| Model name/type | Specific genetic modification | Key cardiac phenotypes observed | Lethality / viability | Relevance to human disease |
|---|---|---|---|---|
| Global **Hcn4** knockout | Constitutive global loss of **Hcn4** | Severely diminished **I<sub>f</sub>**, ~40% reduction in embryonic heart rate, defective sinoatrial node/conduction system development (pqac-00000028, pqac-00000027) | Embryonic lethal; death in utero around E9.5-E11.5 (pqac-00000026, pqac-00000027, pqac-00000028) | Demonstrates that HCN4 is essential for embryonic pacemaker development and baseline cardiac automaticity; models severe loss-of-function end of HCN4 disease biology rather than typical survivable human AD SSS2 (pqac-00000017, pqac-00000026) |
| **Hcn4 R669Q** knock-in / mutant | Single amino-acid substitution abolishing cAMP-dependent regulation while preserving other channel properties (pqac-00000028, pqac-00000029) | Reduced heart rate, loss of catecholaminergic/cAMP responsiveness, impaired physiological chronotropic control (pqac-00000028, pqac-00000029) | Embryonic lethal (pqac-00000028, pqac-00000029) | Mechanistically models human HCN4 variants that disrupt cyclic-nucleotide regulation and supports the importance of cAMP-dependent HCN4 gating in sinus node function (pqac-00000014, pqac-00000028) |
| **Hcn4FEA** knock-in | Three point mutations rendering HCN4 cAMP-insensitive (“silenced” cAMP-dependent regulation) (pqac-00000027, pqac-00000028, pqac-00000029) | Pronounced resting bradycardia, severe sinus dysrhythmia, sinus pauses, chronotropic incompetence/intrinsic sinus node dysfunction, isorhythmic AV dissociation, junctional escape rhythm, excess nonfiring pacemaker cells (pqac-00000005, pqac-00000028, pqac-00000029) | Viable adult model (pqac-00000029) | Considered especially relevant to human HCN4-related sinus node dysfunction because it reproduces moderate bradycardia/dysrhythmia seen with cAMP-regulation-defective HCN4 mutations without developmental lethality (pqac-00000027, pqac-00000028) |
| Inducible **Hcn4** knockout (adult / conditional deletion) | Postnatal or adult inducible deletion of **Hcn4** in the heart (pqac-00000027) | ~75% reduction in sinoatrial **I<sub>f</sub>**, recurrent sinus pauses, mild SND in some settings; in more severe settings marked bradycardia and conduction defects (pqac-00000026, pqac-00000027) | Phenotype ranges from viable with pauses to lethal severe bradycardia/conduction disease depending on degree/timing of deletion (pqac-00000027) | Useful model of acquired intrinsic sinus node dysfunction in established hearts; shows HCN4 is required for maintenance of adult SAN function, not only development (pqac-00000010, pqac-00000026, pqac-00000027) |
| Selective cardiomyocyte **Hcn4** deletion | Cardiac myocyte-restricted ablation of **Hcn4** (pqac-00000026) | Failure of mature pacemaker cell formation with severe conduction/pacemaker dysfunction (pqac-00000026) | Embryonic lethal (pqac-00000026) | Supports cell-autonomous requirement of HCN4 in pacemaker lineage and explains why strong loss-of-function can produce profound sinus node disease phenotypes (pqac-00000017, pqac-00000026) |
| Dominant-negative HCN4 / reduced current transgenic models | Selective reduction of HCN4 current or expression of dominant-negative HCN4 lacking cAMP sensitivity (pqac-00000026) | Progressive severe bradycardia, AV block, reduced spontaneous AVN cell activity under basal conditions; some models progress to cardiac arrest (pqac-00000026) | Variable; some models progress to death/cardiac arrest (pqac-00000026) | Mimics dominant-negative mechanisms described in many human heterozygous HCN4 variants causing autosomal dominant sinus node dysfunction (pqac-00000002, pqac-00000018, pqac-00000026) |


*Table: This table summarizes major mouse models used to study HCN4-related cardiac pacemaker dysfunction, including their genetic design, phenotypes, viability, and translational relevance to autosomal dominant sick sinus syndrome.*