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Chitotriosidase is a newly identified enzyme which is dramatically elevated in symptomatic Gauchers patients. Dr Hans Aerts, Department of Biochemistry, Academic Medical Center, University of Amsterdam, who discovered this marker whilst working with colleagues devoted to helping Gauchers sufferers in Holland, explains the enzymes potential application in the diagnosis and monitoring of enzyme replacement therapy:
The underlying defect in Gauchers disease is an inherited deficiency in the activity of the enzyme glucocerebrosidase. As a result, the capacity of all cells in the body to degrade or breakdown the natural lipid (fatty substance) glucocerebroside is impaired. In the vast majority of cells, this is without consequences since the residual enzyme activity is still sufficient for the required degradation of the lipid glucocerebroside.
Unfortunately, in tissue macrophages (certain cells in the body, notably the spleen, liver and bone marrow and, in the case of Type 2 and 3, the brain), the amount of lipid may sometimes exceed the cellular capacity of degradation, resulting in glucocerebroside accumulation.
The abnormal, lipid laden macrophages are called Gaucher cells. It is generally believed that the formation and accumulation of Gaucher cells in the various body locations is the critical process that ultimately leads to the development of clinical symptoms. The removal and prevention of the ongoing formation of abnormal storage cells can be considered to be the ultimate goal of therapy for Gauchers disease.
In view of the crucial role of Gaucher cells in the course and therapy of the disease, we focused attention to the identification of convenient markers (measurements) for these abnormal cells.
Ideally, one would like to monitor the occurrence of Gaucher cells by measuring some directly related abnormality of the blood. We therefore compared in detail the proteins that are present in the plasma (part of the blood) of Gauchers patients and control subjects. This investigation led to the discovery of a novel enzyme that is dramatically elevated in the circulation of symptomatic Gauchers patients.
The enzyme, which we named chitotriosidase, can be very sensitively measured. In fact, the plasma or serum prepared from less than a droplet of blood is highly sufficient for the chitotriosidase measurement.
In the plasma of almost all symptomatic Gauchers patients, but not pre-symptomatic individuals, chitotriosidase activity is at least 100-fold increased above normal values. At present, data have been obtained for more than 600 patients, with the average activity in patient plasma samples being more than 100-fold the mean normal value. The abnormality in chitotriosidase activity in plasma of Gauchers patients is far more prominent than that of any other parameter known so far.
A considerable number of laboratories in Europe and the USA are already performing the plasma chitotriosidase measurement as an additional test for the biochemical confirmation of the diagnosis of Gauchers disease. It should be stressed however that specific expertise is required to assure accurate measurement. In particular, the existence of other enzymes may complicate measurements.
About 1 in every 20 patients does not show any chitotriosidase activity in plasma. This inherited deficiency occurs with a similar frequency among control subjects and appears not to be linked with a clinical complication.
Recently, we have succeeded to purify chitotriosidase and to clone the genetic material that encodes the enzyme.
The origin of the elevated chitotriosidase in the circulation of symptomatic Gauchers patients is of interest. Evidence has been obtained that Gaucher cells are the most likely source of the enzyme. For example, in a collaborative investigation with Prof Cox and Dr Mistry at the University of Cambridge, we were able to demonstrate that freshly isolated Gaucher cells from the spleen of a Gauchers patient contain large amounts of chitotriosidase.
At present we are investigating the presence of chitotriosidase in the liver, kidney, spleen, lung and bone marrow.
In this way we will be able to determine whether Gaucher cells in various locations are all producing chitotriosidase. Furthermore, we will be able to analyse to what extent the elevations in plasma chitotriosidase correlates with particular clinical symptoms.
Preliminary data suggest that storage cells in all tissues contribute to the elevation in chitotriosidase in the blood. The plasma enzyme level therefore seems to be a reflection of the total body content on Gauchers cells, and not specifically related to one particular clinical sign such as enlarged spleen or skeletal deterioration.
Enzyme Replacement Therapy
Enzyme replacement therapy results in a relatively rapid and pronounced decrease in plasma chitotriosidase activity. Complete correction in plasma chitotriosidase is generally not observed within the first years of treatment.
Comparison of data for several hundreds of patients on different dosage regimens reveals a dose-dependency in average response. Patients who receive higher amounts of Ceredase tend to show a more pronounced reduction.
However, marked differences are noted among individual patients who are identically treated. In our view, these findings stress the importance of individualised dosage for enzyme replacement therapy.
At present the value of changes in plasma chitotriosidase as predictive indicator for clinical improvement upon enzyme therapy is being assessed. Preliminary data suggest that poor responses in plasma chitotriosidase are generally accompanied by a suboptimal clinical response. Further detailed analysis is required to allow definitive statements in this connection.
Bone Marrow Transplants
It is of interest to mention that Dr Vellodi at the Institute of Child Health in London, in collaboration with Dr Winchester and Dr Young, observed for a number of Gauchers patients that successful bone marrow transplantation resulted in a gradual, complete correction in plasma chitotriosidase.
Important applications for plasma chitotriosidase measurement may be found in routine diagnosis and (early) detection of disease manifestations. The potential value of plasma chitotriosidase as additional guideline for optimalisation of therapeutic intervention requires further investigation.
I would like to thank all those who contributed to the investigations on chitotriosidase. Prof Timothy Cox and Dr Pram Mistry are particularly thanked for their help and many stimulating suggestions. Furthermore, the continuous support of the members of the Netherlands Gaucher Society is acknowledged.
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Source: Gauchers News March 1996.
© Copyright Gauchers Association 1996