Launching a multi-level data strategy for rare eye diseases: a methodological case study from national to European scale.

Rare eye diseases bring unique challenges in clinical research and patient care due to their heterogeneity, low prevalence, and dispersed expertise. To address these challenges, an integrated multilevel data strategy has been developed in France and Europe, enabling structured, interoperable data collection and reuse across national and transnational initiatives. In this paper, we present a methodological framework using the metaphor of a spaceship to illustrate the vertical articulation of this ecosystem, from national infrastructures to European platforms. At the base of the spaceship lies BaMaRa, France's national registry for rare diseases, which ensures the systematic collection of core clinical data at the point of care. The next stage is FREDD, a disease-specific data warehouse dedicated to rare eye conditions, developed by the SENSGENE network thanks to the France 2030 RaReTiA project. FREDD integrates granular phenotypic and genotypic data, allowing for in-depth research and cohort building. FREDD is fully interoperable with BaMaRa, ensuring data consistency and minimizing redundancies. The third level of the spaceship is REDgistry, the European registry for rare eye diseases coordinated by ERN-EYE. REDgistry allows for cross-border data harmonization and aligns with FAIR principles to facilitate data sharing and secondary use in international research. At the top of the structure is the European Health Data Space, a future platform enabling secure and standardized access to health data for research and policy-making at the EU level. At the core of this architecture remains the patient, whose data is the very foundation of the system. Although not always actively involved in data collection, the patient remains central to the purpose and structure of each data layer. Building and sustaining such a complex infrastructure requires the coordination of a wide range of stakeholders, including clinicians, research networks, data stewards, hospital IT departments, national authorities, and European institutions. The success of this multilevel model depends on shared standards, transparent governance, and sustained collaboration across all actors. Together, they form the propulsion system of the spaceship, enabling the structured reuse of high-quality data to accelerate research and improve care in the field of rare eye diseases.