PI3K(p110α) as a determinant and gene therapy for atrial enlargement in atrial fibrillation
Authors: Ezeani Martin , Prabhu Sandeep 
Published: July 2022

Atrial fibrillation (AF) is an irregular heart rhythm, characterised by chaotic atrial activation, which is promoted by remodelling. Once initiated, AF can also propagate the progression of itself in the so-called ‘‘AF begets AF’’. Several lines of investigation have shown that signalling molecules, including reactive oxygen species, angiotensin II, and phosphoinositide 3-kinases (PI3Ks), in presence or absence of cardiovascular disease risk factors, stabilise and promote AF maintenance. In particular, reduced cardiac-specific PI3K activity that is not associated with oncology is cardiotoxic and increases susceptibility to AF. Atrial-specific PI3K(p110α) transgene can cause pathological atrial enlargement. Highlighting the crucial importance of the p110α protein in a clinical problem that currently challenges the professional health care practice, in over forty (40) transgenic mouse models of AF (Table1), currently existing, of which some of the models are models of human genetic disorders, including PI3K(p110α) transgenic mouse model, over 70% of them reporting atrial size showed enlarged, greater atrial size. Individuals with minimal to severely dilated atria develop AF more likely. Left atrial diameter and volume stratification are an assessment for follow-up surveillance to detect AF. Gene therapy to reduce atrial size will be associated with a reduction in AF burden. In this overview, PI3K(p110α), a master regulator of organ size, was investigated in atrial enlargement and in physiological determinants that promote AF.Table 1

Transgenic and Knockout Mouse Models of AF

Gene Alteration
Atrial enlargement
Fibrosis
Thrombus
Ventricular dysfunction based on echo and/or catheter
Conduction abnormalities by ECG
APD Alteration
AF pattern/other major cellular and molecular mechanisms
References

Rho GDIα TG
Cardiac-specific overexpression of Rho GDP dissociation inhibitor (GDI)α with α-myosin heavy chain (α-MHC) promoter
Atrial weight 0.6-fold increase vs NTg at 4 months but no changes at 4 weeks
✔ no significant increase in atrial and ventricle
Not reported
 ↔ 
Sinus bradycardia, varying degrees of AV block, prolongation of P-wave duration, and PR interval at 7 months
Not reported
SpontaneousOther mechanismsoreduced Connexin 40 expressionoincreased expression of RhoA, Rac1, and Cdc42
[58]

RhoA
Cardiac-specific overexpression of RhoA with α-MHC promoter
Atrial weight threefold increase vs NTg
✔ inventricle
Not reported
✔
Bradycardia and AV block
Not reported
SpontaneousOther mechanismsoincreased expression of hypertrophic genesoInflammation
[59]

Junction TG
Cardiac-specific overexpression of junctin protein with α-MHC promoter
Atrial weight, more than tenfold increase vs WT for right atrium
✔ in atrial and ventricle
✔ in left and right atria
✔
Bradycardia
Atrial and ventricle APD70,phase 3 ↑
SpontaneousOther mechanismsoreduced triadin, RYR2, diastolic Ca2+, and Ca2+ transient amplitude
[60]

Junctate 1 TG
Cardiac-specific SR-located Ca2+-binding proteinjunctate 1 overexpression with α-MHC promoter
Atrial weight, fourfold increase for left atrium and about fivefold increase for right atrium vs WT
↑ in atria and ventricle
✔ Intra-atrial thrombi
✔
Ventricular bigeminy, sinus pause, and bradycardia
APD90, phase 4 ↑
SpontaneousOther mechanismsoreduced phospholamban phosphorylation, troponin I phosphorylation, Calreticulin, and RyR2 channeloreduced SR Ca2+ content, Ca2+ transient amplitudeoincreased ICa,L
[61]

AMPK TGN488I
Cardiac-specific PRKAG2 (AMPK γ2 subunit) overexpression with missense mutation
Not reported
Not reported
Not reported
✔
Reduced PR interval,persistent sinus bradycardia without AV block
Not reported
Spontaneous and paroxysmalOther mechanismsocardiac glycogen accumulation
[62]

A1AR TG
Cardiac-specific overexpression of A1 adenosine receptor (A1AR) with α-MHC
No difference
No fibrosis
Not reported
✔
Slow AV conduction
APD90, phase 4 ↔ APD50,phase 2 ↔ APD70,phase 2 ↔ 
Spontaneous
[63]

A3tg TG
Cardiac-specific overexpression of A3 adenosine receptor (A3AR) with α-MHC promoter
Atrial size onefold and twofold increase at 12 weeks and 21 weeks, respectively, vs NTg
Not present in atria and ventricle
Not reported
✔
Absence of normal sinus rhythm, bradycardia, and intermittentlycomplete
Not reported
SpontaneousOther mechanismsoreduced SERCA mRNA levels
[64]

RTEF1 TG
Cardiac-specific overexpression of Transcription enhancer factor-1-related factor(RTEF1) with α-MHC promoter
Atrial weight4–sixfold increase vs control
Not present in atria and ventricle
✔ Organised
Not reported
Slow conduction in working myocardium, prolonged PR interval, and QRS duration
Not reported
SpontaneousMechanismsoincreased PP1β phosphataseochronic dephosphorylation of cardiac connexin
[65]

ACE 8/8 TG
Cardiac-restricted angiotensin-converting enzyme (ACE)Overexpression with α-MHC Ang II concentration was 4.3-fold higher in ACE mice compared to WT
Atrial weight, about threefold increase vs WT
✔ in atria but not in ventricle
Not reported
✔
AV block
Not reported
Spontaneous
[66]

Kir2.1 TG
Kir2.1 IK1 channel subunit cardiac-specific overexpression with α-MHC promoter
Atrial weight, left and right atrial to body weight 65% and 141% increase, respectively, vs control
Not reported
Not reported
✔
Absence of T wave and reduced QT interval
APD90, phase 4 ↓APD50,phase 2 ↔ APD75,phase 3 ↔ MAP90Phase 4 ↓MAP75phase 3 ↓MAP50,phase 2 ↔ 
Spontaneous
[67]

Kcne1−/−
K+-channel KCNE1 subunit global protein deletion in mouse
Normal atrial size
Not present in atria and ventricle
Not reported
 ↔ 
AV block
APD50, phase 2 ↓APD90, phase 4 ↓
Spontaneous
[68]

hKCNE1-hKCNQ1 TG
Human (h)KCNE1-hKCNQ1 Cardiac-specific overexpression with α-MHC promoter in mouse
Not reported
Not reported
Not reported
Not reported
Complex atrial and irregular ventricular excitation
β-AR mediatedAPD50,phase 2 ↑APD90, phase 4 ↓
SpontaneousOther mechanismsoIncreased IKs density
[69]

Des−/−
Desmin global knockout
Not reported
Not reported
Not reported
Not reported
Supraventricular premature beats, spontaneous ventricular premature beats, and Wenckebach periodicity
Not reported
SpontaneousOther mechanismsoHypokalemia,oReduced refractory period
[70]

CREM-IbΔC-X
Human cAMP-response element modulator (CREM) heart-directedoverexpression with α-MHC promoter
Atrial weight, about 5–sevenfold increase vs NTg at 12–16 weeks
Not present in left atrium and ventricle
✔ Organised thrombi in left and right atria
✔
Not reported
Not reported
SpontaneousOther mechanismsoReduced phosphorylation of CREB and of PLBoIncreased phosphorylation of SERCA2, PP1, and mRNA levels of ANP
[71]

CREM-IbΔC-X
Human cAMP-CREM heart-directedOverexpression with α-MHC promoter
Left atrial size, twofold increase vs WT at 13–17 weeks
↑ in atria
Not reported
Not reported
Ectopic beats
APD25,phase 1 ↑APD50,phase 2 ↑APD90phase 4 ↑
Spontaneous and persistentOther mechanismsoLeaky SR Ca2+ storesoDownregulation of connexin 40
[72]

CREM-IbΔC-X
Human cAMP- CREM and reduced RyR2-S2814A phosphorylation heart-directedoverexpression with germline transmission and Meox2-Cre crossing
Atrial weight, sixfold increase vs WT at 3 months
↑ in atria and ventricle
Not reported
 ↔ 
Spontaneous atrial ectopy
APD80, phase 4 ↑
Spontaneous at 3-month paroxysmal and persistent at 4–5 monthsOther mechanismsoincreased SR Ca2+ leak and CaMKII activityoreduced connexin 40
[73]

JDP TG
Heart-restricted c-Jun dimerization protein 2 overexpression with α-MHC promoter
Atrial cell diameter 1.4-fold increase vs WT
Not present in the atrial and ventricle
Not reported
 ↔ 
Increased PR interval, AV block andWenckebach periodicity
Not reported
SpontaneousOther mechanismsoreduced expression of connexin 40 and 43oAng II signalling
[74]

RacET
Heart-restricted constitutively active Rac1 RhoGTPase overexpression with α-MHC promoter
Atrial weight, fourfold increase vs WT
↑ in atria and ventricle
Not reported
✔
No observable conduction defects except AF
Not reported
Spontaneous and persistentOther mechanismsoincreased NADPH oxidase activity
[75]

Anxa7−/−
Annexin global knockout
Not reported
Not reported
Not reported
 ↔ at basal
AV block, ventricular tachyarrhythmia, shorter P-wave and QRS duration, and abnormal conduction velocity
Not reported
SpontaneousOther mechanismsoreduced protein expression of SERCA2aoincrease expression of NCX proteinoβ1-adrenergic signalling
[76]

TNF1.6 TG
Heart-directedoverexpression of tumour necrosis factor-α with α-MHC promoter
Isolated atrial area 3.6-fold increase from 6 to 9 months in female vs NTg
✔ in atria
✔ Organised thrombi in atria
Not reported
Episodes of second degree AV block, premature beats, and Ventricular ectopy
APD75Phase 4 ↔ 
SpontaneousOther mechanismsoimpaired Ca2+ loadingoreduced intracellular Ca2+ transients
[77]

MHCsTNF TG
Cardiac-specific overexpression of tumour necroticfactor with α-MHC promoter
Not reported
Not reported
Not reported
✔
AV junctional rhythm, short PR interval and wide QRS complex
Not reported
SpontaneousOther mechanismsoreduced connexion 40 expressionoinflammation
[78]

MURCTG
Cardiac-specific overexpression of muscle-related coiled-coil protein with α-MHC promoter
Enlarged atrial compared to NTg
↑ in atria and ventricle
Thrombus in the left atrial
✔
Complete AV block and prolongation of the PR interval
Not reported
SpontaneousOther mechanismsoreduced SERCA2, increased ANP, BNP, βMHC, TGF-β1, TGF-β2, and TGF-β3
[79]

Nup155±
Reducednuclear envelope permeability by nucleoporin (NUP) 155 gene missense mutation on R391H
Not reported
Not reported
Not reported
Not reported
Irregular RR intervals
APD90, phase 4 ↓
SpontaneousOther mechanismsoreduced HSP70 nuclear localization
[80]

a1D−/−
L-type Ca2+ channel (Cav1.3) subunit global knockout
Not reported
Not reported
Not reported
Not reported
SA andAV nodes conduction defects
Not reported
SpontaneousOther mechanismsolack of Cav1.3, and reduced ICa,L
[81]

LTCC (α1D−/−)
L-type Ca2+ channel α1D subunit global knockout
Smaller compared with WT
Not reported
Not reported
Not reported
Sinus bradycardia and AV block
Not reported
SpontaneousOther mechanismsoreduced ICa,L, Ca2+ transient amplitude, and SR Ca2+ content
[82]

dnPI3K-DCM
Cardiac-specific dominant negative phosphoinositide 3-kinase p110α (dnPI3K) DCM due to overexpression of mammalian sterile 20-like kinase 1 expression with α-MHC promoter
Atrial size 3.45-fold increase vs NTg
↑ in atriaand ventricle
✔ Chronic thrombi in the left atrium
✔
Prolonged PR intervals, double peak P-wave, and second and third degreeAV block
Not reported
SpontaneousOther mechanismsoaltered expression of metabolic genes and K+ channelsoreduced HSP70
[16]

Dct−/−
Melanin synthesisenzyme dopachrome tautomerase global knockout
Not reported
No
Not reported
 ↔ 
No observable conduction defects except for AF
APD50, phase 2 ↔ APD90, phase 4 ↔ 
SpontaneousOther mechanismsoplasma membrane caveolae accumulationoenlargement of mitochondria
[83]

RyR2R176Q/+
R176Q mutation in RYR2 gene through germline transmission and Meox2-Cre crossing
Normal atrial size
No fibrosis in atrial and ventricle
Not reported
Not reported
RR interval variability, absence of P-wave
APD50 phase 2 ↔ APD80 phase 4 ↔ 
SpontaneousOther mechanismsoincreased CaMKII-dependent phosphorylation of RyR2oelevated SR Ca2+ leak
[84]

Gαq TG
Overexpression of activated Gαqcardiac protein with α-MHC promoter
Left atrial size, 2.5-fold increase vs WT
↑ in atria but not in ventricle
✔ Left atrial, unorganised thrombus
Not reported
Premature atrial contraction and irregular RR interval
APD80, phase 4 ↑
Spontaneous
[85]

NppaCre+Pitx2−/−
Atrial and ventricular-restricted loss of function of paired-like homeodomain transcription factor 2 (PITX2)
Atrial length about 1.6-fold increase for left atrium and 1.2-fold increase for right atrium vs WT
↑ in ventricle but not in atria
Not reported
Not reported
AV block
APD20 phase 1, ↔ APD50 phase 2, ↔ APD90 phase 4, ↔ 
SpontaneousOther mechanismsoreduced expression of Pitx2,oreduced expression of Nav1.5oreduced expression of Kir2.1
[86]

AnkB±
Ankyrin-B (ANK2) heterologous null mutation
Not reported
Not reported
Not reported
✔
Spontaneous bradycardia and abnormal ventricular response
APD90 phase 4, ↓
SpontaneousOther mechanismsoreduced ICa,Loreduced Cav1.3 expression,osignalling interaction between ankyrin-B and Cav1.2
[87]

D1275N-Nav1.5
Human sodium channelNav1.5 global missense mutation
Not reported
No
Not reported
✔
prolongation of P-wave and QRS duration PR interval and AV block
APD50, phase 2 ↑APD90, phase 4 ↑
SpontaneousOther mechanismsoreduced peak INaoincreased late INa
[88]

SLN−/−
Sarcolipin global knockout
No difference
↑ in atriabut not in ventricle
Not reported
Not reported
Small oscillatory waves
APD50, phase 2 ↔ APD90, phase 4 ↑
SpontaneousOther mechanismsoSR Ca2+ overloadoDADsoincreased phosphorylation of RyR2
[89]

FKBP12.6−/−
FK506-binding protein deficiency with reduced RYR2 phosphorylation at S2814
Not reported
Not reported
Not reported
Not reported
Absence of P-waves and irregular RR intervals
APD30, phase 2 ↔ APD50, phase 2 ↔ 
SpontaneousOther mechanismsoLack of FK506-binding protein 12.6oDADsoSR Ca2+ leakoincreased INCXoCaMKII phosphorylation of RYR2 and PLB
[90]

MHC-TGFcys33ser
Cardiac-restricted constitutively active TGFβ1 overexpression with αMHC promoter
Not reported
↑ in atria
Not reported
Not reported
Activation wavefront
APD80, phase 4 ↓ for both left and right atria
SpontaneousOther mechanismsoincreased Ca2+ transient
[91]

DN-MSTN TG13 TG
Heart-directed overexpression of the N-terminal pro-peptide with α-MHC promoter
Atrial weight 3.7-fold increase vs NTg
↑ in atria
Appears present
 ↔ 
AV block,BradycardiaIncreased P-waves and QRS duration
Not reported
SpontaneousOther mechanismsoreduced connexin 40 expression
[92]

Casq2−/−
Calsequestrin 2 global knockout
Atria tissue area, about 1.8–2.0-fold increase vs WT
No differences
Not reported
✔
Atrial ectopic activity, bradycardia
APD80, phase 4↑
Spontaneous
[93]

LKB1 knockout
Cardiac-specific AMPK-activating liver kinase B1(LKB1) knockout with α-MHC promoter
Atria size, about twofold increase for paroxysmal at 4–6 weeks and threefold increase for persistent AF over 6 weeks vs WT
↑ in atria
✔ Intra-atrialthrombi
 ↔ 
Increased PR interval andQRS duration in paroxysmal AF
Not reported
Paroxysmal and persistentOther mechanismsoreduced expression of AMPKoincreased in connexin 40 and 43 expressionoROS and inflammation
[94]

F1759A-Nav1.5-dTG
Human sodium channelNav1.5 cardiac-specific expression with α-MHC promoter
Right and left atria area increase by 52% and 54%, respectively, vs control
↑ in atria and ventricle
Not reported
✔
Premature ventricularcomplexes andnon-sustained polymorphic VT
APD80, phase 4 ↑ for both right and left atria
SpontaneousOther mechanismsoincreased late INaoincreased glycogen accumulationomyofibril disorganisationomitochondria injuryoNCX regulation of Na+ entry
[95]

LKB1/CTR
LKB1/CT atrial-specific knockdown
Not reported
↑ in atria
Not reported
 ↔ 
Irregularly irregular R–R intervals
Not reported
SpontaneousOther mechanismsoAtrial cardiomyocyte produces calcitoninoCalcitonin receptor and its ligand signalling governs fibroblast rolesoParacrine signalling between atrial cardiomyocyte released calcitonin and fibroblast
[96]

PLK2 deficiency
PLK2 Knockout
Greater left atrial area
↑ in atria
Not reported
 ↔ 
ventricular tachycardia
APD ↔ ERP ↔ 
SpontaneousOther mechanismsoPLK2/ERK/OPN is a dominant structural remodelling axis for AF generation
[97]

Mouse models that have been used to study the pathophysiology of AF, including atrial enlargement, electrophysiological alterations, apoptosis, functional and molecular underpinnings, and anatomical, transgenic; RYR2, ryanodine receptor 2; SR, sarcoplasmic reticulum; APD, action potential; SERCA mRNA, sarco/endoplasmic reticulum Ca2+-ATPase messenger ribonucleic acid; CTR, calcitonin receptor; KCNE1, potassium voltage-gated channel subfamily E member 1; AV, Atrioventricular block; MAP, monophasic action potential; PLB, phospholamban; ANP, atrial natriuretic peptide; β-AR, beta adrenergic receptor; PPβ1, protein phosphatase type 1β; NADPH, nicotinamide adenine dinucleotide phosphate; CaMKII, Ca2+/calmodulin-dependent protein kinase II; NCX, sodium–calcium exchanger; SERCA2a, Sarco/endoplasmic reticulum calcium (Ca2+) ATPase gene; TGF- β, Transforming growth factor beta; BNP, brain natriuretic peptide; HSP70, heat shock protein 70; DCM, dilated cardiomyopathy; AMPK, 5' adenosine monophosphate-activated protein kinase; PLK2, polo-like kinase 2; OPN, osteopontin; ERK1/2, extracellular signal-regulated kinase ½. ↔ unchanged in that condition; ✔ present in that condition; ↑ increased in that condition; ↓ reduced in that condition