophosphorylase activity could be enhanced by fructose 1-phosphate, a precursor of UDP-GlcNAc. The enzyme showed a broad substrate specificity with sugar 1-phosphates, including glucose 1-phosphate, GlcNAc-1-P and xylose 1-phosphate. The enzyme was therefore named UDP-sugar pyrophosphorylase. The N-terminal and internal peptide sequences were determined and compared with known sequences from various sources. It was found that N-terminal sequence is similar to that of UDP-GlcNAc and UDP-glucose pyrophosphorylases from other bacterial sources.

Amino Acid Sequence↗
PubMed1998

Neuronal stress and injury in C57/BL mice after systemic kainic acid administration.

Kainate-induced seizures are widely studied as a model of human temporal lobe epilepsy due to behavioral and pathological similarities. While kainate-induced neuronal injury is well characterized in rats, relatively little data is available on the use of kainate and its consequences in mice. The growing availability of genetically altered mice has focused attention on the need for well characterized mouse seizure models in which the effects of specific genetic manipulations can be examined. We therefore examined the kainate dose-response relationship and the time-course of specific histopathological changes in C57/BL mice, a commonly used founder strain for transgenic technology. Seizures were induced in male C57/BL mice (kainate 10-40 mg/kg i.p.) and animals were sacrificed at various time-points after injection. Seizures were graded using a behavioral scale developed in our laboratory. Neuronal injury was assayed by examining DNA fragmentation using in situ nick translation histochemistry. In parallel experiments, we examined the expression an inducible member of the heat shock protein family, HSP-72, another putative marker of neuronal injury, using a monoclonal antibody. Seizure severity paralleled kainate dosage. At higher doses DNA fragmentation is seen mainly in hippocampus in area CA3, and variably in CA1, thalamus and amygdala within 24 h, is maximal within 72 h, and is largely gone by 7 days after administration of kainate. HSP-72 expression is also highly selective, occurring in limbic structures, and it evolves over a characteristic time-course. HSP-72 is expressed mainly in structures that also manifest DNA fragmentation. Using double-labeling techniques, however, we find essentially no overlap between neurons expressing HSP-72 and DNA fragmentation. These findings indicate that DNA fragmentation and HSP-72 expression are complementary markers of seizure-induced stress and injury, and support the notion that HSP-72 expression is neuroprotective following kainate-induced seizures.

Animals↗
PubMed1998

Top DNA polymerase from Thermus thermophilus HB27: gene cloning, sequence determination, and physicochemical properties.

A gene, top encoding Thermus thermophilus HB27 (Top) DNA polymerase, was cloned in E. coli and its nucleotide sequence was determined. Based on its deduced amino acid sequence, Top DNA polymerase is a 93.8 kDa protein comprising 834 amino acid residues. Top DNA polymerase showed high amino acid homology with those of other DNA polymerases from the Thermus sp., for example, 87.3% identity with Taq DNA polymerase. Codon usage in the top gene was similar to those of the proteins from other Thermus strains. The G + C content in the third position of the codons was as high as 93%. The top gene under the control of the tac promoter was expressed in E. coli [plasmid pTOP9]. DNA amplification using the recombinant Top DNA polymerase performed the same as other thermostable DNA polymerases from Thermus strains. The optimum temperature for its reaction was 76 degrees C. An interesting observation was that the recombinant Top DNA polymerase was slowly cleaved into two fragments of about 60 kDa and 35 kDa at 4 degrees C and -20 degrees C. The larger fragment possessed polymerase activity like the Klenow fragment of E. coli DNA polymerase I. To prevent the cleavage of the Top DNA polymerase, a variety of protecting agents were examined. Among those examined, (NH4)2SO4 (100 mM) solution demonstrated an outstanding ability to block its cleavage for a prolonged period.

Amino Acid Sequence↗
PubMed1998

Myelopathy resulting from invasive aspergillosis.

Aspergillus, a ubiquitous mold, may cause invasive and fatal disease in immunosuppressed patients. Myelopathy is an uncommon presentation of invasive aspergillosis. This report describes three children admitted to the hospital between 1988 and 1995 who developed myelopathy as the first evidence of invasive aspergillosis. All had advanced leukemia and were profoundly immunosuppressed because of chemotherapy and broad-spectrum antibiotics. Weakness and pain presented first; then, sensation to pain and temperature was lost 2 to 6 days later, followed by complete myelopathy. Multiple brain lesions were seen on magnetic resonance imaging in one patient. Despite antifungal therapy, aspergillosis proved fatal within 1 month of onset of myelopathy in all patients. Physicians caring for immunocompromised children should be aware of myelopathy as a presentation of invasive aspergillosis.

Adolescent↗
PubMed1997

Cloning and expression of the gene for xylose isomerase from Thermus flavus AT62 in Escherichia coli.

The gene encoding xylose isomerase (xylA) was cloned from Thermus flavus AT62 and the DNA sequence was determined. The xylA gene encodes the enzyme xylose isomerase (XI or xylA) consisting of 387 amino acids (calculated Mr of 44,941). Also, there was a partial xylulose kinase gene that was 4 bp overlapped in the end of XI gene. The XI gene was stably expressed in E. coli under the control of tac promoter. XI produced in E. coli was simply purified by heat treatment at 90 degrees C for 10 min and column chromatography of DEAE-Sephacel. The Mr of the purified enzyme was estimated to be 45 kDa on SDS-polyacrylamide gel electrophoresis. However, Mr of the cloned XI was 185 kDa on native condition, indicating that the XI consists of homomeric tetramer. The enzyme has an optimum temperature at 90 degrees C. Thermostability tests revealed that half life at 85 degrees C was 2 mo and 2 h at 95 degrees C. The optimum pH is around 7.0, close to where by-product formation is minimal. The isomerization yield of the cloned XI was about 55% from glucose, indicating that the yield is higher than those of reported enzymes. The K(m) values for various sugar substrates were calculated as 106 mM for glucose. Divalent cations such as Mn2+, Co2+, and Mg2+ are required for the enzyme activity and 100 mM EDTA completely inhibited the enzyme activity.

Aldose-Ketose Isomerases↗
PubMed1997

Cerebellar mutism in children: report of six cases and potential mechanisms.

Cerebellar mutism is a rare finding associated with resection of posterior fossa tumors or cerebellar hemorrhages. We reviewed the medical records of six children, aged 6 to 12 years, who developed cerebellar mutism after resection of a posterior fossa mass or as a result of posterior fossa trauma. From 1989 to 1994, 210 children underwent posterior fossa resection at our institution, and four developed mutism (an incidence of 1.6%). All four patients had primitive neuroectodermal tumors. The fifth patient experienced trauma, and another patient had an arteriovenous malformation (AVM). In four children, hydrocephalus developed as a result of their tumor or AVM. Four developed cerebellar mutism 24 to 48 hours after surgery or trauma, and one developed cerebellar mutism 5 days after surgery, coincident with hydrocephalus. In one, mutism occurred after a second resection was performed for a recurrence of his posterior fossa tumor. Cerebellar mutism lasted 10 days in one patient and 2 to 8 weeks in the other four. Dysarthria was apparent in four patients during the recovery phase. We suggest trauma to the dentate nucleus and/or its outflow tract, the superior cerebellar peduncle, as a cause of reversible mutism. Because posterior fossa tumors are common in children, mutism should be recognized as an important side effect of surgery.

Brain Concussion↗
PubMed1997

Protein C and S deficiency in children with ischemic cerebrovascular accident.

A prothrombotic state caused by a deficiency of protein C or protein S, each an inhibitor of coagulation cascade, increases the risk of ischemic cerebrovascular accident (ICVA) in young adults. This retrospective review studies the relationship of protein C or S deficiency with childhood ICVA. From 1989 to 1995, 37 children were diagnosed by neuroimaging with ICVA unrelated to any known preexisting diagnosis. Protein C deficiency or protein S deficiency was the only identified risk factor for 5.4% (2 patients) and 13.5% (5 patients), respectively, of these 37 children. The data suggest that protein C or protein S deficiency is an important risk factor of ICVA in children.

Adolescent↗
PubMed1997

The resolution of the dative NP ambiguity in Korean.

An eye-movement study examined the ambiguity of the dative noun phrase (NP) in Korean, which may be attached either to the main verb or to a relative verb. Experimental sentences of the form NP-Nom NP-Dat NP-Nom V (dative) NP-Acc V (dative or simple transitive; Nom = nominative; Dat = dative; V = verb; Acc = accusative) were tested. Garden-path theory predicts that comprehension difficulty occurs when the main verb is transitive, forcing the dative NP to be attached to the relative verb, because the dative NP will have already been attached in the clause posited when the first nominative NP is read. In contrast, lexically based theory predicts that comprehension difficulty occurs when the main verb is a dative verb, because the dative NP will be attached to the first verb encountered (the relative verb). The present study did not fully support either theory, because the differences seen in the experimental conditions appeared, at least in part, in the control conditions. However, the results were much different from the prediction of lexically based theory. Reading was faster, and regressive eye movements fewer, in the dative-ambiguous condition than in the transitive-ambiguous condition. Some reasons for the unexpected difficulty of the transitive-unambiguous condition are advanced.

Humans↗
PubMed1997

Antisense expression of the peptide transport gene AtPTR2-B delays flowering and arrests seed development in transgenic Arabidopsis plants.

Previously, we identified a peptide transport gene, AtPTR2-B, from Arabidopsis thaliana that was constitutively expressed in all plant organs, suggesting an important physiological role in plant growth and development. To evaluate the function of this transporter, transgenic Arabidopsis plants were constructed expressing antisense or sense AtPTR2-B. Genomic Southern analysis indicated that four independent antisense and three independent sense AtPTR2-B transgenic lines were obtained, which was confirmed by analysis of the segregation of the kanamycin resistance gene carried on the T-DNA. RNA blot data showed that the endogenous AtPTR2-B mRNA levels were significantly reduced in transgenic leaves and flowers, but not in transgenic roots. Consistent with this reduction in endogenous AtPTR2-B mRNA levels, all four antisense lines and one sense line exhibited significant phenotypic changes, including late flowering and arrested seed development. These phenotypic changes could be explained by a defect in nitrogen nutrition due to the reduced peptide transport activity conferred by AtPTR2-B. These results suggest that AtPTR2-B may play a general role in plant nutrition. The AtPTR2-B gene was mapped to chromosome 2, which is closely linked to the restriction fragment length polymorphism marker m246.

Agrobacterium tumefaciens↗