- LYSO-PROOF – Determine the Prognostic Value of Lyso-Gb1 for Monitoring the Progress of Gaucher Disease
LYSO-PROOF – Determine the Prognostic Value of Lyso-Gb1 for Monitoring the Progress of Gaucher Disease
Gaucher disease is an autosomal recessive inherited lysosomal storage disorder. The disease is caused by the hereditary deficiency of the glucocerebrosidase, a lysosomal enzyme that breaks down glucocerebroside into glucose and ceramide. Gaucher disease is the most common sphingolipidosis and it is among the most frequent inherited diseases among the ethnic group of Ashkenazi Jews. The gene that encodes for glucocerebrosidase is located on the long arm of the chromosome 1 and contains 11 exons. So far, more than 400 different mutations have been described in Gaucher patients, the most frequent of which is the missense mutation but also the frame-shift and splice-site mutation and insertions and deletions are known. The more frequent mutations are N370S, L444P, IVS2+1G>A, c.84insG, R463C, and R496H. Clinical symptoms are numerous. The classical symptoms affect visceral organs (hepatosplenomegaly) and the skeletal system (bone marrow infiltrations up to bone infarcts and pathological fractures) with consecutive changes in the blood panel (anemia, thrombocytopenia) (Sidransky, 2004).A major distinguishing factor is the occurrence of neurological manifestations (myoclonus epilepsy, hydrocephalus, eye motion disorders). Today, it is discussed whether the division into the classical three types of disease (type 1: non-neuronopathic form; type 2: the acute neuronopathic form; type 3: the chronic neuronopathic form) are still applicable because in clinical practice the presenting symptoms are only insufficiently reflected with this classification.
There is no clear genotype-phenotype relationship. The same DNA mutation is seen in patients with a completely different characterization of the disease. An exception is the mutation N370S, which so far has been found only in the visceral form (type1), but still may have a high phenotypic heterogeneity in GD Type 1 (Fairley et al., 2008).
The outcome of the non-neuronopathic diseases could be improved greatly with the introduction and widely available enzyme replacement therapy. Among these are the reductions of the size of the liver and spleen as well as normalization in the blood count (Elstein and Zimran, 2009). The glucocerebrosides that are not metabolized are stored in the entire reticuloendothelial system. The macrophages enriched with glycolipids form the pathoanatomic basis of the so-called "Gaucher cells" which can be verified with the light microscope. The accumulation of Gaucher cells in liver, spleen, and bone marrow is thought to underlie the characteristic hepatosplenomegaly, pancytopenia and bone complications of GD patients (Beutler 2001). There are no causative cures for lysosomal storage diseases and treatment is mostly symptomatic, although mostly enzyme replacement therapy (ERT) has been demonstrated especially for the treatment of non-neuronopathic GD with good success (Elstein and Zimran, 2009). In addition, substrate reduction therapy (SRT), a method used to decrease the accumulation of storage material, is currently being evaluated for some of these diseases. In SRT, oral administration of the iminosugar N-butyldeoxynojirimycin aims to reduce Glucosylceramide (GlcCer) synthesis and thus limit its accumulation. However, SRT is only approved for mildly tomoderately affected patients in whom ERT is not a therapeutic option (Ficicioglu, 2008). Additionally, Eliglustat is currently under investigation for the primary oral treatment of GD type 1 patient (Lukina et al., 2014).
To date a definitive diagnosis of Gaucher's disease can only be made applying biochemical testing measuring the reduced enzymatic activity of the beta-glucosidase together with genetic confirmation. Since numerous different mutations may be the cause of a particular lysosomal storage disease the sequencing of the entire beta-glucosidase gene is applied in Gaucher's disease in order to confirm the genetic diagnosis (Zimran, 2011). Although there are attempts to apply easy diagnostic methods based on associated biochemical abnormalities such as high alkaline phosphatase, angiotensin-converting enzyme (ACE) and immunoglobulin levels, or, in case of Gaucher's disease, by cell analysis showing "crinkled paper" cytoplasm and glycolipid-laden macrophages, there is an unmet need for a simple biochemical test exhibiting highly specific and highly sensitive detection at an early stage, monitoring progression of the disease and early monitoring the efficacy of applied therapies.
A biomarker should be technically feasible in many hands, easy to measure; useful, with a consistent, relative magnitude between patients and controls, or treated and untreated patients; reliable, precise, and accurate clinically, and classifiable as strongly predictive or prognostic. In GD some lysosomal enzymes, used as indirect biomarkers, were found to be elevated, including tartrate-resistant acid phosphatase, hexosaminidase, and a human chitinase, chitotriosidase. Thus, there are attempts to monitor the reduction of storage cells in tissues by measurement of such surrogate markers of Gaucher cells like chitotriosidase and CCL18 (Hollak et al., 1994; Boot et al., 2004). However, the use of plasma chitotriosidase as a Gaucher cell marker is hampered by the fact that patients, including those with Gaucher's disease, may be deficient in chitotriosidase activity due to a 24-base pair (bp) duplication in the chitotriosidase gene. Obviously these individuals cannot be monitored by the measurement of plasma chitotriosidase activity. The frequency of the homozygous 24-bp duplication in the chitotriosidase gene depends on the ethnicity and can vary between 6% and nearly 35% in the Latino population (unpublished data). In these cases the marker CCL18 is used. Furthermore, elevated levels of CCL18 were also found to be associated with a variety of diseases, such as different types of cancer and inflammation of joints, lungs and skin. For example, the ascitic fluid of patients suffering from ovarian cancer contains a significantly elevated level of CCL18 compared to patients without this carcinoma (Budd-Chiari syndrome). Since it attracts and activates specific immune cells CCL18 plays a role in tumor suppression. Furthermore, children having acute lymphocytic leukemia are found to exhibit elevated levels of CCL18. Plasma CCL18 levels do not reflect one particular clinical symptom, but rather are a reflection of the total body burden of Gaucher cells. The use of primary storage molecules as biomarker was assessed for glucosylceramide (Gb1) in plasma of Gaucher's disease patients and compared to the level of Gb1 in healthy individuals (Groener et al. 2008). Already in 1989 Rosengren et al. (1989) showed that in lipidoses not only the catabolism of the major sphingolipid but also its deacylated compound is affected. It is important to note that until today no use of a highly specific and highly sensitive biomarker is routinely used beside the methods described above, that exhibits an unsatisfactory limit of detection, sensitivity and/or specificity and thus proved to be unsuitable for clinical application.