By David N. Leff
Editor¿s note: Science Scan is a roundup of recently published biotechnology-relevant research.
Sickle cell disease (SCD) has scored another pivotal advance since 1957, when it became the first genetic disorder for which a causative mutation was identified at the molecular level. This was a point mutation that swapped two amino acids ¿ valine for glutamic acid. SCD was also the first genetic disease to be detected by prenatal diagnosis, and in 1979 became the first malady for which gene therapy was attempted.
Friday¿s issue of Science, dated Dec. 14, 2001, tells the tale of this latest advance in an article titled: ¿Correction of sickle cell disease in transgenic mouse models by gene therapy.¿ Its senior author is molecular biologist Philippe Leboulch, of Harvard/MIT, and director of Genetix Pharmaceuticals Inc., all of Cambridge, Mass. (See BioWorld Today, May 11, 2000, and April 20, 2001, both p. 1.)
His novel gene therapy strategy abolished the dreaded signs and symptoms from two overlapping strains of mice, each engineered to mimic SCD¿s stigmata. After trying other viral vectors to deliver healthy gene sequences to the red blood cells and hemoglobin, where SCD arises from mutated genes, Leboulch and his co-authors turned to the AIDS virus.
¿Genetix has made a very substantial advance in the treatment of genetic diseases by gene therapy,¿ Leboulch told BioWorld Today, ¿in this case relating to the long-term correction of sickle cell disease in transgenic mouse models by means of a proprietary lentiviral gene vector developed by Genetix ¿ specifically from HIV proteins. Our therapy transfers an anti-sickling variant of the faulty gene to the bone marrow, where it incorporates itself, with unusually high efficiency, into the stem cells that give rise to the erythrocytes.
¿First,¿ he went on, ¿we designed a beta globin gene containing an amino-acid residue that confers anti-sickling action in another globin ¿ gamma globin. Then, we outfitted a vesicular stomatitis retrovirus with a flap of DNA from the HIV-1 virus, as the gene carrier. This was devoid of any potential that might replicate the entire pathogen.¿
Loaded with its cargo of modified beta globin genes, the improved viral vector quickly took up residence in the blood-forming stem cells of the mice, which had been irradiated to kill off their old bone marrow.
¿In two different models of SCD,¿ Leboulch said, ¿the new gene was rapidly expressed in 99 percent of all circulating red blood cells ¿ preventing sickling and other signs of the disease. our gene therapy caused an eightfold reduction in sickled cells in one model, and elimination of irreversible sickled cells in the other. It also corrected such characteristics of the disease as spleen enlargement, a urine concentration defect and dehydration of red blood cells.
¿The new gene also appeared to be permanently integrated in active form into the mouse genome, persisting for at least 10 months,¿ Leboulch noted, adding, ¿This has not generally been the case in past attempts at gene therapy for SCD and other diseases. These results are particularly striking, given the comparatively large size of the globin gene that must be transferred into the genome to correct this condition, and given that we were transfecting stem cells, which tend not to integrate new genes because they are not undergoing cell division. These research results thus clearly demonstrate the superior capabilities of lentiviral gene vectors for gene therapy involving stem cells.
¿We believe that this preclinical success in two animal models of sickle cell disease,¿ Leboulch said, ¿represents a substantial advancement toward successful human trials of gene therapy for SCD. But obstacles remain to be overcome, before we can apply to the FDA for an IND. To make this therapy safe for human clinical trials, we must continue to perfect the viral delivery for safe, large-scale production. It would also be desirable to devise milder ways to rid the body of existing bone marrow without toxic, life-endangering radiation or chemotherapy.¿
Received Wisdom Challenged: Do Amyloid Plaques Really Cause Alzheimer¿s Disease?
An experienced clinical neurologist can diagnose Alzheimer¿s disease (AD) with 90 percent accuracy, telling it apart from the many other senile dementias for which ¿ unlike AD ¿ therapies are available. The remaining 10 percent can only be identified with certitude by brain autopsy. This alone reveals the telltale hallmarks of AD ¿ the neuritic beta-amyloid plaques clumped around the dying neurons that mark the disease.
However, a paper in the Proceedings of the National Academy of Sciences, dated Dec. 4, 2001, questions whether amyloid plaques are the cause, or merely the benign consequence, of AD. It bears the title: ¿Specific spatial learning deficits become severe with age in b-amyloid precursor protein transgenic mice that harbor diffuse b-amyloid deposits but do not form plaques.¿ The article¿s authors are from the Czech Republic, Finland, Spain and Scios Inc., of Sunnyvale, Calif.
Their results suggest that the formation of beta-amyloid plaques is not responsible for the memory impairment and increasing dementia observed in AD patients. To model the disease, the co-authors genetically engineered a strain of mice to produce the human form of beta-amyloid precursor protein (APP), the molecule from which the plaque peptide is cleaved off, within its brain tissue. Although brain levels of APP were similar to those seen in early AD, the mice failed to develop those plaques.
Even absent those ostensible AD hallmarks, the animals showed significant learning and memory impairments, which worsened as they aged. ¿Since the amyloid plaques do not appear to cause the observed memory deficits,¿ the co-authors conclude, ¿this research will help to direct the quest for the mechanism of memory loss in Alzheimer¿s disease.¿