Astra Zeneca Pharmaceutical Research Forum

The Research Forum was moderated by Dr. Christopher Ross who said in his introduction that we are finally at the point where we are taking what we have learned in the lab in animal models and applying it to human clinical trials.

James Gusella was the first speaker. He stressed that Huntington's Disease research is a partnership between researchers and families.

Diagram 1 :

Diagram 2 :

When we first started this work, we started studying HD families and inheritance. We narrowed down the location of the gene, and then found a marker. One-third of HD patients have a common ancestor. By looking closely at that group, we located an area that was different and found the gene. On the gene, there is a stretch of CAG repeats. Where there are more than 35 repeats, this is the Huntington's Disease gene. A key question is what is it about having too many consecutive gluamines that causes cell death?

Frame of Reference

• Most important fact is that HD is a genetic disease.
• Use of genetics: correlation of genetic variation with phenotype and inheritance.
• HD gene was identified using genetics, not candidate guesswork from disease description.
• Truly understanding HD pathogenesis requires understanding its genetic trigger.

On average, the longer the CAG repeat, the earlier the mean age of onset. The length of the CAG repeat explains two thirds of the variation in age of onset. The average age of onset for 50 CAG repeats is 30 and the average age of onset for 40 CAG repeats is 50+, as examples. However, there is variation that is explained by other factors that include other genes, environmental factors, and luck.

Other CAG repeat diseases that affect different parts of the brain have also been discovered, including SCA1, SCA2, SCA3, DRPLA. All follow the same pattern with a negative correlation between number of repeats and age of onset as with HD.

Genetic Criteria for Disease Mechanism

• Shows threshold glutamine number.
• Progressively worse as the glutamine number increases.
• One copy of the gene causes full disease.
• Regions of protein outside the glutamines somehow determine what types of nerve cells die.

Mutant huntington's protein has been examined in a variety of models:

• Test tubes
• Cells
• Mouse
• Fly
• Worm

The discovery of protein aggregates appears to fit the genetic criteria for the disease mechanism. The fragment of huntingtin's that contains the extra repeats clumps in test tubes and in cells but the full protein does not clump. There is no clumping below a threshold of CAG repeats. The longer the repeats, the worse the clumping. Doubling the material doesn't matter.

In the Bates mice where a transgenic fragment has been inserted, there is extensive nuclear clumping. Marcy MacDonald has developed a mouse model where the mouse has extra glutamines inserted into the mouse huntingtin's gene. This model will allow the researchers to examine the early, more subtle changes and find the triggers.

Diagram 3 :

So far we are looking at the 2/3 of the variation that is explained by repeat length. We need to learn more about the unexplained 1/3 of the variation? To do this we go back to the patient population and look for genes that delay onset. So far only one gene has been found, a gene involved in the transmission of signals, and the effect is only a small one. However, the research continues.

Referring to the second diagram, we are currently on the third step of the process, characterizing how the defect causes HD disease features, but we will get to step four, treatment. Dr. Gusella concluded his presentation, "I am hopeful about the progress because I've seen it come so far."