Flemish network on rare connective tissue diseases (CTD): patient pathways in systemic sclerosis. First steps taken

Despite the low prevalence of each rare disease, the total burden is high. Patients with rare diseases encounter numerous barriers, including delayed diagnosis and limited access to high-quality treatments. In order to tackle these challenges, the European Commission launched the European Reference Networks (ERNs), cross-border networks of healthcare providers and patients representatives. In parallel, the aims and structure of these ERNs were translated at the federal and regional levels, resulting in the creation of the Flemish Network of Rare Diseases. In line with the mission of the ERNs and to ensure equal access to care, we describe as first patient pathways for systemic sclerosis (SSc), as a pilot model for other rare connective and musculoskeletal diseases. Consensus was reached on following key messages: 1. Patients with SSc should have multidisciplinary clinical and investigational evaluations in a tertiary reference expert centre at baseline, and subsequently every three to 5 years. Intermediately, a yearly clinical evaluation should be provided in the reference centre, whilst SSc technical evaluations are permissionably executed in a centre that follows SSc-specific clinical practice guidelines. In between, monitoring can take place in secondary care units, under the condition that qualitative examinations and care including interactive multidisciplinary consultations can be provided. 2. Patients with early diffuse cutaneous SSc, (progressive) interstitial lung disease and/or pulmonary arterial hypertension should undergo regular evaluations in specialised tertiary care reference institutions. 3. Monitoring of patients with progressive interstitial lung disease and/or pulmonary (arterial) hypertension will be done in agreement with experts of ERN LUNG.

OP0309 WES ANALYSIS OF RARE FAMILIES POINTS TO A PATHOGENIC ROLE FOR THE cGAS/STING-TYPE I IFN AXIS IN SSc

Background

Systemic sclerosis (SSc) is a rare (prevalence ±20/100,000), chronic condition characterized by vascular, immunological and connective tissue abnormalities. Genetic factors contribute to disease, as evidenced by a 15-fold increased relative risk in relatives of patients. Case-control studies have identified numerous associated loci; the identification and functional validation of risk alleles has however been elusive. No monogenic or oligogenic causes of SSc have been demonstrated, likely due to the infrequency of familial clustering (≤ 1.6% of cases) in this disease. We are fortunate to have access to DNA samples from five such families, with two affected first-degree relatives each: To our knowledge the only series of its kind.

Objectives

The goal of our research is to identify and functionally validate genes that drive SSc pathogenesis in these families, thereby shedding light on fundamental disease mechanisms. In light of the apparent lack of a multi-generational effect, we hypothesize that SSc may be transmitted as a di/oligogenic trait in these families, with two-to-a-few genes acting together to cause disease.

Methods

Whole exome sequencing (WES) was performed on all available blood-DNA (Figure 1). We filtered for variants that are (a) shared by both affected individuals within each family, (b) absent-to-rare in the general population, and (c) predicted to affect protein function by multiple in silico tools. Candidate gene prioritization and pathway analysis is followed by in vitro testing of a subset of SSc-variants, to assess for their effects on protein function. Where two strong candidate genes are identified in the same family, in vitro validation of both variants will be followed by the generation of single- vs. double- knock-in mice, to test for di/oligogenic effects of variant combinations on SSc-related phenotypes.

Figure 1.

SSc families. Filled symbols: affected. Arrows: WES on blood-DNA. lSSc: Limited SSc; lcSSc: Limited cutaneous SSc; dcSSC: Diffuse cutaneous SSc. cGAS/STING (red) and IFN I (black) - related candidate genes.

Results

WES yielded 23-45 genes per family with variants that satisfied the above criteria. Prioritization of candidates was based on systematic annotation of all 180 genes for expression pattern, function, and potential impact of the SSc-variants. Our data suggest familial SSc is genetically heterogeneous, with little sharing of candidates between families. We therefore assessed for pathways represented in gene-lists from multiple families, and found that participants in the cGAS/STING - type I-IFN axis are identified in all (Figure 1). This would suggest a unifying disease mechanism.

cGAS/STING is a sentinel system responsible for the production of anti-viral type I interferons (IFN-α/β) and pro-inflammatory cytokines in the presence of cytoplasmic double stranded DNA (dsDNA). cGAS/STING-dependent self-dsDNA sensing has been implicated in silica-induced inflammation: a risk factor in SSc and pulmonary fibrosis. A type I IFN-response signature is identified in SSc patients and correlates with disease severity. Mutations in two of our candidates cause Mendelian diseases with phenotypic overlaps with SSc, and all but one cause disease-relevant phenotypes when perturbed in mice. Finally, preliminary in vitro data suggest gain-of-function effects of the variant identified in TMEM173/STING, and loss-of-function effects in NT5E/CD73 and PRKD2 (the last with two different variants identified in two families; Figure 1). All would lead to increased IFN-I signaling.

Conclusion

We report, for the first time, WES analysis of a familial SSc series of unprecedented size for this rare disease. The identification of potential “driver” gene variants in cGAS/STING-type I IFN signaling regulators across all families points to an important pathogenic role for this pathway in SSc.

Disclosure of Interests

Pierre Maus: None declared, Vanessa Smith: None declared, Tessa Du Four: None declared, Marie Vanthuyne: None declared, Bernard Lauwerys Shareholder of: UCB Pharma, Employee of: Currently an employee of UCB Pharma, Nisha Limaye: None declared