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NIS is strongly expressed in a significant proportion of triple-negative breast cancer cells, suggesting a potential role for NIS-directed (131)I-radioablative strategies in this patient population.

The enzymes that aminoacylate tRNAs have been studied extensively and can be organized into two distinct classes based on signature sequences and the position of aminoacylation. The class I enzymes have canonical HIGH and KMSKS sequences as part of a Rossman fold nucleotide‐binding site. The tryptophan‐specific enzymes have been placed in class I based on analysis of the cognate genes from Escherichia coli, B. stearothermophilus, B. taurus, and Homo sapiens. An unidentified open reading frame (ORF) on Saccharomyces cerevisiae chromosome XV, HRE342, has 46% identity with the bovine tryptophanyl‐tRNA synthetase and possesses the appropriate signature sequences. The predicted molecular weight of the putative HRE342 protein also closely matched the expected monomer size of the S. cerevisiae enzyme. The HRE342 ORF plus about 250 bp of 5′ and 3′ flanking sequence was amplified by polymerase chain reaction, cloned into a 2 μ based vector, and transformed into a host strain, S. cerevisiae JG369.3B. Nucleotide sequence analysis of the clone confirmed the presence of HRE342. Extracts from transformed yeast have a 30‐ to 100‐fold increase in specific activity of the tryptophanyl‐tRNA synthetase. An HRE342 locus in a diploid strain, PTY33XPTY44, was disrupted with a LEU2 insert. Sporulation and tetrad analysis of the HRE342::LEU2 strain demonstrated that HRE342 is an essential gene. We conclude that HRE342 is the S. cerevisiae gene encoding the cytoplasmic tryptophanyl‐tRNA synthetase, WRS1. A search of the Saccharomyces Genome Database using amino acid sequences from other eukaryotic aminoacyl‐tRNA synthetases suggests there is sufficient similarity to identify both class I and class II genes. © 1997 by John Wiley & Sons, Ltd.

Optical imaging has made it possible to monitor response to anticancer therapies in tumor xenografts. The concept of treating breast cancers with (131)I is predicated on the expression of the Na(+)/I- symporter (NIS) in many tumors and uptake of I- in some. The pattern of (131)I radioablative effects were investigated in an MCF-7 xenograft model dually transfected with firefly luciferase and NIS genes. On Day 16 after tumor cell implantation, 3 mCi of (131)I was injected. Bioluminescent imaging using d-luciferin and a cooled charge-coupled device camera was carried out on Days 1, 2, 3, 7, 10, 16, 22, 29, and 35. Tumor bioluminescence decreased in (131)I-treated tumors after Day 3 and reached a nadir on Day 22. Conversely, bioluminescence steadily increased in controls and was 3.85-fold higher than in treated tumors on Day 22. Bioluminescence in (131)I-treated tumors increased after Day 22, corresponding to tumor regrowth. By Day 35, treated tumors were smaller and accumulated 33% less (99m)TcO(4)(-) than untreated tumors. NIS immunoreactivity was present in <50% of (131)I-treated cells compared to 85-90% of controls. In summary, a pattern of tumor regression occurring over the first three weeks after (131)I administration was observed in NIS-expressing breast cancer xenografts.

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