Dean A. HaycockSpecial To BioWorld Today
It took about 48 hours from the time they found the mutation to thetime they worked out the model. But it was an intense 48 hours.
The result is a clean story _ so clean it almost looks easy the wayresearchers linked a mutated gene for hereditary pancreatitis to itsprotein product and then tied that to the protein's structure, itsfunction and finally to the pathology of the disease.
"I think we got lucky along with having a modest amount of insight,"said Garth Ehrlich, of the University of Pittsburgh.
Ehrlich and his colleagues describe their model in "Hereditarypancreatitis is caused by a mutation in the cationic trypsinogen gene"in this month's issue of Nature Genetics.
Sporadic pancreatitis tortures thousands of new victims each year andis associated with alcoholism, stress and high fat diets, mostlyalcoholism. It produces the same epigastric pain as the hereditaryform of the disease.
The disease is the result of uncontrolled activity by digestiveenzymes. When the body cannot control the activity of enzymes suchas trypsin, the enzymes begin to digest parts of the body. That is whathappens to the pancreas in pancreatitis.
The body usually protects itself from autodigestion by storingdigestive enzymes in inactive, precursor forms which are activatedwhen needed. When their job is done, the active forms of theenzymes are broken down quickly.
It took Ehrlich and his co-authors less than 90 days to identify thegene responsible for hereditary pancreatitis (HP), a rare autosomaldominant disease, after they localized it to chromosome 7. Becauseof progress in the human genome project, other genes in the region ofthe HP gene were on record. This saved the authors time by helpingthem exclude false leads as they searched the long arm ofchromosome 7 for their prey, the trypsinogen gene.
They found the same mutation in all pancreatitis-sufferers in five HPfamilies. The mutations were in the gene that encodes the precursorof trypsin, the cationic trypsinogen gene.
The authors predict that the mutation changes the shape of trypsin.The shape change would appear to prevent inhibitors of trypsin frombinding and inactivating the digestive enzyme. As a result, trypsinbuilds up in the pancreas, disrupts the regulation of other digestiveenzymes and allows the pancreas to be slowly destroyed by theenzymes it produces but can no longer regulate.
The mutation is a simple substitution of one amino acid at position117, arginine, for another, histidine, in trypsin's protein structure.That single change, the authors suggest, occurs right at the spotwhere trypsin is cut apart when it is inactivated. The histidinesubstitution prevents the cleavage and inactivation of the enzyme inHP patients.
As of today, nine HP families have been studied "Five or six havethis mutation. It appears as if this is the most prevalent mutationassociated with hereditary pancreatitis," Ehrlich said.
The researchers have also observed different mutations in the samegene in a couple of other families. The pathological mechanisms tiedto these mutations, however, are not well understood.
"Right now we are working on modeling these other mutations. Theydid not lend themselves to as facile an explanation as the Arg117[mutation]," Ehrlich said.
A Team Of 15 Made It Happen
The effort to identify the gene for HP and explain the pathologyrequired at least 15 scientists in disciplines ranging from medicine togenetics to crystallography.
"That is what I do. I bring people together from many differentdisciplines because one of the things I realized years ago is thatadvances generally happen at crossroads of different disciplines,"Ehrlich told BioWorld Today. "So I have tried to create anenvironment where we are continually in that situation."
The other essential participants in the project were members of theHP families.
"These types of studies are really a collaborative effort between thescientists and the families. We just had incredible enthusiasm andhelp from the families," Ehrlich recalled.
The researchers now are turning their attention to the sporadic formof the disease. They want to know if there are different forms orpolymorphisms of the trypsinogen gene which might makeindividuals more susceptible to environmentally induced pancreatitis.
"If we were to make such observations, it would have pretty broadapplicability because there are certainly more people who suffer fromsporadic pancreatitis than from familial pancreatitis," Ehrlich said.Each year, only a few hundred persons are diagnosed with HP while40,000 or more develop sporadic pancreatitis.
Understanding the genetic basis of pancreatitis provides a strongertheoretical basis for developing potential therapies to treat thedisease.
"[Pancreatitis patients] may derive benefit from treatment withcompounds that shut down pancreatic enzyme production and thensupplying them with enzymes they need in a pill form," Ehrlich said.
The scientists are continuing work to prove their model which issupported by X-ray crystal structure analysis, protein digest data andmolecular modeling.
"We have cloned the mutant gene and are in the process ofdeveloping both in vitro and in vivo expression models in mice tostudy the effect of the mutation," Ehrlich said. n
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