Background and Objectives Women, having two X chromosomes, are more predisposed than men to autoimmune diseases. The X chromosome contains many genes linked to immunity which may contribute to this gender bias. In a mouse model, a duplication of the innate immunity X-linked toll like receptor 7 (Tlr7) gene has been shown to potentiate autoimmunity in males. We then proposed to investigate whether TLR7 gene and its neighboring paralog TLR8 could have variations in their copy numbers, contributing to the pathogenesis of rheumatoid arthritis (RA) in men.
Methods A real-time quantitative PCR protocol was developed to assess copy number variation (CNV) of TLR7 and TLR8 gene, using sensitive and optimised ΔΔCt and standard curve methods, in DNA from peripheral blood mononuclear cells of 60 patients with RA (including 49 men) and 64 healthy controls (including 42 men). Among them, 31 men with RA and 18 healthy men were further screened for TLR7/8 CNV in 4 subpopulations: B cells, T cells, granulocytes and the depleted fraction of the former 3.
Results TLR7/8 copy numbers significantly increased with age in PBMCs from all men (P < 0.0001, Spearman’s rank correlation test), whether they had RA or not. The increase had mean amplitude of 20%, spanning from the age of 20 until 80, according to the linear-regression-curve’s best fit. This age-dependent and disease-independent CNV increase was also observed in all cell subsets. Interestingly, such increase was not observed in women, healthy or with RA, but rather an opposite trend.
Conclusion and Perspectives For the first time we showed an increased CNV in TLR7 and TLR8 genes which is age and sex-mediated. Several hypotheses could explain such phenomenon. For example, somatically acquired duplications can affect some cells over time and result in an increase with age in men. In parallel, X chromosome monosomy, previously described in aging women could account for the opposite trend in those. Another explanation for men can be due to the presence of feminine microchimerism, arising from feto-maternal or twin sister exchange during in utero life, as previously described. Such cells would carry 2 X chromosomes and contribute to the increased pool of X-linked genes among XY host cells. Investigating these hypotheses would provide better understanding of age-associated X-linked genetic modifications and the role of the X chromosome in gender differences in health and disease.