New Targets For Huntington’s Disease

Huntington’s disease (HD) is characterized by the presence of an expanded polyglutamine domain in the gene encoding huntingtin. Transgenic mice expressing exon 1 of the mutant human huntingtin (R6/2) develop a fatal neurological disease, with loss of striatal neurons. Mutant huntingtin has been shown to trigger apoptosis in neuronal cell cultures, presumably by activating a biochemical cascade that involves caspases, particularly caspase-8. Drugs that inhibit caspase have been shown to protect striatal neurons from death both in vitro and in vivo in animal models of HD. Chronic (as would be required for the treatment of HD) blockade of caspases, however, would probably not be well tolerated, given the critical role played by these enzymes in a number of cellular events, particularly in the immune system.

Two new potential drug targets for HD have now been proposed. Normal huntingtin has been shown to interact with a protein called Hip-1. However, huntingtin interacts with many other proteins, and it was therefore unclear whether the interaction with Hip-1 had anything to do with HD. Gervais and colleagues now show that in cultured cells, the longer the CAG repeat, the weaker the interaction of huntingtin with Hip-1. As a consequence, Hip-1 is liberated and binds to another newly identified protein, Hippi (Hip-1 protein interactor). The Hip-1, hippi hetero dinier recruits procaspase-8 and forms a complex Hip-1, hippi, procaspase-8 that sets off the apoptotic cascade. Blocking the binding of Hip-1 to Hippi could thus slow disease progression.

Another report shows that transglutaminase (TGase), an enzyme that cross-links huntingtin, may also be an interesting target for drugs. TGase activity has been reported to be increased by as much as 270% in the brains of HD patients. TGase activity is also elevated in the brain of 11-week old transgenic for the huntingtin mutation (R6/2 mice). However the increase in enzyme activity is modest (21%), possibly because TGase activity was measured early in the life of the transgenic animals. Daily administration of cystamine, a competitive inhibitor of TGase, starting at Week 7 when neurological symptoms first appear, significantly extended survival compared to vehicle-treated animals. In addition, mice treated with cystamine had less tremor and abnormal movements. In the doses used, cystamine caused an approximate 36% decrease in brain TGase activity, suggesting that other mechanisms may also be involved. Interestingly, cystamine treatment increased transcription of HSP40, a gene known to protect from polyglutamine neurotoxicity in Drosophila.

Source. Gervais FG, et al. Recruitment and activation of caspase-8 by the Huntingtin-interacting protein Hip-1 and a novel partner Hippi: Nature Cell Biology 2002 Feb; 4(2): 95-105. Karpuj MV, Becher MW, Springer JE, et al. Prolonged survival and decreased abnormal transgenic model of Huntington’s disease, with administration of the transglutaminase inhibitor cystamine. Nature Medicine 2002; 8:143-149.

Xenotransplantation: alpha-Gal overcome

Studies show that there are five times as many people on the waiting list for organ transplants as will actually receive one. In recent years, transplantation of animal organs or tissues to humans (xenotransplantation) has considerably increased, in part because of shortage of human organs, and in part because of improved treatments to prevent graft rejection. Xenotransplantation could be used for several conditions, including replacement of the heart, lungs, kidney, and liver. The technology is also being studied for the replacement of missing cells, for example for the treatment of Parkinson’s disease. A major barrier to the xenotransplantation of pig organs to humans is a phenomenon known as "hyperacute rejection," a violent reaction which involves the attack and destruction of the transplanted organ within minutes following xenotransplantation. Hyperacute rejection is caused by the destruction of the blood vessels of the transplanted organ, cutting off oxygen, and destroying the transplant within minutes. The reason for the reaction is that animal cells, notably pigs, carry on the surface of their cells a sugar (alpha-gal). Humans and Old World primates lost in evolution the enzyme that adds alpha-gal to the cell surface. Cells that carry alpha-gal on their surface are therefore immediately recognized as foreign. A number of intestinal bacteria express alpha-gal, so most people have circulating antibodies to the protein, explaining why rejection is hyperacute and occurs in a matter of minutes. Several groups are developing strategies to avoid hyperacute rejection. One strategy is to genetically engineer pigs so that, like humans, they do not express alpha-gal. Two groups have now announced that they have succeeded. PPL Therapeutics, the Scottish company that cloned Dolly, has announced the birth on Christmas day of five knock-out female piglets. Each had one inactivated gene for alpha-1,3,-galactosyl transferase. Immerge Biotherapeutics published a similar finding in the January 3 issue of Science. Although this is a great success, it must be stressed that each research team only knocked out one of the two copies of the gene, and all the piglets continue to make alpha-gal with their remaining copy. The companies now intend to breed their piglets in the hope of obtaining offspring that have both genes knocked out.

Source. Butler D. Xenotransplantation experts express caution over knockout piglets. Nature 2002; 415: 103-104.