Only one in a hundred colon cancer patients have the familialpolyposis form of the disease, but these plus others at risk addup to 50,000 people in the U.S. alone. They and their relativeswill welcome what its main discoverer, Kenneth Kinzler, calls "aneat application of state-of-the-art biotechnology in a clinicalsetting."
This application is described in the current New EnglandJournal of Medicine (NEJM) as "Molecular Diagnosis of FamilialAdenomatous Polyposis." Its principal author is oncologistKinzler of Johns Hopkins University School of Medicine.
Contriving a quick and easy method for finding peoplegenetically programmed to inherit familial adenomatouspolyposis (FAP) involved short-circuiting one of the largestgenes in the human genome. The challenge was to detectdozens or scores of mutations PP one-third of them spontaneousPP strung along its 8,500 base pairs. The approach that Kinzlerand his nine co-authors devised proved so reliable andpractical that the university will soon offer the test to allcomers with intimations that FAP runs in their family.
Familial adenomatous polyposis is an insidious, dominantlyinherited syndrome in which hundreds or more tinymushroom-shaped growths carpet the large intestine. Thesepolyps start out benign, but inevitably progress to colorectalcancer, usually during its victim's teens or 20s.
In the late 1980s, FAP was pinpointed to the long arm ofchromosome 5. Its gene, APC (adenomatous polyposis coli),controls the life cycle of colon cells, but mutated variants of itoccur in the germline of patients with the disease. Identifyingthese myriad point, splicing or frameshift errors in such asizable gene sequence was "impractical," to quote Kinzler'sunderstatement.
Instead, the Johns Hopkins team took advantage of one thingthese mutations seemed to have in common: 90 percent of thealterations incorporate a stop codon that truncates their proteinproduct. Finding such a stubby protein in a person at riskwould verify his or her predilection for getting the disease.
To go about this detection, Kinzler and his co-workers choppedthe lengthy APC gene into five segments and fitted the 5'terminal of each sub-gene with custom-made transcription andtranslational sequences. This made it feasible to seek mutationsin their five happiest hunting grounds a segment or two at atime or all simultaneously, rather than attempting to amplifythe 8,500-base-pair APC whole.
"We amplified each surrogate gene segment with PCR, placed itin a one-tube reaction, then transcribed and translated it, usingcommercially available kits," Kinzler told BioWorld. They thenexpressed each sub-gene, "making a surrogate protein, bysynthesis in vitro, to see if it looks normal or not."
This shortcut, he said, "takes about a tenth the time, roughly,that analyzing the entire gene would, maybe even less."
To prove their system, the Johns Hopkins team obtained bloodsamples from the most recent 62 unrelated patients enrolled inthe university FAP registry. All 62 had classic FAP, as definedby more than 100 colorectal polyps discerned by endoscopy, X-ray or surgery. Of the 62, 14 had spontaneous, de novomutations. The team also studied nine unrelated normalsubjects and seven relatives of three FAP patients.
First, they looked at 20 sporadically occurring tumors. In eachthey found, as predicted, a stunted AFP protein. Next, theywent to peripheral blood samples and identified truncatingmutations in 51 of their 62 patients (82 percent). These 51mutations were strewn over the first four surrogate segments.Overall detection of APC gene alterations attained an 87-percent success rate.
These and other tests validated the assay, Kinzler said, whichwill be available in two to four months for diagnosing thelurking disease, primarily in individuals with affected relatives.The university's Office of Technology Transfer, which filed topatent the system a couple of months ago, is now working outthe details of making it available to the public.
"Patients will be able simply to send a blood sample," Kinzlersaid. But "every applicant must first be in contact with ageneticist who can determine whether it's worthwhile for themto have the test, and be sure they understand its significance,"he emphasized.
"In our lab setting, if done as quickly as possible, a test couldbe completed in three days or less," said Kinzler. But in actualroutine practice, the university will require all tests to besupervised by a board-certified geneticist rather than theoncologists who developed it.
One obvious use of the genetic probe will be in prenataldiagnosis so that patients and others at risk can plan families.Detection of the FAP gene mutation in presymptomaticindividuals can help not only them but their kin. The 54 JohnsHopkins patients who revealed abnormal APC have at least 280relatives at risk, who are now testable for the first time.
Besides ending a person's uncertainty, early diagnosis canensure that preventive measures are taken before overt cancerdevelops.
An intriguing aspect of the Johns Hopkins approach, Kinzlerpointed out, is its potential applicability to other geneticdiseases, notably cystic fibrosis. In CF, he noted, 70 percent ofthe mutant genes involve specific triplet deletions, "which ourtest does not easily detect. But of CF's remaining 30 percent, afair amount would be detectable."
Neurofibromatosis II and von Hippel-Lindau syndrome areother candidate disorders.
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.