Background Mutations that map to the KvLQT1 gene on human chromosome 11 account for more than 50% of inherited long QT syndrome (LQTS). It has been discovered recently that the KvLQT1 and minK proteins functionally interact to generate a current with biophysical properties similar to I Ks , the slowly activating delayed-rectifier cardiac potassium current. Since I Ks modulates the repolarization of cardiac action potentials it is reasonable to hypothesize that mutations in KvLQT1 reduce I Ks , resulting in the prolongation of cardiac action potential duration. Methods and Results We expressed LQTS-associated Kv LQT1 mutants in Xenopus oocytes either individually or in combination with wild-type KvLQT1 or in combination with both wild-type KvLQT1 and minK. Substitutions of alanine with proline in the S2-S3 cytoplasmic loop (A177P) or threonine with isoleucine in the highly conserved signature sequence of the pore (T311I) yield inactive channels when expressed individually, whereas substitution of leucine with phenylalanine in the S5 transmembrane domain (L272F) yields a functional channel with reduced macroscopic conductance. However, all these mutants inhibit wild-type KvLQT1 currents in a dominant-negative fashion. Conclusions In LQTS-affected individuals these mutations would be predicted to result in a diminution of the cardiac I Ks current, subsequent prolongation of cardiac repolarization, and an increased risk of arrhythmias.
H-2K/v-jun transgenic mice develop sarcomas at sites of wounding via a multistep process characterized by discrete pathological stages. To study this progression in vitro, cells from different stages of tumorigenesis were cultured and examined for their growth properties. The results show that whereas transgenic fibroblasts do not manifest enhanced proliferative potential in vivo in the absence of wounding, they do show obvious proliferative advantage relative to nontransgenic fibroblasts in vitro, including the capacity for indefinite growth. In addition, relative to nontransgenic fibroblasts, transgenic cells show altered sensitivity to platelet-derived growth factor and tumor necrosis factor alpha, both of which are known to be mobilized during wounding. No obvious differences in growth potential are observed between transgenic fibroblasts and cells cultured from wound-induced premalignant lesions, and confluent cultures of both cell populations give rise to spontaneous foci of transformed myogenic and nonmyogenic cells that resemble those of late-stage malignant wound sarcomas. Relative to transgenic fibroblast cultures, however, premalignant lesion cultures segregate transformed cells at a greater frequency and after shorter intervals of in vitro growth. The results suggest that wound-induced multistage tumorigenesis can be recapitulated in vitro and that cells cultured from different stages of tumorigenesis retain biological properties that reflect the pathological stage from which they are derived.
The clinical features of long QT syndrome result from episodic life-threatening cardiac arrhythmias, specifically the polymorphic ventricular tachycardia torsades de pointes. KVLQT1 has been established as the human chromosome 11-linked gene responsible for more than 50% of inherited long QT syndrome. Here we describe the cloning of a full-length KVLQT1 cDNA and its functional expression. KVLQT1 encodes a 676-amino acid polypeptide with structural characteristics similar to voltage-gated potassium channels. Expression of KvLQT1 in Xenopus oocytes and in human embryonic kidney cells elicits a rapidly activating, K + -selective outward current. The I Kr -specific blockers, E-4031 and dofetilide, do not inhibit KvLQT1, whereas clofilium, a class III antiarrhythmic agent with the propensity to induce torsades de pointes, substantially inhibits the current. Elevation of cAMP levels in oocytes nearly doubles the amplitude of KvLQT1 currents. Coexpression of minK with KvLQT1 results in a conductance with pharmacological and biophysical properties more similar to I Ks than other known delayed rectifier K + currents in the heart.
Genes coding for unique or tissue-specific (differentiated) functions in the liver are induced at different times during development. It has generally been felt that transcriptional control represents the dominant mechanism for regulating expression of these genes. We have determined the relative transcription rates and mRNA steady-state levels for a series of genes specifically or preferentially expressed in rat liver and find examples of transcriptional control (albumin, alpha-fetoprotein, alpha 1-antitrypsin, tyrosine aminotransferase, transferrin, and cytochrome P450, TF-1) and post-transcriptional control (alpha 1-acid glycoprotein, apolipoproteins A-1 and E, malic enzyme, and ATP citrate lyase), as well as "mixed" regulation (ligandin and cytochrome P450, R17). Examples have been identified in which the predominant mode for regulating expression of preferentially expressed genes changes from transcriptional to post-transcriptional at different stages of liver development and some members of multigene families (cytochrome P450s and apolipoprotein genes) also show independent and sometimes contrasting modes of regulation. Therefore, it appears that regulation of specific gene expression in the liver is a dynamic process, far more complex than heretofore suspected, and a much greater contribution of post-transcriptional regulation accounts for changes in expression of genes representing major functions of the liver.
Based on the observation that albumin transcripts accumulate in the liver nuclear RNA fraction of Nagase analbuminemic rats (NAR), it was proposed [Esumi, H., Takahashi, Y., Sato, S., Nagase, S. & Sugimura, T. (1983) Proc. Natl. Acad. Sci. USA 80, 95-99] [corrected] that a 7-base-pair deletion at the splice donor site of intron H-I of the albumin gene in these animals leads to impaired processing of albumin pre-mRNA. To identify the specific splicing abnormality, we examined the primary structure of cytoplasmic albumin mRNA across the junctions of exons G-H-I by RNase protection mapping, Northern blot hybridization, Southern blot analysis of polymerase chain reaction-amplified cDNA, and DNA sequencing. The major albumin mRNA species in NAR showed precise deletion of exon H, suggesting that this exon was skipped during albumin pre-mRNA processing. Since the intron G-H splice donor and acceptor sites and exon H sequence are normal, the finding of exon H skipping in NAR has important implications regarding the mechanism of splice site selection. Moreover, the NAR model provides an excellent system to study splicing in vivo in a higher animal.
In efforts to understand mechanisms of liver dysfunction in cirrhosis, transcription of specific genes important to liver function has been measured in the rat model of CCl 4 -induced hepatic fibrosis. The relative transcription rates of albumin, α-fetoprotein and pro-α 1 -collagen genes were studied during development of fibrosis and after fibrosis was established. During the initial phase of CCl 4 administration, there was a decrease in albumin transcription associated with increased α-fetoprotein transcription, indicative of active liver regeneration. However, later during development of fibrosis, the response pattern of these genes was different, as albumin gene transcription was normal or increased and α-fetoprotein gene transcription was no longer increased. Three weeks after completion of CCl 4 treatment (fully established cirrhosis), albumin genes responded normally or hypernormally to an acute regenerative stimulus, but the α-fetoprotein gene was again not measurably responsive. Pro-α 1 -collagen gene transcription increased during the entire fibrogenic process and remained elevated after cirrhosis was established. These studies suggest that a switch from albumin to α-fetoprotein gene transcription can serve as a marker of liver regenerative capacity, and that this process is altered during and after development of hepatic fibrosis. The fibrogenic process is also associated with elevated transcription of collagen genes.
