Juvenile idiopathic arthritis (JIA) is a complex rheumatic disease with both autoimmune and autoinflammatory components. Recently, familial cases of systemic-onset JIA have been attributed to mutations in LACC1/FAMIN. We describe three affected siblings from a Moroccan consanguineous family with an early-onset chronic, symmetric and erosive arthritis previously diagnosed as rheumatoid factor (RF)-negative polyarticular JIA. Autozygosity mapping identified four homozygous regions shared by all patients, located in chromosomes 3, 6 (n:2) and 13, containing over 330 genes. Subsequent whole exome sequencing identified two potential candidate variants within these regions (in FARS2 and LACC1/FAMIN). Genotyping of a cohort of healthy Moroccan individuals (n: 352) and bioinformatics analyses finally supported the frameshift c.128_129delGT mutation in the LACC1/FAMIN gene, leading to a truncated protein (p.Cys43Tyrfs*6), as the most probable causative gene defect. Additional targeted sequencing studies performed in patients with systemic-onset JIA (n:23) and RF-negative polyarticular JIA (n: 44) revealed no pathogenic LACC1/FAMIN mutations. Our findings support the homozygous genotype in the LACC1/FAMIN gene as the defect underlying the family here described with a recessively inherited severe inflammatory joint disease. Our evidences provide further support to the involvement of LACC1/FAMIN deficiency in different types of JIA in addition to the initially described systemic-onset JIA.

This work has been partially funded by: CERCA Programme/Generalitat de Catalunya (JIA, XE, SO), the PERIS program of the Generalitat de Catalunya grant SLT002/16/00310 (RR), the Spanish Ministry of Economy and Competitiveness co-financed by European Regional Development Fund (ERDF) grant SAF2015-68472-C2-1-R (JIA), the Instituto de Salud Carlos III/Transnational Research Projects on Rare Diseases (JIA) grant AC15/00027, the Spanish Society of Pediatric Rheumatology (JIA), the Secretaria d’Universitats i Recerca del Departament d’Economia grant 2009-SGR-1502 (XE) and the European Union Seventh Framework Programme (FP7/2007-2013) grant agreement no. 262055 (XE).