Human Molecular Genetics Advance Access originally published online on November 13, 2006
Human Molecular Genetics 2006 15(24):3559-3568; doi:10.1093/hmg/ddl432
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Increased SK3 expression in DM1 lens cells leads to impaired growth through a greater calcium-induced fragility
1 School of Biological Sciences, University of East Anglia, Norwich, NR4 7TJ, UK, 2 Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, UK and 3 School of Life Sciences, University of Dundee, Dundee, UK
* To whom correspondence should be addressed. Tel: +44 1603592252; Fax: +44 1603592250; Email: j.rhodes{at}uea.ac.uk
Received August 24, 2006; Revised October 19, 2006; Accepted November 2, 2006
Although cataract is a characteristic feature of myotonic dystrophy type 1 (DM1), little is known of the underlying mechanisms. We generated four lens epithelial cell lines derived from DM1 cataracts and two from age-matched, non-DM cataracts. Small-pool PCR revealed typical large triplet repeat expansions in the DM1 cells. Furthermore, real-time PCR analysis showed reduced SIX5 expression and increased expression of the Ca2+-activated K+ channel SK3 in the DM1 cells. These cells also exhibited longer population doubling times which did not arise through reduced proliferation, but rather increased cell death as shown by increased release of lactate dehydrogenase (LDH). Using 86Rb+ as a tracer for K+, we found no difference in the resting K+ influx or efflux kinetics. In all cases, the ouabain sensitive component of the influx contributed 
50% of the total. However, stimulating internal Ca2+ by exposure to ionomycin not only caused greater stimulation of K+ (86Rb) efflux in the DM1 cells but also induced a higher rate of cell death (LDH assay). Since both the hyper-stimulation of K+ efflux and cell death were reduced by the highly specific SK inhibitor apamin, we suggest that increased expression of SK3 has a critical role in the increased Ca2+-induced fragility in DM1 cells. The present data, therefore, both help explain the lower epithelial cell density previously observed in DM1 cataracts and provide general insights into mechanisms underlying the fragility of other DM1-affected tissues.
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