Recommendations for laboratory workflow that better support centralised amalgamation of genomic variant data: findings from CanVIG-UK national molecular laboratory survey.

BACKGROUND: National and international amalgamation of genomic data offers opportunity for research and audit, including analyses enabling improved classification of variants of uncertain significance. Review of individual-level data from National Health Service (NHS) testing of cancer susceptibility genes (2002-2023) submitted to the National Disease Registration Service revealed heterogeneity across participating laboratories regarding (1) the structure, quality and completeness of submitted data, and (2) the ease with which that data could be assembled locally for submission. METHODS: In May 2023, we undertook a closed online survey of 51 clinical scientists who provided consensus responses representing all 17 of 17 NHS molecular genetic laboratories in England and Wales which undertake NHS diagnostic analyses of cancer susceptibility genes. The survey included 18 questions relating to 'next-generation sequencing workflow' (11), 'variant classification' (3) and 'phenotypical context' (4). RESULTS: Widely differing processes were reported for transfer of variant data into their local LIMS (Laboratory Information Management System), for the formatting in which the variants are stored in the LIMS and which classes of variants are retained in the local LIMS. Differing local provisions and workflow for variant classifications were also reported, including the resources provided and the mechanisms by which classifications are stored. CONCLUSION: The survey responses illustrate heterogeneous laboratory workflow for preparation of genomic variant data from local LIMS for centralised submission. Workflow is often labour-intensive and inefficient, involving multiple manual steps which introduce opportunities for error. These survey findings and adoption of the concomitant recommendations may support improvement in laboratory dataflows, better facilitating submission of data for central amalgamation.

Germline mismatch repair (MMR) gene analyses from English NHS regional molecular genomics laboratories 1996-2020: development of a national resource of patient-level genomics laboratory records.

OBJECTIVE: To describe national patterns of National Health Service (NHS) analysis of mismatch repair (MMR) genes in England using individual-level data submitted to the National Disease Registration Service (NDRS) by the NHS regional molecular genetics laboratories. DESIGN: Laboratories submitted individual-level patient data to NDRS against a prescribed data model, including (1) patient identifiers, (2) test episode data, (3) per-gene results and (4) detected sequence variants. Individualised per-laboratory algorithms were designed and applied in NDRS to extract and map the data to the common data model. Laboratory-level MMR activity audit data from the Clinical Molecular Genetics Society/Association of Clinical Genomic Science were used to assess early years' missing data. RESULTS: Individual-level data from patients undergoing NHS MMR germline genetic testing were submitted from all 13 English laboratories performing MMR analyses, comprising in total 16 722 patients (9649 full-gene, 7073 targeted), with the earliest submission from 2000. The NDRS dataset is estimated to comprise >60% of NHS MMR analyses performed since inception of NHS MMR analysis, with complete national data for full-gene analyses for 2016 onwards. Out of 9649 full-gene tests, 2724 had an abnormal result, approximately 70% of which were (likely) pathogenic. Data linkage to the National Cancer Registry demonstrated colorectal cancer was the most frequent cancer type in which full-gene analysis was performed. CONCLUSION: The NDRS MMR dataset is a unique national pan-laboratory amalgamation of individual-level clinical and genomic patient data with pseudonymised identifiers enabling linkage to other national datasets. This growing resource will enable longitudinal research and can form the basis of a live national genomic disease registry.

Mosaicism in Hartsfield syndrome.

Hartsfield syndrome is a rare condition characterised by the co-occurrence of ectrodactyly and holoprosencephaly spectrum disorders; cleft lip and palate is a common associated feature. This is due to either monoallelic, or less commonly, biallelic variants in FGFR1 with a loss of function or dominant negative effect. To date 37 individuals have been reported, including two instances of germline mosaicism. We report a further family with Hartsfield syndrome due to a novel variant in FGFR1, with two affected fetuses, and somatic and germline mosaicism in the father detected on Sanger sequencing. The father had not come to medical attention prior to this finding. In light of our findings and those in the published literature, we suggest that mosaicism, either germline or germline and somatic, may be a relatively frequent finding, affecting 3 of 35 (9%) reported families, which has important implications for genetic counselling.

Pseudodominant Alport syndrome caused by pathogenic homozygous and compound heterozygous COL4A3 splicing variants.

Alport syndrome is a genetic disorder affecting the basement membranes of the kidney, ear and eye, and represents a leading cause of monogenic kidney disease. Alport syndrome is genetically heterogeneous with three key genes involved (COL4A3-5) and several transmission patterns, including monogenic X-linked, autosomal recessive/dominant and digenic. We report a consanguineous family where 13 individuals presented variable features of Alport syndrome including kidney failure on two generations and male-to-male transmission, suggesting autosomal dominant inheritance. COL4A3-5 gene panel analysis surprisingly reveals two distinct, confirmed splice-altering variants in COL4A3 (NM_000091.4: c.1150+5G>A and c.4028-3C>T) present in homozygous or compound heterozygous state in individuals with kidney failure. This adds a further mode of transmission for Alport syndrome where, in a consanguineous family, the independent segregation of two variants at the same locus may create a pseudodominant transmission pattern. These findings highlight the importance of a molecular diagnosis in Alport syndrome for genetic risk counselling, given the variable modes of inheritance, but also the pitfalls of assuming identity by descent in consanguineous families.

Further delineation of phenotypic spectrum of SCN2A-related disorder.

SCN2A-related disorders include intellectual disability, autism spectrum disorder, seizures, episodic ataxia, and schizophrenia. In this study, the phenotype-genotype association in SCN2A-related disorders was further delineated by collecting detailed clinical and molecular characteristics. Using previously proposed genotype-phenotype hypotheses based on variant function and position, the potential of phenotype prediction from the variants found was examined. Patients were identified through the Deciphering Developmental Disorders study and gene matching strategies. Phenotypic information and variant interpretation evidence were collated. Seventeen previously unreported patients and five patients who had been previously reported (but with minimal phenotypic and segregation data) were included (10 males, 12 females; median age 10.5 years). All patients had developmental delays and the majority had intellectual disabilities. Seizures were reported in 15 of 22 (68.2%), four of 22 (18.2%) had autism spectrum disorder and no patients were reported with episodic ataxia. The majority of variants were de novo. One family had presumed gonadal mosaicism. The correlation of the use of sodium channel-blocking antiepileptic drugs with phenotype or genotype was variable. These data suggest that variant type and position alone can provide some predictive information about the phenotype in a proportion of cases, but more precise assessment of variant function is needed for meaningful phenotype prediction.

Regulation of sinus node pacemaking and atrioventricular node conduction by HCN channels in health and disease.

The funny current, If, was first recorded in the heart 40 or more years ago by Dario DiFrancesco and others. Since then, we have learnt that If plays an important role in pacemaking in the sinus node, the innate pacemaker of the heart, and more recently evidence has accumulated to show that If may play an important role in action potential conduction through the atrioventricular (AV) node. Evidence has also accumulated to show that regulation of the transcription and translation of the underlying Hcn genes plays an important role in the regulation of sinus node pacemaking and AV node conduction under normal physiological conditions - in athletes, during the circadian rhythm, in pregnancy, and during postnatal development - as well as pathological states - ageing, heart failure, pulmonary hypertension, diabetes and atrial fibrillation. There may be yet more pathological conditions involving changes in the expression of the Hcn genes. Here, we review the role of If and the underlying HCN channels in physiological and pathological changes of the sinus and AV nodes and we begin to explore the signalling pathways (microRNAs, transcription factors, GIRK4, the autonomic nervous system and inflammation) involved in this regulation. This review is dedicated to Dario DiFrancesco on his retirement.

Human urine-derived renal epithelial cells provide insights into kidney-specific alternate splicing variants.

The majority of multi-exon genes undergo alternative splicing to produce different mRNA transcripts and this may occur in a tissue-specific manner. Assessment of mRNA transcripts isolated from blood samples may sometimes be unhelpful in determining the affect on function of putative splice-site variants affecting kidney-specific mRNA transcripts. Here we present data demonstrating the power of using human urine-derived renal epithelial cells (hUREC) as a source of kidney RNA. We report clinical and molecular genetic data from three affected cases from two families all with end-stage renal disease by 15 years of age. In both families, heterozygous variants which are predicted to effect function in NPHP3 were found on one allele, in combination with a synonymous SNV (c.2154C>T; p.Phe718=), 18 base pairs from the exon-intron boundary within exon 15 of NPHP3. The only mRNA transcript amplified from wild-type whole blood showed complete splicing out of exon 15. Urine samples obtained from control subjects and the father of family 2, who carried the synonymous SNV variant, were therefore used to culture hUREC and allowed us to obtain kidney-specific mRNA. Control kidney mRNA showed retention of exon 15, while the mRNA from the patient's father confirmed evidence of a heterozygous alternate splicing of exon 15 of NPHP3. Analysis of RNA derived from hUREC allows for a comparison of kidney-specific and whole-blood RNA transcripts and for assessment of the effect on function of putative splice variants leading to end-stage kidney disease.

MEGF10 related myopathies: A new case with adult onset disease with prominent respiratory failure and review of reported phenotypes.

Recessive mutations in MEGF10 (multiple epidermal growth factor 10) have been reported in a severe early onset disorder named Early Myopathy, Areflexia, Respiratory Distress and Dysphagia, and a milder form with cores in the muscle biopsy; and a possible genotype-phenotype correlation determining the clinical presentation has been suggested. We undertook exome sequencing in a 66 year old male with a 20 year history of progressive proximal and distal weakness of upper and lower limbs, facial weakness and dysphagia, who developed respiratory failure requiring ventilation while still ambulant in his 50s. Muscle biopsy demonstrated myopathic changes with aggregation of myofibrillar proteins. Mutations in MEGF10 were identified: a novel essential splice site (c.1426+1G>T) and a novel missense variant (c.352T>C, p.(Cys118Arg)). We performed a detailed review of all reported MEGF10 cases (n = 20), and confirmed the presence of a genotype-phenotype correlation, namely that with ≥1 null mutation onset of respiratory dysfunction occurs in the first year of life, whereas with 2 missense mutations, respiratory dysfunction occurs at 10 years old or much later, as in the patient reported here. Our findings expand the phenotype of MEGF10 mutations to include onset in the 5th decade, and discuss the spectrum of MEGF10 related disease.

Exome sequences versus sequential gene testing in the UK highly specialised Service for Limb Girdle Muscular Dystrophy.

BACKGROUND: Limb girdle muscular dystrophies are a group of rare and genetically heterogeneous diseases that share proximal weakness as a common feature; however they are often lacking very specific phenotypic features to allow an accurate differential diagnosis based on the clinical signs only, limiting the diagnostic rate using phenotype driven genetic testing. Next generation sequencing provides an opportunity to obtain molecular diagnoses for undiagnosed patients, as well as identifying novel genetic causes of muscle diseases. We performed whole exome sequencing (WES) on 104 affected individuals from 75 families in who standard gene by gene testing had not yielded a diagnosis. For comparison we also evaluated the diagnostic rate using sequential gene by gene testing for 91 affected individuals from 84 families over a 2 year period. RESULTS: Patients selected for WES had undergone more extensive prior testing than those undergoing standard genetic testing and on average had had 8 genes screened already. In this extensively investigated cohort WES identified the genetic diagnosis in 28 families (28/75, 37%), including the identification of the novel gene ZAK and two unpublished genes. WES of a single affected individual with sporadic disease yielded a diagnosis in 13/38 (34%) of cases. In comparison, conventional gene by gene testing provided a genetic diagnosis in 28/84 (33%) families. Titinopathies and collagen VI related dystrophy were the most frequent diagnoses made by WES. Reasons why mutations in known genes were not identified previously included atypical phenotypes, reassignment of pathogenicity of variants, and in one individual mosaicism for a COL6A1 mutation which was undetected by prior direct sequencing. CONCLUSION: WES was able to overcome many limitations of standard testing and achieved a higher rate of diagnosis than standard testing even in this cohort of extensively investigated patients. Earlier application of WES is therefore likely to yield an even higher diagnostic rate. We obtained a high diagnosis rate in simplex cases and therefore such individuals should be included in exome or genome sequencing projects. Disease due to somatic mosaicism may be increasingly recognised due to the increased sensitivity of next generation sequencing techniques to detect low level mosaicism.

Importance of gradients in membrane properties and electrical coupling in sinoatrial node pacing.

The sinoatrial node (SAN) is heterogeneous in terms of cell size, ion channels, current densities, connexins and electrical coupling. For example, Nav1.5 (responsible for INa) and Cx43 (responsible for electrical coupling) are absent from the centre of the SAN (normally the leading pacemaker site), but present in the periphery (at SAN-atrial muscle junction). To test whether the heterogeneity is important for the functioning of the SAN, one- and two-dimensional models of the SAN and surrounding atrial muscle were created. Normal functioning of the SAN (in terms of cycle length, position of leading pacemaker site, conduction times, activation and repolarization sequences and space constants) was observed when, from the centre to the periphery, (i) cell characteristics (cell size and ionic current densities) were changed in a gradient fashion from a central-type (lacking INa) to a peripheral-type (possessing INa) and (ii) coupling conductance was increased in a gradient fashion. We conclude that the heterogeneous nature of the node is important for its normal functioning. The presence of Nav1.5 and Cx43 in the periphery may be essential for the node to be able to drive the atrial muscle: Nav1.5 provides the necessary depolarizing current and Cx43 delivers it to the atrial muscle.

Characterization of a factor H mutation that perturbs the alternative pathway of complement in a family with membranoproliferative GN.

Complement C3 activation is a characteristic finding in membranoproliferative GN (MPGN). This activation can be caused by immune complex deposition or an acquired or inherited defect in complement regulation. Deficiency of complement factor H has long been associated with MPGN. More recently, heterozygous genetic variants have been reported in sporadic cases of MPGN, although their functional significance has not been assessed. We describe a family with MPGN and acquired partial lipodystrophy. Although C3 nephritic factor was shown in family members with acquired partial lipodystrophy, it did not segregate with the renal phenotype. Genetic analysis revealed a novel heterozygous mutation in complement factor H (R83S) in addition to known risk polymorphisms carried by individuals with MPGN. Patients with MPGN had normal levels of factor H, and structural analysis of the mutant revealed only subtle alterations. However, functional analysis revealed profoundly reduced C3b binding, cofactor activity, and decay accelerating activity leading to loss of regulation of the alternative pathway. In summary, this family showed a confluence of common and rare functionally significant genetic risk factors causing disease. Data from our analysis of these factors highlight the role of the alternative pathway of complement in MPGN.

Genotype/phenotype correlations in complement factor H deficiency arising from uniparental isodisomy.

We report a male infant who presented at 8 months of age with atypical hemolytic uremic syndrome (aHUS) responsive to plasma therapy. Investigation showed him to have complement factor H (CFH) deficiency associated with a homozygous CFH mutation (c.2880delT [p.Phe960fs]). Mutation screening of the child's parents revealed that the father was heterozygous for this change but that it was not present in his mother. Chromosome 1 uniparental isodisomy of paternal origin was confirmed by genotyping chromosome 1 SNPs. CD46 SNP genotyping was undertaken in this individual and another patient with CFH deficiency associated with chromosome 1 uniparental isodisomy. This showed a homozygous aHUS risk haplotype (CD46GGAAC) in the patient with aHUS and a homozygous glomerulonephritis risk haplotype (CD46AAGGT) in the patient with endocapillary glomerulonephritis. We also showed that FHL-1 (factor H-like protein 1) was present in the patient with aHUS and absent in the patient with glomerulonephritis. This study emphasizes that modifiers such as CD46 and FHL-1 may determine the kidney phenotype of patients who present with homozygous CFH deficiency.

Postnatal development of transmural gradients in expression of ion channels and Ca²âº-handling proteins in the ventricle.

Transmural gradients in myocyte action potential duration (APD) and Ca(2+)-handling proteins are argued to be important for both the normal functioning of the ventricle and arrhythmogenesis. In rabbit, the transmural gradient in APD (left ventricular wedge preparation) is minimal in the neonate. During postnatal development, APD increases both in the epicardium and the endocardium, but the prolongation is more substantial in the endocardium leading to a significant transmural gradient. We have investigated changes in the expression of ion channels and also Ca(2+)-handling proteins in the subepicardial and subendocardial layers of the left ventricular free wall in neonatal (2-7 days of age) and adult male (~6 months of age) New Zealand White rabbits using quantitative PCR and also, when possible, in situ hybridisation and immunohistochemistry. In the adult, there were significant and substantial transmural gradients in Ca(v)1.2, KChIP2, ERG, K(v)LQT1, K(ir)2.1, NCX1, SERCA2a and RyR2 at the mRNA and, in some cases, protein level-in every case the mRNA or protein was more abundant in the epicardium than the endocardium. Of the eight transmural gradients seen in the adult, only three were observed in the neonate and, in two of these cases, the gradients were smaller than those in the adult. However, in the neonate there were also transmural gradients not observed in the adult: in HCN4, Na(v)1.5, minK, K(ir)3.1 and Cx40 mRNAs - in every case the mRNA was more abundant in the endocardium than the epicardium. If the postnatal changes in ion channel mRNAs are used to predict changes in ionic conductances, mathematical modelling predicts the changes in APD observed experimentally. It is concluded that many of the well known transmural gradients in the ventricle develop postnatally.

Remodelling of human atrial K+ currents but not ion channel expression by chronic ß-blockade.

Chronic ß-adrenoceptor antagonist (ß-blocker) treatment in patients is associated with a potentially anti-arrhythmic prolongation of the atrial action potential duration (APD), which may involve remodelling of repolarising K(+) currents. The aim of this study was to investigate the effects of chronic ß-blockade on transient outward, sustained and inward rectifier K(+) currents (I(TO), I(KSUS) and I(K1)) in human atrial myocytes and on the expression of underlying ion channel subunits. Ion currents were recorded from human right atrial isolated myocytes using the whole-cell-patch clamp technique. Tissue mRNA and protein levels were measured using real time RT-PCR and Western blotting. Chronic ß-blockade was associated with a 41% reduction in I(TO) density: 9.3 ± 0.8 (30 myocytes, 15 patients) vs 15.7 ± 1.1 pA/pF (32, 14), p < 0.05; without affecting its voltage-, time- or rate dependence. I(K1) was reduced by 34% at -120 mV (p < 0.05). Neither I(KSUS), nor its increase by acute ß-stimulation with isoprenaline, was affected by chronic ß-blockade. Mathematical modelling suggested that the combination of I(TO)- and I(K1)-decrease could result in a 28% increase in APD(90). Chronic ß-blockade did not alter mRNA or protein expression of the I(TO) pore-forming subunit, Kv4.3, or mRNA expression of the accessory subunits KChIP2, KChAP, Kvß1, Kvß2 or frequenin. There was no reduction in mRNA expression of Kir2.1 or TWIK to account for the reduction in I(K1). A reduction in atrial I(TO) and I(K1) associated with chronic ß-blocker treatment in patients may contribute to the associated action potential prolongation, and this cannot be explained by a reduction in expression of associated ion channel subunits.

Ageing-dependent remodelling of ion channel and Ca2+ clock genes underlying sino-atrial node pacemaking.

The function of the sino-atrial node (SAN), the pacemaker of the heart, is known to decline with age, resulting in pacemaker disease in the elderly. The aim of the study was to investigate the effects of ageing on the SAN by characterizing electrophysiological changes and determining whether changes in gene expression are involved. In young and old rats, SAN function was characterized in the anaesthetized animal, isolated heart and isolated right atrium using ECG and action potential recordings; gene expression was characterized using quantitative PCR. The SAN function declined with age as follows: the intrinsic heart rate declined by 18 ± 3%; the corrected SAN recovery time increased by 43 ± 13%; and the SAN action potential duration increased by 11 ± 3% (at 75% repolarization). Gene expression in the SAN changed considerably with age, e.g. there was an age-dependent decrease in the Ca(2+) clock gene, RYR2, and changes in many ion channels (e.g. increases in Na(v)1.5, Na(v)ß1 and Ca(v)1.2 and decreases in K(v)1.5 and HCN1). In conclusion, with age, there are changes in the expression of ion channel and Ca(2+) clock genes in the SAN, and the changes may provide a partial explanation for the age-dependent decline in pacemaker function.

Anatomical and molecular mapping of the left and right ventricular His-Purkinje conduction networks.

Functioning of the cardiac conduction system depends critically on its structure and its complement of ion channels. Therefore, the aim of this study was to document both the structure and ion channel expression of the left and right ventricular His-Purkinje networks, as we have previously done for the sinoatrial and atrioventricular nodes. A three-dimensional (3D) anatomical computer model of the His-Purkinje network of the rabbit heart was constructed after staining the network by immunoenzyme labelling of a marker protein, middle neurofilament. The bundle of His is a ribbon-like structure and the architecture of the His-Purkinje network differs between the left and right ventricles. The 3D model is able to explain the breakthrough points of the action potential on the ventricular epicardium during sinus rhythm. Using quantitative PCR, the expression levels of the major ion channels were measured in the free running left and right Purkinje fibres of the rabbit heart. Expression of ion channels differs from that of the working myocardium and can explain the specialised electrical activity of the Purkinje fibres as suggested by computer simulations; the expression profile of the left Purkinje fibres is more specialised than that of the right Purkinje fibres. The structure and ion channel expression of the Purkinje fibres are highly specialised and tailored to the functioning of the system. The His-Purkinje network in the left ventricle is more developed than that in the right ventricle and this may explain its greater clinical importance.

Changes in ion channel gene expression underlying heart failure-induced sinoatrial node dysfunction.

BACKGROUND: Heart failure (HF) causes a decline in the function of the pacemaker of the heart-the sinoatrial node (SAN). The aim of the study was to investigate HF-induced changes in the expression of the ion channels and related proteins underlying the pacemaker activity of the SAN. METHODS AND RESULTS: HF was induced in rats by the ligation of the proximal left coronary artery. HF animals showed an increase in the left ventricular (LV) diastolic pressure (317%) and a decrease in the LV systolic pressure (19%) compared with sham-operated animals. They also showed SAN dysfunction wherein the intrinsic heart rate was reduced (16%) and the corrected SAN recovery time was increased (56%). Quantitative polymerase chain reaction was used to measure gene expression. Of the 91 genes studied during HF, 58% changed in the SAN, although only 1% changed in the atrial muscle. For example, there was an increase in the expression of ERG, K(v)LQT1, K(ir)2.4, TASK1, TWIK1, TWIK2, calsequestrin 2, and the A1 adenosine receptor in the SAN that could explain the slowing of the intrinsic heart rate. In addition, there was an increase in Na(+)-H(+) exchanger, and this could be the stimulus for the remodeling of the SAN. CONCLUSIONS: SAN dysfunction is associated with HF and is the result of an extensive remodeling of ion channels; gap junction channels; Ca(2+)-, Na(+)-, and H(+)-handling proteins; and receptors in the SAN.

Changes in the expression of ion channels, connexins and Ca2+-handling proteins in the sino-atrial node during postnatal development.

There are important postnatal changes in the sino-atrial node (SAN), the pacemaker of the heart. Compared with the neonate, the adult has a slower intrinsic heart rate and a longer SAN action potential. These changes may be due to differences in ion channel expression. Consequently, we investigated postnatal developmental changes in the expression of ion channels and Ca(2+)-handling proteins in the SAN to see whether this is indeed the case. Using quantitative PCR, in situ hybridization and immunohistochemistry, we investigated the expression of ion channels, Ca(2+)-handling proteins and connexins in the SAN from neonatal (2-7 days of age) and adult (∼6 months of age) New Zealand White rabbits. The spontaneous beating rate of adult SAN preparations was 21% slower than that of neonatal preparations. During postnatal development, quantitative PCR revealed a significant decline in the SAN of the following mRNAs: HCN4 (major isoform responsible for I(f)), Na(V)1.5 (responsible for I(Na)), Ca(V)1.3 (in part responsible for I(Ca,L)) and NCX1 (responsible for inward I(NaCa)). These declines could be responsible for the slowing of the pacemaker during postnatal development. There was a significant decline during development in mRNA for delayed rectifier K(+) channel subunits (K(V)1.5, responsible for I(K,ur), K(V)LQT1 and minK, responsible for I(K,s), and ERG, responsible for I(K,r)) and this could explain the prolongation of the action potential. In situ hybridization confirmed the changes observed by quantitative PCR. In addition, immunohistochemistry revealed hypertrophy of nodal cells during postnatal development. Moreover, there were complex changes in the expression of Ca(2+)-handling proteins with age. In summary, there are significant postnatal changes in the expression of ion channels and Ca(2+)-handling proteins in the SAN that could explain the established changes in heart rate and action potential duration that occur during normal development.

Ion channel transcript expression at the rabbit atrioventricular conduction axis.

BACKGROUND: Little is known about the distribution of gap junctions and ion channels in the atrioventricular node, even though the physiology and pathology of the atrioventricular node is ultimately dependent on them. METHODS AND RESULTS: The abundance of 30 transcripts for markers, gap junctions, ion channels, and Ca(2+)-handling proteins in different regions of the rabbit atrioventricular node (nodal extension and proximal and distal penetrating bundle of His as well as atrial and ventricular muscle) was measured using a novel quantitative polymerase chain reaction technique and in situ hybridization. The expression profile of the nodal extension (slow pathway into penetrating bundle) was similar to that of the sinoatrial node. For example, in the nodal extension, in contrast to the atrial muscle and as expected for a slowly conducting tissue with pacemaker activity, there was no or reduced expression of Cx43, Na(v)1.5, Ca(v)1.2, K(v)1.4, KChIP2, and RYR3 and high expression of Ca(v)1.3 and HCN4. The expression profile of the penetrating bundle was less specialized. In situ hybridization revealed a transitional zone with reduced expression of Cx43, Na(v)1.5, and KChIP2 that may form the fast pathway into the penetrating bundle. CONCLUSIONS: At the atrioventricular node, the expression of gap junctions and ion channels in the nodal extension (slow pathway) and a transitional zone (putative fast pathway) as well as the penetrating bundle (output pathway) is specialized and heterogeneous and roughly matches the electrophysiology of the different regions.

Distribution of the pacemaker HCN4 channel mRNA and protein in the rabbit sinoatrial node.

Several studies of the pacemaker mechanisms in mammalian cells, most of which were carried out in cells isolated from the rabbit sinoatrial node (SAN), have highlighted the role of the I(f) current. While the distribution of Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels, the molecular correlates of f-channels, is known at the mRNA level, the identification of f-channel proteins in this tissue is still undetermined. Here we investigate HCN protein expression in the rabbit pacemaker region. We found that HCN4 is the main isoform, and set therefore to analyze its distribution within the SAN and surrounding areas with the aim of correlating protein expression and pacemaking function. The analysis was carried out in tissue slices and single cells of the intercaval area, which includes the crista terminalis (CT), the SAN, and the septum interatrialis (SI). Immunolabeling, in situ hybridization, qRT-PCR analysis, and electrophysiological recordings identified the SAN as a region characterized by high HCN4 signal and current levels, while the expression in the CT and in the SI was either negligible or absent. Detailed analysis of the central SAN area showed that cells are predominantly distributed in islets interconnected by cell prolongations, and single-cell HCN4 labeling suggested sites of channel clustering. Our data indicate that in the rabbit SAN, HCN4 proteins are major constituents of native f-channels, and their distribution matches closely the SAN as defined morphologically and electrophysiologically. Until recently, the SAN was identified as the region where Cx43 and atrial natriuretic peptide are not expressed; we propose here that expression of HCN4 is an appropriate tool to map and identify the cardiac SAN pacemaker region.