What is glucocerebrosidase made of?

Why test for glucocerebrosidase enzyme activity?

Gaucher disease is an autosomal recessive disorder secondary to mutations in the gene that encodes for the enzyme glucocerebrosidase (GBA1).1,2 Therefore, a diagnosis of Gaucher disease is often based on clinical history, physical examination and laboratory testing, and confirmed by the finding of a deficiency in the enzyme activity of glucocerebrosidase.1 Genetic mutation studies are also recommended, and a family history of the disease can support the diagnosis of suspected or proven Gaucher disease.1

Previously, a diagnosis of Gaucher disease was often made by the presence of Gaucher cells in a bone marrow aspirate or when a patient presented with an unexplained massive splenomegaly and was treated with splenectomy. However, these diagnostic methods were not accurate because many storage cells, so-called pseudo-Gaucher cells, may be confused with Gaucher cells on the marrow examination. Nevertheless, a bone marrow examination may still be needed in patients where the splenomegaly does not regress on treatment, or if the patient develops enlarged lymph nodes or B symptoms that suggest development of a lymphoma.1

What is the glucocerebrosidase enzyme activity test?

Measurement of glucocerebrosidase enzyme activity in leukocytes or skin fibroblasts on a skin biopsy is considered the ‘gold standard’ for diagnosing Gaucher disease.1,3 Dried blood spots are used as a screening assay for glucocerebrosidase enzyme activity.4 A test using approximately 5 mL of EDTA or heparinised blood is all that is necessary to confirm a diagnosis of Gaucher disease as this allows direct measurement of glucocerebrosidase activity in leukocytes.5,6 If patients have leukopaenia, cultivated skin fibroblasts from a skin biopsy can be assayed instead.5 The determination of Gaucher disease must be made by specialised laboratories with particular experience in the measurement of glucocerebrosidase activity and its interpretation.5-9 In patients with Gaucher disease, glucocerebrosidase activity levels are approximately 10‒30% of normal.8,9

Residual glucocerebrosidase enzyme activity has been shown to significantly correlate with age, chitotriosidase enzyme activity, spleen size and greater disease severity in patients with Gaucher disease.10 Yet, measuring glucocerebrosidase enzyme activity does not distinguish between patients with Gaucher disease who are heterozygote carriers of mutations in the GBA1 gene and individuals who do not have Gaucher disease. Moreover, this diagnostic blood test does not provide histological information on the bone, liver or spleen for diagnosis.1 However, a statistically significant relationship between the residual enzyme activity of glucocerebrosidase and bone involvement has been previously noted.10

Clinical experience in diagnosing Gaucher disease

Results were reported from the biochemical diagnosis of blood samples taken from 5128 patients with suspected Gaucher disease from the Biochemical Genetics laboratory in Egypt. For each patient, 5 mL of whole blood were collected by venous puncture into EDTA tubes. Plasma was obtained for chitotriosidase assay and leukocytes were separated to determine the activity of glucocerebrosidase using synthetic substrate. In all cases, measurement of glucocerebrosidase activity was conducted in parallel with the assessment of chitotriosidase in peripheral leukocytes. In healthy unaffected individuals without Gaucher disease, normal enzyme activity was reported as 1‒5 µmol/g prot/h for glucocerebrosidase and 4‒80 µmol/L/h for chitotriosidase.11

Of the 5128 suspected cases of Gaucher disease, 882 patients (17%) were diagnosed with the disease. Most of these patients (81.5% [719/882]) showed positive parental consanguinity; the male to female ratio was 1.6 to 1. The age range for diagnosis was from 3 months to 45 years, with 97.5% of patients diagnosed between the ages of 1.7 to 8 years. A decrease in the activity of glucocerebrosidase was evident in 99% of patients diagnosed with Gaucher disease. The mean glucocerebrosidase activity value in these patients represented 0.3 µmol/g prot/h, which was 30% of the low normal value. In these patients, chitotriosidase activity levels were increased (mean [standard deviation (SD)] 6243 [20,211] µmol/L/h). However, in nine patients, chitotriosidase levels were zero, and glucocerebrosidase activity was 45% of the low normal value.11

In 103 cases of suspected Gaucher disease, an elevation in mean (SD) chitotriosidase activity (131.8 [24.0] µmol/L/h) was noted; however, mean glucocerebrosidase (3.2 µmol/g prot/h) levels were normal.11

Measuring glucocerebrosidase enzyme activity is considered the ‘gold standard’ for diagnosing Gaucher disease.1,3

C-ANPROM/INT//7567; Date of preparation: September 2020

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The current standard treatment for type 1 Gaucher disease is enzyme replacement therapy using infusions of natural or recombinant forms of glucocerebrosidase, the lysosomal enzyme that is deficient in Gaucher disease. Enzyme replacement therapy is generally well tolerated and has not been linked to serum enzyme elevations or to instances of clinically apparent acute liver injury.

Glucocerebrosidase is a lysosomal enzyme that is deficient or defective in the inherited condition known as Gaucher disease. The enzyme acts in lysosomes upon the sphingolipid glucocerebroside, catalyzing its conversion to glucose and ceramide. Glucocerebrosidase is an important step in the metabolism of glycolipids and its absence leads to accumulation of glycosylceramide in macrophages, giving rise to "foam cells" particularly in the spleen, liver and bone, leading to splenomegaly, hepatomegaly and bone dysplasia. In type 1 Gaucher disease, the most common form of this disease, the tissue damage is mostly limited to liver, spleen and bone, but sometimes involves lung and kidney. In types 2 and 3 Gaucher disease, there is also neurologic accumulation of foam cells and damage, leading to severe neurologic outcomes in infancy or childhood. Infusions of glucocerebrosidase have been shown to increase the activity of this enzyme intracellularly, to decrease foam cells and to ameliorate the signs of symptoms of type 1 Gaucher disease. The initial forms of glucocerebrosidase (alglucerase) used to treat patients were prepared from human tissue, largely placentas. Subsequently, recombinant forms of glucocerebrosidase were developed (imiglucerase, velaglucerase, taliglucerase) that have improved and more prolonged activity, allowing for less frequent infusions with better tolerance. All forms of glucocerebrosidase used clinically have been generally well tolerated with only mild adverse events, largely due to local or hypersensitivity reactions. None of the natural or recombinant forms of the enzyme have been linked to liver injury.

Alglucerase (al gloo' ser ase) is a placenta-derived form of purified glucocerebrosidase that was approved for use in type 1 Gaucher disease in the United States in 1991, the first drug approved as an enzyme replacement therapy. Alglucerase was typically administered three times weekly, and was withdrawn from use when recombinant forms of glucocerebrosidase became available that could be administered every 1 to 4 weeks, and appeared to have equal if not superior efficacy and similar or fewer side effects. Alglucerase is no longer commercially available.

Imiglucerase (im" i gloo' ser ase) was the first recombinant form of glucocerebrosidase approved for therapy of type 1 Gaucher disease. Imiglucerase is prepared by recombinant techniques using Chinese hamster ovary cells. It differs from the native enzyme in one amino acid (histidine at position 495 instead of arginine) and by modification of the glycosylation sites so that they terminate in mannose sugars, which are specifically recognized and taken up by macrophages. Imiglucerase was approved for use in the United States in 1994 and soon became the most widely used enzyme replacement therapy for Gaucher disease. It is available as a lyophilized powder in vials of 200 and 400 Units. The typical dose is 60 Units/kg given by intravenous infusion over 1 to 2 hours every two weeks. Side effects are uncommon and generally mild, but can include local infusion site reactions, fatigue, headache, dizziness, abdominal pain, nausea, diarrhea, back pain, fever and rash. Rare, but potentially severe adverse reactions include hypersensitivity reactions and anaphylaxis.

Velaglucerase (vel" a gloo' ser ase) alfa was the second recombinant form of glucocerebrosidase approved as therapy of Gaucher disease. Velaglucerase is produced in a human cell line using gene activation technology, resulting in production of a recombinant protein with the identical amino acid sequence as the native, human enzyme. The glucocerebrosidase producing cells are treated with enzymes that modify glycosylation and result in high-mannose type glycans which increase uptake of the velaglucerase by macrophages. Velaglucerase was approved for use as enzyme replacement therapy of type 1 Gaucher disease in the United States in 2010. It is available as a lyophilized powder in single use vials of 400 Units under the brand name Vpriv. The typical initial dose is 60 Units/kg intravenously every 2 weeks. Side effects are not common and usually mild, but can include local infusion reactions, fatigue, headache, dizziness, abdominal pain, nausea, back pain, joint pain and fever. Rare, but reported severe adverse reactions include hypersensitivity reactions and anaphylaxis.

Taliglucerase (tal" i gloo' ser ase) alfa was the third recombinant form of glucocerebrosidase approved for therapy of type 1 Gaucher disease. Taliglucerase is prepared in plant cell cultures transfected with the human glucocerebrosidase gene. Taliglucerase differs from the native enzyme by two amino acids at the N terminal and 7 amino acids at the C terminal end. It is glycosylated and the oligosaccharide chains have terminal mannose sugars, which increase its uptake by macrophages. Taliglucerase was approved for use as enzyme replacement therapy for type 1 Gaucher disease in the United States in 2011. It is available as lyophilized powder in single use vials of 200 Units. The typical dose is 60 Units/kg every two weeks administered intravenously over 1 to 2 hours. Side effects are not common and usually mild, but can include local infusion reactions, fatigue, headache, dizziness, abdominal pain, nausea, back pain, joint pain and fever. Rare, but reported severe adverse reactions include hypersensitivity reactions and anaphylaxis.

In preregistration controlled trials, liver test abnormalities were no more common with glucocerebrosidase enzyme replacement therapy than with placebo or other treatments. What abnormalities occurred were mild and resolved spontaneously, usually without need for dose interruption. During premarketing clinical trials and since its more widespread clinical availability, no reports of acute liver injury with jaundice attributable to natural or recombinant forms of glucocerebrosidase have been published.

LIkelihood score: E (unlikely to cause clinically apparent liver injury).

The natural and recombinant forms of glucocerebrosidase are proteins and are metabolized in multiple organs and tissues to polypeptides and amino acids. There is no reason for these proteins to cause liver disease other than by a hypersensitivity reaction or by their direct enzymatic reactivity.

Serum enzyme elevations that occur on enzyme replacement therapies are usually self-limited and mild and generally do not require dose modification or discontinuation of therapy. Persistent or prominent elevations should lead to evaluation for other forms of liver disease. No instances of acute liver failure, chronic hepatitis or vanishing bile duct syndrome due to enzyme replacement therapy of Gaucher disease have been reported.

Drug Class: Gaucher Disease Agents, Enzyme Replacement Therapy

Other Drugs in the Class: Eliglustat, Miglustat