Gene repression is crucial to the maintenance of differentiated cell types in multicellular organisms, whereas aberrant silencing can lead to disease. The organization of DNA into chromatin and heterochromatin is implicated in gene silencing. In chromatin, DNA wraps around histones, creating nucleosomes. Further condensation of chromatin, associated with large blocks of repetitive DNA sequences, is known as heterochromatin. Position effect variegation (PEV) occurs when a gene is located abnormally close to heterochromatin, silencing the affected gene in a proportion of cells. Here we show that the relatively short triplet-repeat expansions found in myotonic dystrophy and Friedreich's ataxia confer variegation of expression on a linked transgene in mice. Silencing was correlated with a decrease in promoter accessibility and was enhanced by the classical PEV modifier heterochromatin protein 1 (HP1). Notably, triplet-repeat-associated variegation was not restricted to classical heterochromatic regions but occurred irrespective of chromosomal location. Because the phenomenon described here shares important features with PEV, the mechanisms underlying heterochromatin-mediated silencing might have a role in gene regulation at many sites throughout the mammalian genome and modulate the extent of gene silencing and hence severity in several triplet-repeat diseases.
Major insights have been attained into the molecular pathology of the trinucleotide repeat neurodegenerative diseases over the past decade. Genetic definition has allowed subclassification into translated polyglutamine diseases, which are due to CAG repeat expansions, and a more heterogeneous group in which the trinucleotide repeat remains untranslated. The polyglutamine disorders are due to a toxic gain of function of mutant expanded proteins. Neuronal intranuclear inclusions (NIIs) characteristically occur. Protein misfolding, interference with DNA transcription and RNA processing, activation of apoptosis and dysfunction of cytoplasmic elements have all been invoked in the toxic process. The end result is apoptotic cell death with many aspects of neuronal function being perturbed. Promising progress has been made into arresting and reversing neurodegeneration in both cellular and animal models. The molecular mechanisms underlying the untranslated group remain less clear. Impedance of gene transcription secondary to abnormal DNA structures formed by repeats, modification of chromatin gene packaging and dysfunction at the RNA level have all been suggested as possible pathological mechanisms. These diseases remain irreversible. It is hoped that clarification of the molecular pathogenic mechanisms will provide the tools for future therapeutic intervention.
Mutations in TTN are a cause of MFM, and titinopathy is more common than previously thought. The finding of the p.C30071R mutation in 3.9% of our study population is likely due to a British founder effect. The occurrence of novel FN3 domain variants, although still of uncertain pathogenicity, suggests that other mutations in this domain may cause MFM, and that the disease is likely to be globally distributed. We suggest that HMERF due to mutations in the TTN gene be nosologically classified as MFM-titinopathy.
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