Rationale:

A structural and functional atrial remodeling plays a key role in the development and especially perpetuation of atrial fibrillation (AF). We previously reported that inactivation of histone deacetylase 2 (HDAC2) reduced atrial remodeling and the onset of atrial fibrillation (AF) in CREM-IbΔC-X transgenic mice (TG), a well-characterized model with extensive atrial remodeling and spontaneous onset of AF. Recently, the VITAL Rhythm Study gave new rise to a discussion about sex differences in atrial fibrillation and indicated that women even have a higher risk for atrial fibrillation after controlling for height and/or body size. Here, we present data showing that AF develops more rapidly in female than in male TG animals and that concomitant inactivation of HDAC2 in TG has a more potent effect on atrial remodeling and AF development in female animals.

Material & methods:

Mice with cardiomyocyte-specific inactivation of HDAC2 (KO) were generated by crossbreeding HDAC2^loxP/loxP and αMHC^Cre+/- mice and were subsequently mated with TG mice. Experiments were performed on CTR (FVB/N^Cre+/-), TG (CREM-IbΔC-X^Cre+/-) and TGxKO mice. Macroscopic spontaneous Ca²⁺ events and Ca²⁺ sparks were recorded in isolated atrial cardiomyocytes (ACMs) with Fluo-4/AM using a myocyte calcium and contractility system (Ionoptix) or a ZEISS LSM7. RNA isolated from atrial tissue was analyzed by qPCR. Thrombus frequency was determined in mouse hearts and the amount of atrial fibrosis by Masson-Goldner trichrome staining.

Results:

Increased atrial fibrosis in TG was reduced in TGxKO in both sexes to a similar extent. The proportion of thrombus-positive atria increased in TG with age (2% at 6-8 weeks to 64%^# at 30 weeks, n=25-42). Simultaneous HDAC2 inactivation reduced thrombus formation exclusively in female mice (TG: 50%, TGxKO: 9%^#; 30 weeks, n=11-14). The occurrence of spontaneous Ca²⁺ oscillations was increased in ACMs from female TG mice (48%*) vs CTR (12%) and was reduced by simultaneous HDAC2-inactivation to 13%^# (TGxKO), while there were no differences between genotypes in male mice (n=9-12, ACMs: 5-10/mice). Ca²⁺ spark frequency was increased in ACMs from TG mice (µm²/s±SEM: CTR: 10±1 TG: 16±2*; n=6, 58-68 ACMs). Spark frequency was also increased in ACMs from male TGxKO mice (µm²/s±SD: CTR: 10±2, TGxKO: 15±3*; n=3-4, 24-36 ACMs) but not in ACMs from female TGxKO mice vs CTR. mRNA levels of genes involved in Ca²⁺ cycling (Slc8a1, Atp2a2, Ryr2, Casq2) were down-regulated in TG by 42-71%* compared to CTR (n=10/genotype). Slc8a1, Ryr2 and Casq2 down-regulation was limited in female TGxKO, while in male TGxKO mRNA expression was unchanged vs TG. TG mice reliably developed AF between 7^th and 13^th week of life. However, female TG mice developed AF on average 2 weeks earlier. Inactivation of HDAC2 delayed the time course of AF development in female TGxKO to that of male TG mice (*p<0.05 vs CTR, ^#p<0.05 vs TG).

Conclusion:

Our data illustrate that arrhythmogenic impairment of Ca²⁺ homeostasis is more pronounced in female TG mice and that inactivation of HDAC2 favorably affects this substrate and thrombus formation in TGxKO female animals, correlating with beneficial effects on the development of atrial fibrillation. This underscores the importance of these substrates in the development of AF and suggests that inhibition of HDAC2 may represent a therapeutic option against atrial remodeling that may have a greater impact on females.

Supported by the DFG