Clinical evaluation of chemokine and enzymatic biomarkers of Gaucher disease

Abstract

Purpose:

Gaucher disease is an exemplary orphan disorder. Enzyme replacement therapy with imiglucerase is effective, but very expensive. To improve the assessment of severity of disease and responses to this costly treatment, we have evaluated several enzymatic biomarkers and a newly-described chemokine.

Subjects and methods:

We studied 48 untreated adults with Type I Gaucher disease: 20 patients were studied after the introduction of enzyme replacement. Disease activity was monitored by serial measurement of platelet count, visceral volumes (spleen and liver) by magnetic resonance imaging, serum activities of total acid phosphatase, angiotensin-converting enzyme (ACE) and the lysosomal chitinase, chitotriosidase. Pulmonary and activation-regulated chemokine (PARC/CCL 18) was also determined in serum by ELISA.

Results:

Serum PARC concentrations were elevated 10–40-fold in patients with Gaucher disease compared with 67 healthy controls, without overlap (P < 0.0001). Unlike chitotriosidase, PARC was detectable in all individuals. Serum PARC was a reliable indicator of splenic (R = 0.53, P < 0.01) and liver (R = 0.65, P < 0.01) volume and platelet count (R = 0.50, P < 0.01). In splenectomized patients and in patients with null alleles of the chitotriosidase gene, serum PARC concentration correlates with visceral volume and other biomarkers of disease activity. Unlike chitotriosidase, serum PARC concentrations showed unbiased covariation with splenic and platelet responsiveness to enzyme replacement.

Conclusion:

Serum PARC concentrations are correlated with visceral Gaucher disease and with key clinical responses to enzyme complementation. Determination of this chemokine is a facile and universally applicable method that permits objective monitoring of enzyme replacement therapy for patients with Gaucher disease.

Introduction

Gaucher disease is a leading orphan disorder: its treatment has been revolutionized by the introduction of enzyme replacement therapy (ERT) [1], [2], [3], [4]. A second-line oral substrate reduction treatment has also been licensed [5].

A major consequence of the introduction of treatment for very rare diseases is the high cost—an issue that applies particularly to Gaucher disease. This cost places a particular responsibility on physicians to prescribe in a cost-effective manner. To meet these goals, there is a need for new biomarkers of the disease that can be monitored simply and which reflect therapeutic responses.

Gaucher disease (OMIM 230800) results from deficiency of glucocerebrosidase [6]. Pathological macrophages (Gaucher cells) engorged with undegraded glycosphingolipids [7], [8], [9] accumulate in liver, spleen and bone marrow, causing hepatosplenomegaly and thrombocytopenia. Bone marrow infiltration is associated with painful episodes of avascular necrosis. Episodic infarction of the spleen and liver followed by fibrotic scarring also occurs frequently in the untreated condition [10]. There is striking variation in the clinical manifestations of Gaucher disease, even in monozygotic twins homozygous for the mild N307S mutation [11]. The systemic manifestations of Gaucher disease are accompanied by evidence of inflammation with prominent immunoglobulin and cytokine responses [9], [12], [13].

Enzyme replacement therapy induces salutary responses in most Gaucher patients [14]. The response of patients to ERT varies and certain tissues appear to be more resistant to enzymatic complementation than others. Clearly established tissue injury and scarring are irreversible, so that although the treatment is beneficial for the systematic manifestations, bone disease may demonstrate a lesser response [14], [15], [16]. In an attempt to maximize cost-effectiveness, some clinicians prescribe doses of enzyme replacement therapy that vary according to disease severity and therapeutic responses [17], [18], [19], [20], [21]. No comparative trials have shown that individualized dosing strategies are more effective than fixed dose regimens. Reliable biomarkers of disease severity and of the responses to treatment are thus highly sought after for the evaluation of therapeutic responses, though any potential biomarker requires a systematic analysis of its clinical usefulness.

An ideal biomarker should be greatly elevated in the disease and show no overlap between untreated patients and healthy subjects. The marker should reflect the total burden of disease. Its concentration or activity should change in response to treatment in parallel to key clinical endpoints. An ideal biomarker should not vary in response to factors unrelated to the disease, such as genetic variation in the population. Finally, an ideal surrogate biomarker should be estimated quickly, reliably and cheaply in easily accessible body fluids.

Although several biomarkers are in widespread use for monitoring Gaucher disease [12], [22] (tartrate-resistant acid phosphatase (TRAP) [23], [24], angiotensin-converting enzyme (ACE) [25] and chitotriosidase [26]), these analytes vary widely in their ability to reflect disease activity and none meets ideal criteria. TRAP is neither specific for Gaucher disease nor greatly elevated [27]; the protein is unstable and is subject to wide analytical variability [28]. ACE activity is subject to variable expression related to a common genetic polymorphism [29] and decreased by the use of frequently-prescribed ACE inhibitors [30]. Chitotriosidase has achieved popularity for monitoring and is often elevated several hundred-fold in patients with active disease [26]. Chitotriosidase activity falls briskly on the introduction of ERT and rises again when treatment is stopped. However, analysis of chitotriosidase activity is technically complex, labor-intensive [31] and not standardized between laboratories. It is also subject to genetic variation [32]: about one-third of the population is heterozygous for a null (insertion) variant and expresses approximately half the activity of chitotriosidase observed in those who are wild-type. Five percent of the population is homozygous for this null chitotriosidase variant and enzyme activity is absent, even in Gaucher disease.

Recently, a novel chemokine, pulmonary and activation-regulated chemokine (PARC/CCL18/macrophage inhibitory protein-4), has been described [33]. Although its function in humans is as yet unknown, this member of the human CC-chemokine family is produced by macrophages and dendritic cells as a product of “alternative” activation [34], which has been implicated in chronic inflammation and fibrotic scarring. Through development of a Gaucher-specific splenic cDNA library, Moran and colleagues [35] demonstrated greatly increased expression of PARC mRNA in Gaucher spleen. Boot and co-workers have recently applied surface-enhanced laser desorption/ionization time-of-flight mass spectrometry (SELDI-TOF MS) to demonstrate marked elevation of PARC/CCL18 in plasma of patients with untreated Gaucher disease [36]. Plasma CCL18 concentrations, like chitotriosidase activity, decreased during the introduction of enzyme therapy.

Since there is a need for reliable surrogate biomarkers of disease activity for the monitoring of treatment in Gaucher disease with expensive orphan drugs, we have conducted a clinical evaluation of PARC/CCL18 and enzymatic markers to identify the most informative biomarkers for clinical application.