Alternative titles; symbols
HGNC Approved Gene Symbol: HCN2
Cytogenetic location: 19p13.3 Genomic coordinates (GRCh38) : 19:589,881-617,159 (from NCBI)
| Location | Phenotype |
Phenotype MIM number |
Inheritance |
Phenotype mapping key |
|---|---|---|---|---|
| 19p13.3 | {Epilepsy, idiopathic generalized, susceptibility to, 17} | 602477 | Autosomal dominant | 3 |
| Febrile seizures, familial, 2 | 602477 | Autosomal dominant | 3 | |
| Generalized epilepsy with febrile seizures plus, type 11 | 602477 | Autosomal dominant | 3 |
The HCN2 gene encodes a voltage-gated cation channel that belongs to a family of similar HCN genes that play a role in generating neuronal and cardiac automaticity. Upon hyperpolarization, these channels carry an inward current mediated by Na+ and K+, termed I(f) in the heart and I(h) in neurons. HCN2 is widely expressed in the brain (summary by Santoro et al., 1997 and Nakamura et al., 2013; review by Rivolta et al., 2020).
Santoro et al. (1997) cloned a member of the voltage-gated potassium channel family from mouse brain (Bcng1) that contained a carboxy-terminal cyclic nucleotide-binding domain (CNBD) and proposed that it is a candidate gene for pacemaker channels. Using heterologous expression of Bcng1, Santoro et al. (1998) demonstrated that Bcng1 codes for a channel with properties indistinguishable from pacemaker channels in brain and similar to those in heart. Santoro et al. (1998) subsequently used sequence homology to clone 2 human genes, BCNG1 (602780) and BCNG2, that are closely related to Bcng1. The mouse and human genes display unique patterns of mRNA expression in different tissues, including brain and heart. Both BCNG1 and BCNG2 contain the conserved motifs of a voltage-gated potassium channel, including the S1-S6 transmembrane segments, a charged S4 voltage sensor, and a pore-lining P loop. In addition, both contain a conserved CNBD in their respective carboxy termini. The core region of the human BCNG2 protein is 98% similar to that of mouse Bcng1. The main sites of BCNG2 expression are brain and heart.
Ludwig et al. (1999) independently cloned HCN2 from a human heart cDNA library. They found that the HCN2 cDNA encodes a protein of 889 amino acids.
The HCN2 gene contains 8 exons spanning about 27 kb (Ludwig et al., 1999).
Ludwig et al. (1999) noted that the HCN2 gene is contained within 2 cosmid clones localized to chromosome 19p13.3.
Crystal Structure
Zagotta et al. (2003) investigated the mechanism underlying the modulation of HCN channels by cyclic nucleotides by studying the C-terminal fragment of HCN2 containing the cyclic nucleotide-binding domain and the C-linker region that connects the cyclic nucleotide-binding domain to the pore. X-ray crystallographic structures of this C-terminal fragment bound to cyclic AMP or cyclic GMP, together with equilibrium sedimentation analysis, identified a tetramerization domain and the mechanism for cyclic nucleotide specificity, and suggested a model for ligand-dependent channel modulation. Because the cyclic nucleotide-gated and eag- (see 603305) and KAT1-related families of channels share amino acid sequence similarity with HCN channels, Zagotta et al. (2003) concluded that they are probably related to HCN channels in structure and mechanism.
Ludwig et al. (1999) observed that when expressed in HEK293 cells, HCN2 gives rise to hyperpolarization-activated cation currents with the hallmark features of the native cation current. HCN2 has fast activation kinetics, and Ludwig et al. (1999) concluded that HCN2 may underlie the fast component of the cardiac hyperpolarization-activated cation current.
By constructing truncation mutants, Wainger et al. (2001) demonstrated that the CNBD inhibits activation of the core transmembrane domain of HCN family members. Cyclic AMP binding relieves this inhibition. Differences in activation gating and extent of cAMP modulation between the HCN1 and HCN2 isoforms result largely from differences in the efficacy of CNBD inhibition.
The rate of action potential firing in nociceptors is a major determinant of the intensity of pain. Possible modulators of action potential firing include the HCN ion channels, which generate an inward current, I-h, after hyperpolarization of the membrane. Emery et al. (2011) found that genetic deletion of HCN2 removed the cAMP-sensitive component of I-h and abolished action potential firing caused by an elevation of cAMP in nociceptors. Mice in which HCN2 was specifically deleted in nociceptors expressing NaV1.8 (604427) had normal pain thresholds, but inflammation did not cause hyperalgesia to heat stimuli. After a nerve lesion, these mice showed no neuropathic pain in response to thermal or mechanical stimuli. Emery et al. (2011) concluded that neuropathic pain is therefore initiated by HCN2-driven action potential firing in NaV1.8-expressing nociceptors.
Dibbens et al. (2010) identified a variant in the HCN2 gene that resulted in a 9-bp deletion (2156delCGCCGCCGC) removing 3 proline residues at 719 to 721 (delPPP) from a 7-proline repeat close to the cyclic nucleotide-binding domain (602781.0001). The deletion was present in 3 (2.3%) of 65 patients with generalized epilepsy with febrile seizures plus (GEFSP11; 602477) and in 3 (2.5%) of 61 patients with febrile seizures (see FEB2, 602477), whereas it was present in only 3 (0.2%) of 772 controls. In vitro functional expression studies in Xenopus oocytes showed that the delPPP variant had a 35% increase in current size in response to hyperpolarization compared to wildtype. This current increase would depolarize the membrane potential, taking the neuron closer to the firing potential, and thus could enhance neuronal excitability. The HCN2 delPPP variant was not observed in patients with idiopathic generalized epilepsy who did not have febrile seizures.
In 2 unrelated Japanese children with febrile seizures, Nakamura et al. (2013) identified a heterozygous missense variant in the HCN2 gene (S126L; 602781.0002). The mutations were found by direct sequencing of the HCN2 gene among a cohort of 160 children with febrile seizures. In 1 case, the S126L variant was inherited from a similarly affected mother. Electrophysiologic studies in HEK293 cells expressing the mutation showed that it had elevated sensitivity to increased temperature compared to controls. The mutant channels showed faster activation, a greater depolarizing shift, and increased current density at higher temperatures compared to wildtype. The findings suggested that S126L mutant channels may augment neuronal excitability during hyperthermia.
In affected individuals from 3 unrelated families with idiopathic generalized epilepsy-17 (EIG17; 602477), Li et al. (2018) identified heterozygous missense mutations in the HCN2 gene (S632W, 602781.0003 and V246M, 602781.0004). The mutations, which were found by direct sequencing of 585 patients with suspected genetic epilepsy, segregated with the disorder in the families from whom DNA was available. In vitro electrophysiologic studies showed that both mutations caused a depolarizing shift in activation, a greater slope, and faster activation kinetics compared to controls, consistent with a gain-of-function effect. These findings were also observed when expressed with wildtype, indicating a dominant effect.
In a 28-year-old man with onset of autosomal recessive idiopathic generalized epilepsy-17 (see EIG17; 602477) at age 12 years, DiFrancesco et al. (2011) identified a homozygous missense variant in the HCN2 gene, resulting in a glu515-to-lys (E515K; 602781.0005). The mutation, which was found by direct sequencing, was present in the heterozygous state in multiple family members who did not have seizures. In vitro functional expression studies in CHO cells showed that the homomeric mutant, but not heteromeric mutant/wildtype channels, were inhibited. There was a large negative shift of activation with lowering of the activation threshold and slowed activation kinetics, effectively abolishing HCN2 contribution to resting activity, consistent with a loss-of-function effect. Transfection of the mutation into rat cortical neurons resulted in similar changes, as well as an increase in cell excitability and firing frequency compared to wildtype.
Chung et al. (2009) identified a spontaneous recessive mouse mutant, 'apathetic' (ap/ap), that showed ataxia, uncoordinated motor movements, and seizures resembling generalized absence and tonic-clonic convulsions. Some heterozygous mice showed absence seizures, and most had enhanced susceptibility to chemoconvulsive seizures, consistent with neuronal hyperexcitability. EEG showed aberrant spike-wave activity. The associated mutation was a frameshift in the murine Hcn2 gene, resulting in a loss of protein expression and function. The findings suggested that I(h) channels are critical for maintaining normal neuronal network oscillations.
Dibbens et al. (2010) identified a variant in the HCN2 gene that resulted in a 9-bp deletion (c.2156_2164delCGCCGCCGC, NM_001194) removing 3 proline residues at 719 to 721 (delPPP) from a 7-proline repeat close to the cyclic nucleotide-binding domain. The deletion was present in 3 (2.3%) of 65 patients with generalized epilepsy with febrile seizures plus (GEFSP11; 602477) and in 3 (2.5%) of 61 patients with febrile seizures (see FEB2, 602477), whereas it was present in only 3 (0.2%) of 772 controls. In vitro functional expression studies in Xenopus oocytes showed that the delPPP variant had a 35% increase in current size in response to hyperpolarization compared to wildtype. This current increase would depolarize the membrane potential, taking the neuron closer to the firing potential, and thus could enhance neuronal excitability.
In 2 unrelated Japanese children with familial febrile seizures-2 (FEB2; 602477), Nakamura et al. (2013) identified a heterozygous c.377C-T transition (c.377C-T, NM_001194.3) in the HCN2 gene, resulting in a ser126-to-leu (S126L) substitution at a highly conserved residue in the intracellular N-terminus region close to the S1 transmembrane segment. One proband inherited the variant from her similarly affected mother, consistent with autosomal dominant inheritance. The variant was found by direct sequencing of the HCN2 gene among 160 unrelated Japanese children with febrile seizures. Electrophysiologic studies in HEK293 cells expressing the mutation showed that it had elevated sensitivity to increased temperature compared to controls. The mutant channels showed faster activation, a greater depolarizing shift, and increased current density at higher temperatures compared to wildtype. The findings suggested that S126L mutant channels may augment neuronal excitability during hyperthermia.
In 4 affected individuals from 2 unrelated families with idiopathic generalized epilepsy-17 (EIG17; 602477) or febrile seizures (FEB2; 602477), Li et al. (2018) identified a heterozygous c.1895C-G transversion (c.1895C-G, NM_001194.3) in the HCN2 gene, resulting in a ser632-to-trp (S632W) substitution at a conserved residue in the cytoplasmic CNBD domain. The mutation, which was found by direct sequencing, was not present in the gnomAD database. It segregated with the disorder in 1 of the families. DNA from the affected sib in the other family was not available for analysis. In vitro electrophysiologic studies showed that the mutation caused a depolarizing shift in activation, a greater slope, and faster activation kinetics compared to controls, consistent with a gain-of-function effect. These findings were also observed when expressed with wildtype, indicating a dominant effect. The phenotype was somewhat variable: 1 family segregated photosensitive epilepsy and the other early-onset absence epilepsy.
In a father and his 2 offspring with idiopathic generalized epilepsy-17 (EIG17; 602477), Li et al. (2018) identified a heterozygous c.736G-A transition (c.736G-A, NM_001194.3) in the HCN2 gene, resulting in a val246-to-met (V246M) substitution at a moderately conserved residue in the second transmembrane domain. The mutation, which was found by direct sequencing, segregated with the disorder in the family. It was present at a low frequency (1.631 x 10(-5)) in the gnomAD database (4 heterozygotes). In vitro electrophysiologic studies showed that the mutation caused a depolarizing shift in activation, a greater slope, and faster activation kinetics compared to controls, consistent with a gain-of-function effect. These findings were also observed when expressed with wildtype, indicating a dominant effect. Two of the patients in the family had photosensitive epilepsy and the other had an unclassified epilepsy.
In a 28-year-old man with onset of autosomal recessive idiopathic generalized epilepsy-17 (see EIG17, 602477) at age 12 years, DiFrancesco et al. (2011) identified a homozygous c.1543G-A transition in exon 5 of the HCN2 gene, resulting in a glu515-to-lys (E515K) substitution at a conserved residue in the cytoplasmic C-linker domain, which is known to affect channel gating. The mutation, which was found by direct sequencing, was present in the heterozygous state in multiple family members who did not have seizures. In vitro functional expression studies in CHO cells showed that the homomeric mutant, but not heteromeric mutant/wildtype channels, were inhibited. There was a large negative shift of activation with lowering of the activation threshold and slowed activation kinetics, effectively abolishing HCN2 contribution to resting activity, consistent with a loss-of-function effect. Transfection of the mutation into rat cortical neurons resulted in similar changes, as well as an increase in cell excitability and firing frequency compared to wildtype.
Chung, W. K., Shin, M., Jaramillo, T. C., Leibel, R. L., LeDuc, C. A., Fischer, S. G., Tzilianos, E., Gheith, A. A., Lewis, A. S., Chetkovich, D. M. Absence epilepsy in apathetic, a spontaneous mutant mouse lacking the h channel subunit, HCN2. Neurobiol. Dis. 33: 499-508, 2009. [PubMed: 19150498] [Full Text: https://doi.org/10.1016/j.nbd.2008.12.004]
Dibbens, L. M., Reid, C. A., Hodgson, B., Thomas, E. A., Phillips, A. M., Gazina, E., Cromer, B. A., Clarke, A. L., Baram, T. Z., Scheffer, I. E., Berkovic, S. F., Petrou, S. Augmented currents of an HCN2 variant in patients with febrile seizure syndromes. Ann. Neurol. 67: 542-546, 2010. [PubMed: 20437590] [Full Text: https://doi.org/10.1002/ana.21909]
DiFrancesco, J. C., Barbuti, A., Milanesi, R., Coco, S., Bucchi, A., Bottelli, G., Ferrarese, C., Franceschetti, S., Terragni, B., Baruscotti, M., DiFrancesco, D. Recessive loss-of-function mutation in the pacemaker HCN2 channel causing increased neuronal excitability in a patient with idiopathic generalized epilepsy. J. Neurosci. 31: 17327-17337, 2011. [PubMed: 22131395] [Full Text: https://doi.org/10.1523/JNEUROSCI.3727-11.2011]
Emery, E. C., Young, G. T., Berrocoso, E. M., Chen, L., McNaughton, P. A. HCN2 ion channels play a central role in inflammatory and neuropathic pain. Science 333: 1462-1466, 2011. [PubMed: 21903816] [Full Text: https://doi.org/10.1126/science.1206243]
Li, M., Maljevic, S., Phillips, A. M., Petrovski, S., Hildebrand, M. S., Burgess, R., Mount, T., Zara, F., Striano, P., Schubert, J., Thiele, H., Nurnberg, P., Wong, M., Weisenberg, J. L., Thio, L. L., Lerche, H., Scheffer, I. E., Berkovic, S. F., Petrou, S., Reid, C. A. Gain-of-function HCN2 variants in genetic epilepsy. Hum. Mutat. 39: 202-209, 2018. [PubMed: 29064616] [Full Text: https://doi.org/10.1002/humu.23357]
Ludwig, A., Zong, X., Stieber, J., Hullin, R., Hofmann, F., Biel, M. Two pacemaker channels from human heart with profoundly different activation kinetics. EMBO J. 18: 2323-2329, 1999. [PubMed: 10228147] [Full Text: https://doi.org/10.1093/emboj/18.9.2323]
Nakamura, Y., Shi, X., Numata, T., Mori, Y., Inoue, R., Lossin, C., Baram, T. Z., Hirose, S. Novel HCN2 mutation contributes to febrile seizures by shifting the channel's kinetics in a temperature-dependent manner. PLoS One 8: e80376, 2013. [PubMed: 24324597] [Full Text: https://doi.org/10.1371/journal.pone.0080376]
Rivolta, I., Binda, A., Masi, A., DiFrancesco, J. C. Cardiac and neuronal HCN channelopathies. Pflugers Arch. 472: 931-951, 2020. [PubMed: 32424620] [Full Text: https://doi.org/10.1007/s00424-020-02384-3]
Santoro, B., Grant, S. G. N., Bartsch, D., Kandel, E. R. Interactive cloning with the SH3 domain of N-src identifies a new brain specific ion channel protein, with homology to Eag and cyclic nucleotide-gated channels. Proc. Nat. Acad. Sci. 94: 14815-14820, 1997. [PubMed: 9405696] [Full Text: https://doi.org/10.1073/pnas.94.26.14815]
Santoro, B., Liu, D. T., Yao, H., Bartsch, D., Kandel, E. R., Siegelbaum, S. A., Tibbs, G. R. Identification of a gene encoding a hyperpolarization-activated pacemaker channel of brain. Cell 93: 717-729, 1998. [PubMed: 9630217] [Full Text: https://doi.org/10.1016/s0092-8674(00)81434-8]
Wainger, B. J., DeGennaro, M., Santoro, B., Siegelbaum, S. A., Tibbs, G. R. Molecular mechanism of cAMP modulation of HCN pacemaker channels. Nature 411: 805-810, 2001. [PubMed: 11459060] [Full Text: https://doi.org/10.1038/35081088]
Zagotta, W. N., Olivier, N. B., Black, K. D., Young, E. C., Olson, R., Gouaux, E. Structural basis for modulation and agonist specificity of HCN pacemaker channels. Nature 425: 200-205, 2003. [PubMed: 12968185] [Full Text: https://doi.org/10.1038/nature01922]