Analysis of C9Orf72 Expansions in Patients with Systemic Lupus Erythematosus and Rheumatoid Arthritis: Preliminary Data

The most frequent genetic cause of Amyotrophic Lateral Sclerosis (ALS) and Frontotemporal Lobar Dementia (FTLD) is a large hexanucleotide expansion (>30, mostly hundred/thousand repeats) within a non-coding region of the C9orf72 gene (1). The cut-off to distinguish normal and pathogenic expansions has not yet been defined, but most healthy individuals have 2-20 repeats on both alleles (usually 2, 5, 8). The pathogenic mechanism of the dominant mutation is most probably toxic gain of functions, through the production of repetitive transcripts and proteins. Nonetheless, C9orf72 reduced expression has been observed in post-mortem brains of mutated ALS/FTLD patients (2). Interestingly, while C9orf72 haploinsufficiency alone seems insufficient to cause neurodegeneration, decreased transcriptional activity with increasing numbers (7-24) of repeats has been demonstrated in vitro (3) and knockout mice exhibit immune dysregulations, developing features of autoimmunity, reminiscent of systemic lupus erythematosus (SLE), suggesting a protective role for the C9orf72 protein against autoimmune diseases (4). Recently, increased prevalence of autoimmune diseases has been observed in C9orf72 expansion-positive FTLD patients (5). We hypothesized that normal but in the upper range C9orf72 expansions could influence the immune system and investigated their size in a cohort of patients with rheumatoid arthritis (RA) and SLE. As a control group we studied a cohort of 49 ALS patients without pathogenic expansion. Methods: 29 SLE and 6 RA pts were screened for C9orf72 expansion, by the use of a PCR-based protocol, validated in our laboratory (6). Clinical and serological data were collected from clinical charts. A cut-off of ≥9 repeat units was considered in our analysis. Results: As expected, no patients with large expansions were found. The average and median values of repeat units were 5.29 (SD 2.87) and 6 in SLE (range 2-11), 5.08 (SD 4.14) and 3.5 (range 2-15) in RA and 4.8 (SD 3.05) and 5 (range 2-19) in the control population. We individuated ≥9 repeat units in 5/30 (16.7%) SLE patients and 2/6 (33.3%) RA patients; a prevalence higher than what found in ALS group (8.16%). We searched for clinical or serological differences among SLE pts with the normal and ≥9 repeat size expansion. Although those differences were not statistically significant, we reported a higher prevalence of kidney involvement in patients with a number of repeats ≥9 (5/6; 83.3% versus 7/23; 30.4%), p=0.056. Conclusion: Conclusion: our preliminary results indicate that ≥9 repeats within the C9orf72 gene are detectable in a non negligible number of patients with systemic autoimmune disease, confirming the possible role of C9orf72 in autoimmune system. The possible association with specific subset of disease must be confirmed in a larger cohort of patients.