By David N. Leff

Therapeutics with a likely market of less than 200,000 consumers are legally termed ¿orphan drugs.¿

One orphan disorder, in spades, is medullary thyroid carcinoma, or MTC. Like do-gooder Little Orphan Annie, MTC has just come up with a welcome oncological surprise.

¿It¿s a very rare cancer,¿ observed molecular biologist Barry Nelkin, of the Johns Hopkins University Oncology Center in Baltimore. ¿In the U.S.,¿ Nelkin told BioWorld Today, ¿MTC is diagnosed in about 1,000 cases a year. In all of Western society, the number given is 3,500. Of these,¿ he pointed out, ¿some 20 percent are hereditary; many of the remaining sporadic cases arise from a mutation in a tyrosine kinase proto-oncogene. Most of MTC¿s familial victims are between the ages of 15 and 35.¿

MTC is not a pain in the neck, which is where the thyroid gland straddles the windpipe. Rather, the physician or his young patient will notice a hard, solid lump in the neck, which turns out to be a painless but malignant medullary tumor.

¿Like almost all cancers,¿ Nelkin observed, ¿MTC is a multistage disease. It progresses slowly from an initial neoplastic hyperplasia [tissue overgrowth] of thyroid C cells to overt malignancy.¿

Nelkin is senior author of a paper in the current Proceedings of the National Academy of Sciences, dated April 13, 1999. Its title: ¿Roles of trk family neurotrophin receptors in medullary thyroid carcinoma development and progression.¿

For reasons connected with its discovery in the 1980s, ¿trk¿ stands for ¿tropomyosin receptor kinase.¿ The family of trk (pronounced ¿track¿) receptors consists of three members, trkA, trkB and trkC genes, each residing on a separate chromosome of the human genome. ¿These receptors,¿ Nelkin explained, ¿are responsible for intracellular signaling based on binding of extracellular ligands, in this case, small peptides.

¿The news here,¿ he went on, ¿is that the trk family of neurotrophin receptors are also found in thyroid C cells, and in the MTC tumors derived from them. In the progression of the cancer stages there are changes in the expression of these receptors. What we found was that normal C cells, as well as the preneoplastic-state, hyperplastic C cells, express the trkB receptor.

¿The earliest stage on the way to tumors is called thyroid C cell hyperplasia,¿ Nelkin said. ¿Those cells are all expressing trkB and, to some extent, A or C. As we followed that progression, we saw in these cells that the level of trkB went down, while the levels of trkA and especially trkC went up. We found that normal thyroid C cells, at least some of them, express the trkB receptor, but not A and not C.

¿At that point,¿ he continued, ¿we wondered whether this had anything to do with driving the progression of the MTC tumor. We could envision that trkA or trkC would make cells grow more, while B would inhibit cellular growth past a certain point. Which was why we only saw it in the early stages.¿

When Nelkin and his co-authors tried to model this progression in vitro, using a human medullary thyroid carcinoma cell line, they found the ¿cell culture didn¿t express much of these trks at all. So we put each of the genes into the cell line separately, as an expression vector. The trkA and trkC made the cells grow faster. And as we might have expected, the trkB had very little effect.¿

That was when the team turned from in vitro to in vivo.

¿We tried injecting these receptor genes into immunodeficient nude mice, in a tumorigenesis assay, to see whether there would be differences in tumor formation.

¿And there was a very dramatic difference, in that the trkA and trkC grew tumors relatively rapidly for this cell line. And the tumors grew larger, consistent with what we had seen. TrkA and trkC made the cells grow faster, as they did in the later, more malignant stages of the disease.

¿On the other hand,¿ Nelkin recounted, ¿the cells that were expressing the trkB gene, which we had put in, generally would not grow tumors in these mice. The few that we got were small and very flat.¿

That lit an electric light bulb.

¿This suggested to us,¿ he recalled, ¿that trkB might be mediating a difference in the tumorigenicity, based on something that we wouldn¿t see in cell culture ¿ such as angiogenesis.

¿So we started looking at the candidate angiogenesis factors that make tumors grow, and the first one we looked at was the best known ¿ vascular endothelial growth factor, VEGF. This is the now-well-known hormone that tumor cells produce to elicit a growth response in endothelial cells, so that more blood vessels will be made to serve the tumor.¿

Nelkin and his team ascertained that the levels of VEGF being produced by the medullary carcinoma cell line containing trkB were, on average, 11-fold lower than those into which he¿d put trkA.

¿So we now suggest,¿ he said, ¿that this difference in VEGF production may limit tumor growth by limiting angiogenesis. We have not proven that yet because to do so we¿d have to put the VEGF gene back into these cells and show that we can rescue the tumorigenicity.¿ That¿s next on his agenda.

Angiogenesis Bodes Well For Other Tumors, Too

¿We¿d like to look at the mechanism by which trkB regulates VEGF. We also need to prove that it¿s really VEGF that¿s responsible for the tumorigenesis, or lack of tumorigenesis, mediated by trkB.

¿And finally, since we in medullary thyroid carcinoma, and others investigating neuroblastoma and medulloblastoma, have shown that there are differences in the responses to the tumor by different trk receptors, we want to look at the differences in intracellular signal transduction that are mediated by the various trk family members.

¿Those pathways,¿ Nelkin concluded, ¿are going to be important far beyond MTC, because if we can show what controls angiogenesis, this will obviously be important for many tumor cell types. Trk receptors are already seen in prostate cancer, gliomas and leukemia.¿ n