Biomedical subjects

U Tantravahi

Publications and source records attributed to U Tantravahi.

42 records · Page 3Linked to original sources

PubMed1981

Evidence for methylation of inactive human rRNA genes in amplified regions.

In two unrelated families, the short arm of a 14p+ marker chromosome contains an increased number of copies of the 18S + 28S rRNA genes without a comparable increase in the transcriptional activity, as shown by silver staining. The DNA in this region is highly enriched in 5-methylcytosine, as shown by specific antibody binding. In contrast, the owl monkey and cat have a single major nucleolus organizer region (NOR) per haploid genome; these NORs contain about the same number of rRNA genes as the 14p+ chromosome but are not methylated. These findings suggest that most of the amplified human rRNA genes on the 14p+ chromosomes have been inactivated by a process involving DNA methylation.

Animals↗

PubMed1981

Amplified ribosomal RNA genes in a rat hepatoma cell line are enriched in 5-methylcytosine.

In a rat hepatoma cell line, H4-IIE-C3, a 10-fold excess of 18S and 28S rRNA genes has been found in amplified chromosome regions. Antibodies to 5-methylcytidine bound extensively to the DNA of these regions, indicating a high level of DNA methylation. Most of the amplified rRNA genes were transcriptionally inactive, as shown by their failure to stain with silver. DNAs from the tumor cells and control rat hepatocytes grown with L-[methyl-14C]methionine were digested with restriction endonuclease EcoRI; the DNA fragments were separated by agarose gel electrophoresis, denatured, transferred to nitrocellulose filters, and hybridized to 32P-labeled rRNA or cDNA. Fragments containing the 18S or 28S rRNA coding sequences occurred in three major size classes; all three were rich in 5-methylcytosine. Analysis of EcoRI fragments of DNA from the tumor and control cells after digestion with Hpa II or Msp I endonuclease indicated that the 5'-C-C-G-G-3' sequences in most of the amplified rRNA genes were methylated. Analysis of the fragments produced by digestion with Hha I endonuclease indicated a high degree of methylation within its recognition sequence in the amplified rRNA genes as well. The association of hypermethylation with restricted transcriptional activity suggests that DNA methylation may regulate the activity of the rRNA genes.

5-Methylcytosine↗

PubMedDate not supplied

Correspondence between effects of 5-azacytidine on SCE formation, cell cycling and DNA methylation in Chinese hamster cells.

The effects of 5-azacytidine (5-Aza-C), alone and in combination with mitomycin C, were measured on sister-chromatid exchange (SCE) formation and DNA methylation in different genomic regions of Chinese hamster ovary cells and in Chinese hamster cells containing amplified, dihydrofolate reductase sequences and resistant to methotrexate. 5-Aza-C, when present for the penultimate preharvest cell cycle, induced SCEs in a manner consistent with a directly measured reduction in deoxycytosine methylation in cellular DNA. At higher 5-Aza-C concentrations, cell cycling was inhibited and both SCE induction and DNA demethylation tended to level off. Under appropriate conditions, 5-Aza-C also potentiated the induction of SCEs by mitomycin C. 5-Aza-C-induced DNA demethylation could also be detected in the vicinity of different DNA sequences with the use of comparative HpaII/MspI digestion, DNA blotting, and molecular probes. The efficiency of an individual demethylation event in inducing SCE induction appeared to be very low, compared with alkylating agents such as 8-methoxypsoralen, suggesting that SCE induction by 5-Aza-C might be an indirect effect from long range changes induced in cellular DNA or chromatin conformation.

Animals↗

PubMedDate not supplied

Analysis of deletions in DNA from patients with Becker and Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is an X-linked recessive genetic disorder for which the biochemical defect is as yet unknown. Recently, two cloned segments of human X-chromosome DNA have been described which detect structural alterations within or near the genetic locus responsible for the disorder. Both of these cloned segments were described as tightly linked to the locus and were capable of detecting deletions in the DNA of boys affected with DMD. In an attempt to determine more precisely the occurrence of these deletions within a large population of DMD patients and the accuracy of one of the segments, DXS164 (pERT87), in determining the inheritance of the DMD X chromosome, the subclones 1, 8 and 15 were made available to many investigators throughout the world. Here we describe the combined results of more than 20 research laboratories with respect to the occurrence of deletions at the DXS164 locus in DNA samples isolated from patients with DMD and Becker muscular dystrophy (BMD). The results indicate that the DXS164 locus apparently recombines with DMD 5% of the time, but is probably located between independent sites of mutation which yield DMD. The breakpoints of some deletions are delineated within the DXS164 locus, and it is evident that the deletions at the DMD locus are frequent and extremely large.

Chromosome Deletion↗

PubMedDate not supplied

Molecular analysis of inhibin A and activin A subunit gene loci in epithelial ovarian cancer.

Inhibin A (alpha-betaA) and activin A (betaA-betaA) are biochemically similar proteins that generally have opposite biologic functions. For example, while inhibin (alpha subunit) is proposed to be a tumor suppressor in some types of ovarian cancer, activin appears to stimulate tumor development. Previous reports suggest that a loss of alpha inhibin subunit expression and elevated serum activin levels are associated with human epithelial ovarian cancer (EOC). Our objective was to examine the alpha inhibin subunit gene locus on chromosome 2q for evidence of loss of heterozygosity (LOH) in cases of EOC and to correlate these results with serum activin A levels measured in the same patients. Ovarian tumor and matched healthy tissue samples were collected from 22 women with EOC. DNA was extracted and subjected to PCR analysis using 10 primers, seven from chromosome 2q (alpha inhibin subunit locus) and, as a control, three from chromosome 7p (inhibin/activin betaA subunit). In addition, each patient had a preoperative serum activin A measurement using an ELISA assay. One (1/22) case of EOC demonstrated LOH for one microsatellite marker at the alpha inhibin gene locus. Thirty-six percent (8/22) of patients had an activin A level that was increased above the normal range. We conclude that loss of heterozygosity at the inhibin/activin alpha subunit locus is not frequently associated with EOC. More direct molecular analyses of the inhibin and activin genes are warranted to rule out mutations in cases of epithelial ovarian cancer.

Activins↗

PubMedDate not supplied

46,XX gonadal agenesis in a neonate with multiple congenital anomalies: case report and review of the literature.

We report a neonate with 46,XX gonadal agenesis, a rare disorder, confirmed by autopsy, karyotype determination, and fluorescent in situ hybridization examination of intact cells. Multiple other anomalies, including diaphragmatic hernia, a doomed bicuspid aortic valve, and müllerian derivative defects, were present. There was no sexual ambiguity. The age of this patient and the presence of anatomically dispersed congenital anomalies are unique among reported examples of 46,XX gonadal agenesis. Review of the literature reveals that all five previously reported cytogenetically confirmed patients with 46,XX gonadal agenesis were 17 to 25 years of age, none were diagnosed before their teens, all had female phenotype with sexual infantilism, three had müllerian derivative anomalies, and none had nongenitourinary anomalies. The abnormalities in this case may represent a polytopic field defect due to unknown insults occurring at approximately 6 weeks of developmental age.

Abnormalities, Multiple↗