Epigenetic alterations on C1‐inhibitor expression may influence hereditary angioedema attack frequency and C4 levels

Abstract

Epigenetic studies reveal how our genes (nature) are influenced by environment (nurture) leading to wide variability in clinical presentations, especially in autoimmune diseases. Patients with C1‐inhibitor deficiency, even within the same family, have diverse clinical presentations that may reflect epigenetic control of gene expression by hormones or inflammatory signals.

The interesting report by Karagianni P et al. on the finding of increased DNA methylation of H19 locus imprinting control region in saliva samples of Sjögren’s syndrome patients correlating with low complement C4 levels [1] may offer insights into how C4 levels may be regulated in serpinopathies such as C1‐inhibitor deficiency. An undetectable or low C4 level in patients with severe angioedema is a feature of C1‐inhibitor deficiency [hereditary angioedema (HAE) type I with low to absent function and antigenic levels; HAE type II with point mutations in the SERPING1 gene that affect the reactive centre loop affecting protein function only]. However, C4 levels do not always clinically correlate with disease activity [2], and up to 6% patients do not have known mutations in the SERPING1 gene [3]. Uncontrolled classical pathway activation and persistently low C4 levels in some HAE patients may lead to development of autoantibodies (Ro‐ and La‐antibodies) and a clinical picture consistent with lupus or Sjögren’s syndrome [4]. Cytokine studies before and during HAE attacks suggest that attacks are associated with changes in the balance of inflammatory and anti‐inflammatory cytokines. Excess interleukin (IL)‐23 signatures noted during attacks could skew the system towards pathogenic T helper type 17 (Th17) and autoimmunity [5]. Such clinical variability suggests that epigenetic alterations and changes leading to functional consequences in other members of the SERPIN superfamily have been described [6]. A clearer understanding of the role and functional consequence of DNA methylation of the SERPING1 gene may lead to newer therapeutic options for patients with HAE.

The mammalian genome has stretches of DNA with an unusually high frequency of cytosine (C) and guanine (G) nucleotides connected by a phosphodiester bond (referred to as CpG islands) that mainly appear at promoter regions of genes and conserved through evolution. The CpG islands are usually unmethylated to allow gene expression. Methylation of CpG by DNA methyltransferases (DNA methylation) decreases gene expression, while the addition of acetyl groups on lysine in histone tails by acetylases or transferases (histone acetylation) allows chromatids to spread out, increasing gene expression, and these are two major processes involved in regulation of gene expression without changes in DNA (epigenetic regulation). To understand epigenetic regulation, scientists commonly use 5′AZA‐2′‐deoxycytidine, which removes methyl groups, and Trichostatin A, that inhibits mammalian classes I and II histone deactylases preventing removal of acetyl groups, both of which would allow more gene expression. The SERPING1 locus has two CpG‐rich islands, and while the proximal island in exon 2 is unmethylated, the distant island in exon 1 is fully methylated. Using Huh‐7 cells exposed to 5′AZA‐2′‐deoxycytidine and Trichostatin A, Lopez‐Lera et al. show that global demethylation can increase SERPING1 mRNA expression and C1‐inhibitor protein expression several‐fold [7], suggesting that products of other genes can exert epigenetic effects on this region.

The product of the H19 gene is a paternal‐imprinted long non‐protein coding RNA (lnc RNA) that has the potential to affect genomewide DNA methylation [8]. The expression of this lnc RNA is developmentally regulated, varying widely in expression in tissues and cells, and is therefore likely to be affected by local hormonal or cytokine milieux, consistent with Karagianni’s comment that regulation of H19 expression can be influenced by stress and inflammatory signals. It is equally interesting to note that both oestradiol and androgen have effects on H19 expression [9]. Oestradiol can significantly promote H19 transcription via ERβ, while androgen stimulation via dihydrotestosterone decreased H19 cellular expression which was reversed by androgen blockade (enzalutamide). C1‐inhibitor deficient patients are exquisitely sensitive to the effects of oestrogen, such that anti‐androgen therapy provokes attacks in males while girls have more attacks during puberty. Androgen therapy has been proven to be effective prophylaxis in HAE. Do epigenetic changes in H19 locus by hormones offer an explanation for some of the notoriously unexplained variability in angioedema attacks or C4 levels in HAE patients? Understanding this complex network would require meticulous and well‐controlled experiments, but the findings would be undoubtedly fascinating.

Disclosures

The authors have no conflicts of interest to declare.