The VSG expression sites of Trypanosoma brucei: multipurpose tools for the adaptation of the parasite to mammalian hosts

The variant surface glycoprotein (VSG) genes of Trypanosoma brucei are transcribed in telomeric loci termed VSG expression sites (ESs). Despite permanent initiation of transcription in most if not all of these multiple loci, RNA elongation is abortive except in bloodstream forms where full transcription up to the VSG occurs only in a single ES at a time. The ESs active in bloodstream forms are polycistronic and contain several genes in addition to the VSG, named ES-associated genes (ESAGs). So far 12 ESAGs have been identified, some of which are present only in some ESs. Most of these genes encode surface proteins and this list includes different glycosyl phosphatidyl inositol (GPI)-anchored proteins such as the heterodimeric receptor for the host transferrin (ESAG7/6), integral membrane proteins such as the receptor-like transmembrane adenylyl cyclase (ESAG4) and a surface transporter (ESAG10). An interesting exception is ESAG8, which may encode a cell cycle regulator involved in the differentiation of long slender into short stumpy bloodstream forms. Several ESAGs belong to multigene families including pseudogenes and members transcribed out of the ESs, named genes related to ESAGs (GRESAGs). However, some ESAGs (7, 6 and 8) appear to be restricted to the ESs. Most of these genes can be deleted from the active ES without apparently affecting the phenotype of bloodstream form trypanosomes, probably either due to the expression of ESAGs from 'inactive' ESs (ESAG7/6) or due to the expression of GRESAGs (in particular, GRESAGs4 and GRESAGs1). At least three ESAGs (ESAG7, ESAG6 and SRA) share the evolutionary origin of VSGs. The presence of these latter genes in ESs may confer an increased capacity of the parasite for adaptation to various mammalian hosts, as suggested in the case of ESAG7/6 and proven for SRA, which allows T. brucei to infect humans. Similarly, the existence of a collection of slightly different ESAG4s in the multiple ESs might provide the parasite with adenylyl cyclase isoforms that may regulate growth in response to different environmental conditions. The high transcription rate and high recombination level that prevail in VSG ESs may have favored the generation and/or recruitment in these sites of genes whose hyper-evolution allows adaptation to a larger variety of hosts.

A single-stranded DNA-binding protein shared by telomeric repeats, the variant surface glycoprotein transcription promoter and the procyclin transcription terminator of Trypanosoma brucei

In Trypanosoma brucei the genes are organised into long polycistronic transcription units and only three promoters for protein-encoding genes and a single terminator have been characterised. These promoters recruit a polI-like RNA polymerase for the transcription units encoding the two major stage-specific antigens of the parasite, the variant surface glycoprotein (VSG) of the bloodstream form and procyclin of the insect-specific procyclic form, while the terminator is that of a procyclin transcription unit. By deletional and mutational analysis we defined the two DNA sequences essential for the activity of the VSG promoter from a bloodstream form transcription unit and one of the functional elements of the procyclin terminator. These three short sequences are similar, and their C-rich strand binds the same protein of 40 kDa. In addition, this factor also binds to the C-rich strand of the telomeric repeats, the consensus target sequence being 5'-CCCTNN-3'. The factor-binding sequences are functionally interchangeable in chimeric promoter or terminator constructs, although additional elements are required for full activity.

Characterization of a transcription terminator of the procyclin PARP A unit of Trypanosoma brucei

The polycistronic procylcin PARP (for procyclic acidic repetitive protein) A transcription unit of Trypanosoma brucei was completely characterized by the mapping of the termination region. In addition to the tandem of procyclin genes and GRESAG 2.1, this 7.5- to 9.5-kb unit contained another gene for a putative surface protein, termed PAG (for procyclin-associated gene) 3. The terminal 3-kb sequence did not contain significant open reading frames and cross-hybridized with the beginning of one or several transcription units specific to the bloodstream form. At least three separate fragments from the terminal region were able to inhibit chloramphenicol acetyltransferase expression when inserted between either the PARP, the ribosomal, or the variable surface glycoprotein promoter and a chloramphenicol acetyltransferase reporter gene. This inhibition was due to an orientation-dependent transcription termination caused by the combination of several attenuator elements with no obvious sequence conservation. The procyclin transcription terminator appeared unable to inhibit transcription by polymerase II.

Phase I study of docetaxel administered as a 1-hour intravenous infusion on a weekly basis

PURPOSE

This phase I study of Taxotere (RP 56976, NSC 628503; docetaxel, Rhône-Poulenc Rorer, Antony, France) was undertaken to determine the maximum-tolerated dose (MTD), toxic effects, and basic pharmacokinetics of a day-1 and -8 schedule of this novel semisynthetic product related to Taxol (paclitaxel; Bristol-Myers Squibb, Wallingford, CT).

PATIENTS AND METHODS

Thirty-two eligible patients with refractory solid malignancies have been treated with a 1-hour infusion of Taxotere on a day-1 and -8 schedule every 3 weeks as long as patients maintained a polymorphonucleotide count > or = 1,500/microL and a platelet count > or = 100,000/microL. Dose levels tested have ranged between 20 and 110 mg/m2 per course.

RESULTS

Considering 128 assessable courses, the main toxicities have been neutropenia (which was dose-limiting), asthenia, alopecia, hypersensitivity reactions, skin toxicity, and edema. No significant cardiac or platelet toxicity has been observed. Seven patients have had aggravation of preexisting paresthesias or new onset of sensory symptoms during Taxotere treatment. The MTD at this schedule appears to be 110 mg/m2 per course, with six of 10 patients at this level experiencing severe toxicity. Five partial remissions have been observed in four heavily pretreated patients with breast cancer and in one patient with adenocarcinoma of unknown origin. Two patients with ovarian cancer have had meaningful decreases in CA125 levels.

CONCLUSION

Like Taxol, this novel chemotherapeutic agent appears to possess promising activity in patients with refractory breast and ovarian neoplasms, with tolerable toxicities. Using this schedule, 100 mg/m2 per course is the recommended dose for future phase II trials.

Alternative splicing within and between alleles of the ATPase gene 1 locus of Trypanosoma brucei

The P-type ATPase gene TBA1 of Trypanosoma brucei belongs to a polycistronic transcription unit. We analyzed the structure and expression of a 4-kb region located immediately downstream from TBA1. This region is unique and contains two large open reading frames transcribed into stable mRNAs. These putative genes, termed ADG1 and ADG2, can respectively encode a 24-kDa and a 81-kDa protein. The intergenic spacings between the polyadenylation sites and the next 3' splice acceptor sites are very short: 148 bp between TBA1 and ADG1, and 127 bp between ADG1 and ADG2. Transcripts from each of the two ADG1 alleles can be detected, indicating that both homologs are transcribed. These transcripts are differentially spliced due to a single base difference which destroys in one homolog the AG acceptor site present in the other. In the 'mutant' allele an alternative downstream splice acceptor site is used. Despite its sequence conservation in both alleles, this splice site is only used in the allele lacking the upstream AG acceptor site. The major population of ADG1 transcripts exhibit a long 5'-untranslated extension and no 3'-terminal tail, but a minor population shows a smaller 5'-untranslated region due alternative splicing closer to the initiation codon of the gene. The steady-state amounts of transcripts from individual genes in this region are differentially stage-regulated.

A phase I trial of a new antiemetic drug--clebopride malate--in cisplatin-treated patients

Clebopride, a new benzamide derivative, has, in common with the other members of this group, antidopaminergic activity. In animals, its therapeutic ratio is superior to that of metoclopramide at doses free of side effects associated with hyperprolactinemia and extrapyramidal symptoms. The present study was designed to define the maximum tolerated dose (MTD) in patients with advanced histologically-proven cancer, treated with cisplatin at a dose of greater than 50 mg/m2. Most of them were pretreated and refractory to standard antiemetics. Clebopride was started at a dosage of 0.10 mg/kg in a group of 6 patients and escalated by 0.2 mg at each dose level. A total of 30 patients were included. Side effects include somnolence, diarrhea and extrapyramidal-like symptoms. The latter occurred at almost all dose levels in 14% of the cycles and limited continuation of the study. Activity in this group of patients was encouraging but, considering the rate of extrapyramidal symptoms, further dose escalation is not indicated and activity at lower, nontoxic levels should be investigated.

A gene from the VSG expression site of Trypanosoma brucei encodes a protein with both leucine-rich repeats and a putative zinc finger

The transcription unit of the gene for the variant specific glycoprotein (VSG) AnTat 1.3A of Trypanosoma brucei contains several associated genes (ESAGs, for Expression Site-Associated Genes), 7 of which have already been described. We report here the characterization of a further ESAG, which we term ESAG 8, present 1 kb downstream from the putative adenylate cyclase gene ESAG 4. ESAG 8 encodes a 70 kd protein whose sequence indicates that it is probably not exposed at the cell surface. With the exception of the N-terminal domain which contains a presumptive DNA-binding zinc finger, the ESAG 8 protein consists exclusively of leucine-rich repeats of 23 amino acids, typical of protein-interacting domains such as the RAS-interacting region of the yeast adenylate cyclase. ESAG 8 transcripts are only found in bloodstream forms, and their level is particularly low, suggesting a high rate of degradation. The ESAG 8 protein may be involved in stage-specific regulatory processes, such as gene expression control or adenylate cyclase activation.