Diagnosis and Monitoring of Gauchers Disease at Great Ormond Street Hospital

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A diagnostic service for lysosomal storage disorders, including Gauchers disease, has been provided by Great Ormond Street Hospital for Children , London since 1970. Over the last 25 years, the Enzyme Laboratory, which is part of the Hospital's Chemical Pathology Services and is located at the Institute of Child Health, has diagnosed 125 cases of Gauchers disease. Professor Bryan Winchester is head of the Division of Biochemistry and Genetics and Elisabeth Youngand Cathy Meaney are responsible for the biochemical and mutational analysis respectively. They explain here what goes on behind the scenes:

A deficiency of the enzyme glucocerebrosidase is diagnostic for Gauchers disease. Usually the enzyme level is measured in white blood cells but occasionally, for instance if the patient has had repeated blood transfusions or has a very low white blood cell count, then the enzyme is assayed (measured) in cells cultured from a skin biopsy.

Assay of the enzyme allows diagnosis of all three clinical sub-types (Types 1, 2 and 3) but unfortunately the test does not distinguish between the sub-types or indicate the severity of the disease. The assay is semi-automated but requires experience in interpretation of the results.

The Enzyme Laboratory currently receives over 1,200 blood samples annually for diagnosis of lysosomal storage diseases and offers a turn-round time of a maximum of four weeks from receipt of the sample to report of result. Culture of skin cells can take up to 6 weeks before enough material is available for enzyme assay.

Carrier Detection

Carrier detection by enzyme analysis is not always clear cut as there is an overlap in the level of enzyme activity in carriers and non-carriers. However the isolation of the gene for glucocerebrosidase and the discovery of many of the underlying mutations allow the reliable detection of carriers in families where the mutation is known.

Mutations

Our laboratory provides a service for the detection of the three most common mutations: N370S, L444P and 84gg, which account for about 90% and 50% of the mutations in the Ashkenazi Jewish and non-Jewish populations respectively. However over 50 different mutations have been discovered and most of them are rare or unique to a particular family.

Although the correlation between the severity of the disease (phenotype) and the mutations (genotype) is not perfect, some mutations are associated with the different types of Gauchers disease. The great majority of patients homozygous (having inherited the same mutation from both parents) for the N370S mutation, which is the most prevalent one in the Jewish population, have the milder Type 1 disease. In contrast, patients homozygous for the L444P mutation, which is more common in the non-Jewish population, generally have a neuronopathic (involvement of the nervous system) form of the disease, usually Type 3.

Patients who are compound hetero-zygotes (have inherited a different mutation from each parent) for the N370S and L444P mutations normally have Type 1 disease as do those who have the N370S and 84gg mutations (the second most common mutation in the Jewish population). There is, however, variation in the severity of the disease among patients with the same genotype, making prediction of the course of the disease very difficult.

So far, we have analysed DNA from 54 patients with Gauchers disease: see the Table in the next column.

The clinical diagnosis of one patient was revised from Type 1 to 3 on the basis of mutation analysis.

At present material has to be sent to laboratories overseas for identification of rare mutations. Our laboratory could and would carry out this research work in collaboration with Anne Harris' laboratory in Oxford if funding was available.

Therapy

Two specific forms of therapy for Gauchers disease are available, enzyme replacement therapy (ERT) and bone marrow transplantation (BMT). BMT has been performed in a number of patients and has been shown to be therapeutic although there is a 5%-10% mortality risk even from a matched sibling. The risk is even greater with transplants from unrelated donors.

ERT with modified glucocere-brosidase purified from natural sources (Ceredase) or produced synthetically (Cerezyme) has become the choice for Type 1 Gauchers disease. There is some evidence that BMT may be the treatment of choice for selected patients with Type 3 disease.

Marker to Monitor Response

As clinical improvement is relatively slow, a biochemical marker to monitor the response of treatment is extremely useful. Dr Hans Aerts and his colleagues in Amsterdam have shown that there is marked increase in the level of another enzyme, chitotriosidase, in the blood plasma of most patients with Gauchers disease and that this activity decreases after ERT. The rate of decrease of the chitotriosidase activity may be useful in deciding the appropriate dose of Ceredase or Cerezyme.

Fortunately we had stored plasma that had been obtained from Gauchers patients prior to commencement of treatment and therefore we have been able to monitor and compare the progress of patients who are having ERT or who have had BMT.

BMT-treated patients show complete correction of the chitotriosidase activity after several years post-transplant. This provides direct evidence, for the first time, that gene therapy using bone marrow cells may be more effective than ERT in the treatment of Gauchers disease, as the mechanism of gene therapy and BMT are very similar.

Pre-natal Diagnosis

Our laboratory also offers prenatal diagnosis of Gauchers disease following chorionic villus sampling at 10-11 weeks gestation or amniocentesis at 15-16 weeks. Glucocerebrosidase can be assayed directly in chorionic villus samples and a result is available within 48 hours. Following amniocentesis, cells have to be cultured for 3-4 weeks before assay.

Technically prenatal diagnosis is also possible by mutation analysis of foetal DNA if the mutations are known in an affected offspring and both parents. However this will not generally offer any advantage over the enzyme-based test. Although several laboratories in the UK offer enzymatic diagnosis of Gauchers disease, only three offer pre-natal diagnosis and two DNA analysis.

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Source: Gauchers News September 1996

© Copyright Gauchers Association 1996