The developmental pattern of homologous and heterologous tRNA methylation in rat brain differential effect of spermidine

Using S -adenosyl- L -[Me- 14 C] methionine, rat cerebral cortex methyltransferase activity was determined during the early postnatal period in the absence of added Escherichia coli tRNA and in its presence. [Me- 14 C] tRNA was purified from both systems and its [Me- 14 C] base composition determined. The endogenous formation of [Me- 14 C] tRNA (homologous tRNA methylation) was totally abolished in the presence of 2.5 mM spermidine, whereas E. coli B tRNA methylation (heterologous methylation) was markedly stimulated. Only [Me- 14 C] 1-methyl guanine and [Me- 14 C] N 2 -methyl guanine were formed by homologous methylation, there being an inverse shift in their relative proportions with age. Heterologous tRNA methylation led, additionally, to the formation of [Me- 14 C] N 2 2 -dimethyl guanine, 5-methyl cytosine, 1-methyl adenine, 5-methyl uracil, 2-methyl adenine, and 1-methyl hypoxanthine. A comparison of heterologous tRNA methylation between the whole brain cortex (containing nerve and glial cells) and bulk-isolated nerve cell bodies revealed markedly lower proportions of [Me- 14 C] N 2 -methyl and N 2 2 -dimethyl guanine and significantly higher proportions of [Me- 14 C] 1-methyl adenine in the neurons. The present findings suggest (1) that homologous tRNA methylation may provide developing brain cells with continuously changing populations of tRNA and (2) that neurons are enriched in adenine residue-specific tRNA methyltransferases that are highly sensitive to spermidine.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45399/1/11064_2004_Article_BF00966229.pd

Cytogenetic Prognostication Within Medulloblastoma Subgroups

PURPOSE: Medulloblastoma comprises four distinct molecular subgroups: WNT, SHH, Group 3, and Group 4. Current medulloblastoma protocols stratify patients based on clinical features: patient age, metastatic stage, extent of resection, and histologic variant. Stark prognostic and genetic differences among the four subgroups suggest that subgroup-specific molecular biomarkers could improve patient prognostication. PATIENTS AND METHODS: Molecular biomarkers were identified from a discovery set of 673 medulloblastomas from 43 cities around the world. Combined risk stratification models were designed based on clinical and cytogenetic biomarkers identified by multivariable Cox proportional hazards analyses. Identified biomarkers were tested using fluorescent in situ hybridization (FISH) on a nonoverlapping medulloblastoma tissue microarray (n = 453), with subsequent validation of the risk stratification models. RESULTS: Subgroup information improves the predictive accuracy of a multivariable survival model compared with clinical biomarkers alone. Most previously published cytogenetic biomarkers are only prognostic within a single medulloblastoma subgroup. Profiling six FISH biomarkers (GLI2, MYC, chromosome 11 [chr11], chr14, 17p, and 17q) on formalin-fixed paraffin-embedded tissues, we can reliably and reproducibly identify very low-risk and very high-risk patients within SHH, Group 3, and Group 4 medulloblastomas. CONCLUSION: Combining subgroup and cytogenetic biomarkers with established clinical biomarkers substantially improves patient prognostication, even in the context of heterogeneous clinical therapies. The prognostic significance of most molecular biomarkers is restricted to a specific subgroup. We have identified a small panel of cytogenetic biomarkers that reliably identifies very high-risk and very low-risk groups of patients, making it an excellent tool for selecting patients for therapy intensification and therapy de-escalation in future clinical trials

Identifi cation of the Tumor Factor of Abnormal Cancer Methylation Enzymes as the Catalytic Subunit of Telomerase

OBJECTIVE The objective was to study the relationship between the tumor factor of cancer MATLT and the catalytic subunit of telomerase. The function of telomerase in the blockade of cell differentiation and in the protection of DNA MT resembles closely the function of the tumor factor of cancer MATLT. Because of this close similarity we made an attempt to examine the possibility that the tumor factor of MATLT might be the catalytic subunit of telomerase. METHODS We used purified MAT isozymes, telomerase antibody, immunoprecipitation, and a selective inhibitor of the tumor factor of MATLT from urine to study the relationship between the tumor factor of MATLT and telomerase.RESULTS We were able to show that the tumor MATLT, but not the liver MATL, was selectively inhibited by the telomerase antibody, and the tumor MATLT, but not the liver MATL, was preferentially immunoprecipitated with the telomerase antibody. The catalytic subunit of telomerase was detectable in the tumor MATLT preparation by immunoblotting, but was undetectable in the liver MATL preparation and the tumor MATL preparation stripped off of the tumor factor. In addition, PP-0.39, which is an effective differentiation inducer purified from urine previously found to selectively antagonize the tumor factor of MATLT, was found in this study to be a potent inhibitor of telomerase. The inhibition of telomerase by PP-0.39 was far more sensitive than the elimination of the tumor factor from MATLT.CONCLUSION All results are consistent with the hypothesis that the tumor factor of MATLT is the catalytic subunit of telomerase. Thus, the blockade of cell diff erentiation by telomerase is mediated through its interaction with MAT to affect methylation enzymes, so that hypomethylation of nucleic acids necessary for the cell to undergo differentiation cannot take place