By David N. Leff
¿When a person suffering from thrombotic thrombocytopenic purpura [TTP] comes to the emergency room,¿ observed hematologist Han-Mou Tsal at the Albert Einstein College of Medicine in the Bronx, N.Y., ¿if the diagnosis is not prompt, and treatment proper, the patient may become comatose, and die within a couple of days.
¿The proximal cause of death in TTP,¿ Tsal pointed out, ¿occurs usually if the individual is not treated properly, or doesn¿t respond to treatment. In such case, eventually he will have a series of neurological malfunctions, get into coma or seizures, eventually leading to death.
¿The onset of TTP,¿ he continued, ¿often occurs in previously healthy individuals. The patient may present with variable, nonspecific symptoms ¿ from anemia, fever, headache, fatigue, to purpura ¿ red-to-purple hemorrhage under the skin. Or he can present with acute neurological manifestations, like mental changes or stroke.¿
Tsal explained, ¿TTP is characterized by intravascular clumping of platelets, with destruction of erythrocytes, resulting in anemia and thrombocytopenia. In a related disorder, hemolytic uremic syndrome [HUS],¿ he explained, ¿neurologic symptoms are less evident and renal failure more prominent. HUS is most often seen in young children, usually following infection with a specific Shiga toxin-producing strain of Escherichia coli. It¿s presumed,¿ he suggested, ¿that the Shiga toxin produced by the microorganism causes endothelial injury, leading to fibrin thrombosis in the small blood vessels.
¿HUS is really a syndrome including multiple entities,¿ Tsal pointed out. ¿And these individuals have a different type of underlying pathophysiology. HUS patients don¿t have a deficiency of this protease, and the pathological features formed in the small blood vessels are different from TTP.¿
Sporadic TTP ¿ unlike the familial form ¿ is only a moderately rare affliction. ¿We have no hard data as to its prevalence,¿ Tsal said. ¿There was a study by a group of investigators at the Centers for Disease Control in 1991, based on death certificates. It estimated four cases per million of the population per year. There has since been a trend toward increase. Some recent and published data based on insurance companies,¿ he noted, ¿report that the incidence of TTP can be up to 30 or 40 per million per year.¿
Tsal is senior author of an article in today¿s Nature, dated Oct. 4, 2001. Its title: ¿Mutations in a member of the ADAMTS gene family cause thrombotic thrombocytopenic purpura.¿
Different TTP Forms, Different Mechanisms
¿What we report,¿ he told BioWorld Today, ¿is, first, that patients with the familial form of TTP have a deficiency of von Willebrand factor-cleaving protease. This deficit is also found in patients who have the acquired form of TTP, but it is caused by a different mechanism. The familial form is constitutional, whereas the acquired form involves autoantibodies. We cloned the gene encoding that protease, and identified it in patients with the familial form of TTP ¿ confirming that the disease is caused by a deficiency of this enzyme. We also determined that this protease deficiency is critical for TTP disease, and we identified the gene mutations.
¿Familial forms of TTP are rare,¿ Tsal pointed out. ¿But the importance of this finding is by studying these patients we could clone this gene, and now we will be able to study the structure and function of the enzyme it encodes. Also, we can make antibodies against this protease, based on its sequence. The antibodies will do more than elucidate the function and biological role of this enzyme. They will allow us to develop a new and easier diagnostic tool for the detection of protease deficiency in patients with this gene. And, of course, we can also try to use the gene¿s cDNA to make a recombinant protease that may eventually replace plasma therapy, which is currently used in TTP patients.¿
As for TTP¿s etiology, Tsal said, ¿What causes the patients to develop autoantibodies is not known. But the familial cases are due to the gene mutation.¿ That gene¿s acronym, ADAMTS, stands for its full name, with is almost as impenetrable as ITT itself, namely: ¿a disintegrin-like and metalloproteinase with thrombospondin motifs.¿
With patient-referral DNA samples from a dozen university medical centers throughout the U.S., Tsal¿s close co-author, molecular geneticist David Ginsburg at the University of Michigan, Ann Arbor, conducted extensive linkage analysis of TTP patients and their unaffected relatives. He focused on seven unrelated individuals in the collection of pedigrees.
¿The linkage analysis showed that the defect causing the protease deficiency mapped to the long arm of human chromosome 9,¿ Tsal recounted. ¿That finding led us to a more refined analysis, and eventually we were able to identify the gene mutations in our specimens. It¿s not a single mutation; different families have different mutations.¿
Tsal and Ginsburg are now ¿attempting to develop antibodies that will serve as a diagnostic assay for the pathogenic protease deficiency. We are trying to make knockout mice,¿ Tsal went on, ¿so we can confirm in vivo that the deficiency of this gene has been caused by clinical TTP disease. Creating such an animal model will be useful when we have a new treatment to verify its efficacy in the disease. We are working on that.¿
Clinical Potential Seeks Protease Inhibitor
¿As for the clinical potential,¿ he went on, ¿we can try to produce recombinant ADAMTS13, which we believe is the critical factor for patients with TTP. And that can replace the current plasma exchange therapy. Treated instead with the enzyme, patients may not even require a catheter, but the medication can be administered by simple injection.
¿Excessive cleavage of the von Willebrand factor by protease,¿ Tsal pointed out, ¿is the underlying cause of bleeding tendency in some patients with von Willebrand disease. So theoretically,¿ he concluded, ¿there may be a therapeutic possibility if we can develop some way of inhibiting this protease.¿
A News & Views commentary in the Nature paper takes this clinical prospect a putative step further. ¿Finally,¿ it concluded, ¿because the exact location of the ADAMTS13 gene within the human genome has been mapped, gene therapy ¿ along the lines being explored for treating hemophiliacs ¿ may be a realistic option in the future for patients with inherited TTP.¿