Validating a New Severity Score System for Adults With Type 1 Gaucher Disease (GD1)

GD1 is a prototypical lysosomal storage disorder and the first disorder to have compelling evidence of successful treatment with enzyme replacement therapy. The common clinical manifestations are hematologic cytopenias, hepatomegaly, splenomegaly, and a spectrum of skeletal pathologies. Disease expression is diverse. The rate and extent of disease progression are variable and often independent of the age at which symptoms are first reported1. Despite a long history of treatment efficacy2, there is significant heterogeneity of response among patients with regard to the maximum improvement in hematologic, visceral, bone, and other manifestations and the dynamic speed of response during therapy1-3. There have been few well-designed studies that comprehensively annotate phenotypic variation over time or measure treatment efficacy and dose response. In part, this is attributable to lack of a validated disease severity scoring system for GD1 to standardize the monitoring of progression and treatment response and to define patient cohorts in clinical studies. DS3 is a method of expressing an integrated assessment of the burden of disease in a given patient. It can be used to monitor patient status, determine endpoints in clinical studies, classify disease phenotypes and compare patients with the same disease. Although frequently referred to as 'disease severity indices,' DS3 instruments may also include measures of disease activity and damage. DS3s utilize a minimal data set to score the patient in a comprehensive manner. They usually are structured as a group of domains (often according to organ system) that are populated with non-redundant items that are valid, reliable, use feasible, standardized methods of assessment, and that are variably weighted based on associated morbidity and mortality. A DS3 for adult GD1 patients was recently developed and subjected to successful preliminary testing for validity, reliability and feasibility4. With respect to changes over time, a minimal clinically important difference was defined. Construct validity has been partially demonstrated. Using 20 patient profiles from the International Collaborative Gaucher Group (ICGG) Gaucher Registry, the instrument was shown to correlate very well with the "gold standard" clinical global impression scale. However, larger scale testing in a population that is representative of the world wide distribution of GD1 phenotypes (including splenectomy patients) is needed and predictive validity has yet to be determined. Moreover, the DS3 has not yet been correlated with disease-specific measures of response such as achievement of therapeutic goals or broadly used biomarkers. Combining retrospective and prospective analysis, this study is designed to address these issues