Each of these diseases can be traced to a different flawed gene, but each shares a common problem in its DNA -- instructions for producing the amino acid glutamine are repeated excessively along a given stretch of a gene coding for an important protein. In normal proteins, the sequence of three nucleotides that constitutes glutamine -- cytosine, adenine, and guanine, or CAG -- is often repeated 15 to 20 times, but in the mutant proteins the repeats increase in number to 50 or even 100, with earlier onset and greater severity of the particular disease linked to higher numbers of the trinucleotide repeats. The cause for the increased repeats is not known.
Now, scientists have discovered that the proteins with glutamine expansions, which appear to function well in most respects, have a new and dangerous characteristic that their normal counterparts do not. In some neurons, the CAG-repeat proteins accumulate in the nuclei of the cells, aggregating into insoluble masses that can crowd and eventually fill much of the nuclear space. And once this aggregation has begun, the mutant proteins appear to recruit normal versions of the protein into the growing mass, called an inclusion, further exacerbating the problem.
"These aggregated proteins take over a significant fraction of the space
in the nucleus and
Contact: Franklin Hoke
University of Pennsylvania School of Medicine