Effects of CAG repeat expansion of the androgen receptor and its truncation on neural and non-neural cells
JM Wong1, JD Zajac2, GL Warne1 & KJ Greenland1
Kennedy's disease (KD), also known as spinal and bulbar muscular atrophy, is one of eight polyglutamine diseases caused by an expansion of CAG repeats within the coding region of the affected gene. In KD, the CAG repeat region within the androgen receptor (AR) gene is increased to over 40 repeats, resulting in an expanded polyglutamine tract. Degeneration of a sub-population of neurones occurs in all polyglutamine diseases due to a toxic gain of function typically unrelated to the normal functions the affected gene.
Current evidence indicates that expansion of the CAG repeats of the AR, when coupled with truncation of the receptor, imposes increased cellular toxicity on selected neuronal subpopulation. To examine the relationship between increased CAG size and truncated AR length on the pathogenesis of KD, we generated an in vitro cell model of KD using chimeric expression vectors containing the AR and enhanced green fluorescent protein (EGFP). Full length and truncated human AR cDNA containing wild type (n=20) and mutant (n=59) CAG repeats were inserted into the EGFP-C1 expression vector (Clontech), and transiently transfected into the neuroblastoma-spinal cord hybrid cell line NSC34, and into green monkey kidney cell line COS. We localised the different AR expression vectors by observing the intracellular distribution of fluorescence using a F900E confocal microscope (Optiscan).
The proportion of viable cells post-transfection differs between the full length and the truncated ARs. In addition, we found that while the wild type AR is uniformly distributed within the cells, mutant AR containing either expanded CAG repeats or when C-terminally truncated are predominantly cytoplasmic. Further, cells containing AR with both expanded CAG repeat and truncation showed marked intracellular aggregation, mainly within the cytoplasm. Hence, our data show that the increased vulnerability for cell death may be determined by the cytoplasmic localisation of aggregates.