Pre-mRNA processing enhancer (PPE) element increases the expression of an intronless thymidylate synthase gene but does not affect intron-dependent S phase regulation

Polymorphisms of the repeated sequences in the enhancer region of the thymidylate synthase gene promoter may predict downstaging after preoperative chemoradiation in rectal cancer

PURPOSE

Thymidylate synthase (TS) is an important target enzyme for the fluoropyrimidines. TS gene promoter possesses regulatory tandemly repeated (TR) sequences that are polymorphic in humans, depending on ethnic factors. These polymorphisms have been reported to influence TS expression. TS expression levels affect tumor downstaging after preoperative fluoruracil (5-FU)-based chemoradiation. Tumor downstaging correlates with improved local control and disease-free survival. The aim of this study is to correlate TR polymorphisms with downstaging and disease-free survival.

PATIENTS AND METHODS

Sixty-five patients with rectal cancer underwent tumor resection after preoperative 5-FU-based chemoradiation. Tumor downstaging was evaluated by comparing the pretreatment T stage with the pathologic stage observed in the surgical specimen. TS polymorphism genotype was determined by polymerase chain reaction amplification of the corresponding TS promoter region, and products of amplification were electrophoresed, obtaining products of 220 bp (2/2), 248 bp (3/3), or both (2/3). The TS polymorphism genotype results were subsequently compared with the downstaging observed and with disease-free survival.

RESULTS

Patients who were homozygous for triple TR (3/3) had a lower probability of downstaging than patients who were homozygous with double TR or heterozygous patients (2/2 and 2/3): 22% versus 60% (P =.036; logistic regression). Furthermore, a trend toward improved 3-year disease-free survival was detected in the 2/2 and 2/3 groups, compared with that in the 3/3 group (81% v 41%; P =.17).

CONCLUSION

This preliminary study suggests that TS repetitive-sequence polymorphisms are predictive for tumor downstaging. TR sequences in TS promoter may be useful as a novel means of predicting response to preoperative 5-FU-based chemoradiation.

Arginine-rich regions mediate the RNA binding and regulatory activities of the protein encoded by the Drosophila melanogaster suppressor of sable gene

The Drosophila melanogaster suppressor of sable gene, su(s), encodes a novel, 150-kDa nuclear RNA binding protein, SU(S), that negatively regulates RNA accumulation from mutant alleles of other genes that have transposon insertions in the 5' transcribed region. In this study, we delineated the RNA binding domain of SU(S) and evaluated its relevance to SU(S) function in vivo. As a result, we have defined two arginine-rich motifs (ARM1 and ARM2) that mediate the RNA binding activity of SU(S). ARM1 is required for in vitro high-affinity binding of SU(S) to small RNAs that were previously isolated by SELEX (binding site selection assay) and that contain a common consensus sequence. ARM1 is also required for the association of SU(S) with larval polytene chromosomes in vivo. ARM2 promotes binding of SU(S) to SELEX RNAs that lack the consensus sequence and apparently is neither necessary nor sufficient for the stable polytene chromosome association of SU(S). Use of the GAL4/UAS system to drive ectopic expression of su(s) cDNA transgenes revealed two previously unknown properties of SU(S). First, overexpression of SU(S) is lethal. Second, SU(S) negatively regulates expression of su(s) intronless cDNA transgenes, and the ARMs are required for this effect. Considering these and previous results, we propose that SU(S) binds to the 5' region of nascent transcripts and inhibits RNA production in a manner that can be overcome by splicing complex assembly.

Transcriptional control elements and complex initiation pattern of the TATA-less bidirectional human thymidylate synthase promoter

The nucleotide sequences that are important for transcription of the human thymidylate synthase gene were analyzed by deletion and site-directed mutagenesis of the promoter region. Deletion analyses from the 5' and 3' ends indicated the presence of multiple positive and negative elements. The promoter had approximately the same strength in the normal or inverted orientation. The region between 161 and 141 nt upstream of the translational start codon was found to be both necessary and sufficient for high-level promoter activity in both directions and was designated the essential promoter region. This region, which is highly conserved in human, mouse and rat TS promoters, contains potential binding sites for Ets, Sp1, and LSF transcription factors. Site directed mutagenesis of each of these elements led to large decreases in promoter strength. However, inactivation of potential Sp1 and E2F elements adjacent to the essential promoter region led to increases in promoter strength. The transcriptional start site pattern was analyzed by S1 nuclease protection assays of mRNA isolated from cells transiently transfected with TS minigenes. Multiple start sites were detected, most of which were between 160 and 120 nt upstream of the AUG codon.

Inactivation of MED-1 elements in the TATA-less, initiator-less mouse thymidylate synthase promoter has no effect on promoter strength or the complex pattern of transcriptional start sites

The mouse thymidylate synthase (TS) promoter is a member of a family of promoters that lack a TATA box as well as an initiator element and that initiate transcription at many sites over a broad initiation window. An element (MED-1) downstream of the initiation window of almost all promoters of this family has been proposed to be important for promoter activity, as well as for multiple start site utilization. Two consensus MED-1 elements are located downstream of the initiation window of the TS promoter. To determine the role of the MED-1 elements in the TS promoter, one or both elements were inactivated by site-directed mutagenesis and the effects on promoter function were determined. We found that inactivation of the MED-1 elements had no measurable effect on promoter strength, the boundaries of the initiation window, or the pattern of transcriptional start sites. Furthermore, inactivation of the elements did not affect the ability of the TS promoter to direct S phase-specific expression of the gene in growth-stimulated cells. We conclude that the MED-1 element does not play a significant role in TS promoter function and therefore is not an essential component of all TATA-less promoters with complex transcriptional initiation patterns.