The ?-subunit of the skeletal muscle sodium channel is encoded proximal to Tk-1 on mouse Chromosome 11

Evolution of voltage-gated Na+ channels

Voltage-gated Na+ channels play important functional roles in the generation of electrical excitability in most vertebrate and invertebrate species. These channels are members of a superfamily that includes voltage-gated K+, voltage-gated Ca2+ and cyclic-nucleotide-gated channels. There are nine genes encoding voltage-gated Na+ channels in mammals, with a tenth homologous gene that has not been shown to encode a functional channel. Other vertebrate and invertebrate species have a smaller number of Na+ channel genes. The mammalian genes can be classified into five branches in a phylogenetic tree, and they are localized on four chromosomes. Four of the branches representing the four chromosomal locations probably resulted from the chromosomal duplications that led to the four Hox gene clusters. These duplications occurred close to the emergence of the first vertebrates. The fifth branch probably evolved from a separate ancestral Na+ channel gene. There are two branches in the invertebrate tree, although members of only one of those branches have been demonstrated to encode functional voltage-gated Na+ channels. It is possible that the other branch may have diverged, so that its members do not represent true voltage-gated Na+ channels. Vertebrate and invertebrate Na+ channels appear to be derived from a single primordial channel that subsequently evolved independently in the two lineages.

Resurgence of sodium channel research

A variety of isoforms of mammalian voltage-gated sodium channels have been described. Ten genes encoding sodium channel alpha subunits have been identified, and nine of those isoforms have been functionally expressed in exogenous systems. The alpha subunit is associated with accessory beta subunits in some tissues, and three genes encoding different beta subunits have been identified. The alpha subunit isoforms have distinct patterns of development and localization in the nervous system, skeletal and cardiac muscle. In addition, many of the isoforms demonstrate subtle differences in their functional properties. However, there are no clear subfamilies of the channels, unlike the situation with potassium and calcium channels. The subtle differences in the functional properties of the sodium channel isoforms result in unique conductances in specific cell types, which have important physiological effects for the organism. Small alterations in the electrophysiological properties of the channel resulting from mutations in specific isoforms cause human diseases such as periodic paralysis, long QT syndrome, and epilepsy.

Evidence for evolutionary conservation of a physical linkage between the human BAF60b, a subunit of SWI/SNF complex, and thyroid hormone receptor interacting protein-1 genes on chromosome 17

The ubiquitously expressed rat BAF60b gene, which codes for a subunit of the multiprotein SWI/SNF complex, was recently identified between the pituitary growth hormone (GH-N) and thyroid hormone receptor interacting protein-1 (TRIP-1) genes. In primates, duplication of the GH-N gene has resulted in the addition of four placenta-specific (GH-V, CS-A, CS-B, and CS-L) genes downstream of the GH-N gene. As part of our study of the effect of remote sequences on the transcriptional regulation of the GH/CS gene family, we showed recently that these genes lie 40 kb upstream of the human TRIP-1 gene. We have now investigated the presence of the human BAF60b gene upstream of the TRIP-1 gene for evidence of evolutionary conservation of this arrangement or its disruption by the recent duplication of the nearby GH-N gene in primates. We report that, as in the rat genome, the human BAF60b gene is in the reverse transcriptional direction relative to the TRIP-1 gene, such that their polyadenylation sites are separated by 93 bp which compares with 92 bp in the rat. Reexamination of reported porcine TRIP-1 sequences also revealed the presence of the BAF60b gene separated by 93 bp, supporting an evolutionary conservation of this arrangement.Key words: P1 clone, gene mapping, downstream gene.

Diversity of mammalian voltage-gated sodium channels

A variety of different isoforms of mammalian voltage-gated sodium channels have been identified. These channels can be classified into three different types. Eight type 1 isoforms have been identified in the CNS, PNS, skeletal muscle, and heart. All of these channels have been expressed in exogenous systems, and all of the genes have been mapped. Three type 2 isoforms have been identified in heart, uterus, and muscle. These channels diverge from the type 1 channels in critical regions, and have not been functionally expressed, so their significance is unknown. A single isoform identified in the PNS may represent a third class of channels, in that it diverges from both type 1 and 2 channels. The type 3 channel has not been functionally expressed.

Cloning and characterization of a mouse sensory neuron tetrodotoxin-resistant voltage-gated sodium channel gene, Scn10a

Small-diameter sensory neurons associated with unmyelinated axons express a tetrodotoxin-insensitive (TTXi) voltage-gated sodium channel (VGSC) that may play an important role in the transmission of nociceptive information to the spinal cord. A TTXi VGSC, named SNS, that accounts for the tetrodotoxin-resistant sodium current described in sensory neurons has been cloned from rat dorsal root ganglia. Using recombinant lambda phage clones encoding a mouse 129/SV genomic library, we have determined the detailed structure of the mouse SNS gene (Scn10a), including the location of exon-intron boundaries and the nucleotide sequence of the exons. The gene consists of 27 exons spanning approximately 90 kb on chromosome 9. Mouse SNS shows 95.3% overall amino acid identity to rat SNS and 98.5% identity throughout the putative transmembrane segments and the intracellular loop linking domains 3 and 4. The sizes of the exons and the exon-intron junction positions of the mouse SNS and the human skeletal muscle VGSC genes are remarkably conserved. These results provide the basis for an evolutionary comparison of sodium channels, the construction and analysis of a mouse SNS null mutant as a direct approach to understanding the biological function of SNS, and the identification of regulatory elements that are responsible for the tissue- and cell-specific expression of SNS.

Mouse homologues of human hereditary disease

Details are given of 214 loci known to be associated with human hereditary disease, which have been mapped on both human and mouse chromosomes. Forty two of these have pathological variants in both species; in general the mouse variants are similar in their effects to the corresponding human ones, but exceptions include the Dmd/DMD and Hprt/HPRT mutations which cause little, if any, harm in mice. Possible reasons for phenotypic differences are discussed. In most pathological variants the gene product seems to be absent or greatly reduced in both species. The extensive data on conserved segments between human and mouse chromosomes are used to predict locations in the mouse of over 50 loci of medical interest which are mapped so far only on human chromosomes. In about 80% of these a fairly confident prediction can be made. Some likely homologies between mapped mouse loci and unmapped human ones are also given. Sixty six human and mouse proto-oncogene and growth factor gene homologies are also listed; those of confirmed location are all in known conserved segments. A survey of 18 mapped human disease loci and chromosome regions in which the manifestation or severity of pathological effects is thought to be the result of genomic imprinting shows that most of the homologous regions in the mouse are also associated with imprinting, especially those with homologues on human chromosomes 11p and 15q. Useful methods of accelerating the production of mouse models of human hereditary disease include (1) use of a supermutagen, such as ethylnitrosourea (ENU), (2) targeted mutagenesis involving ES cells, and (3) use of gene transfer techniques, with production of 'knockout mutations'.

Structure, function and expression of voltage-dependent sodium channels

Voltage-dependent sodium channels control the transient inward current responsible for the action potential in most excitable cells. Members of this multigene family have been cloned, sequenced, and functionally expressed from various tissues and species, and common features of their structure have clearly emerged. Site-directed mutagenesis coupled with in vitro expression has provided additional insight into the relationship between structure and function. Subtle differences between sodium channel isoforms are also important, and aspects of the regulation of sodium channel gene expression and the modulation of channel function are becoming topics of increasing importance. Finally, sodium channel mutations have been directly linked to human disease, yielding insight into both disease pathophysiology and normal channel function. After a brief discussion of previous work, this review will focus on recent advances in each of these areas.

Periodic paralysis, myotonia congenita and sarcolemmal ion channels: a success of the candidate gene approach

The classification of periodic paralyses and myotonic syndromes has been a subject of debates for the last 40 yr. Recent advances in molecular biology have led geneticists to reconsider this old question, using a candidate gene approach. Two groups of disorders have now emerged: (1) muscle sodium channel-associated diseases which include hyperkalemic periodic paralysis and its clinical variants, as well as paramyotonia congenita; (2) muscle chloride channel-associated disorders which comprise both the dominant and recessive form of myotonia congenita. This review is focussed on these recent discoveries.