Few physicians in the world have ever seen a patient with Peutz-Jeghers syndrome (PJS). The familial disorder is so rare that even the acronym of its mutant gene, LKB1, is scarcely known.

"The gene itself and the syndrome were described in 1921 by a Dutch physician, J.L.A. Peutz," observed cancer geneticist Ronald DePinho, "and by an American, Harold Jeghers. But the actual gene in terms of its discovery in humans," DePinho added, "was reported in Finland in 1998-99. The syndrome's parsimonious prevalence is only one in many thousands."

PJS features a double-decker symptomology - one mainly oral, the other intestinal. Its visible manifestations are dark brown melanin pigmentation all over lips, tongue, mouth, palate and nostrils, disseminated to hands, fingers and toes. But beyond these relatively benign stigmata, the disease colonizes the gastrointestinal tract with countless polyps. And unlike the well-known growths of gut polyposis, the polyps of the PJ syndrome are hamartomas. They have low malignant potential and consist of disorganized GI mucosa with prominent branching smooth-muscle components. Although GI carcinoma is an infrequent hallmark of PJS, it's not clear whether hamartomas are to blame for these cancers.

DePinho holds an endowed research professorship at the Harvard-affiliated Dana-Farber Cancer Institute in Boston. He is senior author of a paper in Nature dated Sept. 12, 2002. Its title: "Loss of the Lkb1 tumor suppressor provokes intestinal polyposis but resistance to [malignant] transformation."

"I think the study provided us with insights into some of the most enigmatic aspects of Peutz-Jeghers syndrome, and how its gene works," DePinho told BioWorld Today. "The disease is typified by a very bizarre tissue architecture in the neoplasm - a propensity or lack thereof of those polyps to progress to malignancy, and the curious absence of RAS mutations, which are so common in other GI neoplasms."

Several inherited human diseases are characterized by the formation of polyps in the gut. In some of these diseases, the polyps undergo transformation into malignant gastrointestinal tumors - but this occurs relatively rarely in Peutz-Jeghers syndrome. "In an effort to understand why," DePinho recounted, "we engineered mice carrying mutations in Lkb1, the murine counterpart of the LKB1 gene, which is affected in the human syndrome. These animals came up with some unexpected findings."

Transgenic Mouse Models As Consultants

"What we did here," he continued, "was to exploit the experimental attributes of the mice to ask some very specific questions about some of these features. The animals we engineered contained a mutation that is essentially equivalent to that seen in patients with Peutz-Jeghers. By doing that we were able to learn from different studies in those mice that cells derived from them are resistant to cancer transformation by RAS and a variety of other oncogenes. So this direct genetic evidence provided a rational explanation as to why these neoplasms in human PJS patients rarely have ras mutations, and also rarely progress to full malignant transformation."

Inherited mutations in tumor-suppressor genes, which usually keep the brakes on cell growth, show that some cell types are highly susceptible to malignant transformation, whereas others with the same mutations seem to be resistant. Hereditary polyposis syndromes illustrate those phenomena. Intestinal polyps generally consist of two cell types, epithelial and stromal. Malignant tumors derived from such polyps are composed mainly of transformed epithelial cells. Patients with inherited mutant genes develop familial adenomatous polyposis, which have numerous epithelial-rich polyps that tend to progress to cancer. By contrast, patients with inactivated mutations develop juvenile polyposis, and people with LKB1 mutations incur Peutz-Jeghers syndrome.

"To investigate the role of LKB1 in human PJS," DePinho went on, "we generated mice that lacked the relevant murine gene. Those that were minus both copies of the gene - homozygous animals - died as embryos. Heterozygous mice, with one gene deleted and one normal, survived, but developed gastrointestinal hamartomas with features similar to those in patients with PJS or juvenile polyposis. These animals are valuable models of the syndrome," he pointed out, "but the molecular basis for the limited malignant potential of PJS polyps has remained a roadblock: The early demise of the homozygous embryos prevents cell lines from being isolated and analyzed."

To tackle this puzzler, the team constructed mice that lacked one copy of the Lkb1 mouse gene, plus a "conditional" copy of the gene. That is inactivated under specific experimental conditions, so the animals could develop normally. The authors isolated mouse embryonic fibroblast (MEF) cells from those mice, then switched off the conditional Lkb1 gene in these cells, thus producing heterozygous MEFs in culture. The propensity of cultured normal MEFs to become immortal is limited by senescence - a program of cellular "aging." That fail-safe mechanism helps prevent transformation. Lkb1 deficiency in the group's experiments turned out to be a double-edged sword, somehow promoting perpetual cell growth but preventing malignant transformation. It thus explained why the polyps in PJS patients develop yet remain benign.

Bizarre Genes Simulate Wound Healing

"In addition," DePinho went on, "we looked at the tissues and cells of these animals, and found that there were disturbances in gene expression that also provided a rational explanation for the bizarre tissue architecture that we see in these neoplasms. Many of the genes that were thus inappropriately expressed are often found in wound-healing reactions. Some of the genes are important for cell-cell interaction, or for stimulating stromal cell proliferation, plus a variety of other agents that gave us clues as to why we saw the pathology that we did.

"This preclinical research," DePinho narrated, "enabled us to use the experimental merits of the mouse to ask very specific questions in a controlled way about some of the curious features seen in the human disease that they mimicked." He cited "an example of how we used the mouse to model and understand those complex disorders in humans. Like many rare disorders, it gave us clues about general principles. It was an example of where we had a very, very interesting gene. One reason it may not have been on the radar screen is that LKB1 is not often involved in any other cancers. So it wasn't detected or discovered in tumors," DePinho concluded, "but only in very rare syndromes."