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The disorders of skeletal muscle and cardiac function observed in myotonic dystrophy (DM) occur as a consequence of a CTG repeat expansion (1, 2) located in the 3′ untranslated region of a protein kinase, myotonic dystrophy protein kinase (DMPK; 3, 4), on chromosome 19q13.3. The molecular mechanisms by which expanded CTG sequences produce DM pathophysiology remain unresolved, though three models prevail. First, partial loss of DMPK resulting as a consequence of nuclear retention of the mutant DMPK mRNA may contribute to DM (5-8). Second, decreased transcription of a neighboring homeodomain-encoding gene, DMAHP, occurring possibly due to altered chromatin structure near the CTG expansion, may play a role in DM (9, 10). Lastly, transdominant effects associated with expression of expanded CUG repeats and modulation of splicesite recognition may contribute to DM pathology (11-13).The primary clinical cardiac manifestation in DM is the development of conduction disturbances, with progressive atrioventricular (A-V) block and bradycardia. Prolonged A-V conduction (first-degree A-V block) is common, and higher-grade A-V block is a significant cause of death (14-17). The His-Purkinje system may also be affected, with bundle branch block and intraventricular conduction delay (18,19).To test the hypothesis that partial DMPK loss contributes to DM pathology, we developed a mouse strain lacking functional DMPK (DMPK -/-). We have shown that DMPK -/-mice develop late-onset skeletal myopathy as a consequence of abnormal excitation/contraction coupling (20). In this study we demonstrate that DMPK dosage modulates cardiac conduction. On electrocardiogram (ECG), a prolonged P-R interval (first-degree A-V block) was present in adult DMPK -/-mice compared to wild-type controls. Electrophysiological evaluation of DMPK -/-mice demonstrated more serious conduction disturbances including second and third-degree A-V block. Adult DMPK +/-mice also exhibited firstdegree A-V block similar to DMPK -/-mice. Thus, heterozygous animals have a quantifiable cardiac phenotype reminiscent of DM patients. Our results show that DMPK gene disruption causes A-V conduction abnormalities in both homozygous and heterozygous DMPK-deficient mice, and support loss of DMPK as playing a dominant role in the characteristic DM cardiac phenotype. MethodsAnimal care. A total of 27 male and 20 female adult 129/BS strain mice were studied. The mean age was 63.9 ± 21 weeks, with no differences between genotypes. The average weight was 37.7 ± 9 grams. Another 18 young mice were studied to assess age-related alterations in conduction properties. Death occurred in 6 animals due to procedural complications. Mice were housed in a diurnal facility, in compliance with the American Association for the Accreditation of Laboratory Animal Care and each center's Institutional Animal Care and Use Committee. Mice were anesthetized with intraperitoneal ketamine hydrochloride and pentobarbital (0.033 mg/gm each). Myotonic dystrophy (DM) is the most common form of muscular dystrophy...

While telomere repeat-containing non-coding RNA has been identified in a variety of eukaryotes, its biological role is not yet clear. We have identified telomeric transcripts in fission yeast, a model system that combines precise genetic manipulability with telomeres remarkably similar to those of human. Like human and budding yeast, fission yeast harbours a population of telomeric RNA molecules containing G-rich telomeric repeats transcribed from the subtelomere to the telomere. In addition, we detect substantial levels of C-rich telomeric RNA whose appearance is independent of the RNA-dependent RNA polymerase, suggesting that the telomere repeats themselves serve as promoter sites; multiple distinct subtelomeric RNAs are also present. The regulation of these transcripts depends on the telomere-associated proteins Taz1 and Rap1, as deletion of taz1+ or rap1+ leads to increased levels of both telomere repeat-containing and subtelomeric transcripts. In contrast, loss of the heterochromatin proteins Swi6 or Clr4 or the telomerase regulator Rif1 results in elevated subtelomeric RNA levels while telomere-repeat containing transcript levels remain repressed. Coupled with the large body of knowledge surrounding the functions of telomeric and heterochromatin factors in fission yeast, these in vivo analyses suggest testable models for the roles of TERRA in telomere function.

The dramatic telomerase-dependent overelongation of telomeres in cells lacking Taz1 (ortholog of human TRF1/TRF2) or Rap1 implicates these proteins in restraint of telomerase activity. However, the modes by which these proteins regulate telomerase remain mysterious. Here we show that the mechanisms underlying excessive telomerase activity differ markedly between taz1Δ and rap1Δ strains. Despite allowing elevated telomerase access, rap1Δ telomeres are processed and synthesized in a cell-cycle-constrained manner similar to that of wild-type cells. In contrast, taz1Δ telomeres are processed with little cell-cycle dependency and recruit telomerase over an abnormally wide range of cell-cycle stages. Furthermore, although taz1Δ telomeres experience transient attrition mediated by replication fork stalling, this is balanced not only by temporal expansion of the telomerase activity period, but also by markedly increased recruitment of telomerase and its accessory factor Est1, suggesting that stalled forks generate robust substrates for telomerase.